(echinodermata, stylophora) from the tremadoc of shropshire ... - BioOne

J. Paleont., 74(5), 2000, pp. 890–906 Copyright q 2000, The Paleontological Society 0022-3360/00/0074-890$03.00

A NEW MITRATE (ECHINODERMATA, STYLOPHORA) FROM THE TREMADOC OF SHROPSHIRE (ENGLAND) AND THE ORIGIN OF THE MITROCYSTITIDA BERTRAND LEFEBVRE UMR Domaines oceániques, Sciences de la Terre (paleóntologie), Universite´ de Bretagne Occidentale, 6 avenue Le Gorgeu, BP 809, F-29285 Brest cedex, France. ^[email protected]& ABSTRACT—Plate homologies are identified and discussed in primitive representatives of cornute and mitrate stylophorans. Comparative morphological analysis suggests that: 1) Lagynocystida are digital-bearing mitrates; 2) Peltocystida are glossal-bearing mitrates; 3) in Mitrocystitida, glossal and digital are incorporated into a closed marginal thecal frame or modified into articulated posterior spines (Anomalocystitidae); 4) Ovocarpus? circularis is synonymized with O. moncereti; 5) Chauvelia discoidalis and Mitrocystites riadanensis are both assigned to the genus Aspidocarpus; 6) Mitrocystella barrandei is assigned to the new genus Promitrocystites. The original reconstruction of Vizcainocarpus dentiger proposed by Ruta, (1997a) is modified, as a result of the identification of two additional plates in the posterior portion of the theca. The new species Vizcainocarpus rutai from the Tremadoc (Lower Ordovician) of Shropshire (England) is described and represents the oldest record of mitrocystitidan mitrates. V. rutai differs from V. dentiger in the relatively broader size of its zygal and marginals and in the presence, on the lower thecal surface, of a peripheral fringe of fibrillar stereom. A cladistic analysis of selected stylophoran taxa based on the proposed plate homologies indicates that: 1) cornutes and mitrates are sistergroups, both deriving from a Ceratocystis-like ancestor; 2) Peltocystida and Mitrocystitida are sister-groups; 3) Lagynocystida is sistergroup of (Peltocystida 1 Mitrocystitida); 4) Lobocarpus is not a cornute but a primitive Cambrian mitrate belonging either to the stemgroup of Mitrocystitida or to the stem-group of (Peltocystida 1 Mitrocystitida); 5) Anomalocystitidae represents a family of the suborder Mitrocystitida.

INTRODUCTION

PLATE HOMOLOGY IN STYLOPHORANS

(cornutes and mitrates) are non pentamerous, asymmetrical Paleozoic fossils ranging from the Middle Cambrian to the Upper Carboniferous. Cornutes and mitrates share the same basic anatomical organization. They consist of a single, articulated, highly flexible arm, comparable in morphology to a single crinoid arm, and a flattened theca. Stylophorans have been traditionally considered to be primitive echinoderms, due to the lack of pentamery and the possession of a calcitic skeleton of stereom mesh (Gill and Caster, 1960; Ubaghs, 1975; Chauvel, 1981; Kolata and Jollie, 1982; Parsley, 1988; Sprinkle, 1992; Ruta, 1999d). Another school of thought regards them as primitive chordates (‘‘calcichordates’’) retaining echinoderm-like stereomic plates (Jefferies, 1968; Cripps, 1990; Daley, 1992; Beisswenger, 1994; Ruta and Theron, 1997). The interpretation of these fossils as primitive chordates has been convincingly rejected elsewhere (Ubaghs, 1975, 1981; Philip, 1979; Kolata and Jollie, 1982; Kolata et al., 1991; Peterson, 1995; Parsley, 1997; Lefebvre et al., 1998; Ruta, 1999d). Consequently, stylophorans are here considered to be ‘‘normal’’ echinoderms, following Ubaghs’ interpretation of their morphology (Ubaghs, 1967a). Mitrates are often thought to derive from cornute ancestors (Jefferies and Prokop, 1972; Jefferies, 1973, 1979, 1981, 1986; Derstler, 1979; Cripps, 1989; Cripps and Daley, 1994; Parsley, 1991, 1997; Ruta, 1999d). The aim of this paper is to show through analysis of plate homology that mitrocystitidan mitrates (as other mitrates) do not derive from cornutes, but probably from a Ceratocystis-like ancestor (see also Lefebvre and Vizcaı¨no, 1999). This result is supported by the re-examination of several stylophoran species and by the discovery of a new primitive mitrocystitidan mitrate from the Tremadoc of Shropshire (England). The material examined during the course of this study belongs to the following collections: Institut de Paleóntologie, Museúm National d’Histoire Naturelle, Paris (LP MNHN); Vizcaı¨no (VOMN) and Monceret-Goujon (MG) collections, Carcassonne; Institut de Geólogie, Rennes (IGR); Museúm d’Histoire Naturelle, Nantes (MHNN); The Natural History Museum, London (BMNH).

General anatomical organization in stylophorans.All stylophorans consist of a single articulated arm, or aulacophore, inserted into a flattened theca (see Ubaghs, 1967a, 1981; Parsley, 1988, 1997). The aulacophore is divided into three regions (tripartite organization), referred to as proximal, middle and distal parts, in order of increasing distance from the theca. The proximal part of the aulacophore is broad and highly flexible and consists in most species of a variable number of imbricating, tetramerous rings enclosing a large lumen. This lumen did communicate with the intrathecal cavity. The middle and distal parts of the aulacophore are composed of unpaired elements (stylocone and ossicles, respectively) and of paired cover plates articulated to them. The stylocone is commonly cone-shaped, thus adapting to the variation in width between the proximal region of the aulacophore and its narrower distal portion. The stylocone may result from the fusion of two (or more) distal ossicles (Jefferies, 1973, 1986; Kolata and Jollie, 1982; Parsley, 1991). Numerous structures are present on the internal surface of unpaired elements (stylocone and ossicles), among these, a longitudinal median groove, lateral transverse channels and lateral depressions. In most stylophorans, the cover plates of the middle and distal parts of the aulacophore are paired and each pair is associated to the stylocone and ossicles. The situation is different in some primitive forms (Ceratocystis perneri and Protocystites menevensis) in which the left and right series of cover plates show an alternating pattern, and a variable number of plates are associated to the stylocone and ossicles. This alternating pattern of cover plates and the structures observed on the internal surface of the stylocone and the ossicles strongly support the interpretation of the median and distal portions of the aulacophore as forming together a single brachial process, comparable to a single crinoid arm (Ubaghs, 1961b, 1967a, 1975; Nichols, 1972; Chauvel, 1981; Parsley, 1988, 1997). The longitudinal median groove would correspond to the (unique) ambulacral canal. Transverse channels (lateral ambulacral nerves?) lead to lateral depressions which probably housed podia in life (Ubaghs, 1967a; Nichols, 1972). The mouth is thought to have been internal and to have been located at the proximal end of the longitudinal median groove at, or close to, a deep notch present on

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LEFEBVRE—NEW TREMADOC MITRATE FROM ENGLAND the stylocone of all stylophorans. The orientation of the organism is deduced from the location of the mouth: the ‘‘arm’’ is anterior and the theca is posterior. Other interpretations of the aulacophore as a blastozoan stem (Barrande, 1887; Bather, 1913; Jaekel, 1918; Chauvel, 1941; Gill and Caster, 1960; Philip, 1979; Kolata and Jollie, 1982) or as a chordate-like tail (Jefferies, 1968, 1981, 1986; Cripps, 1989, 1990, 1991; Daley, 1992) have been convincingly refuted elsewhere (Ubaghs, 1975, 1981; Parsley, 1988, 1991) and are not followed here. In stylophorans, the theca is more or less asymmetrical, flattened, with a convex upper surface and a flat to sligthly concave lower surface. It consists of a frame of marginal plates and of upper and lower integumentary areas surrounded by this frame. An opening at the posterior extremity of the theca corresponds to the anus. The anus and its associated structures are variable in morphology; the anus can be surrounded by an anal pyramid, it can be slit-like, or floored by a large anal plate or by toothlike plates or platelets. A remarkable and consistently asymmetrical feature is the association of two plates on the lower surface (Z and M91, see below). These two plates are homologous in all stylophorans and bear on their internal (upper) surface a strong crest, called zygal crest in cornutes and septum in mitrates (Ubaghs, 1967a). Homology of these structures was suggested by Ubaghs (1967a) and demonstrated by Kolata et al. (1991) and Lefebvre et al. (1998). The zygal crest (or septum) runs from the left anterior extremity of the theca (left of the aulacophore insertion) to its right posterior end. The oblique course of the ridge, from near the mouth to near the anus, strongly suggests it was related to the course of the gut in life. In cornutes and mitrates, the two ‘‘zygal’’ plates usually divide the lower thecal surface into two integumentary areas (left and right infracentral areas); a single integumentary area, the supracentral area, is present on the upper surface of the theca. Main differences between cornutes and mitrates relate to the aulacophore (construction and organization of the proximal region, ornamentation on the lower surface of the stylocone and ossicles) and the number, arrangement and extension of thecal plates and integumentary areas. Identification of a system of homologous plates in all stylophorans is consequently necessary before a cladistic or a taxonomic revision is undertaken. Plate terminology in stylophorans.At least five different nomenclatures have been proposed for the marginal plates of stylophorans (see discussion in Lefebvre and Vizcaı¨no, 1999). Four of these systems (Jaekel, 1901; Bather, 1913; Caster, 1952; Jefferies, 1968) are merely descriptive: plates given the same name are not necessarily homologous. The nomenclature proposed by Jefferies and Prokop (1972) assign identical names to marginal plates supposed to be homologous, regardless of their relative position in the thecal frame. Jefferies and Prokop’s system is not adopted here, because of the incorrect mutual orientation of thecal surfaces in cornutes and mitrates following the ‘‘calcichordate theory’’ (see Ubaghs, 1975, 1981; Philip, 1979; Chauvel, 1981; Kolata and Jollie, 1982; Caster, 1983; Parsley, 1988, 1997; Kolata et al., 1991; Lefebvre et al., 1998; Ruta, 1999d). The plate nomenclature system applied in this paper is that proposed by Jaekel (1901), with the important distinction that plates given the same name are regarded as homologous. In this system, the aulacophore insertion is chosen as a landmark; marginal plates located to the right of the aulacophore insertion are labelled as Mn and those to the left of the aulacophore insertion are labelled M9n (where n is a number indicating the position of the marginal plate with respect to the aulacophore insertion, in order of increasing distance from the latter). Basic homologies in stylophorans.The two anteriormost marginals framing the aulacophore insertion on the lower surface of the theca, M1 and M91, possess a thin vertical calcitic

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upfolding, or apophysis, forming a vertical, almost complete wall at the anterior extremity of the theca. Apophyses are present in all stylophorans except two primitive forms, Ceratocystis perneri and Lobocarpus vizcainoi. Both M1 and M91 bear a small cup, or scutula (‘‘pyriform bodies’’ in mitrate internal moulds), lying in upper-lateral position with respect to the apophysis. The anterior branch of the zygal crest is always borne on the internal (upper) surface of M91. Apophyses, scutulae and anterior branch of the zygal crest support the homology of the two most anterior marginals in all stylophorans (see also Ubaghs, 1967a; Lefebvre et al., 1998; Lefebvre and Vizcaı¨no, 1999; Ruta, 1999d). The adoral (or adaulacophoral) plates correspond to the three anteriormost plates framing the aulacophore insertion on the upper surface of the theca. A1 and A91 are in contact with M1 and M91 respectively. A notch, probably corresponding to the hydropore (see Ubaghs, 1967a; Parsley, 1994), often pierces the right adoral A1. The median adoral A0 is sometimes lost in derived forms. In most stylophorans, adorals are clearly distinct from more posterior elements corresponding to supracentral integumentary platelets. The homology of adorals in all stylophorans is, therefore, beyond doubt (see also Ubaghs, 1967a; Lefebvre et al., 1998; Lefebvre and Vizcaı¨no, 1999; Ruta, 1999d). In all stylophorans except for the two primitive forms Ceratocystis perneri and Protocystites menevensis, a strong oblique crest (zygal crest) is borne on the internal surface of two plates. This crest (septum) runs invariably from the left anterior extremity of the theca (left of the aulacophore insertion) towards the right posterior end of the thecal frame, near the anal opening. The most anterior of the two crest-bearing plates is homologous in all stylophorans, as it corresponds to M91 (marginal plate bearing the left apophysis and the left scutula). The posterior zygal plate is invariably sutured with certain thecal plates. It is anteriorly articulated to M91, together these two plates form the ‘‘zygal bar’’ which divides the lower surface into two regions and often separates a left and a right infracentral areas (except in Ceratocystis perneri and such derived forms as Lyricocarpus, Mitrocystella and Anomalocystitidae, all of which have lost their right infracentral area). The posterior zygal plate is always articulated posteriorly to three plates. On the right, it forms a long suture with a narrow marginal plate, mostly represented on the upper thecal surface, but that can extend also on the lower surface. A long suture between the plate in question and the posterior zygal plate can be observed in Ceratocystis, most cornutes (with the exception of advanced Cothurnocystida that tend to lose this narrow marginal plate) and most mitrates (with the exception of Lagynocystis, Chinianocarpos and probably Kirkocystidae). The narrow marginal plate is homologous in all stylophorans and corresponds to M4 (see Lefebvre and Vizcaı¨no, 1999). In addition, the posterior zygal plate is articulated posteriorly to a single process, which corresponds to the glossal of Ceratocystis, cornutes, peltocystidan (see Lefebvre et al., 1998) and mitrocystitidan mitrates (see below). Finally, the posterior zygal plate often forms a short suture with a small marginal, along its left posterior margin. This suture can be observed in Ceratocystis and all cornutes with a ‘‘closed’’ marginal thecal frame (in ‘‘open’’ forms, this plate has been lost, see Lefebvre and Vizcaı¨no, 1999). The small plate corresponds to M5 (following the nomenclature adopted in Lefebvre and Vizcaı¨no, 1999). The posterior zygal plate, always showing the same orientation, the same internal structures and the same contacts with surrounding plates in all stylophorans, is considered to be homologous in these (see also Kolata et al., 1991; Lefebvre et al., 1998) and is called the zygal plate, Z (see Chauvel and Nion, 1977; Lefebvre and Vizcaı¨no, 1999). Z corresponds to M5 of Lefebvre et al. (1998). A left posterior element, present to the left of the anus, is

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JOURNAL OF PALEONTOLOGY, V. 74, NO. 5, 2000

FIGURE 1—Plate homology in primitive stylophorans and cornutes (not to scale). 1, Ceratocystis perneri, Skryje Shales, Middle Cambrian, Bohemia; 2, Protocystites menevensis, St. David’s Series, Middle Cambrian, Wales; 3, Arauricystis primaeva, Saint-Chinian Formation, Lower Arenig, Montagne Noire, France; 4, Galliaecystis lignieresi, Saint-Chinian Formation, Lower Arenig, Montagne Noire, France; 5, Amygdalotheca griffei, Saint-Chinian Formation, Lower Arenig, Montagne Noire, France.

observed in Ceratocystis, most cornutes (with the exception of derived forms such as Phyllocystis and Scotiaecystis), lagynocystidan and mitrocystitidan mitrates. The element in question is the digital; being always posterior to M94, it is considered to be homologous in all stylophorans (see Jefferies, 1969; Lefebvre and Vizcaı¨no, 1999). Plate homology in primitive stylophorans.The morphology of Ceratocystis perneri Jaekel (Figs. 1.1, 2.1) from the Skryje Shales of Bohemia (early Middle Cambrian) is known in great detail thanks to the works by Pompeckj (1896), Jaekel (1901), Ubaghs (1967b), and Jefferies (1969). Ceratocystis is generally considered to be the most primitive stylophoran known so far (Ubaghs, 1967a, 1967b; Jefferies, 1969, 1986; Derstler, 1979; Parsley, 1988, 1997; Cripps, 1991; Lefebvre and Vizcaino, 1999). It represents a key-species for assessing plate homology in stylophorans. In Ceratocystis, both thecal surfaces are composed of large plates and the upper surface is strengthened by a strong triradiate ridge. The aulacophore insertion is framed by six plates; this is a unique condition among stylophorans. The three plates framing the anterior margin of the upper surface correspond to the adorals A1, A0, and A91. This identification is confirmed by the presence of the right adoral orifice (hydropore) on A1 (straddling the A1-M1 suture). On the lower surface, a small additional median marginal plate M0 is inserted between the much larger marginal plates M1 (on the right) and M91 (on the left). The two posterior thecal processes can be identified with confidence as the glossal (on the right) and the digital (on the left), as suggested by Ubaghs (1967b), Jefferies (1969), and Lefebvre and Vizcaı¨no (1999). This identification is supported by the much greater extension of these plates observed in young specimens of Ceratocystis, in which the relative development of the glossal and the digital is comparable with that of most derived cornutes. The large central plate on the lower surface is identified as the zygal plate, Z. A zygal crest has not been observed on the internal surface of M91 and Z. The identification of Z is nevertheless confirmed by the fact that it contacts anteriorly M91, and by the fact that it is posteriorly sutured with three plates: along its right margin, Z forms an elongate suture with a narrow plate mostly developed on the upper surface (M4); Z is articulated posteriorly to the glossal and is also in contact with a small element corresponding to M5. The two marginals located between M4 and M1 correspond to M2 and M3. As in more derived forms (Protocystites, Galliaecystis, Arauricystis), a spinal blade is borne by M3. As in other stylophorans, the digital is sutured with M94. The two marginals located between M94 and

M91 correspond to M92 and M93, whereas the small plate between M94 and M5 can be identified as M95. The huge polyplated anal lobe which appears in the reconstruction provided by Jefferies (1969, fig. 2) does not exist (see Ubaghs, 1967b, fig. 1, pl. 1, and discussion in Lefebvre and Vizcaı¨no, 1999). No right infracentral area is present between the enlarged marginals M1, M2, M3, M4, and Z. Two small separated central plates can be homologized to the left infracentral area of other stylophorans. The anterior small central plate, Ia, is surrounded by M91, M92, and M93; the posterior small central plate, Ip, is surrounded by Z, M93, and M94. The supracentral area is composed of seven enlarged plates. The morphology of Ceratocystis displays an array of cornute and mitrate features and suggests that this primitive form belongs to the stem-group of the group encompassing both orders (see Derstler, 1979; Lefebvre and Vizcaı¨no, 1999). Ceratocystis shares with mitrates the possession of enlarged adoral and marginal plates, an aulacophore insertion cavity delimited by adorals and anterior marginals, the presence of a large zygal plate Z in a central position, the posterior suture of M93 with M94 (absence of M93-M95 suture) and a slit-like anal opening (absence of anal pyramid). Ceratocystis resembles cornutes in the strong asymmetry of the theca, in the presence of protuberances on the lower surface and in the possession of a spinal blade borne by M3. Pore-structures on the upper surface of Ceratocystis are typical of cornutes, but may also be present in Lobocarpus (most primitive representative of mitrates, see Ubaghs, 1998, fig. 2). The ctenoid organ of Lagynocystis can also be interpreted as a modified, internal equivalent of cornute pore-structures. Finally, presence of both glossal and digital is a plesiomorphic condition both in cornutes and in mitrates (see below). Protocystites menevensis Hicks (Fig. 1.2), from the St. David’s Series (late Middle Cambrian) of Wales, is very similar to Ceratocystis in many respects. Despite the very poor preservation of the type material, Jefferies et al. (1987) provided a complete reconstruction. Comparison between Ceratocystis and Protocystites highlights the occurence of homologous skeletal elements. On the lower surface, the small intercalary plate M0, present in Ceratocystis, has been lost in Protocystites, and the aulacophore insertion in the latter is formed by the two apophysis-bearing marginals M1 and M91. The two posterior thecal processes are readily identified as the glossal and the digital. As in Ceratocystis, the right infracentral area is absent in Protocystites and the zygal plate Z is consequenly in contact with the same plates as in Ceratocystis: M91, M1 and M3, anteriorly, and

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FIGURE 2—Plate homology in primitive stylophorans and mitrates (not to scale). 1, Ceratocystis perneri, Skryje Shales, Middle Cambrian, Bohemia; 2, Lobocarpus vizcainoi, Upper Cambrian, Montagne Noire, France; 3, Vizcainocarpus dentiger, Saint-Chinian Formation, Lower Arenig, Montagne Noire, France; 4, Lagynocystis pyramidalis, Sa´rka Formation, Llanvirn, Bohemia; 5, Peltocystis cornuta, Saint-Chinian Formation, Lower Arenig, Montagne Noire, France.

M4, glossal and M5, posteriorly. M4 is almost completely located on the upper surface. As in other stylophorans, the suture between Z and M4 is much longer than that between Z and M5. The digital is sutured to M94, and the two plates inserted between M94 and M91 correspond to M92 and M93. In comparison with Ceratocystis, the M94-Z contact has been lost in Protocystites, whereas M93 is in contact posteriorly with marginals M94 and M95. On the lower surface of the theca, the two large central plates surrounded by M91, Z, M5, M95, M93, and M92 correspond to the small Ia and Ip elements in Ceratocystis. These two plates, separated in Ceratocystis are sutured in Protocystites, forming a left infracentral area; this area is framed by the same marginals in most other stylophorans. On the thecal upper surface of Protocystites, three small adorals frame the aulacophore insertion, and the right adoral orifice (hydropore) is present across the A1M1 suture. Protocystites is here considered as a stem-group cornute (see discussion in Lefebvre and Vizcaı¨no, 1999), as it shows evident cornute features. These include a large anal pyramid, a triple junction between M93, M94, and M95, reduced adorals on the upper surface and the absence of an aulacophore insertion cavity. This taxon exhibits such plesiomorphic characters (lost in more advanced forms) as the presence of a triradiate ridge on its upper surface, the presence of rather large marginals (much narrower in more derived cornutes) and the absence of a right infracentral area. It also lacks such apomorphies of cornutes as the occurence of extensive, polyplated integumentary areas on both thecal surfaces and the difference in size between smaller upper plates (tectals) and larger lower plates (inferolaterals) in the proximal aulacophore. Morphologies of the representatives of three main groups of cornutes, Amygdalothecidae, Hanusiidae and Cothurnocystida can be thought of as deriving from that of Protocystites menevensis (see Lefebvre and Vizcaı¨no, 1999). The main distinction between Amygdalothecida (Amygdalothecidae plus Hanusiidae) and Cothurnocystida consists in the expansion of M4 on the lower surface in the former. In amygdalothecidan cornutes, M4 extends on both thecal faces and consequently Z is in a medially displaced, central position on the lower thecal surface. In cothurnocystidan cornutes, M4 is restricted to the upper surface, in contact with and above Z; consequently, the zygal plate constitutes part of the marginal thecal frame. Another difference concerns the posterior opening of the thecal frame on the lower surface of amygdalothecidan cornutes, resulting from the loss of both M5 and M95. A similar opening of the posterior end of the

thecal frame occured independently in some cothurnocystidan cornutes, the Chauvelicystinae (see discussion in Lefebvre and Vizcaı¨no, 1999). Arauricystis primaeva (Thoral) (Fig. 1.3), from the Saint-Chinian Formation (Lower Arenig) of Montagne Noire (France) is one of the most primitive cothurnocystidan cornutes (Lefebvre and Vizcaı¨no, 1999). The aulacophore insertion is framed by the apophysis-bearing plates M1 and M91 on the lower surface. The bar formed by M91 and Z (zygal bar) divides the lower thecal surface into two infracentral areas. Contrary to the situation observed in more primitive forms (e.g., Ceratocystis, Protocystites), a right infracentral area is present, surrounded by M1, M2, M3, Z, and M91. The zygal plate is in contact anteriorly with M91 and M3, forms an elongate suture along its right edge with M4 (located on the upper thecal surface) and is articulated posteriorly to the glossal and to a small element corresponding to M5. As in Ceratocystis and Protocystites, a spinal blade is borne by M3. The digital is articulated to M94. As in Protocystites, M94 and M95 are both in contact with M93, and the left infracentral area is framed by M91, Z, M5, M95, M93, and M92. Galliaecystis lignieresi Ubaghs (Fig. 1.4), from the Saint-Chinian Formation (Lower Arenig) of Montagne Noire is the most primitive representative of the family Hanusiidae (see Lefebvre and Vizcaı¨no, 1999). The aulacophore insertion is framed by the two apophyse-bearing plates, M1 and M91. The zygal bar is formed by the association of M91 and Z and separates the left and right infracentral areas. The left infracentral area extends to the posterior extremity of the theca, because of the loss of M5 and M95. As in Protocystites, Z forms an elongate suture along its right edge with M4 (extending on both thecal surfaces) and is posteriorly articulated to the glossal. The two plates inserted between M1 and M4 correspond to M2 and M3. The spinal blade is borne by M3. In comparison with Ceratocystis, Protocystites and Arauricystis, and because of the loss of M95, the four plates framing the left side of the theca can be identified as M92, M93, M94, and the digital (see Lefebvre and Vizcaı¨no, 1999). As with other hanusiid cornutes (Hanusia, Reticulocarpos, Prokopicystis), Galliaecystis is characterized by the presence of a posterior bar, formed by the medial expansion of M4 and M94 on the upper surface. Amygdalotheca griffei Ubaghs (Fig. 1.5) from the Saint-Chinian Formation (Lower Arenig) and Nanocarpus dolambii

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Ubaghs from the Landeyran Formation (Lower Arenig) of Montagne Noire are the oldest and most primitive known representatives of the amygdalothecid cornutes (see Lefebvre and Vizcaı¨no, 1999). Their morphology is readily comparable with that of the hanusiid cornutes, except that the amygdalothecids do not carry a posterior bar on the upper surface. Contrary to the situation in Nanocarpus, in which glossal and digital are single plates, glossal and digital are subdivided into serial elements in Amygdalotheca griffei (extra frame plates). Lobocarpus vizcainoi Ubaghs (Figs. 2.2, 3.1) from the ‘‘niveau a` Barroubiocystis’’ (Upper Cambrian) of Montagne Noire, assigned to the? Phyllocystidae (sensu Derstler, 1979) by Ubaghs (1998), is likely to represent a primitive mitrate, probably belonging to the stem-group of mitrocystitidans or to the stem-group of (mitrocystitidans 1 peltocystidans) (see cladistic analysis below). Lobocarpus would, therefore, represent the oldest and only known Cambrian mitrate (see Vizcaı¨no and Lefebvre, 1999). Plate homology allows a direct comparison between Ceratocystis and Lobocarpus to be made. The two plates framing the aulacophore insertion on the lower surface in the latter form can be identified as M1 and M91. As in Ceratocystis, apophyses are absent, which probably represents the primitive condition in stylophorans. The large central plate posterior to M91 can be identified as the zygal plate Z: its position, its internal structures (presence of a strong ridge running on the internal surface of M91 and Z, corresponding to the zygal crest; Ubaghs, 1998) and its contacts with surrounding plates confirm this identification. The zygal plate is sutured along its right margin with M4. As in Ceratocystis, the two marginals located between M1 and M4 correspond to M2 and M3. A small central platelet surrounded by M1, M2, M3, M4, Z, and M91 represents the only element of the right infracentral area. Comparison with the plating pattern in Ceratocystis suggests that the posterior plate sutured to both M4 and Z and lying posterior to these two plates, corresponds to the glossal. The three marginals framing the left side of the theca can be identified as M92, M93, and M94. As in Ceratocystis, M94 is in contact with Z. The two small platelets surrounded by Z, M91, M92, M93, and M94 are likely to be homologous with Ia and Ip in Ceratocystis, because Ia is in contact with M91, M92, and M93, and Ip is in contact with Z, M94, and M93. These two platelets, also observed in Protocystites (Fig. 1.2), form the left infracentral area of Lobocarpus. The small plate articulated to M94 and lying posterior to it can be identified as the digital by comparison with the situation in Ceratocystis. In Lobocarpus, a large central plate surrounded by Z, the glossal, the digital and M94 is likely to correspond to M95, as in Ceratocystis. The morphology of Lobocarpus is thus very similar to that of Ceratocystis. Main differences are the smoothly rounded outline of the theca (loss of protuberances on lower surface and of outward projections of M92, M2, M3, glossal, and digital), the loss of two small elements of the lower surface (M0 and M5), and the delimitation of a left (suture between Ia and Ip) and a right infracentral area (compare Figs. 2.1 and 2.2). As mentioned by Ubaghs (1998) the thecal frame of Lobocarpus is reminiscent of that of such mitrates as Mitrocystites. The morphology of Lobocarpus is much more similar to that of mitrocystitidan mitrates than to that of the ‘‘advanced phyllocystids’’ of Derstler (1979), which correspond to the Amygdalothecida of Lefebvre and Vizcaı¨no (1999). Such symmetrical cornutes as Nanocarpus or Amygdalotheca cannot be related to Lobocarpus in spite of their rather similar outlines. Nanocarpus and Amygdalotheca share with other cornutes the possession of narrow marginals and of large integumentary areas, the loss of an aulacophore insertion cavity delimited by adorals and by anteriormost marginals, the organization of the proximal aulacophore (with tectals smaller than inferolaterals), and the possession of reduced adorals. They also

are characterized by the possession of a posteriorly open thecal frame between glossal and digital, whereas the latter plates are sutured in mitrocystitidan mitrates and Lobocarpus. On the other hand, Lobocarpus shares with mitrocystitidan mitrates the possesion of expanded marginals and adorals, the presence of an aulacophore insertion cavity and the presence of both glossal and digital plates incorporated into the marginal frame. The phyletic position of Lobocarpus among mitrates is nevertheless difficult to assess (see cladistic analysis below). It certainly represents a primitive form, because of the retention of plesiomorphic features such as pore structures on the upper surface (see Ubaghs, 1998), a smooth stylocone, a Z-M94 suture and the lack of apophyses. The proximal region of the aulacophore is poorly known in Lobocarpus, and comparisons with other stylophorans cannot be made. Vizcainocarpus (Fig. 2.3) is one of the most primitive mitrocystitidan mitrates known so far (see cladistic analysis below). Re-examination of the type (and unique) specimen of V. dentiger Ruta from the Saint-Chinian Formation (Lower Arenig) of Montagne Noire and examination of new material from the Tremadoc of Shropshire (described below), indicate that the reconstruction of the posterior part of the theca of V. dentiger as proposed by Ruta (1997a, figs. 2–3) is incorrect (Figs. 10.3, 10.4). A small element is intercalated between his plates D12, n and c whereas his plate b corresponds to two marginals. The morphology of Vizcainocarpus is very similar to that of Lobocarpus. M1 and M91 frame the aulacophore insertion (apophyses are present). The zygal bar, formed as usual by M91 and Z, is interposed between a left and a right infracentral area. As in Lobocarpus, the zygal plate is posteriorly in contact with M4 (elongate suture) and with the glossal (short suture). M2 and M3 frame the right side of the theca, and are located between M1 and M4. The left side of the theca shows the same arrangement of marginals as in Lobocarpus: M92, M93, M94 and digital. Unlike the situation observed in Lobocarpus, M94 is not in contact with Z. Comparison with Lobocarpus also suggests that the small element intercalated between Z, the glossal and the digital certainly corresponds to M95. On the upper surface, two large adorals, A1 and A91, frame the aulacophore insertion. The right adoral orifice is present and straddles the M1-A1 suture (Ruta, 1997a); the median adoral A0 is likely to be represented by a small element intercalated between the postero-median angles of A1 and A91. Lagynocystis pyramidalis (Barrande) (Fig. 2.4) is the only known representative of the suborder Lagynocystida. It has been reported from the Landeyran Formation (Lower Arenig) of Montagne Noire (Ubaghs, 1991), from the Pontyfenni Formation (Upper Arenig) of Wales (Jefferies, 1987), from the Pierre Melie`re and Traveusot formations (Middle Ordovician) of Brittany, France (Chauvel and Nion, 1977; Henry et al., 1997) and from the Sa´rka and Dobrotiva formations (Middle Ordovician) of Bohemia (Barrande, 1887; Jaekel, 1918; Jefferies, 1973). The proximal aulacophore of Lagynocystis is not composed by tetramerous rings and can be compared with that of such primitive stylophorans as Ceratocystis and Protocystites (see Ubaghs, 1967b, p. 16). Two large and elongate marginals, M1 and M91, frame the aulacophore insertion on the lower surface. The zygal plate Z is sutured with the right posterior margin of M91. As in other stylophorans, identification of M91 and Z is based by the presence of the zygal crest on their internal surface (see Jefferies, 1973, fig. 6). Two marginals are present between Z and M1; by comparison with Ceratocystis, these plates probably correspond to M2 and M3. The posterior process borne by M94 is probably homologous to the digital of Ceratocystis, Lobocarpus and cornutes. A single plate present between M94 and M91 corresponds either to M92 or to M93; comparison with Ceratocystis suggests that this marginal is more likely to be M93. A transverse


895

FIGURE 3—Plate homology in primitive mitrates (not to scale). 1, Lobocarpus vizcainoi, Upper Cambrian, Montagne Noire, France; 2, Chinianocarpos thorali, Saint-Chinian Formation, Lower Arenig, Montagne Noire, France; 3, Ovocarpus moncereti, Landeyran Formation, Lower Arenig, Montagne Noire, France; 4, Aspidocarpus riadanensis, Riadan Formation, Caradoc, Brittany, France; 5, Aspidocarpus discoidalis, OuineInirne Formation, Anti-Atlas, Morocco.

articulation was noticed by Jefferies (1973, fig. 5) between M93, M94 and Z; the position of this articulation in Lagynocystis is similar to that of Ip in Ceratocystis. A small plate associated with the anus and articulated to M94 and the digital may correspond to M95. Three large adorals frame the aulacophore insertion on the upper surface. The right adoral orifice (hydropore) is absent in Lagynocystis. As in most other mitrates (e.g., Peltocystis, Kirkocystidae, Ovocarpus, Aspidocarpus, Mitrocystites), the left and right adorals of Lagynocystis present anteriorly a small transverse crest. This crest is reminiscent of the sharp anterior margins of A1 and A91 extending onto the lower thecal surface in Ceratocystis. As in Ceratocystis, the right adoral orifice of mitrates (when present) is always anterior to this transverse crest. In conclusion, the morphology of Lagynocystis resembles that of Ceratocystis in the absence of tetramerous rings in the proximal aulacophore and in the absence of infracentral areas. Comparison with Ceratocystis suggests that four plates have been lost in Lagynocystis: one on the anterior part of the left margin (probably M92) and three in the right posterior part of the theca (M4, glossal, and M5). Peltocystis cornuta Thoral (Fig. 2.5), from the Saint-Chinian Formation (Lower Arenig) of Montagne Noire is the most primitive member of the suborder Peltocystida. It was described in great detail by Thoral (1935), Ubaghs (1969), and Jefferies (1986). The two apophysis-bearing marginals M1 and M91 framing the aulacophore insertion on the lower surface are particularly large. A strong zygal crest occupies the internal surfaces of M91 and of the large plate lying posterior to it, which is consequently identified as the zygal plate, Z (see Lefebvre et al., 1998). Comparison with Ceratocystis suggests that the process articulated posteriorly to Z is homologous to the glossal. On the right side of the theca, the three plates inserted between Z and M1 correspond to M2, M3, and M4 (notice the elongate suture between Z and M4). The right infracentral area of Peltocystis is reduced to a single platelet surrounded by M1, M91, Z, M3, and M2 (see Lefebvre et al., 1998). On the left side of the zygal bar, a second central platelet corresponds to the left infracentral area. Again, comparison with Ceratocystis suggests that this small plate corresponds to the Ip platelet. Assuming this identification is correct, the two marginals on the left side of the theca may be identified as M93 and M94. As in Ceratocystis, Ip is surrounded by Z, M94, and M93. As in Ceratocystis, Lobocarpus, and Lagynocystis, M94 is sutured to Z. The subanal plate, posteriorly articulated to both Z and M94, could represent a highly modified

marginal M95, as suggested by its location and its close association with the anal opening (as in Ceratocystis and Lagynocystis). Plate homology suggests that three plates have been lost in Peltocystis: one marginal (probably M92) on the left side of the theca, and two plates posteriorly (M5 and digital). On the upper surface, the aulacophore insertion is framed by two expanded adorals A1 and A91. The median adoral A0 and the right adoral orifice are probably secondarily lost. Plate homology in mitrocystitidan mitrates.Chinianocarpos thorali Ubaghs (Fig. 3.2) is a primitive mitrocystitidan mitrate from the Saint-Chinian Formation (Lower Arenig) of Montagne Noire. Its morphology is well-known through the works of Ubaghs (1961a, 1969) and Jefferies (1986). Plate homology shows that Chinianocarpos is a close relative of Lobocarpus and Vizcainocarpus (see cladistic analysis below). The two anterior, apophysis-bearing marginals can be identified as M1 and M91. The large plate lying posterior to M91, contributing to part of the right margin of the theca and bearing the posterior portion of the zygal crest on its internal surface corresponds to the zygal plate, Z. Comparison with Lobocarpus and Vizcainocarpus shows that both the right infracentral area and the marginal M4 have been lost in Chinianocarpos. The small plate articulated posteriorly to Z corresponds to the glossal, and the two marginals between Z and M1 can be identified as M2 and M3. As in Lobocarpus and Vizcainocarpus, the left side of the theca is framed by the same plates, M92, M93, M94, and digital. A large polyplated infracentral area is present on the lower surface of theca, surrounded by Z, M91, M92, M93, and M94, as in Lobocarpus, Vizcainocarpus, and most stylophorans. A large plate (subanal plate), closely associated to M94 and digital, is present at the posterior end of the left infracentral area. Comparison with Lobocarpus and Vizcainocarpus suggests that the subanal plate of Chinianocarpos corresponds to M95. Small platelets separate M95 from the glossal and from Z. On the upper surface, the aulacophore insertion is framed by the two enlarged adorals A1 (showing the right adoral orifice) and A91. The median adoral A0 corresponds to a plate intercalated between the postero-median angles A1 and A91. The morphology of Chinianocarpos is, in some respects, more primitive than that of Vizcainocarpus. The polyplated aspect of the proximal aulacophore of Chinianocarpos is reminiscent of the situation in Ceratocystis (Vizcainocarpus shows ‘‘typical’’ mitrocystitidan proximal rings; see Ruta, 1997a, fig. 2). Chinianocarpos shares with Lobocarpus the possession of an expanded M95. This plate is comparatively

896


FIGURE 4—Plating diagrams of lower surfaces of Ovocarpus moncereti, Landeyran Formation, Lower Arenig. All specimens from Les Rocs Ne`gres, Landeyran valley, Cessenon (He´rault, southern France). 1, MG-IV; 2, LP MNHN R55 404; 3, VOMN 2502. Scale bar 5 1 cm.

smaller in Vizcainocarpus. Chinianocarpos apparently shares with Mitrocystites the presence of paripores on its lower surface. Such pores are, nevertheless, absent in Vizcainocarpus, Ovocarpus, and Aspidocarpus and do not open on the surface of homologous plates in Chinianocarpos (M3 and M92) and Mitrocystites (M2 and M92). The supposed homology between the paripores of Chinianocarpos and those of Mitrocystites is therefore questioned. In addition, the supposed homology of the subanal plates in Chinianocarpos, Peltocystis, and Kirkocystidae (Derstler, 1979; Jefferies, 1986; Parsley, 1997) is refuted in the light of plate homology (see also Ubaghs, 1969, p. 86). Subanal plates correspond to homologous elements (M95) in Chinianocarpos and Peltocystis, but have been developed independently in the two lineages. In Chinianocarpos, the subanal plate opens in central position on the lower surface because of the posterior closure of the frame (glossal-digital suture); in Peltocystis, the subanal plate is posterior to the marginal thecal frame (it is posterior to Z and M94), possibly because of the expansion of adorals on the upper surface. In kirkocystids, the corresponding plate (M95) is either vestigial (Balanocystites) or lost (Anatifopsis), and the subanal plate is formed by a highly modified element from the left infracentral area.

Ovocarpus Ubaghs (Fig. 3.3), from the Arenig of Montagne Noire, corresponds to primitive mitrocystitidan mitrates with a marginal frame composed of eleven plates (one more than in Lobocarpus and Vizcainocarpus). Re-examination of Ubaghs’ original material and study of well-preserved new specimens suggest that O? circularis should be synonymised with O. moncereti (Figs. 3.3, 4, 6.1, 6.2). Supposed differences in shape between the two forms (Ubaghs, 1994) result from deformation of the fossils, which are preserved in shales. Both forms have the same number of marginal plates; re-examination of the type material of O? circularis shows that the alleged suture between M6 and M96 (Ubaghs, 1994, fig. 7) corresponds to a fracture. Comparison with Lobocarpus and Vizcainocarpus indicates that the additional marginal element of Ovocarpus corresponds to M95, inserted between the glossal and the digital. As in Lobocarpus and Vizcainocarpus, Z is articulated to M91 anteriorly, and to M4, glossal and M95, posteriorly. As in Vizcainocarpus, M95 is in contact with glossal, Z, the left infracentral area and digital. Ovocarpus shows lateripores on its lower surface, at the M1-M2 and the M91-M92 sutures. Lateripores might be equivalent to the peripheral grooves described in Chinianocarpos (Jefferies, 1986) and Vizcainocarpus (Ruta, 1997a).

FIGURE 5—Plating diagrams of Aspidocarpus riadanensis, Riadan Formation, Caradoc, western France. 1, Holotype IGR 15207, lower surface, from Riadan, near Poligne´ (Ille et Vilaine); 2, MHNN L4T4, upper surface, from Andouille´ (Mayenne); 3, IGR 15210, lower surface, from Riadan, near Poligne´ (Ille et Vilaine). Scale bar 5 1 cm.


897

FIGURE 6—Mitrocystitidan mitrates. 1–2, Ovocarpus moncereti, Landeyran Formation, Lower Arenig, Les Rocs Ne`gres, Landeyran valley, Cessenon (He´rault, southern France); 1, LP MNHN R55 404, lower surface, 33; 2, LP MNHN R55 402, upper surface, 33. 3–5, Aspidocarpus riadanensis, Riadan Formation, Caradoc, western France; 3, holotype IGR 15207, lower surface, from Riadan, near Poligne´ (Ille et Vilaine), 32; 4, MHNN L4T4, lower surface, from Andouille´ (Mayenne), 32; 5, IGR 15210, lower surface, from Riadan, near Poligne´ (Ille et Vilaine), 32.

The genus Aspidocarpus was erected by Ubaghs (1979) for a mitrocystitidan mitrate from the Letna´ Formation (Caradoc) of Bohemia, A. bohemicus (Fig. 7). The diagnosis of the genus is here emended so as to include all mitrocystitidans with lateripores, a thecal frame made of twelve marginals and two infracentral areas composed of a variable number of platelets. Chauvelia discoidalis Cripps (Fig. 3.5) from the Ouine-Inirne Formation (Llandeilo) of Morocco and Mitrocystites riadanensis Chauvel (Figs. 3.4, 5, 6.3–6.5) from the Riadan Formation (Caradoc) of Brittany, France, are therefore assigned to the genus Aspidocarpus. The origin and position of an additional posterior marginal element in Aspidocarpus can be determined in the light of the ontogenetic sequence documented by Ubaghs (1979, fig. 2) in A. bohemicus. The smallest and, therefore, ontogenetically youngest specimen of A. bohemicus differs from larger individuals in possessing eleven marginals (as in Ovocarpus), instead of twelve. Examination of the posterior extremity of the theca shows that the additional element, PP1, corresponds to the posteriorly rounded plate which develops between growth stages 1 and 2, to the right of M95 (Fig. 7.1–7.2). M95 can be easily identified in all ontogenetic stages by its small posterior spike.

Marginal PP1 cannot originate from the right or the left infracentral areas, which are extremely reduced in early growth stages and do not extend further posteriorly. The origin of this plate remains unclear (reminiscence of M5?). The zygal plate Z is sutured posteriorly to four plates in Aspidocarpus, M4, glossal, PP1 and M95 (Fig. 7.1–7.4), as opposed to three in Lobocarpus, Vizcainocarpus, and Ovocarpus. The posterior expansion of the right infracentral area causes the loss of the Z-M4 contact in adult forms (growth stage 5, Fig. 7.5; see also A. discoidalis and A. riadanensis, Fig. 3.4–3.5). The posterior expansion of the left infracentral area causes the loss of the Z-M94 suture (primitive condition, as in Lobocarpus) in most adult forms, with the single exception of growth stage 4, probably due to individual variation (Fig. 7.4). The morphology of Mitrocystites mitra Barrande (Fig. 8.1), from the Sa´rka and Dobrotiva formations (Middle Ordovician) of Bohemia, is well known through the contributions of Barrande (1887), Jaekel (1901, 1918), Chauvel (1941), Ubaghs (1967a), Jefferies (1968), and Parsley (1994). It is characterized by the presence of lateripores and paripores, a thecal frame realized by thirteen marginals, and infracentral areas composed of

FIGURE 7—Growth stages of Aspidocarpus bohemicus, Letna´ Formation, Lower Caradoc, Bohemia, redrawn after Ubaghs (1979, fig. 2), lower surfaces, 36. Note that eleven marginals are present in the youngest specimen (growth stage I), instead of twelve in other specimens (growth stages II to V). The arrow indicates the location of the additional marginal element PP1. Note the expansion of infracentral areas towards the posterior extremity of the theca during growth.

898


FIGURE 8—Plate homology in mitrocystitidan mitrates (not to scale). 1, Mitrocystites mitra, Sa´rka Formation, Llanvirn, Bohemia; 2, Promitrocystites barrandei, Sa´rka Formation, Llanvirn, Bohemia; 3, Mitrocystella incipiens, Traveusot Formation, Llandeilo, Brittany, France; 4, Eumitrocystella savilli, Ouine-Inirne Formation, Llandeilo, Morocco; 5, Barrandeocarpus jaekeli, Letna´ Formation, Lower Caradoc, Bohemia; 6, Ateleocystites guttenbergensis, Guttenberg Formation, Champlainian, Upper Mississipi Valley Region, USA.

a reduced and constant number of enlarged platelets (one on the right infracentral area, three on the left infracentral area). Mitrocystites differs from Aspidocarpus mainly in possessing standardized infracentral areas and an additional element PP2, incorporated into the marginal frame. Comparison with the situation in Aspidocarpus shows that PP2 corresponds to the small plate intercalated between M95 and PP1, and is likely to originate from the left infracentral area. In Mitrocystites, the insertion of PP2 into the posterior margin of the thecal frame results in the loss of the Z-M95 contact, so that the zygal plate is sutured posteriorly to three plates: M4, glossal and PP1. (Remarks: A certain variability has been observed in the number of elements of both infracentral areas in M. mitra. A form described as M. osekensis is characterized by a right infracentral area composed of two platelets (Chauvel, 1941); in M. lata, the left infracentral area exhibits a small additional element (Jaekel, 1901, 1918; Chauvel, 1941). As suggested by Ubaghs (1967a, p. 520), these forms should be synonymised with M. mitra.) ‘‘Mitrocystella’’ barrandei Jaekel (Fig. 8.2) co-occurs with Mitrocystites mitra in the Middle Ordovician of Bohemia (Sa´rka and Dobrotiva´ formations). The most relevant works on ‘‘M.’’ barrandei are those of Jaekel (1901, 1918), Chauvel (1941), and Ubaghs (1967a). The morphology of ‘‘Mitrocystella’’ barrandei is reminiscent to that of Mitrocystites in the occcurence of the same number of marginal elements, but differs in the morphology of the infracentral areas. The right infracentral area as well as the most anterior platelet of the left infracentral area are lost in ‘‘M.’’ barrandei. Lateripores and paripores are also lost (although they might be vestigial in some cases; see Chauvel, 1941; Parsley, 1994). Faint cuesta-shaped ribs are developed on A1 and A91. The right adoral orifice is present. Examination of several Bohemian specimens indicates the existence of individuals whose morphology is intermediate between that of Mitrocystites mitra and that of ‘‘Mitrocystella’’ barrandei; some of them show a left infracentral area composed of three platelets but the right infracentral area is absent (see also Barrande, 1887, pl. 4, fig. 22). Mitrocystella incipiens (Barrande) (Fig. 8.3) from the Sa´rka and Dobrotiva formations (Middle Ordovician) of Bohemia, the Guindo Shales (Llandeilo) of Spain, the Valongo Formation (Llandeilo) of Portugal and the Traveusot Formation (Llandeilo) of Brittany, France, is together with Mitrocystites mitra one of the best known mitrocystitidan mitrates (see Barrande, 1887; Chauvel, 1941, 1981; Jefferies, 1968, 1986; Guttie´rrez-Marco and Meleńdez, 1987; Parsley, 1994). This species is most similar

to ‘‘M.’’ barrandei; differences concern the loss of the digital from the left posterior corner of the theca and a stronger ornament of cuesta-shaped ribs on A1 and A91. Loss of the digital is an important morphological distinction between ‘‘M.’’ barrandei and M. incipiens that justifies the assignment of ‘‘M.’’ barrandei to the new genus Promitrocystites. Results of the cladistic analysis also suggest to distinguish these two forms so as to avoid a possible paraphyly of the genus Mitrocystella (see cladistic below). Eumitrocystella savilli Beisswenger (Fig. 8.4), from the Ouine-Inirne Formation (Llandeilo) of Morocco, is a close relative of Mitrocystella. In comparison with Mitrocystella, two plates have been lost in Eumitrocystella: the more posterior of the two infracentral platelets of the left infracentral area and the posterior plate PP2. The resulting morphology is somewhat similar to that of the poorly known Paranacystis petrii Caster from the Lower Devonian of Brazil (see Caster, 1954; Caster and Eaton, 1956; Ubaghs, 1967a; Ruta, 1999b). The genus Barrandeocarpus Ubaghs (Fig. 8.5) has been described from the Caradoc of Bohemia (Ubaghs, 1979) and Shropshire, England (Ruta, 1997b), as well as from the Ashgill of Norway (Craske and Jefferies, 1989) and perhaps Morocco (see Chauvel, 1971; Ruta, 1997b). Morphology of the lower surface of Barrandeocarpus is most similar to that of Promitrocystites and Mitrocystella, with two left infracentral platelets, no right infracentral area and a large central zygal plate Z. Barrandeocarpus possesses eleven marginal plates, as opposed to thirteen and twelve in Promitrocystites and Mitrocystella respectively. Plate homology reveals that the two plates lost in Barrandeocarpus are glossal and digital. Another fundamental difference concerns the upper surface and requires comparisons with other mitrocystitids. The upper surface of Mitrocystites is composed of two fundamental types of integumentary platelets arranged in two sets of rows: 1) anterior rows consisting of subhexagonal elements arranged regularly and lying posterior to the adorals; 2) posterior rows of posteriorly rounded, overlapping elements forming an irregular, unorganized arrangement and lying anterior to the anal opening. The two types of platelets occupy areas of comparable extension in Mitrocystites. In Promitrocystites, we observe only two or three anterior rows of regularly arranged platelets. In Mitrocystella, only a single row of platelets is observed. In Eumitrocystella, the adorals are comparatively much smaller and the whole upper surface is paved by few large overlapping elements. The situation is clearly different in Barrandeocarpus, in

LEFEBVRE—NEW TREMADOC MITRATE FROM ENGLAND which the upper surface is composed of few large, subhexagonal elements organized in four to five approximatively transverse rows of plates (see Ubaghs, 1979, fig. 2; Craske and Jefferies, 1989, fig. 11). Some overlapping elements are perhaps present near the anus. One of the most remarkable features of the upper surface of Barrandeocarpus is the presence of a small rounded element, the placocystid plate, in contact with the most anterior row of platelets. Ateleocystites guttenbergensis Kolata and Jollie (Fig. 8.6), from the Middle Ordovician (Champlainian) of the upper Mississipi Valley region (Illinois, Missouri and Wisconsin), is one of the oldest and most primitive known anomalocystitid mitrates. Its upper surface shows three adorals, a placocystid plate and four rows of organized, subhexagonal platelets and is in most respects similar to that of Barrandeocarpus. The morphology of the lower surface of Ateleocystites strongly resembles those of Promitrocystites, Mitrocystella, and Barrandeocarpus. As in Barrandeocarpus, the thecal frame consists of eleven plates. Plate homology indicates that these marginals correspond to those of Barrandeocarpus. In Ateleocystites, as in all other Anomalocystitidae, two plates modified into posterior processes are articulated to the thecal frame, and lie posterior to M4 and M94. Comparison with Promitrocystites and Barrandeocarpus shows that these two plates correspond to the glossal and the digital (see also Kirk, 1911; Ubaghs, 1967a; Jefferies and Lewis, 1978; Parsley, 1991, 1997). These two plates, incorporated into the marginal frame in more primitive mitrocystitidan mitrates, are convergently similar to those observed in many cornutes, in which they are likewise elongate and join M4 and M94, on each side of the anus. Consequences of plate homology.Two main scenarios have been proposed for the evolution of the Stylophora and for the relationships between the two orders Cornuta and Mitrata. In the first scenario (Chauvel, 1941; Gill and Caster, 1960; Ubaghs, 1967a, 1967b), cornutes and mitrates represent monophyletic taxa of unclear affinities, sharing an unknown common ancestor. More recently, following the hypothesis of Jefferies (1968), a second scenario has been proposed; mitrates are thought to derive from symmetrical cornute ancestors, close to Phyllocystis (Jefferies, 1968, 1969), Amygdalotheca or Reticulocarpos (Jefferies and Prokop, 1972; Jefferies, 1973, 1979, 1981, 1986; Ubaghs, 1975, 1991, 1994; Derstler, 1979; Parsley, 1988, 1991, 1997, 1998; Cripps, 1988, 1989, 1991; Woods and Jefferies, 1992; Daley, 1992; Cripps and Daley, 1994; Ruta, 1999d). Stylophorans are characterized by a general evolutionary trend towards acquisition of bilateral symmetry. Plate homology shows that nearly symmetrical thecal outlines have evolved independently in different stylophoran groups (at family to subfamily level) and derived from originally distinct architectures (see discussion in Lefebvre and Vizcaı¨no, 1999). For example, the two cornutes Lyricocarpus and Prokopicystis are rather similar in outline and both characterized by long, narrow, nearly symmetrical thecae; however, plate homology shows that these forms belong to two distinct groups of stylophorans. Lyricocarpus belongs to the Chauvelicystinae and derives from asymmetrical forms close to Chauvelicystis, whereas Prokopicystis is a hanusiid cornute which derives from asymmetrical forms close to Galliaecystis (see Lefebvre and Vizcaı¨no, 1999). Therefore, the degree of symmetry/asymmetry of stylophoran thecae thus represents a highly convergent character, and should be used only at family or subfamily level; the definition of natural groups can only rely on the identification of homologous elements. Plate homology shows that cornutes and mitrates are monophyletic assemblages and that both derive from a Ceratocystislike ancestor (see also Lefebvre and Vizcaı¨no, 1999). Cornutes and mitrates are sister-groups (see cladistic analysis below).

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Morphologies of the most primitive representatives of each stylophoran order or suborder are invariably very similar to that of Ceratocystis. Identification of homologous elements in all these forms confirms the existence of ‘‘traditional’’ subdivisions within stylophorans (Cornuta, Mitrata) and within mitrates (Lagynocystida, Peltocystida, Mitrocystitida) and strengthens the boundaries between these subdivisions in the light of character analysis (contrary to Parsley, 1997 and Lefebvre et al., 1998). In cornutes, the Z-M94 contact is lost and M93 is posteriorly sutured with both M94 and M95; an anal pyramid is present; the adorals are extremly reduced in size; the marginals are narrower elements framing large polyplated integumentary areas; pore structures are present; protuberances on the lower surface and a spinal blade are often retained; the aulacophore is not inserted into a cavity defined by adorals, M1 and M91; in the proximal aulacophore, the upper elements (tectals) are much smaller than the lower elements (inferolaterals). In mitrates, we observe an aulacophore insertion cavity, enlarged adoral and marginal plates, a Z-M94 contact, whereas lower protuberances and spinal blade are lost. Lagynocystida are characterized by the absence of both infracentral areas, the retention of a primitive, unorganized proximal aulacophore, the presence of the ctenoid organ (modified pore structure?) and the loss of four marginal elements (M92, M4, M5, and glossal). In Peltocystida, the adorals are particularly enlarged, three marginal plates are lost (M92, M5, and digital), the right infracentral area is present but the left infracentral area may be reduced to Ip. In mitrocystitidan mitrates, glossal and digital are incorporated into the thecal frame, the right infracentral area is present, the left infracentral area results from the joining of Ia and Ip, and new elements are sometimes inserted between glossal and digital (PP1, PP2). A well-defined morphocline can be established within mitrocystitidan mitrates in the light of the identification of homologous elements (see also Derstler, 1979; Jefferies, 1986; Craske and Jefferies, 1989; Cripps, 1990; Beisswenger, 1994; Parsley, 1997; Ruta, 1997a). Mitrocystitidan mitrates derive from a Lobocarpus-like ancestor. The following evolutionary steps can be recognized: 1) loss of the Z-M94 contact in Vizcainocarpus; 2) insertion of M95 between glossal and digital in Ovocarpus; 3) insertion of an additional platelet PP1 between M95 and the glossal, in Aspidocarpus; 4) insertion of a left infracentral platelet PP2 between PP1 and M95 and regularization of the number of infracentral platelets, in Mitrocystites; 5) left infracentral area composed of two plates and loss of the right infracentral area, in Promitrocystites. From a Promitrocystites-like ancestor, evolution of the mitrocystitidans may have proceeded along two distinct paths: 1) upper surface consisting of a progressive larger number of overlapping elements and loss of marginal plates from the left posterior thecal border (digital in Mitrocystella, followed by PP2 and the posterior infracentral platelet in Eumitrocystella); 2) upper surface formed by subhexagonal elements organized in rows, with differentiation of the ‘‘placocystid’’ plate; glossal and digital are excluded from the thecal frame and are either lost (Barrandeocarpus), or transformed into posterior spines (Anomalocystitidae). An independent origin of the anomalocystitids from a Reticulocarpos-like ancestor (see for example Ubaghs, 1967a; Parsley, 1988, 1991) is rebutted in the light of plate homology arguments. The anomalocystitids share with some more primitive mitrocystitidan mitrates (Mitrocystella, Barrandeocarpus) the possession of two additional posterior platelets (PP1, PP2), the organization of the lower surface (large central plate Z; loss of the right infracentral area; left infracentral area composed of two platelets) and ornamented adorals. The transformation of both glossal and digital into posterior processes articulated to M4 and

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M94, on each side of the anus, is a convergent character found also in Cothurnocystida. Consequently, ‘‘anomalocystitid’’ and ‘‘mitrocystitid’’ mitrates (as defined by Caster, 1952) do not form two distinct groups derived from different ancestors, but represent a monophyletic assemblage, with ‘‘anomalocystitids’’ deriving from ‘‘mitrocystitids’’ (see also Chauvel, 1941; Gill and Caster, 1960; Jefferies and Lewis, 1978; Derstler, 1979; Jefferies, 1984, 1986, 1991; Craske and Jefferies, 1989; Cripps, 1990; Beisswenger, 1994; Parsley, 1994, 1997, 1998; Ruta, 1997a, 1999a, 1999c; Ruta and Theron, 1997). ‘‘Mitrocystitid’’ mitrates (as defined by Caster, 1952) therefore form a paraphyletic group (see also Craske and Jefferies, 1989; Parsley, 1997; Ruta and Theron, 1997; Ruta, 1997a). The suborder Anomalocystitida (Caster, 1952) is here considered as a family Anomalocystitidae of the suborder Mitrocystitida. SYSTEMATIC PALEONTOLOGY

Phylum ECHINODERMATA Bruguie`res, 1791 Class STYLOPHORA Gill and Caster, 1960 Order MITRATA Jaekel, 1918 Emended diagnosis.Stylophoran echinoderms with asymmetrical to nearly bilaterally symmetrical theca; lower thecal surface plane or slightly concave; upper thecal surface invariably convex; both thecal surfaces covered by relatively large plates; lower surface divided into two by a very large strut (‘‘septum complex’’) resulting from the joining of M91 and Z; zygal crest (septum) present on the internal surface of the strut; zygal plate Z typically in central position; infracentral areas reduced and composed of few large plates, or absent; when present, infracentrals relatively broad and reduced in number; absence of knobs or protuberances on lower surfaces of marginals; no spinal process; a single plate, M94, posteriorly to M93; glossal and/ or digital present as exothecal processes or incorporated into marginal frame; broad adorals covering a more or less large area on anterior part of upper surface; right adoral orifice sometimes present and continuing into a slit-like opening; one or two pairs of pores sometimes present on lower surface of the theca; anus opening above an anal plate, a rake-like anal pyramide or a slit; aulacophore inserted in a cavity delimited by adorals and by marginals M1 and M91; proximal aulacophore generally with upper plates (tectals) subequal to lower ones (inferolaterals); longitudinal median groove, on upper surface of stylocone and ossicles, with transverse channels and lateral depressions very shallow; cover plates not imbricating in extended position of aulacophore; inferior surface of stylocone and ossicles usually carrying knobs or spines. Discussion.Parsley (1997, 1998) erected the new order Ankyroida to include most derived cornutes and all mitrates (see also Ruta, 1999d). The concept of the Ankyroida is not followed here, as Cornuta and Mitrata are sister-groups and mitrates do not derive from cornutes but from a Ceratocystis-like ancestor (see Lefebvre and Vizcaı¨no, 1999). In the tree proposed by Parsley (1997, 1998), certain characters are obvious convergences, as for example the degree of symmetry of the animals (Parsley’s characters 4, 5, 18, 41); as argued above, acquisition of symmetrical outlines occurs several times independently in Stylophora (see Lefebvre and Vizcaı¨no, 1999). Some of Parsley’s homologies are, in my opinion, erroneous: thus the septum is homologous to the zygal crest (Parsley’s characters 8, 9, 10, 11, 12) and always borne by the two same plates: M91 and Z (see Lefebvre et al., 1998); glossal and digital (Parsley’s characters 13, 14, 19, 20) are borne by homologous plates in stylophorans (see Lefebvre and Vizcaı¨no, 1999) and are often incorporated into the marginal frame; Diamphidiocystis and Lagynocystisare both characterized by a single exothecal process (digital); plate

homology shows that the glossal has been lost independently in these two taxa, that Lagynocystis is a primitive mitrate deriving from a Ceratocystis-like ancestor, and that Diamphidiocystis is a highly derived anomalocystitid (glossal and digital are both present as posterior spines in Diamphidiocystis sp. from Brittany); spines are developed independently in cornutes and mitrates on the lower surface of the stylocone (Parsley’s character 1); as argued above, subanal plates in Chinianocarpos, Peltocystis, and Anatifopsis are not homologous (Parsley’s character 29). Some of Parsley’s characters are highly disputable such as the ‘‘normal length’’ of the aulacophore (Parsley’s character 24). There is evidence that the length of the aulacophore often reflects preservational bias as pointed out by Lefebvre et al., (1998). Suborder MITROCYSTITIDA Caster, 1952 Emended diagnosis.A suborder of mitrates with asymmetrical to nearly bilaterally symmetrical theca; additional PP1 and PP2 elements sometimes present at the posterior extremity of the thecal frame; plate Z in central position; glossal and digital incorporated into the thecal frame or modified into posterior spines; no pore-structure; right adoral orifice often present; one or two pairs of pores sometimes present on lower surface; anus opening above an anal plate, a rake-like anal pyramid or a slit; median adoral A0 usually posterior to A1 and A91. Discussion.As argued above, the suborder Mitrocystitida is here considered to be a monophyletic group. The suborder Anomalocystitida Caster, 1952, is here regarded as a family of the Mitrocystitida, which therefore constitutes a broader group than that originally proposed by Caster (1952) and Ubaghs (1967a). Genus VIZCAINOCARPUS Ruta, 1997a Type-species.Vizcainocarpus dentiger Ruta, 1997a. Emended diagnosis.A genus of mitrocystitid mitrates with large left and right polyplated infracentral areas and without PP1 and PP2 elements; M95 not inserted into marginal frame; Z in central position and posteriorly articulated to M4, M95, and the glossal; marginal frame composed of ten plates; posteriorly pointing spike on some supracentral platelets; skeletal stereom structure highly porous; right adoral orifice present; rake-like anal opening at the posterior end of the upper surface, roofed over by few broad platelets; ridges of fibrillar stereom on the stylocone blades and ossicles. Discussion.Re-examination and production of new latex casts of the type (and unique) specimen of Vizcainocarpus dentiger from the Lower Arenig of Montagne Noire clarifies the morphology of the poorly preserved left posterior margin of the theca, wrongly interpreted by Ruta (1997a). Unlike Ruta’s reconstruction (1997a, fig. 2), two more plates can be identified in this region, because his marginals b and n encompass, in fact, four distinct elements: M93, M94, M95, and the digital (Fig. 10.3, 10.4). VIZCAINOCARPUS RUTAI new species Figures 9, 10.1, 10.2, 11 Diagnosis.Oblong, sub-oval theca with broad marginals showing a finely fibrillar peripheral fringe but no denticulations on their outer margin. Description.Both specimens are disarticulated, showing slightly displaced plates (Figs. 9, 11). Reconstruction of the complete theca was obtained by making successive camera lucida drawings of each single plate. Drawings of the plates were scanned in and re-assembled on computer, following their articulation surfaces and respective positions in the fossils. The resulting morphology (Fig. 10.1) is somewhat more elongate and oval than the general outline of the disrupted holotype. The other


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FIGURE 9—Plating diagrams of Vizcainocarpus rutai, from Coundmoor Brook, near Bull Hill Cottage, Harnage Grange, Cressage (‘‘Arenaceous Beds,’’ Tremadoc, Shropshire, England). 1–2, Holotype, BMNH E63475; 1, lower surface; 2, upper surface. 3–4, BMNH E63471; 3, lower surface; 4, upper surface. Scale bar 5 5 mm.

specimen is slightly larger and more disarticulated than the holotype, and the anterior portion of its theca is missing (marginals M91, M1, and M2). Both specimens are smaller than the single known specimen of Vizcainocarpus dentiger from Montagne Noire, with an estimated thecal length of less than 4 mm for a width of less than 3 mm. The lower surface of the theca is nearly flat, except at the level of the anterior margin of M1 and M91, where the lower surface is slightly curved downwards. The upper surface is not preserved, but the holotype shows a few supracentral platelets and an isolated element that may correspond to an adoral. The upper surface was probably gently convex, as suggested by the fact that the flexure of the marginals is less pronounced towards the posterior extremity of the theca. Therefore, the height of the theca is likely to have decreased regularly from its anterior margin (aulacophore insertion) towards the anal opening. The aulacophore insertion is in a deep cavity (visible

in plane view on upper surface) framed by M1 and M91, on the lower surface, and probably by the adorals, on the upper surface. The lower surface skeleton is composed of ten marginals, two ‘‘central’’ plates (Z and M95) and two polyplated infracentral areas. All marginal plates show a fibrillar fringe near their outer margins. No denticulations have been observed on the anterior border of M1 and M91. The stereom of the marginals is highly porous. The anterior margin of the theca is entirely formed by M1 and M91, both possessing well developed apophyses. M1 and M91 are subequal in size, with M91 slightly larger, owing to the presence of a small posterior expansion in contact with Z (anterior portion of the zygal bar). All marginals are divided into a larger, lower part and a smaller part, folded towards the upper surface. The upfolded portion diminishes in height posteriorly. Marginals M2, M3, and M4 are robust, subrectangular in shape and subequal in size, with M3 slightly smaller than M2 and M4.

FIGURE 10—Comparison of the two described species of the genus Vizcainocarpus (lower surface). Scale bar 5 5 mm. 1–2, Vizcainocarpus rutai, ‘‘Arenaceous Beds,’’ Tremadoc, Shropshire, England; 1, reconstruction of V. rutai; 2, identification of the plates in V. rutai. 3–4, Vizcainocarpus dentiger, Saint-Chinian Formation, Lower Arenig, Montagne Noire, France; 3, uncomplete reconstruction of V. dentiger, after Ruta (1997a, figs. 2–3); 4, modified reconstruction of V. dentiger. Note that two plates have been omitted in the previous reconstruction of Ruta (1997a).

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FIGURE 11—Vizcainocarpus rutai, from Coundmoor Brook, near Bull Hill Cottage, Harnage Grange, Cressage (‘‘Arenaceous Beds,’’ Tremadoc, Shropshire, England), 310. 1–2, Holotype, BMNH E63475; 1, lower surface; 2, upper surface; 3, Specimen BMNH E63471, lower surface.

The medial margin of M4 is sutured for most of its length with Z. The next marginal (glossal) is smaller than the other right marginals and tapers distally in a direction parallel to its suture with the digital. As in most other stylophorans, the Z-glossal suture is much shorter than the Z-M4 suture. On the left border of the theca, M92 is a long and narrow plate. The next two posterior marginals, M93 and M94, are much smaller and closely associated. M93 extends mostly on the upper surface and is partly overlapped along its right margin by M94. On the contrary, M94 extends only in part on the upper surface and contributes little to the thecal frame. The thecal frame is closed posteriorly by a large plate (digital) with a rounded posterior margin. As in V. dentiger, the digital floored the anus, which opened just above it at the posterior end of the theca. The zygal plate Z occupies a central position on the lower thecal surface. With the posterior expansion of M91, it forms the zygal bar (‘‘septum complex’’). The bar separates two polyplated infracentral areas. The left infracentral area is much larger than the right infracentral area. The zygal plate is narrow, elongate and widens posteriorly where it contacts M4, M95, and glossal. A strong ridge is present on the upper (inner) surface of Z and M91; the ridge corresponds to the zygal crest (septum). M95 is a small element inserted between Z, glossal and digital. The upper surface of the theca is poorly known in both specimens. In the holotype, few supracentrals are preserved. As in V. dentiger, these elements show a strong, posteriorly pointing spike. A large isolated plate, at the right anterior edge of BMNH E63475 may correspond to the right adoral A1. The anus has not been observed. As in V. dentiger, it likely opened at the posterior extremity of the upper surface. The proximal aulacophore is not preserved in V. rutai. An incomplete stylocone is present in the holotype. It shows a strong anterior spine, with fibrillar external ornament. A second, posterior blade might have been present, but this portion of the stylocone has not been preserved. A single ossicle is also preserved in the holotype. It also shows a strong spine with fibrillar ornamentation. Etymology.After Dr. Marcello Ruta, who described the type-species of the genus and for his important contribution to the knowledge of mitrates. Types.Holotype, specimen BMNH E63475a, b, showing the external moulds of the upper and lower thecal surface, as well as the stylocone and one isolated ossicle (Figs. 9.1–9.2, 11.1– 11.2). Other material examined.A second, less complete specimen (BMNH E63471, Figs. 9.3–9.4, 11.3). Occurrence.—‘‘Arenaceous Beds’’ (Tremadoc), in the vicinity of Coundmoor Brook, near Bull Hill Cottage, Harnage Grange, Cressage, Shropshire, England (see map in Daley, 1992,

fig. 2). The two specimens were found in association with brachiopods (Eurytreta and lingulids), a rich trilobite fauna (see Fortey and Owens, 1991) and other stylophorans, such as Prochauvelicystis semispinosa (Daley, 1992), an undescribed cornute close to the genus Chauvelicystis (M. Martı´ Mus personal commun.) and a kirkocystid mitrate close to Anatifopsis trapeziiformis. Discussion.Vizcainocarpus rutai is very similar to the type species of the genus, V. dentiger, from the Lower Arenig of Montagne Noire (Southern France). The two species apart from slight differences in age (Tremadoc/Arenig) and size, also show small morphological differences. Marginals in V. rutai are comparatively much broader than those in V. dentiger. The ornament is different in both cases, with marginals showing a fibrillar fringe near their outer margins in V. rutai. This fringe has not been observed in V. dentiger (a similar fringe, described in Ovocarpus moncereti, is not observed in all specimens, because of differences in preservation of the various specimens; see Ubaghs, 1994, p. 22). Denticulations on the anterior margins of M1 and M91, present in V. dentiger, have not been observed in V. rutai, although this portion of the theca is poorly preserved in the latter species (BMNH E63475). Finally, marginals M1 and M91 are relatively wider and Z is much narrower in V. dentiger, than in V. rutai. CLADISTIC ANALYSIS

Phylogenetic analysis.Numerous cladistic analyses of mitrates (and particularly mitrocystitid mitrates) have been published recently (Derstler, 1979; Jefferies, 1979, 1984, 1986; Craske and Jefferies, 1989; Cripps, 1990; Beisswenger, 1994; Ruta, 1997a, 1999a, 1999c; Ruta and Theron, 1997; Parsley, 1997, 1998). Most authors regard cornutes as a paraphyletic assemblage and mitrates as a monophyletic group deriving from ‘‘advanced’’ symmetrical cornutes. According to Derstler (1979), mitrates are polyphyletic and derive from two distinct lineages of cornutes (see also Parsley, 1991). The aims of the present analysis are to show that mitrates and cornutes are sistergroups, both deriving from a Ceratocystis-like ancestor (see Lefebvre and Vizcaı¨no, 1999); to investigate the phylogenetic relationships among primitive mitrocystitid mitrates; and to show that the anomalocystitid mitrates derive from forms traditionally included in the suborder Mitrocystitida Caster, 1952. The choice of the eighteen taxa retained for the present analysis was motivated by these three main objectives. Primitive stylophorans such as Ceratocystis and Protocystites, as well as basal taxa belonging to the three main groups of cornutes (Amygdalothecidae, Hanusiidae, and Cothurnocystida) and to the two other suborders of mitrates (Lagynocystida, Peltocystida), are included in the present study to test the hypothesis of a cornute-mitrate

LEFEBVRE—NEW TREMADOC MITRATE FROM ENGLAND sister-group relationship and to determine the phylogenetic position of the Mitrocystitida among stylophorans. In order to investigate the phyletic relationships among primitive mitrocystitids (Mitrocystitida sensu Caster, 1952; Ubaghs, 1967a), the study has been undertaken at genus level (intrageneric differences are here irrelevant). Consequently, only type-species of each genus of primitive mitrocystitids have been considered. Finally, in order to assess the phylogenetic position of the anomalocystitid mitrates, a primitive representative of this group has also been included. Ceratocystis perneri (Figs. 1.1, 2.1) has been chosen as outgroup (see also Parsley, 1997). According to most authors, this form represents the most primitive stylophoran known (Derstler, 1979; Jefferies, 1986; Cripps, 1991; Daley, 1992; Parsley, 1997; Lefebvre and Vizcaı¨no, 1999). The primitive stylophoran Protocystites menevensis (Fig. 1.2; a stem-group cornute, for Lefebvre and Vizcaı¨no, 1999) and cornutes are introduced in the analysis, so as to test the supposed paraphyly of cornutes. The most primitive species known of each major cornute group have been included in the study (see discussion in Lefebvre and Vizcaı¨no, 1999): 1) Amygdalotheca griffei (Fig. 1.5) is the type-species, oldest and most primitive described member of the family Amygdalothecidae; 2) Galliaecystis lignieresi (Fig. 1.4) is the most primitive known representative of the family Hanusiidae; and 3) Arauricystis primaeva (Fig. 1.3) is one of the most primitive known cothurnocystidan cornutes. Type-species of the two other mitrate lineages have been retained in the present study, Lagynocystis pyramidalis (Lagynocystida, Fig. 2.4) and Peltocystis cornuta (Peltocystida, Fig. 2.5). Lobocarpus vizcainoi represents a primitive stylophoran considered either as a close relative of ‘‘advanced phyllocystids’’ (Ubaghs, 1998) or as a primitive mitrate (Vizcaı¨no and Lefebvre, 1999; see above and discussion of the cladistic analysis). As far as mitrocystitid mitrates are concerned, type-species of each genus of primitive forms (mitrocystitids sensu Caster, 1952) have been included in the present analysis: Vizcainocarpus dentiger (Figs. 2.3, 10.3, 10.4), Chinianocarpos thorali (Fig. 3.2), Ovocarpus moncereti (Figs. 3.3, 6.1, 6.2), Aspidocarpus bohemicus (Fig. 7), Mitrocystites mitra (Fig. 8.1), Promitrocystites barrandei (Fig. 8.2), Mitrocystella incipiens (Fig. 8.3), Eumitrocystella savilli (Fig. 8.4), and Barrandeocarpus jaekeli (Fig. 8.5). Finally, Ateleocystites guttenbergensis (Fig. 8.6), representing one of the oldest and most primitive anomalocystitid mitrates known so far, has also been included. The number of characters considered in the present study (22) is lower than that in other cladistic analyses of mitrates (38 in Ruta and Theron, 1997, 42 in Parsley, 1997, 76 in Ruta, 1997a, 106 in Ruta, 1999c). Choice of the characters is mainly justified by the focus (primitive mitrocystitids) and level (generic level) of this study; only characters of primary taxonomic significance have been used. Consequently, all characters dealing with autapomorphies in cornutes or in mitrate suborders other than mitrocystitids have been omitted (e.g., Mc plate of Cothurnocystidae, M4-M94 bar on the upper surface of Hanusiidae, etc. . . ). All characters used in the present analysis are discrete and based on the recognition of plate homologies in stylophorans, as discussed above. Continuous characters (e.g., those relative to the size of the theca, length of the aulacophore, etc. . . ) as well as characters based on the general outline of the theca have been purposely excluded because identification of plate homologies shows that the general trend towards acquisition of symmetrical thecae occurs convergently in several stylophoran groups (see discussion above). The morphological data are based on direct observations of

903

the specimens when possible (Ceratocystis, Galliaecystis, Arauricystis, Lagynocystis, Peltocystis, Lobocarpus, Vizcainocarpus, Mitrocystites, Promitrocystites, Mitrocystella, Eumitrocystella). These data have been complemented by information gleaned from relevant papers by Ubaghs (1961a, 1969, 1979), Kolata and Jollie (1982), Jefferies et al. (1987) and Beisswenger (1994). The twenty-two characters (see appendix). concern: 1) presence of a triradiate ridge on the upper thecal surface; 2) expansion of adoral plates; 3) ornamentation on the adorals; 4) glossal; 5) digital; presence of 6) a spinal process and 7) knobs on the lower surface of marginal plates; 8) width of marginal plates and their extension on the lower surface; 9) periproct; 10) M95; 11) M5; 12) posterior opening of the marginal frame; 13) organization of the proximal aulacophore; 14) occurence of a proximal cavity for the aulacophore insertion; 15) presence and nature of respiratory structures; 16) Z-M94 suture; 17) presence of PP1 and 18) PP2 elements; 19) left infracentral area, 20) organization of supracentrals on the upper surface; 21) right infracentral area; and 22) presence of pores (paripores, lateripores) on the lower surface. The cladistic analysis was performed using PAUP 3.1.1. (Swofford, 1993), under the ACCTRAN optimization. All characters are unordered and unweighted (see appendix). A branchand-bound search found twenty shortest trees (59 steps) with a consistency index of 0.627. Differences between these twenty shortest trees are small and concern the respective positions of Promitrocystites, Mitrocystella, and Eumitrocystella, and those of Lobocarpus and Peltocystis. The strict consensus of these twenty trees is presented in Fig. 12 and discussed below. Discussion.As far as the respective phylogenetic positions of cornutes and mitrates are concerned, the results of the present cladistic analysis confirm those obtained by Lefebvre and Vizcı¨no (1999), but differ from those of most previous workers, according to whom mitrates derive from paraphyletic cornutes. As mentioned by Cripps (1991), this is probably the result of the choice of mitrates as terminal units included in the outgroups in cladistic analyses of cornutes. Examination of the consensustree (Fig. 12) shows that mitrates and cornutes are monophyletic sister-groups, deriving from a Ceratocystis-like ancestor. Mitrates do not derive from ‘‘symmetrical’’ Reticulocarpos-like cornutes, for two main reasons: 1) stratigraphically, Reticulocarpos and other ‘‘symmetrical’’ cornutes (Beryllia, Prokopicystis) are Middle Ordovician in age, whereas mitrates are recorded from the Upper Cambrian (Lobocarpus) and the Lower Ordovician; 2) morphologically, mitrates do not possess a bar uniting M4 and M94 on the upper surface (as in Reticulocarpos or Prokopicystis), and show three expanded adoral plates instead of two small ones, broad marginal plates, a large zygal plate in connection with marginal plates of the frame, a closed marginal frame, a proximal aulacophore with subequal tectals and inferolaterals, an aulacophore insertion cavity framed by adorals and anterior marginals. Symmetric outlines developed in mitrates and some highly derived cornutes are the result of a general trend in stylophorans and of convergent evolution in distinct lineages. Other important results of the analysis concern the respective positions of the three sub-orders of the Mitrata: the Peltocystida as sister-group of the Mitrocystitida; the Lagynocystida as sistergroup of the (Peltocystida 1 Mitrocystitida). These results are in good agreement with those proposed by calcichordate workers (see Jefferies, 1986). Lobocarpus is reinterpreted as a primitive mitrate (but see Ubaghs, 1998), that either belongs to the stemgroup of the Mitrocystitida or to the stem-group of (Mitrocystitida 1 Peltocystida). The cladistic analysis presented here shows that primitive mitrocystitids (‘‘mitrocystitids’’ sensu Caster, 1952) form a morphological series from Vizcainocarpus and Chinianocarpos to

904


FIGURE 12—Cladistic analysis of stylophoran echinoderms. Strict consensus of the eight shortest trees.

Anomalocystitidae (see also Craske and Jefferies, 1989; Cripps, 1990; Ruta, 1997a; Ruta and Theron, 1997; Parsley, 1997, 1998). This series is characterized mainly by the appearance of additional PP elements in the posterior part of the frame, with a regular increase in the number of marginals from ten to thirteen (from Lobocarpus to Mitrocystites) and by the bilateralization of the theca, involving loss or transformation of plates at its posterior extremity (all taxa more derived than Mitrocystites). The present study shows that the anomalocystitid mitrates derive from primitive mitrocystitids, which is in agreement with the conclusions reached by several previous workers (Chauvel, 1941; Gill and Caster, 1960; Jefferies and Lewis, 1978; Derstler, 1979; Jefferies, 1984, 1986, 1991; Craske and Jefferies, 1989; Cripps, 1990; Kolata et al., 1991; Beisswenger, 1994; Parsley, 1997, 1998; Ruta, 1997a, 1999a, 1999c; Ruta and Theron, 1997). Therefore, the suborder Mitrocystitida as defined by Caster (1952) is paraphyletic; taxa assigned to the suborder Anomalocystitida are here regarded as forming a family in the suborder Mitrocystitida. As redefined here, the suborder Mitrocystitida forms a monophyletic group. The cladistic analysis indicates that anomalocystitids derive from forms close to Promitrocystites, Mitrocystella, or Eumitrocystella, as already suggested by Craske and Jefferies (1989), Beisswenger (1994), Ruta (1997a, 1999a, 1999c), Ruta and Theron (1997), and Parsley (1997, 1998). Comparative morphological analysis suggests that anomalocystitids would rather derive from Promitrocystites (see discussion above). This result is mainly a consequence of the nature and organization of the upper surface in Mitrocystella and Eumitrocystella on the one hand, and Barrandeocarpus and Anomalocystitidae on the other hand. The genus Barrandeocarpus is sister-group of the Anomalocystitidae. ACKNOWLEDGMENTS

This paper is a contribution of the UMR CNRS 6538 ‘‘Domaines oceániques.’’ The author is particularly grateful to P. Rachebœuf and D. Vizcaı¨no for critical comments on early stages of the manuscript, M. Ruta, D. R. Kolata, and T. Hazen for reviewing the completed manuscript and making many helpful suggestions, O. Otero for her precious help in the undertaking

of the cladistic analysis, N. Podevigne for the photographs, P. Daley, and R. A. Fortey for important discussions on the stylophoran material collected in the Tremadoc of Shropshire, E. & S. Monceret and J.-P. Kundura for collecting and lending essential material from Montagne Noire, and R. P. S. Jefferies, J. Plaine, and S. Regnault for access to important specimens. The reconstruction of Vizcainocarpus rutai was mostly carried out at The Natural History Museum, London, during a six-months stay supported by a British Council Grant. REFERENCES

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1 11111 11112 22 12345 67890 12345 67890 12

Taxa Ceratocystis perneri Protocystites menevensis Amygdalotheca griffei Galliaecystis lignieresi Arauricystis primaeva Lagynocystis pyramidalis Peltocystis cornuta Lobocarpus vizcainoi Chinianocarpos thorali

00000 00000 00000 01000 00011 00010 11011 11117 1111? 11011 00117 1111? 11000 00111 00110 10120 11002 10001 10102 11023 10202 ?0?11 110?0 10?00 10111 11022 10002

00000 10000 10010 10010 10010 00030 00000 00000 10010

00 00 10 10 10 00 10 10 01

Vizcainocarpus dentiger 10111 11004 10202 10010 10 Ovocarpus moncereti 10111 11004 10202 10010 11 Aspidocarpus bohemicus 10111 11005 10202 11010 11 Mitrocystites mitra 10111 11005 10202 11120 11 Promitrocystites barrandei 10211 11005 10202 11120 02 Mitrocystella incipiens 10212 11005 10202 11120 02 Eumitrocystella savilli 10112 11003 10202 11020 02 Barrandeocarpus jaekeli 10222 11006 10202 11121 02 Ateleocystites guttenbergensis 10200 11006 10202 11121 02 List of characters used in the phylogenetic analysis. The characterstates are indicated by numerals in brackets (?, missing information). 1. Presence (0) or absence (1) of a triradiate ridge on the upper surface. 2. Adoral plates large and expanding on the upper thecal surface (0) or small and restricted to the thecal frame (1). 3. Adoral plates smooth (0) or with a crest (1) and cuesta-shaped ribs (2). 4. Glossal as external process (0), incorporated into marginal frame (1) or lost (2). 5. Digital as external process (0), incorporated into marginal frame (1) or lost (2). 6. Presence (0) or absence (1) of a ‘spinal’ process, on the right side of the theca. 7. Presence (0) or absence (1) of protuberances on the lower surface of marginal plates. 8. Large marginal plates expanding on the lower surface (0) or narrow marginals forming a narrow thecal frame (1). 9. Anus opening in a slit or a slit bordered by toothlike platelets (0), in an anal pyramid or an anal lobe (1), or floored by an anal plate (2) whatever its nature (M95 or infracentral platelet). 10. M95 sutured to M94, Z, glossal and digital (0), to M94 and M93 (1), to M94 and digital (2), to M94 and Z (3), to Z, glossal and digital (4), to digital only (5), to M94 only (6) or absent (7). 11. Presence (0) or absence (1) of M5. 12. Marginal frame posteriorly closed (0) or open (1). 13. Proximal aulacophore unorganized (0), or organized with tectals much smaller than (1) or subequal to (2) inferolaterals. 14. Presence (0) or absence (1) of a cavity (provided by M1, M91 and adoral plates) for the aulacophore insertion. 15. Respiratory structures present on the upper surface (0), or on the inside of the upper part of the anterior portion of the thecal cavity (1) or absent (2). 16. Zygal plate Z articulated to M94 (0) or not (1). 17. Absence (0) or presence (1) of a PP1 element. 18. Absence (0) or presence (1) of a PP2 element. 19. Left infracentral area composed of Ia and/or Ip (0), numerous unorganized platelets (1), few central elements in fixed number (2) or absent (3). 20. Upper thecal surface unorganized (0) or supracentrals organized in rows, with a placocystid plate (1). 21. Absence (0) or presence (1) of the right infracentral area. 22. Pores on lower thecal surface originally absent (0), present (1), vestigial or lost (2).