a new late silurian (pridolian) naraoiid - CiteSeerX

J. Paleont., 78(6), 2004, pp. 1138–1145 Copyright q 2004, The Paleontological Society 0022-3360/04/0078-1138$03.00

A NEW LATE SILURIAN (PRIDOLIAN) NARAOIID (EUARTHROPODA: NEKTASPIDA) FROM THE BERTIE FORMATION OF SOUTHERN ONTARIO, CANADA—DELAYED FALLOUT FROM THE CAMBRIAN EXPLOSION JEAN-BERNARD CARON,1 DAVID M. RUDKIN,2

AND

STUART MILLIKEN3

Department of Zoology, Ramsay Wright Zoological Laboratories, University of Toronto, Ontario M5S 3G5, Canada, ,[email protected]., 2 Department of Palaeobiology, Royal Ontario Museum, Toronto M5S 2C6, and 3148 East Avenue, Brantford, Ontario N3S 3M4

1

ABSTRACT—The discovery of a new naraoiid nektaspid in the Upper Silurian (Pridolian) of southeastern Ontario significantly extends the range of this unusual group. Nektaspids are nonmineralized arthropods typical of Early and Middle Cambrian soft-bottom communities, but were thought to have become extinct in the Late Ordovician. The unique holotype specimen of Naraoia bertiensis n. sp. comes from a Konservat–Lagersta¨tte deposit renowned for its eurypterid fauna (the Williamsville Member of the Bertie Formation). Naraoia bertiensis lacks thoracic segments and is morphologically similar to Naraoia compacta from the Middle Cambrian Burgess Shale, save for the presence of a long ventral cephalic doublure and a subtly pointed posterior shield. To examine the phylogenetic relationships of the new naraoiid, we coded characters of the holotype specimen and of nine previously described nektaspids. The results confirm a sister taxon relationship between Naraoia compacta and Naraoia bertiensis and the monophyly of nektaspid forms lacking thoracic segments (family Naraoiidae). This latter group may have arisen from an ancestral segment-bearing form through heterochronic loss of thoracic segments early in the Cambrian. The disjunct occurrence of a naraoiid nektaspid in the Late Silurian resembles the reappearance of other ‘‘Lazarus taxa’’ that were thought to have been eliminated during mass extinction events. The naraoiid lineage survived the Late Ordovician biotic crisis, but in this case the ‘‘Lazarus effect’’ seems likely to be taphonomic in origin.

INTRODUCTION

Liwiidae and Naraoiidae, which together include fewer than 10 species of noncalcified, Cambro–Ordovician trilobitomorph arthropods (Fig. 1.1), have recently been assigned to a resurrected order Nektaspida (Budd, 1999). Nektaspids, along with helmetiids and tegopeltids, comprise a small group of early Palaeozoic euarthropods lacking calcified exoskeletons that figure prominently in discussions on the origin and composition of the class Trilobita (Ramsko¨ld and Edgecombe, 1991). In particular, the Naraoiidae, as previously conceived, has been variously regarded as sister group to the calcified Trilobita (Briggs et al., 1992), as representing a higher level taxon (‘‘order uncertain’’) within the Trilobita (Fortey, 1997), or as a more restricted trilobitomorph group excluded from Trilobita 1 a helmetiid-tegopeltid clade. Because the exoskeleton of adult nektaspids consists of two main parts, a cephalic shield and a posterior shield of similar length separated by variable numbers (0–6) of thoracic segments, some authors have argued that one way of potentially resolving nektaspid phylogeny would be to explore heterochronic variations in thoracic segment release during ontogeny (Fortey and Theron, 1994). Based upon the presence or absence of an articulated thorax in the nektaspids, two opposing phylogenetic scenarios have been proposed (Fortey and Theron, 1994): the absence of thoracic segments could be a derived condition within the nektaspids and other arthropods (Briggs and Fortey, 1992) (Fig. 1.2), or, the absence of thoracic segments could be primitive (Fig. 1.3). In the first scenario, the stratigraphical order of appearance of nektaspid genera is not fully congruent with the phylogenetic hypothesis; the articulated segment-bearing forms are both Cambrian and Ordovician in age, but the non-segment-bearing forms are limited to the Cambrian. The second scenario, in which the absence of thoracic segments is primitive, implies a secondary expression of thoracic segments in a descendant of a form that lost the articulated thorax by a process of terminal progenesis or neoteny (rather than hypermorphosis, even though both peramorphic and paedomorphic processes are possible, see Budd, 1999). This second scenario is less parsimonious and the stratigraphical argument is weakened by the presence of thorax-bearing forms in the Early Cambrian (Fortey and Theron, 1994). This paper has three main goals: to describe the first known

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HE FAMILIES

post–Ordovician nektaspid, based on a single specimen recovered from the Upper Silurian (Pridolian) Bertie Formation (Williamsville Member) of southern Ontario; to analyze its phylogenetic relationships; and to make some inferences concerning the mode of segment release in the nektaspids. GEOLOGICAL AND PALEOENVIRONMENTAL SETTING

The Bertie Formation in southern Ontario consists of about 17 m of dark brown-to-buff, finely laminated, argillaceous dolostones with abundant bituminous partings (Fig. 2) (Johnson et al., 1992; Brett et al., 1998). The sequence represents deposition in a range of environments from brackish estuarine, through shallow evaporitic and sabkha settings, to near-normal marine lagoonal and subtidal conditions. The development of these marginal environments is associated with restricted circulation across a broad carbonate platform (Algonquin Arch) on the northern flank of the Appalachian Basin during the Late Silurian (Brett et al., 1998). The Williamsville Member is a thin (1.25–1.5 m) unit of massive, buff grey, planar-laminated dolostone (‘‘waterlime’’) that contains elements of the renowned Bertie eurypterid fauna (Clarke and Ruedemann, 1912; Braddy, 2001). In addition to at least four species of eurypterids, the unit also yields phyllocarid crustaceans (Copeland and Bolton, 1985), xiphosurans, nautiloid cephalopods, ostracodes, chlorophyte algae, and cooksoniid rhyniophytes. The Williamsville Member was likely deposited in an estuarine setting, where brackish conditions resulted from flushing of freshwater streams into shallow hypersaline lagoons. The often superb preservation of the unbiomineralized biota in this Konservat–Lagersta¨tte can be attributed to rapid burial in exceptionally fine carbonate muds, and exclusion of benthic scavengers and bioturbators by virtue of a combination of periodically elevated salinities and bottom anoxia (Kluessendorf and Mikulic, 1991; Brett et al., 1998). SYSTEMATIC PALEONTOLOGY

Order NEKTASPIDA Raymond, 1920 Family NARAOIDAE Walcott, 1912 Genus NARAOIA Walcott, 1912 NARAOIA BERTIENSIS new species Figure 3 Diagnosis.A species of Naraoia characterized by cephalic and posterior shields of subequal length (sagittal), ventral cephalic

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CARON ET AL.—A NEW SILURIAN NARAOIID FROM CANADA

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FIGURE 1—Hypotheses of relationships of the nektaspids based on the condition of segment release. 1, Stratigraphical order of appearance of nektaspids. 2, Segments are plesiomophic. 3, Segments are apomorphic. Modified from Fortey and Theron, 1994.

doublure approximately 25 percent length (sagittal) of cephalic shield, and posterior shield with slightly pointed rear margin. Description.Articulated cephalic and posterior shield preserved in dorsal aspect and low relief (Fig. 3 and see reconstruction Fig. 4). No traces of appendages or soft internal anatomy are preserved. Dorsal surface of carapace with faint linear wrinkles, parallel to and concentrated around the margins of both shields (see Discussion). External surface bears no features that can be interpreted as elements of original prosopon and the cuticle was probably smooth. Cephalic shield suboval in plan view with rounded posterolateral (genal) angles. Posterior margin of cephalic shield almost

FIGURE 2—Geological setting of southern Ontario, with standard stadial nomenclature of the Bertie Formation modified from Haynes and Parkins, 1992. The site of the discovery is emphasized by a star symbol (Ridgemount Quarries Ltd.).

straight (transverse) overlapping anterior margin of the posterior shield. Cephalic shield is 20 mm in length (sagittal). Maximum width (transverse), just anterior of the midlength, of 18 mm. Maximum height of cephalic shield about 4 mm in the midaxial region. Height decreases regularly and concentrically from the midaxial region to the margins. Marginal rim thin (approximately twice the cuticular thickness), as demonstrated by the very faint impressions of the mutually overlapping shields. Cephalic doublure about 5 mm long (approximately 25 percent of the length of the dorsal cephalic shield) at the midline. Inner edge of doublure roughly parallel to margin of the shield to about midlength, then narrows gradually toward posterolateral angle. Exposed doublure convex ventrally. Original cross-sectional shape (sagittal and transverse) of the entire shield roughly lenticular. Presence of sediment in the narrow space between dorsal exoskeleton and doublure. Posterior shield slightly shorter (17 mm in sagittal length) and narrower (17 mm in transverse width) than the cephalic shield. Anterior margin of the posterior shield impressed beneath the posterior part of the cephalic shield and is nearly straight (transverse). Lateral margins parallel to sagittal plane and curving posteromedially to a slightly pointed termination. Posterior shield comparatively less convex axially in cross section. Possible ventral doublure reflected by faint linear depression parallel to margins. Left anterolateral corner with a short angular projection, but this is not nearly as well shown on the right side. Etymology.The name is based on the source of the holotype and only known specimen, the Upper Silurian Bertie Formation in Ontario. Type.Holotype and only known specimen, Royal Ontario Museum, Toronto, Department of Palaeobiology (Invertebrate Section), ROM 56013 (part and counterpart). Occurrence.Williamsville Member of the Bertie Formation (Upper Silurian, Pridolian), Ridgemount Quarries Ltd. (Walker Industries), Ridgemount, Ontario, Canada. Discussion.The effect of minor compaction has been to imprint the margins of the two shields where they overlap mutually along the articulation, as in Naraoia compacta (Whittington, 1977). The natural convexity of the shield has been little altered by compaction of enclosing sediment, resulting in a minimal degree of deformation and associated wrinkling. The wrinkles resemble the ‘‘concentric crush marks’’ on Buenaspis forteyi (in Budd, 1999). Wrinkles are more numerous on the cephalic shield, especially along the region of articulation. The left lateral region of the cephalic shield is more wrinkled than the right, due to the

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TABLE 1—Data matrix of 12 characters from different nektaspid taxa (see character list in the text). 0 5 plesiomorphic state, 1 5 apomorphic state, ? 5 unknown. Character 2 is unknown or plesiomorphic for all taxa (?) except Olenoides where the lateral eyes are dorsal. Character 6 is polymorphic for N. compacta (genal spines present or absent). Agnostus and Olenoides are designed as outgroups (OG) (see text for details). Characters Taxa

1

2

3

4

5

6

7

8

9 10 11 12

N. bertiensis n. sp. N. compacta N. spinifer N. spinosa Misszhouia Buenaspis Liwia Soomaspis Tarricoia Agnostus (OG) Olenoides (OG)

0 0 0 0 0 0 0 0 0 1 1

? 0 ? 0 0 ? ? ? ? 0 1

0 0 0 0 0 0 0 0 0 1 1

1 0 0 0 0 0 0 1 1 0 0

0 0 1 0 0/1 1 0 1 0 0 1 0 0 0 0 1 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0 1 0

1 1 1 1 1 1 1 1 1 0 0

1 1 1 1 1 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 1 1

0 0 0 0 0 1 ? 1 ? 0 1

0 0 0 0 0 1 0 1 1 0 0

slightly oblique orientation of the exoskeleton with respect to the plane of exposure. PHYLOGENETIC ANALYSIS

Ingroup.The following nektaspids are included in this study: Naraoia compacta Walcott 1912, known from the Middle Cambrian Burgess Shale (Whittington, 1977), the Early to Middle Cambrian of Idaho and Utah (Robison, 1984), and from the Early Cambrian Emu Bay Shale in Australia (Nedin, 1999); N. spinifer Walcott, 1931, from the Middle Cambrian Burgess Shale (Whittington, 1977); N. spinosa Zhang and Hou, 1985 and Misszhouia longicaudata (Chen et al., 1997), from the Lower Cambrian Chengjiang biota of China; Liwia sp. (L. convexa and L. plana) Dzik and Lendzion, 1988, from the Lower Cambrian of Poland; Buenaspis forteyi Budd, 1999, from the Lower Cambrian Sirius Passet Fauna; Tarricoia arrusensis Hammann et al., 1990, from the Lower Ordovician ‘‘Puddinga’’ sequence of Sardinia; and Soomaspis splendida Fortey and Theron, 1994, from the Late Ordovician Soom Shale Formation of South Africa. Maritimella rara Repina and Okuneva, 1969 and Orientella rotundata Repina and Okuneva, 1969, from the Cambrian of Russia, are excluded from this study because they are too poorly known (interpreted as possible mudflakes in Robison, 1984, p. 2). Outgroups.Two outgroups are used in this analysis: the trilobitomorph Agnostus Brongniart, 1822 and the trilobite Olenoides Meek, 1877. Agnostids have been regarded as closely related to naraoiids (Babcock, 1994), as members of the Trilobita (Briggs and Fortey, 1992; Fortey and Theron, 1994), as allied to the Crustacea (Bergstro¨m, 1992), or as possible members of the stemlineage Crustacea (Walossek and Mu¨ller, 1990). The controversy surrounding the systematic position of agnostids has led some authors to remove this taxon from any phylogenetic analysis regarding the relationships of Trilobita (Edgecombe and Ramsko¨ld, 1999). In this paper, agnostids are considered relevant to discussions on trilobite-allied arachnates and are thought to be an appropriate terminal taxon (see Briggs and Fortey, 1992; Wills et al., 1998). Olenoides is regarded as a useful taxon to estimate the basal node of Trilobita and is selected for use in this study (Briggs

FIGURE 4—Reconstruction of Naraoia bertiensis n. sp. with main axes of symmetry.

and Fortey, 1992; Fortey and Theron, 1994; Wills et al., 1998; Edgecombe and Ramsko¨ld, 1999). A brief description of the 12 characters used herein as well as the codings for the included taxa is presented below. The data matrix is presented in Table 1. Remarks.The following characters and their states were recorded from extensive published phylogenetic studies of modern and extant Arachnata, and only the most recent publications are referred to in brackets. Most of the nektaspids, including the new form, are only known from their exoskeletons. Consequently, only characters of the exoskeleton are used, with the exception of the character ‘‘lateral eyes,’’ whose absence is inferred from the lack of any external cuticular feature on the dorsal exoskeleton. Plesiomorphic characters are coded with 0, and apomorphic characters are coded with 1. Unknown and inapplicable characters are coded ‘‘?.’’ Character list. 1. Calcified dorsal cuticle.—0-absent; 1-present. This character is widely recognized as an important apomorphy of the Trilobita (Fortey and Whittington, 1989; Ramsko¨ld and Edgecombe, 1991; Briggs and Fortey, 1992; Wills et al., 1998; Edgecombe and Ramsko¨ld, 1999). 2. Lateral eyes.—0-absent; 1-present. Within the nektaspids blindness is usually considered to represent a reversal from an ancestral arthropod that had eyes (see Edgecombe and Ramsko¨ld, 1999). However, the discovery of blind forms in other groups of arthropods suggests that blindness could be interpreted as a symplesiomorphy (Budd, 1999). 3. Ecdysial sutures.—0-absent; 1-present. This character refers to the prominent suture lines on the marginal or dorsal surface of arthropods that have calcified cuticle, and which facilitated molting. In this analysis, only Olenoides and Agnostus have this type of suture (Wills et al., 1998). 4. Doublure on cephalic shield.—0-short (maximum sagittal

← FIGURE 3—Naraoia bertiensis n. sp. (ROM 56013) from the Bertie Formation. 1–3, Part, overall dorsal view (low-angle light). 1, Specimen covered with ammonium chloride. 2, Specimen photographed in natural conditions. 3, Ventral view, detail of the wide cephalic doublure (high-angle light). 4, Counterpart, specimen photographed in natural conditions. Arrows indicate disarticulated sclerites of eurypterid. 5, Composite camera lucida drawing of both part and counterpart.

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5.

6.

7.

8.

9.

10.

11.

12.


length less than 30 percent length of cephalic shield); 1-long (maximum length more than 30 percent length of cephalic shield). This character is derived from character 24 of Edgecombe and Ramsko¨ld (1999, p. 276), but is applied here to the cephalic doublure only. The cephalic doublure is usually better preserved than that of the posterior shield with the exception of Tarricoia (Hammann et al., 1990), where both the cephalic and posterior shield doublures are well preserved. A long doublure was considered an apomorphic trait of Tarricoia and Soomaspis (Fortey and Theron, 1994) in previous studies (Chen et al., 1997; Edgecombe and Ramsko¨ld, 1999). Posterior overlap of cephalic shield.—0-no overlap; 1-overlapping trunk segment. This character refers to the extension of the cephalic shield that overlaps the first trunk segment. The apomorphic state defines a liwiine group (family Liwiidae) according to Edgecombe and Ramsko¨ld (1999, p. 272, character 9). Genal spines.—0-absent; 1-present. Genal spines are absent in the nektaspids with the exception of N. spinosa and N. spinifer. Findings of possible sexual variants of N. compacta with genal spines (see Whittington, 1977) are coded in this study as a multistate character (0 or 1). Posterior shield1thorax length.—0-exceeding 1.5 times the length of the cephalic shield; 1- less than or subequal. Some authors have argued that the length of the posterior shield in the nektaspids is a function of the number of segments released from the front of the posterior shield (Budd, 1999). In this scenario, there is an implicit choice that thorax 1 posterior shield in the liwiine nektaspids and other trilobitomorph arthropods is homologous with the posterior shield of the Naraoiidae (Budd, 1999). Thus, the posterior shield of the Naraoiidae could be considered to represent the ‘‘transitory posterior shield’’ of trilobites during early larval stages (meraspid) and implies a heterochronic origin (see Budd, 1999, p. 109). Narrowing of the anterior part of the trunk relative to the head shield and posterior part of the trunk.—0-absent; 1-present. This character is considered a distinctive feature of the nektaspids (Chen et al., 1997; Edgecombe and Ramsko¨ld, 1999, p. 273, character 14). Free thoracic segments.—0-presence of segment boundaries; 1-complete absence of trunk segments. It is considered that the release of thoracic segments in trilobite-related arthropods occurs from the front of the posterior shield during ontogeny. The presence of a thorax is then considered symplesiomophic (Briggs and Fortey, 1992; Fortey and Theron, 1994; Edgecombe and Ramsko¨ld, 1999). The absence of articulated trunk segments is an apomorphy of the family Naraoiidae that distinguishes it from all other nektaspids. The choice of a binary coding (0 or 1) instead of multistate characters is justified on the premise that the presence or absence of thoracic segments is probably a better reflection of heterochronic processes in the nektaspids than the actual number of segments released (see Budd, 1999, p. 109). Segment articulations.—0-extensive overlap of segments; 1edge-to-edge articulations. This character is considered an apomorphy of the Trilobita and helmetiids; see character 18 in Edgecombe and Ramsko¨ld (1999, p. 273–74). Articulating half-rings.—0-absent; 1-present. Half-rings are important in permitting enrollment of the exoskeleton and are present in the Trilobita. In the nektaspids, this character has been recognized in the liwiines such as Buenaspis (Budd, 1999) and Soomaspis. Median keel on posterior shield.—0-absent; 1-present. This character is present in Buenaspis, Tarricoia, and Soomaspis,

and could reasonably be interpreted as a synapomorphy of these three taxa, despite some uncertainties as to whether it represents a true morphological character or an artifact of fossilization (Chen et al., 1997; Budd, 1999; Edgecombe and Ramsko¨ld, 1999). Analysis.The data matrix (Table 1) was subjected to a parsimony analysis incorporating the methodology of phylogenetic systematics as described by Hennig (Hennig, 1966). A data matrix was constructed using MacClade Version 3.04 for Macintosh Osx (Maddison and Maddison, 1992). Most parsimonious trees were calculated using PAUP (Phylogenetic Analysis Using Parsimony) Version 4.0.b10 for Macintosh (Swofford, 2002). Because of the small size of the data matrix, an exhaustive search was performed to find the shortest tree topology. Characters were equally weighted and unordered, and multistate taxa were treated as polymorphic. Characters were optimized using DELTRAN optimization methods. Olenoides and Agnostus were used as outgroups. RESULTS AND DISCUSSION

The phylogenetic analysis resulted in a single most parsimonious tree of 15 steps (CI 5 .81, RI 5 .85, RC 5 .69) (Fig. 5). The tree confirms the previously well-supported monophyly of the nektaspids (characters 1, 3, 8, 10). The family Naraoiidae is supported by the lack of thoracic segments (character 9). This clade is composed of two sister pairs but the position of Misszhouia (Chen et al., 1997) is ambiguous. In previous studies when internal and soft part characters were taken into account, Misszhouia seemed to be excluded from Naraoia within the Naraoiidae by at least three synapomorphies (see Chen et al., 1997). N. bertiensis n. sp. is found to be the sister taxon of N. compacta and this clade is supported by one synapomorphy (character 7). The monophyly of the family Liwiidae is supported by one synapomorphy (character 5). The character state, long doublure (character 4) originated independently in the clade Tarricoia 1 Soomaspis, and in N. bertiensis. Two other characters seem to have originated independently in different lineages: the genal spines in the trilobites and in a clade grouping N. spinifer 1 N. spinosa (character 6), and the articulating half-rings in the trilobites and in a clade grouping Tarricoia 1 Soomaspis 1 Buenaspis (character 11). The clade Tarricoia 1 Soomaspis 1 Buenaspis is supported by the presence of a median keel (character 12). The absence of lateral eyes is considered a symplesiomorphy and is phylogenetically uninformative for this analysis (presence in the outgroup Olenoides, character 2). Phylogenetic relationships.Despite the limited number of characters available, the phylogenetic analysis supports a sister group relationship between the Naraoiidae and the Liwiidae. Overall, the phylogenetic hypothesis seems to match the stratigraphical order of taxa quite well (Fig. 6). For example, in the Liwiidae, Soomaspis and Tarricoia are the youngest taxa and the most derived in the tree, which gives an additional source of support for the relationships. This phylogenetic tree suggests that the Naraoiidae seems to have evolved from a segment-bearing form relatively early in the Cambrian. Addition of internal and soft part characters could be necessary to test this phylogenetic hypothesis of the nektaspids, when new specimens with preservation of soft parts become available. Another useful test could be to use heterochronic processes as proxies of character change (especially in the Liwiidae), should ontogenetic data for this group ever become available (see Budd, 1999). However, the usefulness of studying heterochronic processes based on segment release from the posterior shield has also been disputed by Budd (1999), who contended that the number of segments released is not necessarily constant and is especially highly variable in some trilobites. The sequence of segment addition is not known in the Liwiidae, given that only adult forms have been recovered. The


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FIGURE 5—Singlemost parsimonious tree produced from analysis of 12 characters coded in Table 1 (15 steps, CI 5 0.81, RI 5 0.85, RC 5 0.69). Homoplasies are indicated by the asterisk symbol (*). Character changes are considered unambiguous except for character 11.

FIGURE 6—Nektaspid relationships and stratigraphical order of appearance of taxa. Dotted lines emphasize the family Naraoiidae, which is characterized by the loss of thoracic segments (arrow). The new nektaspid supports a putative ‘‘Lazarus effect’’ within the lineage Naraoiidae. The phylogenetic tree is from Figure 5 (see text for details).

ontogeny of the Naraoiidae is comparatively poorly understood. The ‘‘giant protaspid’’ traditionally attributed to Naraoia in the Chengjiang biota (Hou and Bergstro¨m, 1991) has been recently reconsidered as an adult form of a new arthropod distinct from the Nektaspids (Zhang et al., 2003). The study of heterochronic processes based on a number of thoracic segments in the nektaspids cannot be tested without making strong assumptions concerning the mode of segment release (Budd, 1999). Despite this problem, Budd admits that heterochrony is a mechanism by which the differences in segment release among the nektaspids might have arisen (Budd, 1999). In Budd’s view, the presence or absence of an articulated thorax (rather than the absolute number of thoracic segments) is a direct expression of heterochrony that has phylogenetic implications. The phylogenetic tree presented in this paper supports the proposal that the Naraoiidae may have experienced a heterochronic loss of thoracic segmentation, but not in the sequence described in scenarios either 2 or 3 from Figure 1. In fact, it appears that the evolution of thoracic segments has been nonlinear in the Naraoiidae 1 Liwiidae. ‘‘Lazarus effect’’.The inclusion of the new species within the genus Naraoia, based on its close relationship with N. compacta, extends the range of the family Naraoiidae well beyond the Cambrian, and that of the Nektaspida from the Late Ordovician to the Late Silurian (Fig. 6). A similar phenomenon is observed for a number of other relict Cambrian morphotypes that appear sporadically in later Palaeozoic Konservat–Lagersta¨tten, for example: Waukesha–Brandon Bridge Fauna—Late Silurian (Mikulic et al., 1985); Hunsru¨ck slate—Lower Devonian (Bartels et al., 1998); and Mazon Creek biota—Pennsylvanian (Shabica and Hay, 1997). The reappearance of the nektaspid lineage after the Late Ordovician extinction events resembles that of so-called ‘‘Lazarus

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taxa’’ documented from the fossil record following major biotic crises (Flessa and Jablonski, 1983; Rickards and Wright, 2002). In this case, however, the ‘‘Lazarus effect’’ is dubious because the nektaspid lineage contains few taxa and most come from localities characterized by exceptional preservation (Konservat– Lagersta¨tten). Given that nektaspids have a nonmineralized cuticle, this is not surprising and supports the view that taphonomic biases could be at the origin of the apparent ‘‘Lazarus effect’’ encountered in this study (Butterfield, 1995; Fara, 2001). ACKNOWLEDGMENTS

We are very grateful to G. Budd and G. Edgecombe who reviewed the manuscript and suggested many helpful improvements. We thank T. Carr and A. Ngo who offered support and advice in running PAUP, and F. Santini for critical review. Caron thanks D. McLennan for helpful comments of an earlier version of parts of this manuscript which was written for a graduate course assignment at the University of Toronto. The paper has been improved by production editor L. Vermaas, and associate editor S. Westrop. This research was supported by doctoral fellowships given to J. B. Caron from the University of Toronto (Department of Zoology). Caron also acknowledges the support of a Natural Sciences and Engineering Research Council of Canada grant given to his supervisor (D. Jackson, University of Toronto). REFERENCES

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