J. Paleont., 78(4), 2004, pp. 795–797 Copyright 䉷 2004, The Paleontological Society 0022-3360/04/0078-795$03.00
NACRE IN LATE CRETACEOUS SENSUITROCHUS FERRERI—IMPLICATIONS FOR THE TAXONOMIC AFFINITIES OF THE CIRRIDAE (GASTROPODA) STEFFEN KIEL
AND
´ DA JIRˇI´ FRY
Freie Universita¨t Berlin, Fachrichtung Pala¨ontologie, Institut fu¨r Geologische Wissenschaften, Malteserstraße 74–100, Haus D, 12249, Germany, ⬍
[email protected]⬎, and Czech Geological Survey, Kla´rov 3, 118 21 Praha 1, Czech Republic, ⬍
[email protected]⬎
ASTROPODS BELONGING to the extinct family Cirridae Cossmann, 1916 are relatively rare elements of Mesozoic marine communities. The unusual change in shell coiling from dextral to sinistral during their ontogeny represents a character which makes their recognition in the fossil record easy. This shell feature is also known from members of the Paleozoic family Porcelliidae Koken in Zittel, 1895, uniting the planispirally coiled Porcelliinae with the sinistrally coiled Agnesiinae Knight, 1956. This heterostrophy is not homologous with that of the Heterobranchia (Fry´da and Blodgett, 2001). Shells of Porcelliinae and Agnesiinae bear an apertural slit, in contrast to the Mesozoic Cirridae. According to Bandel (1993a), slit-bearing members of the Porcelliidae evolved into the Cirridae without slit during the Triassic. Therefore Bandel (1993a) united both families into one natural group, the Cirroidea. That name has now been changed into Porcellioidea, according to the older family name (see discussion in Bandel and Fry´da, in press). The oldest representatives of the Porcelliidae were reported from Silurian strata (Fry´da, 1997). The youngest Cirridae are known from Late Cretaceous strata (Kiel and Bandel, 2002). Fry´da and Blodgett (1998) described an Emsian (Early Devonian) porcellioid—Alaskacirrus bandeli—without slit as in the Mesozoic Cirridea, and pointed out that phylogenetic relationships of the Paleozoic Porcelliidae and Mesozoic Cirridae are still uncertain. The placement of the Porcellioidea within the Archaeogastropoda is based on the discovery of an archaeogastropod-type protoconch in the Paleozoic Porcelliidae (Yoo, 1994) as well as in the Mesozoic Cirridae (Bandel, 1993a). In addition, Bandel (1993a) mentioned the presence of a nacreous inner layer in a Mesozoic cirrid. This observation was from an unpublished Master’s thesis (Ha¨nsel, 1992) supervised by him. Ha¨nsel (1992) demonstrated that nacre is present in shells of the Late Cretaceous cirrid Sensuitrochus ferreri Quintero and Revilla, 1966 (Fig. 1.1), using peels of polished and etched shells to determine the shell structure. Although this observation is important for the interpretation of the higher taxonomic position of the Cirridae, it was never published. The aim of present study is to document the presence of nacreous shell in the Mesozoic cirrid Sensuitrochus ferreri, using scanning electron microscopy.
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MATERIAL AND METHODS
The specimen used in this study was collected from the gray marls of the Puimanyons Olisthostrome of the Vallcarga Formation in the Tremp basin in the Spanish Pyrenees, from outcrops in the valley system between Torallola, Toralla, and Sensui near Pobla de Segur in the Province of Lleida. The locality was described in more detail by Kiel and Bandel (2001) and the sediments are of late Campanian (Late Cretaceous) age (Simo´, 1986). To investigate the shell structure, we selected a well-preserved specimen of Sensuitrochus ferreri and broke several pieces of shell off the base and the whorl’s flank. These pieces were mounted on stubs, sputtered with gold and the fracture zones were photographed using a Leo 1455VP scanning electron microscope. The specimen and its fragments are deposited in the GeologischPala¨ontologisches Institut und Museum, University of Hamburg
(GPI), Type Kat. Nr. 4021. The taxonomic history of Sensuitrochus ferreri was outlined by Kiel and Bandel (2002). SHELL STRUCTURE OF SENSUITROCHUS FERRERI Observations.A fragment of the whorl’s flank shows three layers (Fig. 1.2). The thick, nacreous inner layer is made of high, slender stacks of polygonal nacre tablets (Fig. 1.3, 1.5). These stacks are straight or slightly bent, and are oriented almost perpendicular to the inner shell surface, with deviations from the right angle up to eight degrees. The outer layer shows no distinct structure. The two layers are separated from each other by a thin layer with homogenous appearance (Fig. 1.4). Measurements: Width of the specimen: 28 mm Shell thickness at flank: about 380 m Thickness of the nacreous layer: about 260 m Thickness of the outer layer: about 120 m Thickness of the central layer: 2 m Width of the nacre tablets: 5–10 m Height of the nacre tablets: about 0.5 m DISCUSSION
The nacreous layer with the nacre tablets arranged in high, slender stacks resembles that of Recent Vetigastropoda (Wise, 1970; Erben and Krampiz, 1972; Hedegaard, 1990). Also the dimensions of the nacre tablets of Sensuitrochus ferreri agree well, for example, with those of Tegula funebralis (Wise, 1970, pl. 5, figs. 1, 2). The outer, non-nacreous shell layers in vetigastropods are quite variable and thus of low taxonomic value (Hedegaard, 1990). Therefore, they are here not studied in greater detail. The higher taxonomy of many of the extinct gastropod groups is still uncertain. The limited number of available shell features as well as frequent homoplastic similarities of their shells considerably restricts the possibilities to determine phylogenetic relationships between modern and fossil gastropods. Evidence for a nacreous layer in the Late Cretaceous cirrid Sensuitrochus ferreri, based on new SEM data, enables us to discuss the higher taxonomic position of Mesozoic Cirridae. Nacre is considered a primitive shell structure (Ponder and Lindberg, 1997) and it is present in the molluscan classes Bivalvia, Cephalopoda, Gastropoda, and Monoplacophora (⫽Tryblidiida). Within the Gastropoda, nacre is present only in certain groups of the Archaeogastropoda. Considering that the extinct Cirridae and Porcelliidae had an archaeogastropod type protoconch (Bandel, 1982, 1993a), they should be placed in one of the two modern gastropod groups with such an early ontogenetic shell, i.e., within Patellogastropoda (⫽Docoglossa) or Archaeogastropoda (sensu Hickman, 1988). Ponder and Lindberg (1997) suggested that the Patellogastropoda probably had sinistrally coiled ancestors in the Paleozoic. In this respect, the Porcellioidea could be considered as either the sister group to the Patellogastropoda, or as a paraphyletic group containing the most basal patellogastropods (Hedegaard et al., 1997; Fry´da, 2001). However, evidence for nacre in the Cirridae testifies against such interpretation. Modern Patellogastropoda have no nacre but a shell
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FIGURE 1—Sensuitrochus ferreri Quintero and Revilla, 1966. 1, Well-preserved specimen showing sinistral coiling (from Kiel and Bandel, 2002), ⫻3; 2, cross section of a fragment of shell from the shell’s flank, top: shell’s inner side, bottom: shell’s outer side (GPI Type Kat. Nr. 4021); 3, inner surface of the shell, showing the polygonal nacre tablets; 4, close-up on the transition between the nacreous inner layer and the homogenous outer layer, arrow indicates the thin, homogenous layer in between (the light band above the thin, homogenous layer is the surface of that layer, not another layer, compare with 2); 5, close-up on the nacreous layer.
structure that is very special amongst the Gastropoda (MacClintock, 1967; Hedegaard, 1990; Bandel and Geldmacher, 1996). This patellogastropod shell structure has not changed essentially since the first occurrence of that group in the Triassic (Hedegaard et al., 1997). Also testifying against the porcellioid/patellogastropod relationship is the fact that sinistral coiling in the Porcellioidea is an advanced shell feature (Bandel, 1993a; Fry´da, 1997). Thus, the Archaeogastropoda (s. Hickman, 1988) is the only possible extant gastropod group in which the extinct Cirridae can be placed. Among the modern Archaeogastropoda, nacre is present in the Haliotoidea Rafinesque, 1815, Pleurotomarioidea Swainson, 1840, Seguenzioidea Verrill, 1884, and most Trochoidea, but is absent in members of the Fissurelloidea Fleming, 1822, Phasianellinae Swainson, 1840, and Scissurelloidea Gray, 1847. In addition, nacre is also evident in some of fossil archaeogastropod groups of the Triassic (Bandel, 1991, 1993b) and the Pennsylvanian (Batten, 1972; Bandel et al., 2002). CONCLUSIONS
The presence of the archaeogastropod-type protoconch in the Cirridae and the Porcelliidae, as well as the presence of a nacreous inner shell layer in the Cirridae suggests that the most probable taxonomic position of the extinct Porcellioidea is close to the
nacreous groups of the Archaeogastropoda. The presence of nacre in the Cirridae testifies against the hypothesis that the Porcellioidea are ancestral to the Patellogastropoda. The stratigraphic range of the Porcellioidea is at least from Silurian to Cretaceous and thus about 350 Ma. Development of sinistral heterostrophy (i.e., dextrally coiled early teleoconch whorls followed by sinistrally coiled whorls) is a good apomorphic character defining the Porcellioidea. These facts suggest that this superfamily represents an independent, long-lived natural group within the Archaeogastropoda. ACKNOWLEDGMENTS
We would like to thank K. Bandel, Hamburg, for discussion and material, J. Hartmann, Hamburg, for his help with the SEM, and A. Nu¨tzel, Erlangen, J. Schneider, and an anonymous reviewer for their critical and helpful comments on the manuscript. This study was financially supported by the Alexander von HumboldtStiftung, and grant 205/04/0600 from the Grant Agency of the Czech Republic to JF. REFERENCES
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