Shell Microstructure of a Triassic Patellogastropod Limpet
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Shell microstructure of a Triassic patellogastropod limpet CLAUS HEDEGAARD, DAVID R. LINDBERG AND KLAUS BANDEL Hedegaard, C., Lindberg, D.R. & Bandel, K. 1997 12 15: Shell microstructure of a Triassic LETHAIA patellogastropod limpet. Lethaia, Vol. 30, pp. 331-335. Oslo. ISSN 0024-1 164. The St. Cassian formation of northern Italy contains an unsurpassed diversity ofTriassic organ- isms, often with original or only slightly recrystallized shells. The shell microstructure ofPatella costulata Munster, 1869, is examined and compared with the shell microstructures of 14 extant gastropod limpet taxa. The distribution of shell structure characters supports the assignment of P. costulata to the patellogastropod taxon Patellidae. St. Cassian facies also include near-shore environments that are consistent with the habitat of extant Patella species. The corroboration of Patella costulata as a member of the Patellogastropoda reduces the hiatus between estimated origination and the previous unequivocal first occurrence by a minimum of 116 Ma. OPatellogastropoda, shell structure, Triassic, ghost lineage. Clam Hedegaard [[email protected]],Institute of Biology, University of Aarhus, Aarhrrs, Den- mark; David R. Lindberg [ [email protected]], Department of Integrative Biology and Museuni of Paleontology, University of California, Berkeley, California 94720-4780, USA; Klaus Bandel, Geologisch-PaliiontologischesInstitut der Universitiit, BundesstraJe 55, D-2000 Hambiirg 13, Germany; 19th June, 1996; revised 31st October, 1997. Limpet morphology in the Gastropoda is infamously bonate and organic material of the shell - are diagnostic homoplastic and has independently evolved among in the Patellogastropoda and can be used to allocate indi- numerous gastropod taxa (e.g., Patellogastropoda, Coc- vidual fossils within this clade (MacClintock 1967; Lind- culinidae, Phenacolepadidae, Hipponicacea, Siphonarii- berg 1988; Hedegaard 1990; Lindberg & Squires 1990; dae) (Ponder & Lindberg 1997). Limpet morphologies are Kase 1994; Kase & Shigeta 1996). also widely distributed throughout the fossil record -the The St. Cassian formation, which outcrops around Monoplacophora of the earliest Cambrian (Yu 1979) to Cortina d'Ampezzo in the Dolomites of northern Italy, the giant Anisomyon limpets of the Cretaceous (Kanie contains an unsurpassed diversity of Triassic organisms. 1975).While anatomy provides an independent means by Zardini (1978) described over 300 species of molluscs as which to allocate living limpets to respective taxa, allocat- well as impressive numbers of echinoderms, bryozoa and ing fossil limpets is problematic. other taxa. The fossils occur in clay and marl beds in a The Patellogastropoda are hypothesized to be the sister dolomitic limestone (Fursich & Wendt 1976), which taxon of all other gastropods and one of the earliest allows for the preservation of fossils with original or only branches of gastropod evolution (Golikov & Staroboga- slightly recrystallized aragonitic tests (Bandel 1988, 1991). tov 1975; Lindberg 1988; Haszprunar 1988; Ponder & The excellent preservation of skeletal material from this Lindberg 1996, 1997). They are widely distributed in formation produces a more complete sample of this Tri- Phanerozic sediments, but their limpet morphology assic fauna and provides additional characters, such as imparts few clues and landmarks to distinguish them shell structures, for the taxa present. Two putative patel- from limpet forms of crown groups. When available, logastropods have been reported from this fauna - Patella muscle scars and associated impressions can be used to costulata Munster, 1844, and Patella crateriformis Broili, diagnose taxa (Lindberg & Squires 1990), but even these 1907. Here we report the results of a scanning electron traces of soft anatomy become tentative in deeper time as microscopy study of the shell microstructures of Patella comparisons with living taxa fail (e.g., Yochelson 1988). If costulata to establish if this species is a patellogastropod the fossil limpet shells are not recrystallized, shell micro- limpet, and demonstrate it as the earliest member of this structures - the microscopic arrangement of calcium car- clade yet reported. 332 C.Hedegaarti, D.R. Litidberg S'K. Batidel LETHAIA 30 (1997) Material and methods artifacts associated with cutting, etching, and polishing, md leave more specimen unaltered and available for We examined the shell structures of 11 specimens of future research. This latter consideration is especially Patellrz costdata (Fig. 1A) from Campo, Cortina important when dealing with fossils. For descriptions of d'hmpezzo, Italy ( GeoLPaliiont. Instit. Univ., Hamburg, structures and orientation of fractures we follow the ter- Germany). The shell microstructures of 14 extant limpet minolog of Carter & Clark (1985) with modifications taxa were also examined and tabulated for comparison and additions of Hedegaard (1990) and Lindberg & with the St. Cassian specimens (Table 1 ). Fractured shells Hedegaard (1996). were examined using an Electroscan Environmental Scanning Electron Microscope ( ESEXI) at the University of California hluseum of Paleontology, Berkeley Results (LJCMP).We used fractured sections rather than cut and polished thin sections, because we believe that fractured The specimens are relatively small, rarely exceeding 4 mm sections produce more reproducible results. Patellogas- in length. The original outer shell structures, which are tropod shells tend to break along contours parallel to composed of calcite, are well preserved, but the inner first-order elements of the shell microstructures, and we aragonitic shell structures are largely recrystallized. The find this characteristic much easier to control than the outermost layer has a calcitic comarginal crossed-lamellar angle at which a specimen is cut and polished. Xloreover, structure (crossed-foliated of MacClintock 1967). This if we were not successful in fracturing the specimen in the layer is followed by a thin layer of calcitic prisms, a thick desired plane, or natural breaks were neither transverse or recrystallized layer with possible remains of an aragonitic radial relative to the shell aperture, the orientation rela- comarginal crossed-lamellar layer, a myostracal layer, and tive to growth lines plane of deposition) is readily appar- another recrystallized shell layer immediately below the ent and serves as an index from which to orient the micro- niyostracuni (Figs. lB, C). structures. Fractured sections are also less prone to Aragonitic and calcitic crossed-lamellar structures may be identified by their mode of deposition. Calcitic crossed-lamellar structures are deposited on a plane par- allel to the first-order lamellae, i.e. the first-order lamellae are parallel to the inside of the shell (Fig. 1B). In contrast, Table 1. Shell microstructure sequence of P. costtilata compared nith aragonitic crossed-lamellar structures are deposited on a summary ddtJ for extant linipets. Entries in parentheses indicdte htruc- tures. which may he dhjent in some hpecies; ' =shell niicrcistructure plane perpendicular to the first-order lamellae, i.e. the equivalents. I1yostrac.i [Llyoi 'ire not necessarily humolognus shcll first-order lamellae are perpendicular to the inside of the 5tructures hut serve a\ landmarks tor alignment. Data from Boggild shell (Fig. 1D).This difference provides criteria by which ( 19301. ,\lacClintock j 19671, tledegaard 1990, unpublishedl. to distinguish between these two structures and reduces ~ ~~ ~ ~~ ~~~ ~~ ~ ~~~~ Tdson Exterior Interior the problems associated with determining whether a cal- ~ ~ ~ ~~~~~ ~ ~ 1'. ,-o~tlrlfltll ZConiCL ACL LIyo ACL citic crossed-lamellar layer is a diagenetic outcome of the :\imaca (CHoni PrCCL Fol ACL Sly0 ACL alteration of aragonite to calcite. In the case of the St. Cas- Cocculinidac AHom ACI. Llyo ACL sian specimens, the deposition is parallel to the first-order Emdrginulci hCL lIX1 ACL lamellae, and therefore we identify the structure as origi- 1.epeteIlidx ACL LIVO ACL or ICP nally calcitic. I.epetidae i CH~iiiI Fol XCL Sly0 ACL Table 1 summarizes the shell structures of Patella costu- Lottiidae ((:H(II~IJPrCCI ACL Llvo ACL hta and extant limpet-shaped gastropods, excluding Ci.Ihtl[l ' (CHomi CComCL Llvo XCL some ofthe deep-sea and hot-vent forms (e.g., Hirtopelti- or Fol Hzlsiorr C Horn Fol ACL 51yo ACI. dae 1. Patella rostulata possesses a calcitic comarginal Pnte/lci * CComCL ACL llvo ACL crossed-lamellar structure that is restricted in living gas- or Fol tropods to the Patellogastropoda clades. Within the Patel- S~-lcte/llzstrr?' ( CHom 1 CComCL ACL .\I\.@ ACL logastropoda this shell microstructure is found in the or Fol Phenacolepadidae CHoni ACL. extant taxa Sciitellastra and Patella (Patellidae), and Cel- Pseudocosculinidae AHom ICP hrin (Nacellidae). There are no other structures diagnos- Siphonariidae ACL ACL ~~ ~~~ ~ ~ tic of any of the other limpet groups, such as the prismatic AHom = aragonitic homugencou\ complex crossed-lamellar ones of Acmaeidae and Lottii- CHom = calcitic homo~encou~ dae, or the regularly foliated ones of some Nacellidae, .ACL = drdgoilitlc croshed lamcllar 1 comargind or rddial) Acmaeidae, Patellidae and Lepetidae, or the shell com- Fol = regularly foliated of (:ComCL = calcitic coni,irgin,il cru\wd lamellar posed purely aragonitic structures as it is in the Coccu- ICP = intersected croAsed platy linidae, Fissurellidae, Lepetellidae, Pseudococculinidae, PrCCI = prismatic complex crosd laniell~r.