LATE CRETACEOUS ACTINOPTERYGIANS FROM VEGA ISLAND, ANTARCTICA: PRELIMINARY REPORT

SOLEDAD GOUIRIC CAVALLI; O'GORMAN, JOSÉ; REGUERO, MARCELO; THOMAS, MÖRS; LAURA, CHORNOGUBSKY; MAGALÍ CÁRDENAS; GUILLERMO, LÓPEZ

Simposio; XII International Symposium on Antarctic Earth Sciences; 2015

Antarctic fossil actinopterygians are of particular interest because of their scarcity. The extant Antarctic ichthyofauna diversity is low but it could have been higher during the Mesozoic. Up to date, few Cretaceous localities bearing both chondrichthyan and actinopterygian remains were known from Seymour (Marambio), Vega, and James Ross islands. The material recovered includes several isolated teeth, scales, vertebrae, and skull bones. It is noteworthy to mention that the highest diversity is performed by chondrichthyans meanwhile only few scattered actinopterygians were recovered. There are only two informative actinopterygian specimens described so far, both are acanthomorphs: the beryciform, Antarctiberyx Grande and Chatterjee, represented by a poorly preserved and incomplete anterior region of a head recovered in Maastrichtian levels of the López de Bertodano Fm and one tiny and partially preserved specimen designed as cf. Sphenocephalidae recovered from Maastrichtian levels of the Cape Lamb Mbr of the Snow Hill Island Fm. Here we report bony fish material collected during the last Antarctic Expedition (January and February 2015) in the lower part of the Cape Lamb Mbr of the Snow Hill Island Fm (lower Maastrichtian) at Cape Lamb, southwestern part of Vega Island. The material consists of four concretions containing actinopterygian remains. Due to the incomplete nature and poor preservation of the specimens, we are unable to assign them to any finer classification. Nevertheless, due to the dearth of Cretaceous fish material from Antarctica, the putative difference of some of the new findings from the known specimens and the good temporal control, we feel that the description of the material at hand is justified. Furthermore, even in its current state, the new material adds considerable morphological information. Presently, the more informative material seems to be MLP 15-XI-7-11 and MLP 15-XI-7-12. MLP 15-XI-7-11 consists of disarticulated skull bones associated with articulated, highly imbricated, and large scales, plus part of a vertebral column. The scales are cycloid, oval-shaped, and seem to be longer than deep. Their external surfaces show numerous concentric circulii and scarce radiating radii plus a central focus with numerous central pits. The central focus is markedly large in the lateral line scales. Most of the scales are articulated. Part of the opercular series has been recognized i.e. opercle, subopercle, and preopercle. The opercle is large, more or less triangular, and ornamented with fine radiating lines. The preopercle lacks the distal part of its ventral limb and the posterior margin seems to be smooth and slightly convex. The dorsal limb is elongated and thin. The ventral limb is more or less expanded. There are at least thirteen sensory tubules branching from the main preopercular sensory canal. The tubules are closely arranged in the ventral limb. They are moderately wide, long and seem to reach the ventral margin of the preopercle. The interopercle is broken but it seems to have the same length of the preopercular ventral limb. The subopercle is large. Remains of branchiostegal rays are preserved but their total number is unknown. The material is preliminary assigned here to Ichthyodectiformes due to similarity in the scales and preopercle morphologies. Ichthyodectiformes is an extinct group of basal teleosts that appeared in the Middle Jurassic, became diverse and successful during almost all the Cretaceous. They have been collected from marine deposits of North America, Europe, Lebanon, Australia, South America, Asia, Antarctica, and Africa. The group includes big-sized fishes (e.g., Xiphactinus Leidy, Cladocyclus Agassiz), medium sized forms (e.g., Allothrissops Nybelin, Thrissops Agassiz) and small fishes (e.g., Ascalabothrissops Arratia). MLP 15-XI-7-12 consists of a caudal endoskeleton that, as far as it can be observed, is composed of four caudal vertebrae, three preural centra, and one compound ural centra. The vertebrae are heavily ossified, ornamented, and strongly constricting the notochord. The dorsal flexure of the tail begins in the mid-posterior part of the ural centra. There are two hypurals, the first is well developed and the second is comparatively much reduced. Both hypurals are separated by a marked diastema. There are at least three partially preserved uroneurals. The specimen is preliminary interpreted here as an indeterminate teleost. However it is under study in order to arrive to a more accurate taxonomic designation. MLP 15-XI-7-20 consists of a section of a fish of about 8 cm long and 0.8 cm wide that corresponds to the posterior region of its body. The vertebrae correspond to the posterior abdominal ones. There are at least 10 poorly preserved vertebrae that are longer than broad. The notochord is strongly constricted by the autocentra and the vertebrae are amphycoelus. The autocentra are thick and have their external surface ornamented. There are few slightly posteriorly inclined, long neural spines preserved. Where the neural arch is preserved it is possible to observe that it is positioned at the mid-posterior portion of the autocentra. The isolated vertebra MLP 15-XI-7-19 is composed by a thick amphycoelus autocentra that strongly constrict the notochorda. Nor hemal and neural arches neither its spines are preserved. The latter two specimens are very poorly preserved and incomplete. Because a thick autocentrum that strongly constrict the notochord is present in most fossils above the phylogenetic level of Leptolepis coryphaenoides (Bronn) and in all extant teleosts, the material is designed here as belonging to an indeterminate teleost. The specimens reported were collected from levels that also yielded other marine and continental vertebrates (i.e., plesiosaurs, mosasaurs, and dinosaurs) together with marine invertebrates (i.e., ammonoids, nautiloids, bivalves, gastropods, bryozoans, crustaceans, and equinoids), and palynomorphs. The actinopterygians described here correspond to medium and/or small sized predators. The putative ichthyodectiform is much more complete and better preserved than the previously reported for Upper Cretaceous levels being the first record of the group recovered from the Cape Lamb Member. No acanthomorphs were recovered. It is expected that, through comparative studies, a more accurate taxonomic assignment of the material can be made. This study will lead to a better understanding of the diversity and paleobiogeography of the Cretaceous Antarctic ichthyofauna.