Upper Triassic semionotids from western Gondwana
Final Report Abstract
The Project was meant to carry out a fieldwork campaign to obtain new material and to study the anatomy and phylogenetic relationships of Semionotus vallejensis the oldest record of Semionotiformes in Gondwana, in the Late Triassic Cacheuta Formafion of the Guyana Basin, Argenfina. This was the only possible record of semionotiforms in the Triassic of Gondwana. The project was extended to include a critical revision of all the acfinopterygian fish taxa named from the Triassic of the Cuyana Basin. Among the studied fishes, we revised all the specimens that have been referred to Semionotus, including "S." vallejensis, and concluded that they do not represent semionotiforms, but more primitive non-neopterygian actinopterygians. Therefore, there is no evidence for the presence of Semionotiformes in the Triassic of Gondwana and the oldest record of the group in this supercontinent is Semionotus capensis from the Eariy To date, a total 29 actinopterygian taxa were named from this basin, but these fishes are almost unknown since most of them were exclusively published in local Argenfinean journals of rather restricted circulation. Only four of these taxa have been studied in detail and the taxonomic status ofthe remaining species is doubtful. My co-authors, CerdePio and Rauhut, and me have completed a thorough taxonomic revision of these fishes, after which only eight among the 29 named species are accepted as valid. Among these valid species, Guaymayenia paramillensis is a very primitive actinopterygian probably related to the Earty Triassic Reronisculus. Challaia magna is also a primitive actinopterigian member of the Group A of the Acrolepidae, which othenvise includes Palaeozoic taxa and probably also some Australian Early Triassic taxa {Myriolepis, Leptogenichthys, and Mesembroniscus). The genus Challaia is otherwise known from the late Middle Triassic (Ladinian) Los Rastros Formation ofthe Del Bermejo Basin where it is represented by a different species, Challaia elongate. Neochallaia tellecheai is another primitive . acfinopterygian, which might be a member ofthe Ghondrostei. This taxon show some very interesting features also present in acipenseriforms, but further study is necessary to be certain about its systematic relationships. All the other five valid taxa represent and a single family of perieidiform fishes, the Pseudobeaconiidae, which is endemic of Western Gondwana. Within this family they represent two genera, Pseudobeaconia (with P. bracaccini, P. elegans, P. celestae, and P. cuyana new combination), and Mendocinichthys brevis. Although only a few species were found to be valid, clarifying their systematic relafionships, as well as the possible relafionships ofthe other, undiagnosable material, led us to the recognifion of two faunal associafions: the one of Potrerillos and Santa Clara, dominated by perieidiforms, and the one of Cerro Bayo / Agua de la Zorra, dominated by more primitive actinopterygians. The first associafion is rather typical ofthe Late Triassic, which is in agreement with the age proposed for the bearing sediments. The second however, suggest an older age that currently accepted for the bearing strata. Siudying the literature we found geological information supporting this last hypothesis, in particular, the radiometric dating of 235 ± 5 Ma and 240 ± 10 Ma (Late Middle Triassic) of the basalts sills in the Agua de la Zorra.
Publications
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The Triassic Fish Faunas of the Cuyana Basin, Western Argentina
in: Palaeontology Vol. 53,2, pp.249-276
López-Arbarello, Adriana / Rauhut, OIliver W. / Cerdeno, Esperanza:
