NEW FOSSILS AND CORRELATIONS IN THE ISCHIGUALASTO FORMATION AT THE POORLY KNOWN HOYADA DEL CERRO BOLA LOCALITY (EARLY LATE TRIASSIC), LA RIOJA PROVINCE, NORTHWESTERN ARGENTINA

DESOJO, J.B.; EZCURRA, M.; FIORELLI, L.; ROCHER, S. ; VON BACZKO, M.B.; MARTINELLI, A.; ESCOBAR, J. ; MAMANI, C.; ULLOA-GUAIQUIN, K. ; CARDILLO, A. ; FERNANDEZ BLANCO, M.V.; PAULINA CARABAJAL, A.

La Rioja

Jornada; Jornadas Argentinas de Paleontología de Vertebrados; 2023

UNLR-crilar

The Hoyada del Cerro Bola (HCB; La Rioja Province) is a historical locality where the LosRastros/Ischichuca, Ischigualasto, and Los Colorados formations of the Ischigualasto-Villa Unión Basin crop out. However,Although the Ischigualasto Formation at HCB was formally reported by Yrigoyen and Stover in 1970 with 926 meters exposed, the Ischigualasto Formation at this locality has been sporadically explored in the last decades. Indeed, its fossil record was limited to a single occurrence indicated in a geological profile, without further information. Here, we report the results of a fieldtrip conducted to the Ischigualasto Formation at HCB by the Archosauriform Research Group (ARG) in April-May of 2022. We built a new detailed 850 meter-high stratigraphic profile, took samples of three different tuff levels for radioisotopic analyses, and collected 14 new tetrapod fossils. The presence of multiple, stratigraphically congruent, tuff levels throughout the sequences of the Ischigualasto Formation in the HCB and the near Hoyada del Cerro Las Lajas (HCL) locality allowed establishing lateral correlations between them. We found significant lateral facies changes between HCB and HCL sections, in which HCB has an important interval of amalgamated sandy and sand-conglomeratic channels (100–300m above the base) that are laterally replaced by facies dominated by floodplain deposits and intercalated sandy channels in HCL. The levels of HCB that probably correlate with those of the Hyperodapedon Biozone in HCL yielded only a fragmentary indeterminate rhynchosaur so far (100m). Most fossils were collected at 265−420m in the sequence and probably correlate with the upper Teyumbaita Biozone and overlying levels in HCL. The most abundant taxon of these levels is the cynodont Exaeretodon (n=3), whereas other specimens are a rhynchosaur (non-Hyperodapedon-Teyumbaita Hyperodapedontinae) and indeterminatearchosauromorphs and tetrapods, under preparation. Finally, an indeterminate hyperodapedontine rhynchosaur and an aetosaur were found higher in the sequence (520−550m). Contrasting with HCL, it is striking the absence, so far, of the rhynchosaur genus Teyumbaita in HCB. The abundance of Exaeretodon suggests that the 265−420m of the Ischigualasto Formation at HCB could be correlated with the uppermost levels of the Hyperodapedon-Exaeretodon-Herrerasaurus and the Exaeretodon biozones of the Ischigualasto Provincial Park (IPP; San Juan Province). Future radioisotopic dates ofHCB levels would improve correlations and faunistic comparisons of the Ischigualasto Formation at HCL and IPP. This fieldtrip yielded promising results and further fossil sampling in HCB would help reaching a better understating of faunistic differences within the Ischigualasto-Villa Unión Basin around the Carnian-Norian boundary.*Financial support provided by: ANPCyT PICT 2018-0717, 2018-0853, 2020-SERIEA-02909, Sepkoski Grants, Subsecretaría de Patrimonio, Secretaría de Cultura de La Rioja.