New tetrapod specimens and chronostratigraphy of the Late Triassic Ischigualasto Formation at Cerro Las Lajas (La Rioja Province), northwestern.

LANGER, MAX; DESOJO, JULIA BRENDA; ATILA DA ROSA; MIGUEL EZPELETA; EZCURRA, MARTÍN D.; AGUSTIN MARTINELLI; BELEN VON BACZKO; FIORELLI, LUCAS E.; FELIPE MONTEFELTRO; RAMEZANI, J

Congreso; 5th International Palaeontological Congress; 2018

The early diversification of most modern amniote clades, such as testudinatans,lepidosauromorphs, archosauromorphs, and mammaliaforms, started during the Early?Middle Triassic, andby the Late Triassic most of them were abundant and diverse. The Ischigualasto Formation of northwesternArgentina (late Carnian?early Norian), mainly explored at the Hoyada de Ischigualasto (San Juan Province),is one of the continental units that has best documented this transition. Recent fieldwork on theIschigualsto Formation at Hoyada del Cerro Las Lajas (La Rioja Province) allowed us to considerably improvethe knowledge of the fossil record of this unit. Only three archosaur taxa based on three specimens werepreviously reported from Cerro Las Lajas by José Bonaparte in the 70?s: the holotypes of the ornithosuchidVenaticosuchus rusconii and the dinosauromorph Pisanosaurus mertii, as well as a specimen recentlyreferred to the crocodylomorph Trialestes romeri. Here, we report additional fossil remains from the lowerthird (70m to 200m) of the Ischigualasto Formation at Cerro Las Lajas that includes: skeletons of fourdifferent species of hyperodapedontine rhynchosaurs (two of which are new), few partial skeletons of thetraversodontid Exaeretodon sp., plus incomplete skeletons of an aetosaur, a proterochampsidarchosauriform (Proterochampsa sp.), a non-shuvosaurid poposauroid, and a probainognathian cynodont(cf. Ecteninion). These taxa, except for three of the rhynchosaur species and probably the poposauroid,were previously reported from the two lowermost members of the Ischigualasto Formation at Hoyada deIschigualasto. Our new high-precision U- Pb geochronology by the CA-ID-TIMS method suggests that theCerro Las Lajas captures a near-complete succession of the Ischigualasto Formation and that the faunalassemblage presented here allows an assignment to the Scaphonyx -Exaeretodon -Herrerasaurus biozoneof the Ischigualasto Formation at Hoyada de Ischigualasto. However, no diagnostic fossil remain has yetbeen discovered from the upper two-thirds of the unit at Cerro Las Lajas to be able to compare to theentire faunal assemblage of the Ischigualasto Formation in San Juan.*