INVESTIGADORES
EZCURRA Martin Daniel
congresos y reuniones científicas
Título:
A new early dinosaur from the Carnian Ischigualasto Formation (NW Argentina) and the origin of dinosaurs
Autor/es:
EZCURRA, M. D.
Lugar:
Neuquen
Reunión:
Congreso; III Congreso Latinoamericano de Paleontologia de Vertebrados; 2008
Resumen:
A new basal dinosaur coming from the lower levels of the Ischigualasto Formation (Valle Pintadolocality, San Juan Province, early-middle Carnian; Rogers et al. 1993) is reported here. The IschigualastoFormation has a diverse tetrapod sample, including temnospondyls, proterochampsids, rhyncosaurs, cynodonts,dicynodonts and archosaurs. Nevertheless, this outcrop is famous because it is one of the oldest known dinosaurbearing assemblages. Until now, only three valid species of early dinosaurs were known from the outcrop: thesaurischians Eoraptor lunensis and Herrerasaurus ischigualastensis from the lower third of the unit and theornithischian Pisanosaurus mertii from its middle beds. The new specimen (PVSJ 845) is based on the posteriorregion of the right jaw, caudal vertebrae, reamains of right ilium, ischium, tibia, fibula and both femora, leftmetatarsals II and V and pedal phalanges. Albeit the overall morphology of PVSJ 845 is reminiscent to that ofbasal saurischians, it differs from other known dinosauriforms in the presence of a poorly laterally projectedsupraacetabular crest, a femoral lateral surface with deep and large fossa immediately below the trochantericshelf, and strongly asymmetric distal condyles of metatarsal II. These autapomorphies indicate that PVSJ 845belongs to a new species of early dinosaur.A cladistic analysis (371 characters and 49 taxa) was performed in order to assess the phylogentic position ofthe new dinosaur among basal dinosauriforms. The results unequivocally depicted PSVJ 845 as a basalsauropodomorph, supported by the presence of an ilium with partially open acetabulum, an elongated pubicpeduncle, supraacetabular crest contacting the distal end of the pubic peduncle as a well developed crest, and aproximal tibia with lateral condyle anterior to the lateral one. Within Sauropodomorpha, PVSJ 845 was found asmore closely related to Saturnalia than to any other basal dinosaur, by the presence of an iliac postacetabularprocess with a pointed posteroventral corner and a rounded posterodorsal margin, and with a strong andanterodorsally tapering trapezoidal muscle scar. It is interesting to note, that within Saurischia, Herrerasauridae(including Herrerasaurus, Staurikosaurus, MACN-Pv 18649a) was obtained as the sister group of Eusaurischia,the latter including Theropoda (Eoraptor + Neotheropoda) and Sauropodomorpha. Agnosphitys was depicted asthe most basal sauropodomorph, being more basal than a politomy composed by Guaibasaurus, (Saturnalia +PVSJ 845) and more derived forms.The new basal dinosaur reported here led to modify our understanding on the early dinosaur faunas.Until now, the few oldest known dinosaur bearing assemblages reflected a poor dinosaur diversity, whereasdinosaur diversification and numerical abundance over other tetrapod groups were recorded in Norian beds. Thus,the traditional view was that the first dinosaur radiation occurred after the Carnian-Norian boundary (Bonaparte1982, Benton 1983). This key shift in the early dinosaur evolution was mainly explained as a single process, evenas through a competitive (Bonaparte 1982) or a non-competitive model (Benton 1983). PVSJ 845 and a newspecies of herrerasaurid (MACN-Pv 18649a, Ezcurra and Novas 2007) drastically increase the dinosaur diversityof the lower Ischigualasto Formation, as well as for the entire Carnian global record. In this regard, with the newremains, the dinosaur diversity of the lower Ischigualasto Formation resembles that of typical Norian dinosaurbearing assemblages (e.g. upper Los Colorados and lower Caturrita Fms.). Thus, contrasting with previousstatements, the oldest recorded dinosaur faunas were well diversified and the early radiation of dinosaurs did notoccur as a single process. It is recognized here two distinct diachronous processes, first, a wide diversification,but presenting a low abundance, during the early/middle Carnian; and lately an increase in abundance during theNorian, probably promoted by the end-Carnian extinction event.