A REVIEW OF THE DINOSAUR DIVERSITY OF THE ISCHIGUALASTO FORMATION (CARNIAN, NW ARGENTINA): INSIGHTS ON EARLY DINOSAUR EVOLUTION

EZCURRA, M. D.; NOVAS, F. E.

Ribeirao Preto

Simposio; VI Simposio Brasilero de Paleontologia de Vertebrados; 2008

Universidade do Sao Paulo

The Ischigualasto Formation from Argentina and the Santa Maria Formation from Brazil are among the oldest dinosaur bearer outcrops of the world (middle Carnian). The current available information from these beds shows that dinosaurs were a minor and poorly diverse component of their faunas. In fact, only three dinosaur genera are currently valid from the Ischigualasto Formation (Herrerasaurus ischigualastensis, Eoraptor lunensis and Pisanosaurus mertii) and two from the Santa Maria Formation (Staurikosaurus pricei and Saturnalia tupiniqium). However, here is reported two new dinosaur species exhumed from the lower third of the Ischigualasto Formation (Hoyada de Ischigualasto). The first is based on a partial forelimb, including an articulated and almost complete manus, of a small herrerasaurid saurischian (MACN-Pv 18649a). Its affinities with Herrerasauridae are supported by a large lateral-most distal carpal, lateral condyle of metacarpal I smaller than pulley of phalanx I-1, manual digits II-III with elongated penultimate phalanx, and metacarpals IV-V palmar to the others. Albeit the last two characters are present in coelophysoids, a cladistic analysis interpreted them as homoplasies due to the stem-saurischian position of Herrerasauridae. Otherwise, MACN-Pv 18649a differs from other dinosaurs in the presence of phalanx II-1 with conspicuous longitudinal ridge on its proximolateral border and manual unguals with posteriorly bifurcated lateral groove. The new herrerasaurid differs from Herrerasaurus ischigualastensis in the presence of trochlea of metacarpal I with strongly proximodistally asymmetric condyles, ulnar articular surface subequal to the distal articular end of the ulna, radiale tapering medially, well developed dorsal lip of phalanx I-1 and metacarpal V proportionally longer. MACN-Pv 18649a is distinct from Eoraptor lunensis in the presence of a long and gracile metacrapal I and first phalanx of the first digit, medial distal condyle of metacarpal I smaller than that of phalanx I-1, a lateral flange on the proximal end of the metacarpal III, metacarpals IV and V setted palmar to metacarpals I-III, and strongly recurved and laterally compressed manual unguals. The second new form from the Ischigualasto Formation corresponds to a new species of eusaurischian dinosaur (PVSJ 845), represented by proximal caudal vertebrae, ilium, femora, tibia, fibula, metatarsals, and pedal phalanges. A preliminar cladistic analysis depicted PVSJ 845 as a stem-sauropodomorph more closely related to the Brazilian Saturnalia tupiniqium than to other taxa. The affinities of the new Argentinean form with Sauropodomorpha are supported by the presence of the posterior end of the proximal lateral condyle of the tibia anterior to the medial condyle and an ilium with a long pubic peduncle. Furthermore, this form shares with Saturnalia (contrasting with other dinosauriforms) the presence of the following combination of apomorphies: length of postacetabular process that exceeds the length of the acetabulum, an incipient perforation of the acetabular wall and trochanteric shelf on proximal femur. The Argentinean form differs from other known dinosauriforms (including Saturnalia) in the presence of a poorly laterally projected supraacetabular crest of the ilium, femoral lateral surface with deep and large fossa immediately below the trochanteric shelf and strongly dorsoventrally asymmetric distal trochlea of metatarsal II. The close relationship between the new Argentinean dinosaur and Saturnalia gives more support to previous biostratigraphical correlations between the lower fauna of the Ischigualasto Formation and the Hyperodapedon-zone of the Santa Maria Formation. These two new forms increase the Ischigualastian and world-wide diversity of Carnian dinosaurs, showing that dinosaurs although being a minor faunistic component, they were well diversified before their numerical dominance in post-Carnian times.