THE OLDEST THEROPODS FROM THE NEUQUÉN BASIN: PREDATOR DINOSAUR DIVERSITY IN THE BAJADA COLORADA FORMATION (BERRIASIAN-VALANGINIAN), NEUQUÉN, ARGENTINA

JUAN I. CANALE; SEBASTIÁN APESTEGUÍA; PABLO A. GALLINA; ALEJANDRO HALUZA; FEDERICO A. GIANECHINI; LEONARDO J. PAZO

Buenos Aires

Jornada; XXX Jornadas Argentinas de Paleontología de Vertebrados; 2016

In this contribution an updated summary of the theropod materials collected from the Bajada Colorada Formation (Lower Cretaceous) at its type locality is offered. At present, this includes an axis, two caudal centra, a proximal tibia, and several isolated teeth. The axis shows synapomorphies of Ceratosauria, like the pleurocoels in the centrum, long epipophyses; and from Abelisauroidea, like invaginated spinopostzygapophyseal laminae. The mid-distal caudal centra (MMCh-PV-76-77) are long and gently amphicoelous; they lack pleurocoels and show tall-suboval, articular surfaces. Given the lack of diagnostic traits they were regarded as Theropoda indet. The tibia (MMCh-PV-66) is referable to Ceratosauria after the hypertrophied cnemial crest, the deep lateral fossa, and the fibular crest reaching the proximal end. It is also referable to Abelisauridae becauseof the anteriorly curved fibular crest, running along the dorsal margin of the cnemial crest. Among teeth we recognize at least two morphotypes, for which we ran phylogenetic and multivariate (Principal Components) analyses in order to approach a classification. Morphotype 1, represented by several small, not curved crowns (MMCh-PV-68-2 to 5 and 8 to 10), was not clearly recovered in any theropod subclade, but Morphotype 2, represented by two large, ziphodont, curved crowns, with denticles in mesial carina restricted to the apical half (MMCh-PV-68-6, 7), was recovered by both analyses as belonging to megalosaurid tetanurans. The Bajada Colorada theropod association shows large abelisaurids, small to middle abelisauroids, and large megalosaurid tetanurans, which coexisted at the beginning of the Cretaceous in South America.