The oldest theropods from the Neuquen Basin: Predatory dinosaur diversity from the Bajada Colorada Formation (Lower Cretaceous: Berriasian-Valanginian), Neuquen, Argentina

Abstract

Here, we describe new theropod materials (several isolated teeth, an axis, two caudal centra and a proximal left tibia) from the type locality of the Bajada Colorada Formation (Lower Cretaceous, Berriasian eValanginian). Although fragmentary, the recovered material shows a diverse association of meat-eating dinosaurs for this poorly understood period of time. Three techniques were used to assess the phylogenetic position of the isolated teeth: multivariate (PCA), discriminant and phylogenetic analyses. The morphotypes 1, 2 and 3 (small non-recurved isolated crowns) were regarded as Theropoda indet., as our analyses failed to support a more precise classification. Two large almost complete ziphodont crowns, considered morphotype 4, were identified as belonging to megalosaurid tetanurans by phylogenetic, discriminant and multivariate analyses, thus likely representing the first record of this ancient family in South America, and the youngest worldwide. We refer the axis to a small abelisauroid ceratosaurian based on the following suite of characters; long and pointed epipophyses, a pneumatic foramen in the centrum, the invaginated spinopostzygapophyseal lamina, and the anteroposteriorly long, dorsally convex, and dorsally/posteriorly unexpanded neural spine. The caudal centra can be referred to a theropod, and tentatively to Abelisauroidea, suggesting that they may belong to the same taxon than that represented by the axis. The tibia lacks the incisura tibialis, presents a low, sharp and proximally positioned fibular crest, and has a deep lateral fossa, which is limited dorsally by a coarse, rounded in section and anteriorly directed crest. This combination of features allows to refer it to a large abelisaurid. The Bajada Colorada dinosaur record includes so far a small abelisauroid, a large abelisaurid, a probably medium to large megalosaurid tetanuran, diplodocid and dicraeosaurid sauropods. It shows some similarities with Middle and Upper Jurassic units in central Patagonia, Africa and Portugal, suggesting that no significant dinosaur faunal turnover took place through the Jurassic-Cretaceous event in the southern part of South America.