How diversified was the sauropod fauna at the base of the Neuquén Group?: New evidence from Candeleros Formation (Cenomanian) of southern Neuquén Basin (Patagonia, Argentina)

Abstract

The Neuquén Group represents a thick sequence of Upper Cretaceous terrestrial sediments that were deposited over more than 20 million years in the Neuquén Basin, in northern Patagonia (Argentina). The Neuquén Group has yielded an abundant and diversified vertebrate fossil record, especially of reptile fauna, allowing us to reconstruct part of the evolutionary history of different archosaur lineages, such as sauropod dinosaurs. In this contribution we report new sauropod findings from the lower section of the Candeleros Formation outcropping at El Sauce locality (Neuquén Province, Argentina), the earliest lithostratigraphic unit of the Neuquén Group, to explore the taxonomic and morphological diversification of the sauropod fauna in southern Neuquén Basin during the Cenomanian. The new fossil record is composed of: a middle‐to‐posterior incomplete cervical vertebra (58‐MES‐PV‐04), the anterior portion of a cervical vertebra (58‐MES‐PV‐03), an incomplete anterior caudal vertebra (58‐MES‐PV‐02), a proximal epiphysis of a radius (58‐MES‐PV‐01), a distal epiphysis of a pubis (58‐MES‐PV‐05), and an almost complete coracoid (58‐MES‐PV‐ 07). The cervical elements show opisthocoelic articulation, dorsoventrally low and transversely wide articular surfaces, and camellate internal bone texture, as in derived Titanosauriformes. The anterior caudal centrum has compact internal bone structure and amphicoelic articulation, with the posterior articular surface more concave than the anterior one; moreover, the neural pedicels are tall and the incomplete transverse process, apparently dorsally directed, bears wide pneumatic fossae. The specimen 58‐MES‐PV‐02 shares these conditions with most rebbachisaurids. The radius is triangular in proximal view, with the anterior margin slightly convex and the medial projection elongated and tapered, as in several Titanosauriformes. The distal pubis fragment is crescent in ventral view and anteriorly prominent in lateral view, as in Ligabuesaurus. The coracoid is quadrangular, with the posterior margin slightly convex and the anterior one straight, as in Tapuiasaurus and Ligabuesaurus. The preliminary morphological analysis of new findings from El Sauce locality suggests a diversified neosauropod fauna, composed of different specimens of both Diplodocoidea and Macronaria lineages, such as Rebbachisauridae and Somphospondylii. This composition resembles the sauropod fauna recorded in the nearby Cerro de los Leones locality, where the underlying Lohan Cura Formation (Albian) yielded the non‐titanosaur somphospondylan Ligabuesaurus, the rebbachisaurid Agustinia and the indeterminate neosauropod MCF‐PVPH‐882. This new record suggests the persistence of specific paleoecological conditions in the southern Neuquén Basin during the Early–Late Cretaceous transition, which allowed sustaining diversified sauropod faunae, probably supported by a niche partitioning via feeding high stratification.