A new and well-preserved early-diverging abelisaurid (theropoda: ceratosauria: abelisauroidea) from the early late cretaceous of northern patagonia

Abstract

Abelisaurid theropods were the most diverse and abundant medium-sized to large-bodied carnivorous dinosaurs in many Gondwanan and European paleoecosystems during the Late Cretaceous, frequently occupying the apex predator niche in these environments. Although several derived abelisaurids (e.g., the Patagonian brachyrostrans Ekrixinatosaurus novasi, Skorpiovenator bustingorryi, Viavenator exxoni, Aucasaurus garridoi, and Carnotaurus sastrei and the Madagascan majungasaurine Majungasaurus crenatissimus) are represented by well-preserved skeletons, earlier-diverging members of the clade (e.g., the African Kryptops palaios and Rugops primus) are known from much less complete material. Consequently, the early evolutionary history of Abelisauridae remains poorly understood. Here we report a new taxon of medium-sized (body length ~5 m) basal abelisaurid collected from an exposure of the lowermost Upper Cretaceous (Cenomanian) Candeleros Formation in an area known as Aguada Pichana near the town of Añelo in Neuquén Province, northern Patagonia, Argentina. The new form is known from two individuals, one of which is represented by a largely complete, partially articulated skeleton (including much of an articulated skull and multiple teeth plus dorsal, sacral, and caudal vertebrae, dorsal ribs, hemal arches, a scapula, the forelimb lacking the manus, the pelvis, and several hind limb elements); the second individual is known only from a partial skull (consisting of the maxilla, lacrimal, jugal, quadratojugal, quadrate, pterygoid, ectopterygoid, palatine, and dentary with teeth). Notable osteological features include: (1) maxilla with 14 tooth positions (as in Carnotaurus and some generically unidentified abelisaurids but 3–5 fewer than in Rugops, Skorpiovenator, and Majungasaurus); (2) lacrimal with prominent anterior process; (3) postorbital with ‘inflated’ dorsal terminus and suborbital flange; (4) anterior caudal transverse processes with well-developed anterior projection at distal end; (5) humerus and metatarsals proportionally slender, recalling those of non-abelisaurid abelisauroids; and (6) radius and ulna ~34% length of humerus (proportionally longer than in Aucasaurus, Carnotaurus, and Majungasaurus). Phylogenetic analysis using two independent datasets recovers the new Candeleros form as a basal (i.e., non-brachyrostran, nonmajungasaurine) abelisaurid. As such, the new taxon is herein regarded as the earliest-branching abelisaurid that is known from the greater part of the skeleton.