Silesaurus

Abstract: The South American Late Triassic offers the most comprehensive window to the early radiation of dinosaurs. This is enhanced by the discovery of Sacisaurus agudoensis, a new dinosauriform from the Caturrita Formation of Brazil. Various morphological features suggest its close phylogenetic affinity to Silesaurus, and both may be basal ornithischian dinosaurs. Sacisaurus has a pair of elements forming the tip of its lower jaw, hypothesized to be equivalent to the ornithischian predentary. This suggests that during an initial stage of their evolution, those dinosaurs had a paired predentary, which later fused into a single structure. As an originally paired bone, the predentary is comparable to elements that more often form the vertebrate mandible, such as the mentomeckelian bone. Although synapomorphic for ornithischians, the predentary does not seem neomorphic for the group, but primarily homologous to parts of the symphyseal region of the lower jaw of other vertebrates.

Abstract: Silesaurus opolensis Dzik, 2003 from the Late Triassic (late Carnian) of Poland is a key taxon for understanding the evolution of early dinosaurs. High intraspecific variation observed in the S. opolensis braincase brings caution in taxonomic and diversity studies of early dinosauromorphs. The external and internal osteology of three almost complete braincases of S. opolensis show that this taxon shares several similarities with other early dinosauriforms, which supports a close relationship among these forms. However, the paroccipital processes of S. opolensis are directed ventrally like in birds, reaching the level of the ventral margin of the basioccipital condyle. In dinosauromorphs, these processes usually have an almost horizontal orientation (presumed to be the plesiomorphic condition). Modifications observed in birds and S. opolensis have resulted in the dorsoventral expansion of M. complexus and M. depressor mandibulae, which occupy the dorsolateral part of the posterior side of the skull...

Abstract: Eucoelophysis batdwini Sullivan & Lucas, 1999 is represented by several postcranial elements from the Petrified Forest Formation (Norian), New Mexico, USA. Eucoelophysis Sullivan & Lucas, 1999 was widely considered as a coelophysoid dinosaur by several authors, but the hindlimb anatomy of this genus clearly indicates that it belongs to neither of these groups. The following features exclude Eucoelophysis from Neotheropoda: absence of oblique ligament groove on caudal surface of femoral head, femoral medial epicondyle small and smoothly rounded, absence of caudal cleft between medial part of the proximal end of the tibia and fibular condyles, cnemial crest low, and fibular crest absent. Moreover, Eucoelophysis lacks dinosaurian synapomorphic characters, but has a plesiomorphic slightly inturned femoral head that prevents its assignment to Dinosauria. Interestingly, the morphology of the femur of Eucoelophysis is extremely similar to that of the basal dinosauriform Silesaurus opolensis Dzik, 2003 from the Late Triassic of Poland. In order to determine the phylogenetic position of Eucoelophysis, a cladistic analysis was carried out, which depicts Eucoelophysis as a non-dinosaurian dinosauriform. Thus reinterpreted, Eucoelophysis constitutes the youngest record of a non-dinosaurian dinosauriform, indicating their survival into the Norian, being co-eval with early dinosaurs.