Silesaurus opolensis Dzik, 2003 from the Late Triassic (late Carnian) of Poland is a key species for understanding the evolution of early dinosaurs. Specimens of Silesaurus offered rich evidence about the origin of the dinosaur body plan (Smith et al., 2007; Langer et al., 2009; Martinez & Alcober, 2009; Nesbitt et al., 2010, 2017; Martinez et al., 2011, 2013; Langer, 2014), which becomes especially important in the dispute of paraphyly (Langer & Ferigolo, 2013) or monophyly (Kammerer et al., 2012) of the Silesauridae and its relationship to the Ornithischia (Dzik, 2003; Dzik & Sulej, 2007; Ferigolo & Langer, 2007; Niedźwiedzki et al., 2009; Cabreira et al., 2016). This study presents postcranial anatomy and some aspects of the skull of Silesaurus that permit myological reconstructions and estimation of the range of its skeletal variability. The objective of the present contribution is to use of this knowledge to evaluate similarities of Silesaurus with early dinosaurs. A find of partially articulated skeleton of Silesaurus, with the skull, neck, pectoral girdle and thorax, supplemented by additional specimens, enabled complete restoration of the vertebral column and associated skeletal parts. Cervical ribs of Silesaurus, well preserved in their original disposition, are parallel to the neck and extend backward for a few vertebral lengths. There is a sudden change in their morphology behind the seventh vertebra, although otherwise the transition from the cervical to the dorsal vertebrae is very gradual. Parapophyses slowly migrate upward along the anterior margin of the centrum and leave the centrum at the sixth or seventh dorsal vertebra. Narrowing of the dorsal extremities of the neural spines of the fourth and neighboring vertebrae suggests the ability of this region of the vertebral column to bent upward. There is thus a disparity between the structural and functional neck–thorax transition. The presence of three sacrals firmly connected by their ribs with the ilia and the long tail of Silesaurus, providing a counterbalance to the weight of the body in front of the pelvis, suggest the ability for fast bipedal running. However, unusually long but gracile forelimbs of Silesaurus indicate quadrupedal stance. It is widely accepted that ornithodirans (bird lineage) and some pseudosuchians (crocodilian lineage) achieved fully erect limb posture in different ways. Ornithodirans have buttress-erected hindlimbs, while some advanced pseudosuchians have pillar-erected hindlimbs. Analysis of the musculoskeletal apparatus of the early dinosauriform Silesaurus challenges this view. This ornithodiran had pillar-erected hindlimbs like some pseudosuchians. This condition could be plesiomorphic or represents a transitional state between adductor-controlled limb posture of early dinosauromorphs and the buttress-erected hindlimbs of dinosaurs. This sequence of changes is supported by Triassic tracks left by animals of the dinosaurian lineage. It was associated with the strong development of knee

1 flexors and extensors. Furthermore, the forelimbs of Silesaurus were fully erect analogously to those of early sauropods. Members of both lineages had reduced muscles related to the protraction, retraction, and bending of the limb. They used forelimbs more as a body support and less for propulsion. A similar scapula and humerus construction can be found in the Lagerpetidae and Lewisuchus suggesting that long, slender, fully erected forelimbs are primitive for all Dinosauromorpha, not just the Silesauridae. Early dinosaurs redeveloped several muscle attachments on the forelimb probably in relation with bipedality. A principal component analysis (PCA) performed for a set of 24 measurements on 33 femora and 15 measurements on 20 ilia of Silesaurus, shows that this sample is highly variable but probably monospecific. Most of the morphological variation is concentrated in the muscle attachments and proportions of bones, which significantly change in both size and position during ontogeny. Despite the small sample size, femora of smaller individuals have less flattened shafts and a more sinusoidal appearance. In many large specimens, proximal parts of muscle tendons are ossified at their attachment site on femora and remain attached to the bone in the largest specimens. The specimens with attached ossifications are interpreted as mature females that were statistically larger than proposed males. It is suggested that ossifications developed in females under the calcitonin control. The intrapopulation variability of ilia is high, but less dependent on ontogeny. High intraspecific variation observed in the Silesaurus braincase calls for caution in taxonomy and diversity studies of early dinosauromorphs. The osteology of three almost complete braincases of Silesaurus shows that this taxon shares several similarities with other early dinosauriforms, which supports a close relationship among them. However, the paroccipital processes of Silesaurus 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 Silesaurus have resulted in the dorsoventral expansion of Musculus complexus and M. depressor mandibulae, which occupy the dorsolateral part of the posterior side of the skull. In adult birds, these muscles act strongly on the initial upstroke of the head during drinking. Therefore, the inferred condition of these muscles in Silesaurus may imply that Silesauridae evolved toward bird-like feeding behaviour. The obtained osteological data and their functional interpretation have demonstrated the crucial position of Silesaurus opolensis in early evolution of dinosaurs.

2