Silesaurus opolensis Dzik, 2003

Nesbitt, Sterling J., 2011, The Early Evolution Of Archosaurs: Relationships And The Origin Of Major Clades, Bulletin of the American Museum of Natural History 2011 (352), pp. 1-292 : 49

Silesaurus opolensis Dzik, 2003 (fig. 12F)

AGE: Late Carnian ( Dzik, 2001).

OCCURRENCE: Krasiejów, Opole, Silesia, Poland.

HOLOTYPE: ZPAL Ab III/361, dentaries, braincase, pterygoid, frontals, quadrate, surangular, nearly complete presacral column, sacrum, caudal vertebrae, scapulocoracoid, radii, ulnae, complete pelvic girdle, and hind limbs.

REFERRED MATERIAL: ZPAL AbIII/362, braincase, cervical, dorsal, sacral, and caudal vertebrae, partial pectoral girdle and forelimb, partial pelvic girdle and hind limbs; ZPAL AbIII/363, associated pelvic girdle; ZPAL AbIII/364, braincase, presacral vertebrae, ribs, partial forelimbs, complete articulated hind limbs.

REMARKS: Since Dzik’s (2003) initial description, Silesaurus has revolutionized the understanding of the systematics of basal avian-line archosaurs. Silesaurus is known from well-preserved material from nearly all parts of the skeleton ( Dzik, 2003) except some of the more delicate bones of the skull ( Dzik and Sulej, 2007). The material derives from a single horizon in a single locality (Krasiejów) and occurs as both isolated elements and nearly complete skeletons. The well-preserved three-dimensional specimens allow a nearly unparalleled examination of morphological features.

The divergent morphology of Silesaurus strongly contrasts with the typical basal dinosaurian and avian-line archosaur bauplan. The elongated forelimbs are proportionally longer than those of basal dinosaurs. The manus is largely missing, but fragments of metacarpals and phalanges suggest the manus was quite small. Furthermore, the dentition closely resembles that of ornithischians, and the anterior portion of the dentary tapers to a sharp point.

The odd mixture of features elsewhere present in herbivorous dinosaurs features has led to controversy concerning the systematic position of Silesaurus . Dzik (2003) did not place Silesaurus in a phylogenetic analysis, but suggested that it was closely related to, but did not represent, a true dinosaur. Subsequently, Dzik and Sulej (2007) suggest- ed that Silesaurus represents a basal ornithischian based on new material. However, this was not based on a phylogenetic analysis either. In explicit phylogenetic analyses, Langer and Benton (2006), Ezcurra (2006), and Irmis et al. (2007a) found Silesaurus as the sister taxon to Dinosauria . Given this important systematic position, Silesaurus polarizes dinosaurian synapomorphies and is of extreme interest.

Silesaurus differs from all other archosaurs by the combination of the following characters: (1) edentulous anterior portion of the dentary that tapers to a point well above the dental margin; (2) maxillary and dentary tooth crowns expanded above root with small denticles; and (3) femur with notch on the proximal end.

KEY REFERENCES: Dzik, 2003; Ezcurra, 2006; Nesbitt et al., 2007; Irmis et al., 2007a.