Rauisuchus tiradentes, von Huene, 1938

Lautenschlager, Stephan & Rauhut, Oliver W. M., 2015, Osteology of Rauisuchus tiradentes from the Late Triassic (Carnian) Santa Maria Formation of Brazil, and its implications for rauisuchid anatomy and phylogeny, Zoological Journal of the Linnean Society 173 (1), pp. 55-91 : 82-83

RAUISUCHUS TIRADENTES

Rauisuchoid ilium

The only known ilium ( BSPG AS XXV 88; Fig. 17A, B ) originally assigned to R. tiradentes is a left element and comes from the secondary fossil site as part of ‘ Find 1025’. Compared with the pubis of the lectotype it shows a notable difference in size and is from an individual that was at least 20–25% smaller than the lectotype.

The ilium consists of an acetabular area and the iliac blade with two processes: an anterior preacetabular process and a posterior postacetabular process. The lateral surface of the main body is nearly entirely occupied by a deep acetabulum. A prominent supraacetabular rim surrounds the dorsal margin. Anteriorly, the main body possesses a spike-like, dorsally flattened projection, which forms the pubic process. In this region the supra-acetabular rim is strongly developed. Anteroventrally, below the pubic process, the ilium bears a longitudinal groove with a depression on the ventralmost part, forming the articulation socket for the pubis. The parallel acetabular rim is not interrupted, indicating that the pubis only marginally contributed, or did not contribute at all, to the acetabulum. The posteroventral part of the ilium is partly broken. The area where the ischiadic process would be located is missing. The preacetabular process is short and projects anteriorly, but does not extend anterior to the pubic process. It has a blunt outline and is only slightly separated from the main body. The postacetabular process is considerably elongated and projects posteriorly and slightly laterally, although the latter might be a preservational artefact. Its cross section is triangular with a rounded lateral side. The medial(?) side is covered by a ventromedially excavated depression. Posteriorly it tapers to a blunt tip. Above the supra-acetabular rim, located between the preacetabular and the postacetabular process, the ilium possesses a prominent supra-acetabular ridge or buttress. It is perpendicular to the longitudinal axis of the iliac blade and contacts the dorsal margin to the supra-acetabular rim. According to Colbert (1961) and Chatterjee (1985), this ridge represents the attachment point for the iliofemoralis or the iliotibialis muscle, although this inference has been debated ( Gower, 2000).

On the medial side of the ilium, four distinct depressions are visible, which most probably represent the articulation sockets for the two-pronged sacral ribs of the sacral vertebrae. As no corresponding or overlapping elements are preserved in the lectotype, the ilium cannot be referred to R. tiradentes , although the following characteristics support its classification as a rauisuchoid ilium: (1) the ilium contributes to the major part of the acetabulum; (2) presence of a supraacetabular rim; (3) a short preacetabular and an elongated postacetabular process; (4) presence of a supraacetabular ridge connecting the supra-acetabular rim and the dorsal margin of the ilium; (5) closed acetabulum ( Gower, 2000; Nesbitt, 2011). The fact that the acetabulum is almost entirely or even entirely placed on the ilium might indicate that it was correctly assigned to R. tiradentes , as the latter lacks an acetabular facet on the pubis; however, this can only be confirmed by additional material.

Figures

Figure 17. Left ilium (BSPG AS XXV 88) of Rauisuchoidea indet. (formerly assigned to Rauisuchus tiradentes) in lateral view (A) and medial view (B). Abbreviations: a.sr, articulation with sacral ribs; ac, acetabulum; app, anterior preacetabular process; d, depression; pep, pelvic process; ppp, posterior postacetabular process; sab, supra-acetabular buttress; sar, supraacetabular rim.