partially to completely "open" acetabulum formed by open space instead of bone. The only other archosaurs with open hip sockets are dinosaurs and (to a lesser extent) crocodylomorphs. The upper edge of the acetabulum is formed by a pronounced ridge on the ilium, known as a supraacetabular rim or crest. Although all poposauroids possessed open acetabula, most other specializations of the ilium did not evolve until the clade containing Poposaurus and the shuvosaurids. For example, the supraacetabular crest projects downwards, rather than outwards in this clade. This trait is rare in archosaurs, only evolving independently in a few early theropod dinosaurs such as Coelophysis and Dilophosaurus. Another feature is the presence of an additional diagonal crest which branches upwards from the supraacetabular rim. Although such a crest evolved independently in a number of different archosaurs, this specific subset of poposauroids is unique in having the crest be inclined forwards (rather than vertical) and confluent with the elongated preacetabular blade, which is another derived feature of the clade. The pubis and ischium were also specialized in poposauroids. In every other archosaur, the two bones contact each other on the lower edge of the acetabulum. However, in poposauroids other than Qianosuchus and Lotosaurus, the bones do not touch, leaving the acetabulum open not only from the sides, but also from below. The width of the pubis is variable at different parts of its shaft. The portion near the acetabulum is thickened, but the tip of the bone (except in Qianosuchus) is very thin when seen head-on. In most other archosaurs, the pubis has a consistent width. However, theropod dinosaurs and a few other archosaurs have a distal part of the pubis which is thinner than the proximal part. Shuvosaurids and Lotosaurus also possessed ischia (on either side of the body) which were fused to each other at the midline of the body. Sacrum Although the ancestral archosaur only had two sacral (hip) vertebrae, many different archosaur groups acquired additional sacral vertebrae over the course of their evolution. Nesbitt (2011) argued that additional sacral vertebrae formed between these two 'primordial' vertebrae. He gave the well-preserved sacrum of the poposauroid Arizonasaurus as evidence to this process. Poposauroids have three to four sacral vertebrae, with the last and third-to-last vertebrae articulating with the ilium in a way similar to the two primordial vertebrae of more primitive archosauriformes such as Euparkeria and phytosaurs. The second-to-last vertebra