Poposauroidea

The skull of Qianosuchus, the earliest-diverging genus of poposauroid. Note the arrangement and structure of skull bones around the enlarged nares.

Poposauroids can be differentiated from other pseudosuchians by the structure of the tip of the snout, particularly the premaxillary bone which lies in front of the nares (nostril holes). This bone possesses two bony extensions ("processes") which wrap around the nares. The anterodorsal process, which wraps above the nares to contact the nasal bones on the top edge of the snout, is typically quite short in pseudosuchians. However, poposauroids have elongated anterodorsal processes, longer than the main premaxillary body. The posterodorsal process, which wraps below the nares to contact the maxilla on the side of the snout, possesses the opposite state. It is much shorter in poposauroids (compared to other pseudosuchians), restricted to only a portion of the lower edge of the nares. This has the added effect of allowing the maxilla to form the rest of the hole's lower and rear edge, with the front edge of the maxilla becoming concave as well. Although these snout features are rare among pseudosuchians, they are much more common in certain avemetatarsalians (bird-line archosaurs) such as pterosaurs and saurischian dinosaurs. The rear branch of the maxilla tapers in most poposauroids, with the exception of Qianosuchus. This contrasts with loricatans, in which this branch is rectangular in shape.[1]

However, poposauroids also possess several features which are unusual compared to archosaurs in general. For example, in most archosaurs each side of the braincase possesses a pit from where the internal carotid arteries may exit the brain. In early poposauroids, these pits migrated to the underside of the braincase, thereby resembling the primitive condition seen in archosaur relatives such as Euparkeria and proterochampsians. Nevertheless, this reversion is undone in shuvosaurids (and possibly earlier, although no braincase material is known in Poposaurus or Lotosaurus). In addition, most poposauroids possessed elongated necks, and all of them had long and thin cervical ribs. This second neck trait contrasts with the condition in other pseudosuchians, phytosaurs, and pterosaurs, which have short and stout cervical ribs. The neural spines of the dorsal (back) vertebrae are thin and plate-like, even in members of Poposauroidea without sails. This differs compared to the vertebrae of most other early pseudosuchians (as well as Euparkeria and phytosaurs), which have neural spines that expand outwards to form a flat, rectangular surface when seen from above.[1]

Like other reptiles, the pelvis of poposauroids is formed by three plate-like bones: the ilium which lies above the acetabulum (hip socket), the pubis which is below and in front of the acetabulum, and the ischium which is below and behind the acetabulum. The ilium is a large, complex bone, with a forward-pointing (preacetabular) process, a rear-pointing (postacetabular) process, and a lower portion which forms the upper edge of the acetabulum. In most archosaurs, the lower portion of the ilium is wedge-shaped, forming the inner face of a "closed" acetabulum. However, in poposauroids the lower portion of the ilium is concave, creating a 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.[1]

The pelvis of Poposaurus, showing the many traits unique to poposauroid hips.

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.[1]

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.[1]

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 has a form unlike the vertebrae of these archosauriforms, and Nesbitt concluded that it was an 'insertion', formed from the innermost sections of the two primordial vertebrae. Although this process is not unique to poposauroids, it is only known in a few other archosaur lineages, such as Batrachotomus, silesaurids, and dinosaurs.[1]

Basal poposauroids such as Arizonasaurus and Qianosuchus only had three sacral vertebra, with the second vertebra being the 'insertion'. However, more advanced poposauroids such as Poposaurus and shuvosaurids have four sacral vertebrae, with the third vertebra being recognizable as the 'insertion'. This means that the first vertebra must have been another addition, seemingly the last dorsal vertebra which had been repurposed and transformed into a sacral vertebra. This incorporated dorsal vertebra is known as a 'dorsosacral'. The presence of dorsosacrals is a trait with an irregular distribution among archosaurs, being known in a few ornithosuchids and aetosaurs as well as a variety of dinosaurs (most commonly in ornithischians and theropods)[1][2]

In almost all archosariforms, the sacral ribs of the first 'primordial' sacral vertebra contact the ilium near the base of that bone, close to its contact with the pubis. However, poposauroids have first 'primordial' sacral ribs with additional forward branches, which lie on the inner edge of the ilium's preacetabular blade. In poposauroids more advanced than Qianosuchus, the sacral vertebrae fuse into a single bone, the sacrum. This fusion occurred incrementally, at different portions of the vertebra. For example, the zygapophyses fused together as early as the ctenosauriscids. The centra (main cylindrical portion) of the sacral vertebrae also may have fused as early as the ctenosauriscids.[1] The bases of the neural arches (the portion of the vertebrae above the spinal cord) were fused in some ctenosauriscids (Arizonasaurus) but not others (Bromsgroveia), and were also fused in all poposauroids more advanced than the ctenosauriscids.[3]

Unlike most pseudosuchians, poposauroids lack bony scutes known as osteoderms. The only exception to this is Qianosuchus, which possessed numerous tiny osteoderms, lying in a row extending down the neck and body.[4] In all poposauroids, the tip of the fibula (outer shin bone) is symmetrical and straight when seen from the side, rather than slanted as in other non-crocodylomorph pseudosuchians. Poposauroids more advanced than ctenosauriscids had flattened, "hoof-like" pedal unguals (toe claws). Some poposauroids had very short arms compared to the length of their legs, although disarticulation in Qianosuchus and a lack of limb material in ctenosauriscids means that it is unknown whether this trait was basal to the group as a whole. Missing data for ctenosauriscids also obscures when certain traits of the caudal (tail) vertebrae and ankle bones were gained or lost within Poposauroidea.[1]