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Poposauroidea is a clade of advanced Pseudosuchian. It includes , Shuvosaurinae, , and other unusual pseudosuchians such as Qianosuchus and Lotosaurus. It excludes most large predatory quadrupedal "Rauisuchia" such as and "". Those reptiles are now allied with Crocodylomorpha (crocodile ancestors) in a clade known as Loricata, which is the Sister group to the poposauroids in the clade Paracrocodylomorpha. Although it was first formally defined in 2007, the name "Poposauroidea" has been used for many years. The group has been referred to as Poposauridae by some authors, although this name is often used more narrowly to refer to the family that includes Poposaurus and its close relatives. It was phylogenetically defined in 2011 by Sterling Nesbitt as Poposaurus gracilis and all taxa more closely related to it than to Postosuchus kirkpatricki, Crocodylus niloticus (the Nile crocodile), Ornithosuchus woodwardi, or Aetosaurus ferratus.
Poposauroids went extinct at the end of the Triassic period along with other non-crocodylomorph pseudosuchians. They were among the most diverse and longest lasting members of non-crocodylomorph Pseudosuchia, with Xilousuchus (a ctenosauriscid) living near the very beginning of the Triassic and Effigia (a shuvosaurid) surviving up until near the end of the Triassic. Despite the high level of diversity and anatomical disparity within Poposauroidea, certain features of the clade can be determined, particularly in the structure of the snout and pelvis (hip). Many of these features are examples of convergent evolution with Dinosaur, with bipedal poposauroids such as Poposaurus and shuvosaurids having been mistaken for Theropoda dinosaurs in the past.
Poposauroids also possess several features which are unusual compared to archosaurs in general. For example, in most archosaurs each side of the Neurocranium 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 Proterochampsia. 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 rib. This second neck trait contrasts with the condition in other pseudosuchians, Phytosaur, and pterosaurs, which have short and stout cervical ribs. The neural spines of the dorsal (back) Vertebra 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 outward to form a flat, rectangular surface when seen from above.
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. In poposauroids other than Qianosuchus and Lotosaurus, the bones did not touch, leaving the acetabulum open from the sides and 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. 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.
Basal poposauroids such as Arizonasaurus and Qianosuchus only had three sacral vertebra, with the second vertebra being the 'insertion'. More advanced poposauroids such as Poposaurus and shuvosaurids have four sacral vertebrae, the third 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 called a dorsosacral. They were irregularly distributed among archosaurs, known in a few Ornithosuchidae and Aetosaur as well as a variety of dinosaurs (most commonly in Ornithischia and theropods)
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. Poposauroids had 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. 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.
Sankar Chatterjee reclassified poposauroids as theropod dinosaurs with his description of the new genus Postosuchus in 1985. Chatterjee even considered poposauroids to be the ancestors of . Postosuchus was widely considered to be a poposauroid for the next ten years and was included in many phylogenetic analyses of Triassic archosaurs. In 1995, Robert Long and Phillip A Murry noted that several specimens referred to Postosuchus were distinct from the holotype, and so they assigned those specimens to the new genera Lythrosuchus and Chatterjeea.
In 2005, Sterling Nesbitt noted that "ctenosauriscids" such as Arizonasaurus, Bromsgroveia, and Lotosaurus shared many similarities with "poposaurids" such as Poposaurus, Sillosuchus, and " Chatterjeea" (now known as Shuvosaurus). He proposed that they formed a clade (informally named " Group X") to the exception of other pseudosuchians.
"Group X" was formally given the name "Poposauroidea" by Jonathan C. Weinbaum and Axel Hungerbühler in 2007. In their paper, Weinbaum and Hungerbühler described two new skeletons of Poposaurus and incorporated several new characters of the genus into a phylogenetic analysis. Poposauroidea was recovered as a monophyletic grouping, while other rauisuchians (namely Rauisuchidae and Prestosuchidae) were placed as basal forms of a new group called Paracrocodyliformes.
Brusatte et al. (2010) conducted a phylogenetic study of archosaurs that resulted in a grouping referred to as Poposauroidea. Unlike many recent studies, they found Rauisuchia to be monophyletic, consisting of two major clades: Rauisuchoidea and Poposauroidea. The monophyly of Rauisuchia was not strongly supported in Brusatte et al.'s analysis. They noted that if their tree was enlarged by one step, Poposauroidea fell outside Rauisuchia to become the sister group of Ornithosuchidae, which is thought to closely related to, but outside, Rauisuchia. In their tree, Poposauroidea included genera usually classified as poposauroids as well as several other genera that were not previously placed in the group. One of these genera, Qianosuchus, is unique among pseudosuchians in its semiaquatic lifestyle.
In his massive revision of archosaurs which included a large cladistic analysis, Sterling J. Nesbitt (2011) found Xilousuchus to be a poposauroid most closely related to Arizonasaurus. Nesbitt's analysis did not recover a monophyletic Rauisuchia or monophyletic Rauisuchoidea. Poposauroidea was found to be monophyletic, and more resolved than in previous analyses, with Qianosuchus as the most basal member of the group and Lotosaurus grouping with shuvosaurids instead of ctenosauriscids. The cladogram below follows Nesbitt (2011) with clade names based on previous studies.
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