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. History Franz Nopcsa first used the term Poposauridae in 1923 to refer to poposauroids. At this time, the sole member of the group was Poposaurus, which was considered to be a theropod dinosaur. Over the following years, poposauroids were placed in various groups, including Saurischia, Theropoda, and Carnosauria. This classification existed up until the 1970s, when better remains indicated that Poposaurus was a pseudosuchian rather than a dinosaur. Other genera such as Sillosuchus and Shuvosaurus were later erected. Like Poposaurus, Shuvosaurus was originally thought to be a theropod dinosaur. 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 tyrannosaurs. 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. However, 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