recommended that the term "Rauisuchia" be abandoned. In a study of the ctenosauriscid Arizonasaurus, paleontologist Sterling Nesbitt defined a clade of rauisuchians called "Group X". This group includes Arizonasuchus, Lotosaurus, Sillosuchus, Shuvosaurus, and Effigia. One distinguishing feature of Group X is their lack of osteoderms, which are common among many other crurotarsans. Many more features are found in the pelvis, including fully fused sacral vertebrae and a long, thin crest on the ilium called the supra-acetabular crest. Additionally, many members of Group X have smooth frontal and nasal bones, which make up the upper portion of the rostrum. In other "rauisuchians" and many other crurotarsans, this area has bumps and ridges. "Group X" is now termed Poposauroidea. Nesbitt later erected another clade, "Group Y", in 2007. Group Y falls within Group X to include Sillosuchus, Shuvosaurus, and Effigia. Group Y is diagnosed by the presence of four or more sacral vertebrae with fully fused neural arches, which is also seen in theropod dinosaurs (a case of evolutionary convergence). In addition, the cervical vertebrae that make up the neck are strongly amphicoelus, meaning that they are concave at both ends. The fourth trochanter, a ridge of bone on the femur for muscle attachment seen in nearly all archosaurs, is absent in Group Y. "Group Y" is now termed Shuvosauridae. Although not placed within Group Y, Lotosaurus shares many similarities with members of the clade, foremost of which is edentulous, or toothless, jaws. Edentulism is also seen in Shuvosaurus and Effigia, which have beak-like jaws. Nesbitt suggested that the derived characters of Lotosaurus may indicate that it is a transitional form between basal members of Group X and members of Group Y. Below is the cladogram from Nesbitt (2007): In their phylogenetic study of archosaurs, Brusatte et al. (2010) found only weak support for Rauisuchia as a monophyletic grouping. As a result of their analysis, two clades were found to be within Rauisuchia, which they named Rauisuchoidea and Poposauroidea. Rauisuchoidea included Rauisuchidae and Prestosuchidae, as well as several basal taxa that were once assigned to the families, including Fasolasuchus and Ticinosuchus. Poposauroidea included poposaurids and ctenosauriscids, but the phylogeny had a large polytomy of genera in both groups that was difficult to resolve, which included Arizonasaurus, Poposaurus, and Sillosuchus. However, the characters linking these two groups were weak, and the question as to whether or not "Rauisuchia" forms a natural group remains