Crurotarsans are some pretty amazing animals, having occupied almost every major ecological niche during the Triassic Period, a time that lasted almost 50 million years (251 Ma to 201.6 Ma). They still survive today as crocodiles, alligators, and their relatives but are nowhere near as diverse and impressive as their ancestors.
Named by Paul Sereno in 1991, Crurotarsi means "cross-ankles" based on the way their ankles articulate compared to their sister taxon, Avemetatarsalia (Dinosaurs, Pterosaurs, etc).
Although the taxonomy is still disputed some of the groups that make up this clade include Phytosaurs, Aetosaurs, Rauisuchians, Poposaurids, and Crocodylomorphs.
Project 552: Brusatte, S. L., M. J. Benton, J. B. Desojo, and M. C. Langer. 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida). Journal of Systematic Palaeontology. 8 (1):3-47.
Crown group Archosauria, which includes birds, dinosaurs, crocodylomorphs, and several extinct Mesozoic groups, is a primary division of the vertebrate tree of life. However, the higher-level phylogenetic relationships within Archosauria are poorly resolved and controversial, despite years of study. The phylogeny of crocodile-line archosaurs (Crurotarsi) is particularly contentious, and has been plagued by problematic taxon and character sampling. Recent discoveries and renewed focus on archosaur anatomy enable the compilation of a new dataset, which assimilates and standardizes character data pertinent to higher-level archosaur phylogeny, and is scored across the largest group of taxa yet analysed. This dataset includes 47 new characters (25% of total) and eight taxa that have yet to be included in an analysis, and total taxonomic sampling is more than twice that of any previous study. This analysis produces a well-resolved phylogeny, which recovers mostly traditional relationships within Avemetatarsalia, places Phytosauria as a basal crurotarsan clade, finds a close relationship between Aetosauria and Crocodylomorpha, and recovers a monophyletic Rauisuchia comprised of two major subclades. Support values are low, suggesting rampant homoplasy and missing data within Archosauria, but the phylogeny is highly congruent with stratigraphy. Comparison with alternative analyses identifies numerous scoring differences, but indicates that character sampling is the main source of incongruence. The phylogeny implies major missing lineages in the Early Triassic and may support a Carnian-Norian extinction event
…..great many examples of Archosauriformes have been found in the Newark Supergroup and the Chinle Group. To put these forms into context we need to look (again) at the cladogram (see right) of basic amniote relationships and specifically relationships within the Archosauriformes.