Crocodiles, alligators

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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 .

Key words

  • Crocodylomorpha,

  • Crurotarsi,

  • Dinosauria,

  • Mesozoic,

  • rauisuchians,

  • Triassic

..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.

  • As described before, the Archosauriformes have the shared derived character of an antorbital fenestra (see left; 1).

    A good representative is Euparkeria who you have seen in previous lectures. Euparkeria lacks specializations that bar it from the ancestry of the archosaurs (3). Almost all of the Archosauriformes from the Newark and Chinle, however, belong to a more advanced sub-group within.

    The archosaurs

  • have the shared derived character of the loss of teeth on the palate (2). They consist of crocodilians and birds and all of the descendants of that common ancestor (such as dinosaurs in general).

    Archosaurs can be divided into two groups: the Ornithodira and the Crurotarsi, with shared derived conditions of the ankles being the most important characters. Ankle types within the Archosauriformes fall into three broad categories. First the primitive condition in which the ankle beds at several places in a rather flexible and loose way. The second type is a derived condition called crurotarsal in which the two major bones of the ankle (the astragulus and calcaneum) form a hinge joint (4 above). Usually the calcaneum develops a large backward heel not unlike our own. Crocodiles have this kind of ankle. The third form is another derived form in which a roller-type hinge develops between the astragalus plus the calcaneum and the distal tarsals or metatarsals (5 above). This is called a mesotarsal joint and it is the type seen in birds and other dinosaurs.

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