Nesbitt, 2011

Archosauria ("ruling reptiles") is one of the most diverse and successful groups of land animals. Of these ruling reptiles, dinosaurs and pterosaurs tend to get the most attention, especially in popular culture. Archosaur research has blossomed in recent years, but the public and even well-trained biologists know little about the diversity of extinct archosaurs. This blog is meant to highlight that diversity.

A Brief Note on Archosaur Names & Definitions

Archosauria (“ruling reptiles”, Cope 1869) was originally proposed as an order of Reptilia including the suborders Sauropterygia, Crocodylia, Thecodontia, Dinosauria, Anomodontia, and Rynchocephalia, specifically excluding Testudinata, and united based on the mode of articulation of the quadrate bone. Gauthier and Padian (1985) provided the first explicit phylogenetic definition for Archosauria as a monophyletic taxon, comprised of living birds and crocodiles and all taxa descended from their most recent common ancestor. Currently (Nesbitt, 2011), Archosauria is defined as the least inclusive clade containing Crocodylus niloticus Laurenti, 1768, and Passer domesticus Linnaeus, 1758 (sensu Sereno et al., 2005) and contains two major clades: Avemetatarsalia (the bird-lineage) and Pseudosuchia (the crocodile-lineage) (Figure 1.2).

Several names for crocodile-line archosaurs have been proposed, including Pseudosuchia (Zittel 1890), Crurotarsi (Sereno and Arcucci, 1990), and Crocodylotarsi (Benton and Clark, 1988). Until recently, all three names comprised the same taxa and were thus redundant (Brochu 1997). Pseudosuchia, defined as “all archosaurs closer to crocodiles than to birds” (Gauthier and Padian 1985), retains priority despite the unfortunate consequence that “true crocodiles” (Eusuchia) are “false crocodiles” (Pseudosuchia). Crocodylotarsi was proposed as a more appropriately descriptive name (referring to the characteristic articular configuration of the ankles of taxa considered to belong to the clade) for crocodile-line archosaurs to replace Pseudosuchia directly (Benton and Clark 1988), and is therefore a junior synonym. Crurotarsi, also describing the characteristic ankle configuration of the group, was then proposed as a node-based replacement for the stem-based Pseudosuchia and was defined as “Parasuchia, Ornithosuchidae, Prestosuchus, Suchia, and all descendants of their common ancestor” (Sereno and Arcucci 1990). Pseudosuchia and Crurotarsi have been redundant but not completely synonymous for most of taxonomic history. However, a recent phylogenetic analysis (Nesbitt 2011) places phytosaurs (Parasuchia/Phytosauria) outside of crown-group Archosauria, resulting in different taxonomic compositions for Pseudosuchia and Crurotarsi. Under the new framework, Crurotarsi now comprises phytosaurs and all crown-group archosaurs (both crocodile lineage and bird lineage) and therefore, Pseudosuchia is nested within Crurotarsi and the two terms are no longer redundant.