1st Symposium on the evolution of crocodyliforms

Abstract

Crocodiles have long been regarded as living fossils, animals that entered an evolutionary stasis and barely changed since the Mesozoic Era. Furthermore, with the possible exception of the Indian gharial (Gavialis gangeticus), living crocodiles are usually characterized by their relatively uniform body plan and amphibious mode of life. This generalization was probably based in that the three major living lineages of crocodiles (crocodylids, alligatoroids, and gavialoids) appeared in the Cretaceous and have maintained relatively unchanged morphologies since then. The evolution of these modern lineages of crocodiles (the crown-based clade Crocodylia sensu Benton & Clark, 1988; see Fig. 1), however, has far more diversity and disparity than usually reported (Brochu, 2001) and much of the assumed homogeneity is based on a lack of detailed studies and a disregard of bizarre fossil taxa, such as the gigantic, duck-billed nettosuchids (Brochu, 1999) or the high-snouted terrestrial pristichampsines (Rossmann, 2000). In addition to crown-group Crocodylia, a large number of closely-related fossil taxa form a more inclusive clade, Crocodyliformes (sensu Benton & Clark, 1988; see Fig. 1). This group had previously been regarded as the Order Crocodilia (Kalin, 1955; Romer, 1956; Kuhn, 1968) and has an outstanding diversity in the anatomy and ecological niches of its members. The most ancient record of Crocodyliformes, Hemiprotosuchus leali (Bonaparte, 1972), from the Late Triassic of Argentina, represents the most basal clade of crocodyliforms (Protosuchidae). Protosuchids (and other basal crocodyliforms previously thought to form a paraphyletic Protosuchia; Fig. 1) were small-bodied animals (usually less than 1 metre long) that achieved a worldwide distribution by the Early Jurassic (and survived to the Cretaceous in Central Asia). The Early Jurassic seems to have been a critical period in the evolution of Crocodyliformes, given that ghost lineages of multiple clades extend back to this time, suggesting the existence of a major radiation of this group during this epoch. Much of the Jurassic record of crocodyliforms is, however, restricted to two groups: the above mentioned basal crocodyliforms and the marine thalattosuchians (teleosaurids and metriorhynchids). This group is particularly interesting given it represents the only group of Archosauria completely adapted to the marine environment, having paddle-like forelimbs, a hypocercal tail, and excretory salt glands (Fernandez & Gasparini, 2000; Fernandez & Herrera, 2009). In spite of the wealth of fossils in this clade, its phylogenetic affinities are highly debated and constitute the major unsolved problem in crocodyliform systematics (Clark, 1994; Larsson & Sues, 2007; Jouve, 2009; Pol & Gasparini, 2009). The Jurassic fossil record of other terrestrial groups of Crocodyliformes is scarce (Tykoski et al., 2002) until the Late Jurassic, when members of the large clade Neosuchia become abundant in fluvial and lacustrine deposits in the northern hemisphere. This record includes the dwarf species of Atoposauridae (Buscalioni & Sanz, 1988) and the often gigantic Goniopholididae (Fig. 1). The Cretaceous, in contrast, was undoubtly the time when crocodyliforms were most abundant and diverse. During this period, the crocodyliform fauna of the northern hemisphere was basically composed of relictual basal taxa that survived in Central Asia (Wu, Brinkman & Lu, 1994; Wu, Sues & Dong, 1997; *Corresponding author. E-mail: dpol@mef.org.ar Zoological Journal of the Linnean Society, 2011, 163, S1–S6. With 3 figures