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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 sensuBenton & 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).

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Figure 1. Summary of current points of agreement and disagreement on the phylogenetic relationships of Crocodyliformes based on recent phylogenetic studies.

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In addition to crown-group Crocodylia, a large number of closely-related fossil taxa form a more inclusive clade, Crocodyliformes (sensuBenton & Clark, 1988; see Fig. 1). This group had previously been regarded as the Order Crocodilia (Kälin, 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 (Fernández & Gasparini, 2000; Fernández & 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 & Lü, 1994; Wu, Sues & Dong, 1997; Pol et al., 2004) and representatives of numerous groups of neosuchians (Buffetaut, 1982; Clark & Norell, 1992; Buscalioni et al., 2001), including the first members of each of the three main extant lineages (crown-group Crocodylia; Brochu, 1999, 2004). The crocodyliform fauna of the southern hemisphere was much more diverse. The vast majority of this diversity is in Notosuchia (Fig. 1); a radiation with a wide range of dental morphologies that include hypercarnivory, potential insectivory, omnivory, and even herbivory (Clark, Jacobs & Downs, 1989; Buckley et al., 2000; Riff & Kellner, 2001; Nobre et al., 2008; Montefeltro, Laurini & Langer, 2009). Notosuchians are unique also in being remarkably abundant in some Cretaceous units of Gondwana (Buckley, Brochu & Krause, 1997; Pol & Gasparini, 2007; Sereno & Larsson, 2009) and their diverse feeding and presumed ecological roles may have been responsible for the relatively low abundances and diversities of mammals in the southern continents (O'Connor et al., 2010).

Three other Cretaceous clades that appear to be restricted to Gondwana may represent further radiations within Notosuchia or radiations along stem Neosuchia. Peirosauridae, Sebecidae, and Mahajungasuchidae each contain diverse carnivorous taxa that range from the dog-faced terrestrial Sebecus with serrated, bladed teeth, the long-snouted, piscivorous Stolokrosuchus, to the carnivorous, hippopotamus-like Mahajungasuchus. The phylogenetic relationships of each of these clades is also under dispute and their placements weigh heavily on the reconstruction of the origins of Neosuchia.

Before the end-Cretaceous mass extinction, most of the remarkable diversity of Crocodyliformes disappears from the fossil record, leaving crown-crocodylians as one of the few crocodyliform lineages to survive into the Cenozoic. In addition to crown-crocodylians, two other groups of crocodyliforms survived the mass extinction: Dyrosauridae and Sebecidae. The former is a group of longirostrine taxa that is mostly recovered in estuarine deposits of the Palaeogene of Africa and South America (Jouve, 2007; Barbosa, Kellner & Sales Viana, 2008). Sebecids are high-snouted species with theropod-like teeth that inhabited South America during the Palaeogene and early Neogene (Gasparini, 1996).

The three major lineages of Crocodylia diversified throughout the Paleogene (Brochu, 1997), extending their distribution into geographic regions where they were presumably absent before (e.g. South America, Australia) and evolving into endemic clades that thrived during the Cenozoic (Gasparini, 1996; Salisbury & Willis, 1996; Brochu, 1999). The biogeographic histroy of crocodylians is particularly interesting because it has been largely influenced by the limited range of temperatures tolerated by most modern lineages of crocodylians, which has prompted the use of this group for estimating palaeotemperatures (Markwick, 1998), and the limited saltwater tolerance of many clades.

The remarkable diversity and abudance of fossil crocodyliforms contrasts with the limited attention this group received during most of the 20th century. During the 1970s and 1980s, contributions from a relatively small number of authors increased our knowledge on this group, including descriptions of new taxa and evolutionary reviews of major groups (Gasparini, 1971, 1972, 1984; Langston, 1973; Buffetaut, 1974, 1982). During the 1980s, the use of cladistic methodology began to impact our understanding of the evolution of crocodyliforms based on contributions from a few authors that began to set the basis of modern classification schemes for Crocodyliformes (e.g. Clark, 1986; Benton & Clark, 1988; Buscalioni & Sanz, 1988; Norell, 1988). During the 1990s, our knowledge of the diversity of crocodyliformes increased with the description of new taxa and phylogenetic studies of previously unstudied groups (Gasparini, Chiappe & Fernández, 1991; Wu et al., 1994, 1997; Ortega, Buscalioni & Gasparini, 1996; Wu & Sues, 1996; Gomani, 1997). During this time, however, two seminal papers were published by Clark (1994) and Brochu (1999) that provided comprehensive and detailed phylogenetic studies of Crocodyliformes and Crocodylia, respectively. These two studies are, in our understanding, the most influential papers on phylogenetic relationships of crocodiles, as shown by the large number of characters provided by these studies and the citation record of these papers (100 and 78 citations in Google Scholar, respectively, as of June 2011). During the first decade of the 21st century, the diversity and evolution of crocodyliforms began to receive much more attention and the number of papers on crocodyliforms published per year has been steadily growing in the last twenty years (Fig. 2). This was in part generated by the discovery of a large number of new taxa. A considerable proportion of these new species come from the Cretaceous of Gondwana, a time and place during which crocodyliforms were more diverse and abundant than ever, including a few formations with ‘hyperdiverse’ crocodyliform faunas (Buckley et al., 1997; Pol & Gasparini, 2007; Sereno & Larsson, 2009). In particular, studies in the last decade have shown us that the ecological and dietary diversity of crocodiles was far beyond what was known 20 years ago, and clearly challenged the generalized view that crocodyliforms were a morphologically and ecologically homogeneous group. Contributions in this aspect have broadened the diversity throughout Crocodyliformes, including basal taxa (e.g. Pol et al., 2004), notosuchians (e.g. Buckley et al., 2000; Ortega et al., 2000; Novas et al., 2009), baurusuchids (e.g. Campos et al., 2001; Carvalho, Campos & Nobre, 2005), peirosaurids (e.g. Larsson & Gado, 2000; Sereno & Larsson, 2009), metriorhynchids (e.g. Gasparini, Pol & Spalletti, 2006; Fernández & Herrera, 2009), dyrosaurids (e.g. Jouve, Bouya & Amaghzaz, 2005, Jouve et al., 2006), and crocodylians (e.g. Buscalioni et al., 2001; Brochu, 2004; Brochu et al., 2010). The studies that described this new morphological and taxonomic diversity of crocodiles certainly involved the participation of many new researchers and formed part of the increase in publications on crocodyliforms during the last decade (Fig. 2). Phylogenetic studies have become a standard for presenting new taxa and this has increased the available phylogenetic hypotheses of the group. A general consensus exists for the interrelationships of the major lineages of Crocodyliformes but there are some outstanding points of incongruence among published phylogenetic hypotheses. Some major current conflicts in crocodyliform evolution are: the monophyly (Wu et al., 1997) versus paraphyly (Clark, 1994; Pol & Norell, 2004) of Protosuchia, the phylogenetic position of Thallatosuchia as basal crocodyliforms (Benton & Clark, 1988; Jouve, 2009; Sereno & Larsson, 2009; Young & Andrade, 2009) or as neosuchians (Clark, 1994; Pol & Gasparini, 2009), the inclusion (e.g. Pol & Apesteguía, 2005; Turner & Buckley, 2008; Sereno & Larsson, 2009) or exclusion of Araripesuchus from Notosuchia (e.g. Turner, 2006; Larsson & Sues, 2007), and the affinities of Sebecidae with Baurusuchidae (e.g. Ortega et al., 2000) or with peirosaurids (Larsson & Sues, 2007; Andrade & Bertini, 2008; Sereno & Larsson, 2009).

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Figure 2. Number of papers published per year on Crocodyliformes (counting papers only on fossils and anatomical studies of extant taxa). Data modified from the literature compiled by Brochu et al. (2009).

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During the Third Latin American Congress of Vertebrate Paleontology held in Neuquén (Argentina) in September 2008, Diego Pol and Lucas Fiorelli organized the First Symposium on the Evolution of Crocodyliformes, to gather a large number of specialists working on the systematics and phylogenetic relationships of this group. The symposium was a sucess, with participants from numerous countries (Argentina, Canada, Brazil, Mexico, Spain, United States, and Uruguay) presenting more than thirty contributions and a lively exchange of ideas (Fig. 3).

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Figure 3. Participants of the First Symposium on Crocodyliform Evolution at the Third Latin American Congress of Vertebrate Paleontology (Neuquén, Argentina), September 2008. From left to right, top row: F. Iori, M. Soto, D. Riff. Middle row: M. B. Andrade, R. Figueiredo, D. Fortier, A. Turner, F. Vasconcellos, P. Nascimento, C. Brochu, unidentified, G. Sobral, unidentified, L. Fiorelli. Bottom row: M. Ezcurra, G. Lio, A. Buscalioni, D. Pol, J. Leardi, P. O'Connor, H. Larsson, W. Nava, J. Clark.

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The present special issue of the Zoological Journal of the Linnean Society publishes extended papers on some of the work presented at the symposium. Fourteen papers are gathered that cover different aspects of crocodyliform evolution and morphology. These range from descriptions of new basal crocodyliforms, notosuchians, sebecids, neosuchians, and basal eusuchians to the biomechanics of the postcrania of baurusuchids to taphonomy and pathologies in a unique crocodyliform postcranium. This special issue provides a wealth of new anatomical and phylogenetic information. The fourteen papers span a large portion of the entire range of crocodyliform diversity. Five new taxa are introduced and formally named, and one fragmentary specimen is described but not yet named. Six taxa are redescribed on the basis of new material and/or more detailed study. The postcranial and functional morphology of a baurusuchid is described along with details of its pathologies and taphonomy. A total of nine new phylogenetic analyses are presented and each offers a wealth of novel interpretations of crocodyliform evolution and morphological characters. These contributions will surely impact crocodyliform research for many years to come and provide a leap forward toward understanding the evolution of some of the major groups of crocodyliforms.

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