Crocodyliform Biogeography During the Cretaceous: Evidence of Gondwanan Vicariance from Biogeographical Analysis Alan H
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Received 11 April 2004 Accepted 14 June 2004 Published online 7 September 2004 Crocodyliform biogeography during the Cretaceous: evidence of Gondwanan vicariance from biogeographical analysis Alan H. Turner Department of Geoscience, University of Iowa, Iowa City, IA 52242, USA ([email protected]) Explanations of the distributions of terrestrial vertebrates during the Mesozoic are currently vigorously con- tested and debated in palaeobiogeography. Recent studies focusing on dinosaurs yield conflicting hypoth- eses. Dispersal, coupled with regional extinction or vicariance driven by continental break-up, have been cited as the main causal factors behind dinosaur distributions in the Mesozoic. To expand the scope of the debate and test for vicariance within another terrestrial group, I herein apply a cladistic biogeographical method to a large sample of Cretaceous crocodyliform taxa. A time-slicing methodology is employed and a refinement made to account for the divergence times of the analysed clades. The results provide statistically significant evidence that Gondwana fragmentation affected crocodyliform diversification during the Mid– Late Cretaceous. Detection of a vicariant pattern within crocodyliforms is important as it helps corroborate vicariance hypotheses in other fossil and extant groups as well as furthers the move towards more tax- onomically diverse approaches to palaeobiogeographical research. Keywords: crocodyliforms; Cretaceous; palaeobiogeography; vicariance; tree reconciliation analysis; Gondwana 1. INTRODUCTION dinosaurian clades (Cox 1974; Galton 1977; Colbert 1984; The relative roles that vicariance and dispersal have played Le Loeuff et al. 1992; Russel 1993; Le Loeuff & Buffetaut in shaping the biogeographical patterns seen during the 1995; Upchurch 1995; Fastovsky & Weishampel 1996; Mesozoic remain controversial, owing in large part to the Sampson et al. 1998; Sereno et al. 1998, 2004; Pereda- poor understanding of the biogeographical histories of Suberbiola & Sanz 1999; Pe´rez-Moreno et al. 1999; most terrestrial vertebrate clades living at the time. While Upchurch et al. 2002). Moreover, few comparisons some modern biotas, such as Nothofagus (Swenson et al. between fossil taxa can be made, owing to the paucity of 2001), ratites and other neornithine birds (Van Tuinen et analytical biogeography work on other groups (though see al. 1998; Cracraft 2001) and fishes (cichlid fish (Sparks Buffetaut & Taquet 1979; Buffetaut & Rage 1993; Gaspar- 2004); and killifish (Murray & Collier 1997)) suggest that ini 1996; Krause & Grine 1996; Krause et al. 1999; Krause vicariant processes may have been significant, ambiguity 2001; Pol et al. 2002). Dinosaur palaeobiogeographical from fossil data persists (also see Sanmartı´n & Ronquist work has led to a wealth of biogeographical hypotheses, 2004). Failures to uncover palaeobiogeographical patterns many proposing continent-level vicariance processes as the and/or disagreements among workers on the interpretation dominant causal factor (Milner & Norman 1984; Russel of those patterns that are recovered are prevalent and stem, 1993; Sampson et al. 1998; Upchurch et al. 2002). Sereno in part, from methodological deficiencies and dataset size (1997, 1999) and Sereno et al. (1998, 2004) have con- (Upchurch et al. 2002). tested these claims, suggesting that current dinosaur data Palaeobiogeographical analysis has consisted pre- does not satisfy the minimal conditions necessary to be dominantly of narrative approaches (Cox 1974; Buffetaut interpreted as vicariance and therefore cannot reject a ‘null’ & Taquet 1979; Colbert 1984; Buffetaut & Rage 1993; scenario in which dispersal and regional extinction are the Upchurch 1995; Gasparini 1996; Sereno et al. 1996, 2004; driving factors. However, a cladistic biogeographical analy- Pol et al. 2002). Generally, these methods are based on sis by Upchurch et al. (2002) indicates that at least for por- either a literal reading of fossil distributions or scenario tions of the Mesozoic, dinosaur distributions are consistent construction constrained by phylogenetic data. Although with vicariant origins. effective for hypothesis building, this yields scenarios that Crocodyliform phylogeny indicates a late Gondwanan are difficult to evaluate for consistency with the biogeo- division among many of its constituent clades and therefore graphical data. Cladistic biogeographical methods provide would be subject to any biogeographical events occurring an alternative, analytically rigorous means of inferring his- and affecting other groups at the time (e.g. dinosaurs). torical patterns. These methods, coupled with statistical Additionally, crocodyliforms possess many of the same and topological evaluation of the recovered biogeo- attributes that make dinosaurs ‘an almost ‘ideal’ case study graphical patterns, facilitate comparison of patterns in Mesozoic biogeography’ (Upchurch et al. 2002, between different biotas and different analyses. Most ana- p. 613)—namely high diversity, widespread geography and lytical studies of Mesozoic palaeobiogeography have been a largely terrestrial habit, and thus represent an intuitive taxonomically limited, with attempts focused primarily on next step in examining Mesozoic palaeobiogeography. Proc. R. Soc. Lond. B (2004) 271, 2003–2009 2003 # 2004 The Royal Society doi:10.1098/rspb.2004.2840 2004 A. H. Turner Crocodyliform biogeography during Cretaceous Sebecus Iberosuchus Bretesuchus Crocodylia Trematochampsa Peirosauridae Malagasy form Mahajangasuchus Pabwehshi Baurusuchus 65 Myr ago Simosuchus Maastrichtian Uruguaysuchus g Campanian Notosuchus 80 Myr ago Comahuesuchus Santonian South America form Coniacian h s s a Turonian u a i w a Araripesuchus patagonicus Cenomanian Araripesuchus gomesii Anatosu a i a Albian Ararripesuchus wegeneri Malawisuc o m e 115 Myr ago Aptian Bernissartia r Barrimian o t Hauternian g o 135 Myr ago s u Alligatorium A Ther Th Valanginian Gonio Eutretauranos Berriasian Hsisosu H Tithonian Kimmeridgian Oxfordian Callovian Bathonian Figure 1. Temporally calibrated portion of taxon–area cladogram of crocodyliform taxa. The shapes above the names denote the area in which the taxon occurs. Thickened lines are observed ranges, with thin lines marking ghost lineages inferred from phylogeny. Unfortunately, crocodyliform biogeography remains am- separation event, then a hypothesis of vicariance is only poorly biguous, with much of the work drawing attention to faunal supported. similarities between continents (Buffetaut & Taquet 1979; The crocodyliform phylogeny was ‘time-sliced’ for three differ- Buffetaut & Rage 1993; Gasparini 1996; Buckley & Brochu ent intervals (figure 2); the first time-slice (Late Jurassic–Late 1999; Pol et al. 2002), but having yet to apply rigorous Cretaceous: figure 2a) incorporates the entire crocodyliform methods and statistical evaluation. The present study now phylogeny, the second time-slice (Late Jurassic–Early Cretaceous: considers the crocodyliform data from a cladistic biogeo- figure 2c) incorporates the early biogeographical history of the graphical perspective. A phylogeny of 29 crocodyliforms clade, and the third time-slice (Mid–Late Cretaceous: figure 2e) (figure 1) was examined using ‘tree reconciliation analysis’ incorporates the time-frame in which most of the Gondwanan (TRA)—a method that tests for the presence of repeated divisional events are proposed to have occurred (Smith et al. 1994; patterns of area relationships (Page 1988, 1990a, 1993, Smith & Rush 1997; Scotese 1998; Hay et al. 1999). Searches for 1994a,b; see also Hunn & Upchurch 2001). Evaluation of the optimal area cladogram were conducted in COMPONENT, v. 2.0 the resultant pattern can indicate the presence of a vicariant (Page 1993). Randomization tests, which determine the prob- biogeographical signal (Nelson & Platnick 1981; Page ability that the observed biogeographical pattern could have 1988). occurred by chance alone, were run using TREEMAP’s ‘randomize parasite tree’ function with 10 000 random topologies generated (Page 1994a, 1995). 2. MATERIAL AND METHODS Topological sensitivity and taxon-sampling effects were The philosophy of cladistic biogeography has been discussed in explored for the Mid–Late Cretaceous time-slice using sequential detail by numerous authors (Nelson & Platnick 1981; Patterson pruning and addition (grafting) protocols implemented in PRUNE 1981; Grande 1985; Page 1988, 1994a; Lieberman 2000; Hunn ( J. A. Callery, A. H. Turner and N. D. Smith, unpublished soft- & Upchurch 2001; Upchurch & Hunn 2002; Upchurch et al. ware). First, taxa were sequentially pruned to yield the set of all 2002) and thus will not be covered here. TRA was conducted on the data shown in figure 1. A time-slicing protocol was adapted possible n À 1 trees (where n equals the number of taxa present in from that described by Upchurch et al. (2002) and a refinement the original taxon cladogram). Subsequently, two randomly selec- made in which time-slicing pruned not only the taxa absent during ted taxa are pruned. This is repeated 100 times to approximate the the time-slice, but also those taxa that did not diverge during the range of n À 2 trees. The frequencies at which these data recover interval (Turner 2003). This refinement accounts for divergence the optimal area relationships are then recorded. These data rep- timing in cladistic biogeographical analysis, thus incorporating a resent an asymmetrical test; a high recovery percentage using this crucial data source necessary for