Brochu, 2001

564

AMER. ZOOL., 41:564–585 (2001)

Crocodylian Snouts in Space and Time: Phylogenetic Approaches TowardAdaptive Radiation1

CHRISTOPHER A. BROCHU2

Department of Geology, Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605

SYNOPSIS. Recent phylogenetic analyses of fossil and living crocodylians allow usto compare the taxonomic, geographic, and temporal distributions of morpholog-ical features, such as snout shapes. A few basic snout morphotypes—generalized,blunt, slender, deep, and excessively broad (‘‘duck-faced’’)—occur multiple timesin distantly-related lineages. Some clades—especially those found in the NorthernHemisphere or with minimum origination dates in the Cretaceous or lower Ter-tiary—are morphologically uniform, but geographically widespread; crocodylianfaunas of the early Tertiary tend to be composite, with sympatric taxa being dis-tantly related, and similar-looking taxa on different continents being close relatives.In contrast, crocodylian faunas of the later Tertiary tend to be more endemic, withlocal adaptive radiations occurring in Africa and Australia containing members ofmost basic snout shapes. Endemic radiations in Africa and Australia have largelybeen replaced by Crocodylus, which can be divided into subclades that may indi-vidually represent endemic adaptive radiations.

INTRODUCTION

Crocodyliforms are ubiquitous in conti-nental deposits in much of the worldthroughout the latest Cretaceous and Ce-nozoic, and they occasionally occur in mar-ginal marine deposits as well. Extinctcrown-group crocodylians outnumber theirliving relatives by a wide margin, and therewere times in the past where worldwidecrocodyliform diversity clearly exceededlevels seen today, with only 23 living spe-cies (Taplin, 1984; Hutchison, 1982, 1992;Markwick, 1998; Vasse and Hua, 1998).And yet, in any given period of time, in-dividual crocodylian faunas were probablynot much more diverse than today. Thereare few places now with more than two orthree species occurring in the same generalregion (Thorbjarnarson, 1992), and most‘‘sympatric’’ crocodylians segregate them-selves ecologically (Magnusson, 1985;Magnusson et al., 1987; Webb et al., 1987;Kofron, 1992; Ouboter, 1996; Meshaka et

1 From the Symposium Beyond Reconstruction: Us-ing Phylogenies to Test Hypotheses About VertebrateEvolution presented at the Annual Meeting of the So-ciety for Integrative and Comparative Biology, 4–8January 2000, at Atlanta, Georgia.

2 Present address: Department of Geoscience, Uni-versity of Iowa, Iowa City, Iowa 52242; E-mail:[email protected]

al., 2000). Global crocodyliform diversitymay have been higher in the past, largelybecause warmer climates allowed a broadergeographic distribution for the group, butthe number of crocodyliforms in a singledeposit (and presumably living sympatri-cally or in close proximity) is usually fiveor less, with notable exceptions (e.g., Buck-ley et al., 2000).

Where they co-occur (now or in thepast), crocodylians tend to differ morpho-logically (Fig. 1). The three crocodyliansliving in western Africa (Osteolaemus te-traspis, Crocodylus niloticus, Crocodyluscataphractus), for example, look radicallydifferent from each other—C. niloticus is alarge-bodied stereotypical ‘‘crocodile,’’with a flat toothy snout; Osteolaemus issmall-bodied at adulthood (2 m totallength), with a blunt snout and stout pos-terior teeth; and C. cataphractus has slenderjaws and a tubular rostrum. Fossil faunasare similar in this regard—one usually findsone or two ‘‘generalist’’ crocodiles, one ortwo blunt-snouted forms, one or two taxawith long and slender jaws, and perhaps arepresentative of a morphotype not seen to-day. Not all morphotypes may be repre-sented, but we do not see, for example, fau-nas with four or five generalist crocodiles.These morphological divergences presum-

565PHYLOGENY AND ADAPTIVE RADIATION

FIG. 1. Representative examples of the snout shapecategories used in this paper. A. Leidyosuchus cana-densis, a ‘‘generalized’’ alligatoroid from the Late Cre-taceous of North America. B. Thoracosaurus macror-hynchus, a slender-snouted gavialoid from the Paleo-cene of Europe. C. Alligator mcgrewi, a blunt-snoutedalligatorid from the Miocene of North America (draw-ing adapted from Schmidt, 1941). D. Mourasuchus, aduck-faced caiman from the Miocene and Pliocene ofSouth America (drawing adapted from Price, 1964 andLangston, 1965). E. Pristichampsus vorax, a ziphodontcrocodylian from the Eocene of North America (draw-ing adapted from Langston, 1975). Drawings not toscale; A through D in dorsal view, E in right lateralview.

ably reflect ecological separation, thoughgiven how little we know about the ecologyof many living crocodylian species, thiscannot be stated with certainty. It is evenmore hazardous to infer ecological traits onextinct crocodylians, but the morphologicaldifferences between these crocodiles, whichpresumably lived sympatrically (or at leastsyntopically), are striking.

The focus of this paper is the interfacebetween phylogenetic hypothesis, morpho-logical evolution, and larger-scale temporalpatterns. Are the crocodiles found in a sin-gle unit or region close relatives, with sim-ilar skull shapes arising independently indifferent places? Or did clades of crocodilessharing a similar skull shape disperse wide-ly, such that individual crocodile speciesare distantly related to geographic neigh-bors but closely related to species livingelsewhere? In other words, have crocody-lian lineages formed geographically-re-

stricted adaptive radiations in the strictestsense of that term as first coined by Osborn(1902)?

Definitions and meanings of the phrase‘‘adaptive radiation’’ vary among authors,but contemporary studies of the phenome-non uniformly understand that questions ofadaptive radiation are inherently phyloge-netic (e.g., Guyer and Slowinski, 1993; Lo-sos and Miles, 1994; Larson and Losos,1996). Thus, before we can answer thisquestion, we must first obtain a phyloge-netic hypothesis. Recent explorations ofcrocodylian phylogenetics, including a va-riety of data sets (both morphological andmolecular) and considering both extant andextinct members of the group (Brochu andDensmore, 2001), give us a phylogeneticbackbone for the study of adaptive radia-tion.

Phylogenetic systematics is only begin-ning to play the focal role it should adoptin the study of large-scale paleobiologicalpatterns. A phylogenetic hypothesis givesus a map of the hierarchy of putative char-acter state changes within the group. Thechronicle of morphological evolution with-in a clade is revealed on this map. We canadd geographic or temporal information,which lets us isolate adaptive radiationsand, with enough information, discuss thebeginnings and possible endings of theseclades.

METHODS

Nomenclature and hierarchy ofrelationships

Crocodylia is a crown-group name basedon the last common ancestor of Gavialis,Alligator, Paleosuchus, Caiman, Melano-suchus, Tomistoma, Osteolaemus, and Cro-codylus, and all of its descendents (Clark,1986, 1994; Benton and Clark, 1988; Bro-chu, 1997, 1999). I will apply the phylo-genetic nomenclatural system for Crocod-ylia established by Norell et al. (1994) andexpanded by subsequent authors (Salisburyand Willis, 1996; Brochu, 1997, 1999). Thissystem no longer uses Linnean ranks. With-in Crocodylia, there are three primary stem-based group names established on the basisof living end members: Gavialoidea (Gav-

566 CHRISTOPHER A. BROCHU

FIG. 2. Phylogenetic relationships among basal eusuchians, with geographic regions and snout shapes of in-group taxa indicated. Abbreviations: Af, Africa; As, mainland Asia; Eu, Europe; NA, North America; SA, SouthAmerica. Line width and shading indicate optimized snout shape for that particular lineage.

ialis gangeticus and all crocodylians closerto it than to Alligator mississippiensis orCrocodylus niloticus), Alligatoroidea (Alli-gator mississippiensis and all crocodylianscloser to it than to Crocodylus niloticus orGavialis gangeticus), and Crocodyloidea(Crocodylus niloticus and all crocodylianscloser to it than to Gavialis gangeticus orAlligator mississippiensis). Within each ofthese, we may recognize a node-basedcrown group name on the basis of the lastcommon ancestor of living members; Alli-gatoridae, for example, is the last commonancestor of Alligator, Caiman, Melanosu-chus, and Paleosuchus, and all of its de-scendents.

Eusuchia is also a node-based groupname, in this case referring to the groupincluding the last common ancestor of Hy-laeochampsa and Crocodylia and all of its

descendents (Brochu, 1999). These are ef-fectively the advanced crocodyliforms withprocoelous vertebrae and internal choanaecompletely surrounded by the pterygoids,although character state distributions withinCrocodyliformes are somewhat more com-plex (Brochu, 1999). The only demonstra-ble non-crocodylian eusuchian in this anal-ysis is Hylaeochampsa; Stomatosuchus maybe a eusuchian, and several enigmatic cro-codyliforms (e.g., Dolichochampsa, Aigi-alosuchus) probably belong to this group,though we do not know if they lie outsidethe crown group Crocodylia.

The phylogenetic hypothesis used in thisstudy is reflected in Figures 2–4 and isbased on several parsimony analyses ofmorphological characters for the crown-group (Norell, 1989; Salisbury and Willis,1996; Brochu, 1997, 1999, 2000). Gavialo-


FIG. 3. Phylogenetic relationships among alligatoroid crocodylians, with geographic regions and snout shapesof ingroup taxa indicated. Abbreviations: As, mainland Asia; Eu, Europe; NA, North America; SA, SouthAmerica. Line width and shading indicate optimized snout shape for that particular lineage.

idea today includes one species (the Indiangharial, Gavialis gangeticus), but also in-cludes several extinct taxa from all over theworld. Alligatoroids and crocodyloids forma clade named Brevirostres; sequential out-groups to Brevirostres include the deep-snouted pristichampsines (such as Pristi-champsus) and more generalized Borealo-suchus.

In this study, Tomistoma will be included

with the crocodylids. This is in contrast toseveral recent molecular analyses (Dens-more, 1983; Densmore and Owen, 1989;Densmore and White, 1991; Gatesy andAmato, 1992; Gatesy et al., 1992; Hass etal., 1993; White and Densmore, 2001), inwhich Tomistoma and Gavialis are closestliving relatives and Tomistoma is a gavi-aloid. The reasons for disagreement be-tween various data sets, which otherwise


FIG. 4. Phylogenetic relationships among crocodyloid crocodylians, with geographic regions and snout shapesof ingroup taxa indicated. Abbreviations: Af, Africa; As, mainland Asia; Au, Australia; Eu, Europe; IP, Indo-pacific region (e.g., Indonesia, Philippines, New Guinea); NA, North America; SA, South America. Line widthand shading indicate optimized snout shape for that particular lineage.


agree with each other very strongly (Poe,1996; Brochu, 1997; Brochu and Dens-more, 2001), remain unknown. As will bediscussed later, accepting the preferred mo-lecular tree does not change the broad con-clusions discussed herein.

Temporal calibration follows Salisburyand Willis (1996) for mekosuchine crocod-yloids and Brochu (1997, 1999, 2000) forall other crown-group lineages. All threeextant stem-based groups first appear asfossils in the Late Cretaceous. Crown-groupalligatorids first appear with certainty in thePaleocene, although undescribed fragmen-tary fossils may extend the group’s knownrecord into the Late Cretaceous (Brochu,1999). Crown-group crocodylids first ap-pear in the Early Eocene. Borealosuchusranges from the Late Cretaceous to the Ear-ly Eocene. The first appearance of Pristi-champsinae is in the Paleocene (Li, 1984;Gingerich, 1989), but based on the phylog-eny in Figures 2–4, we would expect pris-tichampsine fossils to be discovered in theLate Cretaceous.

Snout shape categories

It is widely believed that in crocodylians(and, more generally, crocodyliforms), mostphylogenetic action is in the skull. Thepostcranium is viewed as relatively staticwith comparison to the phylogeneticallyplastic skull, which has morphologicallyfluctuated all over the evolutionary map.This is an oversimplication—importantchanges can be documented within thegroup in most postcranial skeletal systems(e.g., Frey, 1988), and in recent phyloge-netic analyses, postcranial information hasproved pivotal in diagnosing several clades(Salisbury and Willis, 1996; Brochu, 1999,2000). And yet, the interspecific differencesin snout shape are much more striking; it iseasy to see why crocodile systematists havebeen much more fascinated with the group’scraniology, given the relative uniformity ofthe limb skeleton against the obvious dif-ferences in the rostrum when we compare,for example, a living piscivore like Gavialiswith an extinct crushing form such as Al-lognathosuchus.

We face several challenges when study-ing the evolution of snout shape in crocod-

ylians. Some of these are common to anystudy applying fossil information. Becauseof the fossil record’s incompleteness, we areassessing the minimum age (and not abso-lute age) for a lineage, as the first appear-ance in the fossil record will postdate thelineage’s actual divergence time; the differ-ence between first appearance and true agecannot be known. The relationships of somegroups (living or extinct) are ambiguous,which further causes ambiguity in our in-terpretations of evolutionary patterns andforcing us to consider multiple equally-sup-ported scenarios. Many fossils are incom-plete or distorted, and characterizing snoutshape can be difficult or impossible; andeven if we accurately reconstruct phylogenyand accurately map snout shape on the tree,we cannot always be confident that snoutshape (which can be preserved) and behav-ior (which is almost never preserved) areactually correlated—the number of livinggroups with derived snout morphologies issmall, and our understanding of living cro-codylian ecology is biased toward thosewith more generalized skulls.

Another important consideration is thecontinuous nature of snout variation be-tween crocodylian species, especially whenfossils are considered. It is easy to distin-guish the snouts of a very derived gavialoidand a very derived alligatorid—one will betubular, and the other will be very blunt.But the line between a truly long and slen-der snout and that of a more generalizedcrocodylian that happens to be rather nar-row is arbitrary. Furthermore, snout shapechanges during ontogeny—snouts are gen-erally shorter relative to skull length inhatchlings than in adults (Mook, 1921; Kal-in, 1933; Dodson, 1975; Webb and Messel,1987; Hall and Portier, 1994; Busbey,1994). For this reason, snout shape will beassessed on the basis of the snouts of ma-ture individuals.

Busbey (1994) applied different sets ofterms to describe cross-sectional shape andlength in crocodyliform snouts. Snoutscould be platyrostral (dorsoventrally com-pressed) or oreinirostral (mediolaterallycompressed) in cross-section; and theycould be short, medium, or long. Platyros-tral snouts were further subdivided into


broad, tubular, and narrow categories on thebasis of the ratio of mediolateral and dor-soventral diameter—it approaches 1 in tu-bular snouts, and the mediolateral diameteris at least twice the dorsoventral in broadsnouts. The shape terms applied here willcombine the shape and length categoriesapplied by Busbey (1994), but with someexceptions as discussed below.

Generalized crocodylians—Most crocod-ylians have a familiar dorsoventrally com-pressed snout that tapers gradually towardthe narial region (Fig. 1a), and most livingforms are dietary generalists as adults, eat-ing any animal matter they can swallow,with dietary differences between taxa relat-ed to differences in available food supply(e.g., Reese, 1915; Cott, 1961; Gorzula,1978; Magnusson et al., 1987; Banerjee etal., 1988; Pooley, 1989). ‘‘Generalized,’’ asused here, is a very heterogeneous assem-blage of anything not having any of the spe-cialized snout shapes discussed below. It istempting to consider these forms as ‘‘un-specialized,’’ and use of the term ‘‘gener-alized’’ reflects the assumption that the oth-er morphologies are derived specializations.This may not be the case; in at least onegroup (the alligatorids), ‘‘generalized’’forms and duck-faced forms may ultimatelyderive from blunt-snouted forms, andamong modern crocodylians, sympatricgeneralized forms tend to segregate them-selves ecologically (e.g., Magnusson, 1985;Kofrron, 1992; Herron, 1994; Ouboter,1996).

Most of the animals in this category havebroad platyrostral snouts of medium lengthsensu Busbey (1994). However, many ofthose he characterized as narrow-snouted(e.g., most extant Crocodylus) or long-snouted (e.g., some Borealosuchus) alsofall within this category, as the snout shapeis not markedly different from close rela-tives with more obviously broad snouts.Most of these fell close to the dividing linebetween ‘‘medium’’ and ‘‘long’’ in Bus-bey’s scheme (1994: Fig. 10.2).

Longirostrine and slender-snouted cro-codylians—Some crocodyliform snouts re-semble a pair of toothed forceps (Fig. 1b).The snout itself is very narrow and may betubular, and the teeth are reduced in size.

These fall within the platyrostrate tubularcategory of Busbey (1994). In a few forms,the snout is not only narrow, but also elon-gate, and there may be an increased numberof teeth—such taxa are not only slender-snouted, but longirostrine. Other morpho-logical transformations have been correlat-ed in the past with this morphology, includ-ing the evolution of large tubera on the ba-sioccipital, enlargement of thesupratemporal fenestrae and reduction ofthe palatal fenestrae, and homodonty(Langston, 1973; Busbey, 1994; Clark,1994). This kind of morphology is usuallythought to reflect piscivory (Pooley, 1989),but we lack good ecological data for manyrelevant living forms, and at least some liv-ing slender-snouted crocodylian popula-tions are not strictly piscivorous (Webb etal., 1983).

The slender-snouted crocodylians havebeen a major thorn in the side of crocodilesystematists for nearly two centuries. Howmany clades are there? Molecular analysesgenerally unite the two living crocodylianswith the most derived snout morphologies(Gavialis gangeticus and Tomistoma schle-gelii), but morphological analyses drawthem apart, and though most data sets agreethat the slender-snouted species of Crocod-ylus (C. cataphractus, C. johnstoni, and C.intermedius) are not particularly closely re-lated to Gavialis or Tomistoma (Densmoreand Owen, 1989; Densmore and White,1991; Hass et al., 1993; Poe, 1996; Brochu,1997, 2000; White and Densmore, 2001),there is little agreement about the relation-ships within Crocodylus.

Outside Crocodylia, the situation growsworse. Historically, long, slender snoutswere thought to have evolved at least threetimes exclusive of crown-group forms: thethalattosuchians, pholidosaurids, and dyro-saurids. Some analyses continue to supportthat conclusion (Buffetaut, 1982b; Norelland Clark, 1990), but others recover amonophyletic group containing these exclu-sively (Clark, 1994). Are these analyses be-ing misled by correlated characters relatedto the snout? Clark (1994) tried to eliminatesnout-related characters and still recovereda monophyletic longirostrine clade, andmorphological analyses of crown-group


crocodylians do not generally recoverclades including all of the long-snoutedforms. Nevertheless, more work is neededto resolve this issue. Of these groups, theonly one relevant to the present discussionis Dyrosauridae, which occurs in latest Cre-taceous and Early Tertiary deposits, espe-cially in the Tethyan region (Buffetaut,1982b).

Blunt-snouted Crocodylians—The blunt-snouted morphotype (Fig. 1c) is representedtoday by the dwarf caimans (Paleosuchus)of South America and the African dwarfcrocodile (Osteolaemus). These are smallcrocodylians, rarely exceeding two metersin length, and the rostrum is shortened rel-ative to skull length. These have enlarged,compound palpebral ossifications and on-togenetic closure of the supratemporal fe-nestrae. The skull table is flattened and hasabrupt margins. All of these taxa havebroad platyrostrate snouts, and all wouldfall within the short or low-end mediumlength categories of Busbey (1994).

Characterizing this group is harder thanit might seem on the surface, and more de-tailed quantitative approaches will doubt-less refine the morphospatial assignmentsmade here. Many extinct crocodylians are‘‘blunt-snouted,’’ but are very differentfrom Paleosuchus and Osteolaemus. This ismost apparent in the basal members ofGlobidonta, such as Brachychampsa, Stan-gerochampsa, and most alligatorines; theseforms do not close off the supratemporalfenestrae, and there is no evidence for en-larged palpebrals. Indeed, Paleosuchus andOsteolaemus are somewhat different fromeach other—the snout of Paleosuchus isdeeper than that of Osteolaemus, and somelarge individual Paleosuchus approach theoreinirostral condition. Although the pos-terior teeth of Osteolaemus are relativelylarge in comparison with their counterpartsin other crocodylids, they are not like theexpanded, bulbous dentition seen in manyextinct globidontans. These taxa often havea roughened region on the maxilla oppositethe posterior dentary teeth, and some au-thors have suggested a specialization onturtles or mollusks (Abel, 1928; Weitzel,1935; Carpenter and Lindsey, 1980; Aoki,1989; Brinkmann, 1992).

We cannot say that the blunt-snoutedmorphology, as defined here, is restricted toa common set of ecological factors. Paleo-suchus and Osteolaemus tend to prefer for-est-bound rivers and forest floors (Medem,1958; Magnusson and Lima, 1991; Kofron,1992; Ouboter, 1996), but this may not betrue for most extinct forms. Certainly, theextremely bulbous cheek teeth seen in suchtaxa as Allognathosuchus are not found inany modern taxon. Hence, as used here,‘‘blunt-snouted crocodylian’’ refers to arange of morphological similarity and notnecessarily to a presumed ecological role.

In this analysis, I consider Hylaeochamp-sa vectiana—the basalmost known eusu-chian—to be blunt-snouted. One of the out-groups used in this analysis (the Glen RoseForm, see Langston, 1974) is blunt-snouted,and although the other (Bernissartia) is cat-egorized here as generalized, it shares somefeatures more commonly seen in blunt-snouted forms, such as enlarged posteriorteeth (Buffetaut and Ford, 1979). Clark andNorell (1992) tentatively suggested that thiskind of morphology might be ancestral forCrocodylia.

Ziphodont crocodylians. The ziphodontsare characterized by deep, laterally com-pressed (oreinirostral) snouts (Fig. 1e).Most have flattened, serrated teeth, and theterm ‘‘ziphodont’’ refers to the teeth. Thesehave been interpreted as terrestrial carni-vores (Kuhn, 1938; Berg, 1966; Langston,1975; Rauhe, 1995; Rossmann, 1998,1999). As this morphotype does not existtoday, we cannot refer to a modern examplefor functional studies.

Duck-faced crocodylians. There is littlewe can say about these animals, which arebizarre enough to belong in a Saturdaymorning cartoon (Fig. 1d). These taxa havelong, very broad, platyrostrate snouts thatresemble tombstones or kickboards in dor-sal view. The skull table is reduced in size,and the teeth are small but very numerous.We have no idea what they did, and thereis nothing alive today resembling theseforms.

One of these is Stomatosuchus, whichwas described from a large skull destroyedduring the Second World War (Stromer,1925). It is from the Late Cretaceous of


Egypt, and its placement within Eusuchia isnot certain. Its placement within Eusuchiais not certain, and it is tentatively placed atthe root of this group in Figure 2. The lowerjaw of this form may have been edentulousand supported a gular sac, like that of agiant pelican or baleen whale (Nopcsa,1926).

I include Purussaurus as a duck-facedcrocodylian, but this may not be entirelyappropriate. Purussaurus is a close relativeof Nettosuchidae (members of which areunambiguously duck-faced) and has a skullthat is long, broad, but deep (Bocquetin etal., 1991). This is in contrast to the broad,but extremely flat, snout found in netto-suchids (Price, 1964; Langston, 1965; Boc-quetin, 1984). Purussaurus is bizarre forother reasons—in particular, the externalnaris is greatly enlarged and in one form(P. brasiliensis) literally covers the snout.No other crocodylian, living or extinct,closely resembles Purussaurus, althoughthe external nares of a few broad-snoutedcrocodylids (such as Crocodylus palaein-dicus) are enlarged.

RESULTS

Evolution of snout shape in Crocodylia—are similar snout shapes phylogeneticallyrestricted?

Mapping snout shape over phylogeny(Figs. 2–4) confirms the suspicions of near-ly all previous workers—that snout shapehas been very labile within Crocodylia, andsimilar snout morphologies have arisenmultiple times (Kalin, 1955; Langston,1973; Busbey, 1994; Russell and Wu,1997). A few clades are dominated by oneor two categories; most (but not all) includegeneralized forms, and other categories ap-pear sporadically throughout the tree.

Within the crown group, a slender snoutarose at least six times based on the phy-logeny applied here—Gavialoidea, Tomis-tominae, Euthecodon, and three times with-in Crocodylus (C. cataphractus, C. johnsto-ni, and C. intermedius). Accepting the mo-lecular hypothesis of phylogeny, it wouldhave arisen only six times, as gavialoidsand tomistomines would have been de-scended from a slender-snouted ancestor,

but we would still be faced with multipleoccurrences of a slender snout.

Actual longirostry—having a slender andelongate snout—arose only three timeswithin the crown group. Gavialoids and Eu-thecodon both have extremely long rostraand an increased number of teeth; whereasmost basal crocodylians have approximate-ly 17 maxillary teeth, extant Gavialis hasmore than 20. Derived tomistomines (Tom-istoma, Old World and New World Gavi-alosuchus) do not increase the number ofteeth, but the spacing between alveoli is in-creased and the rostrum can be described aselongate.

There are several other possible indepen-dent derivations of slender-snouted mor-phology among crocodylians and close rel-atives. One of these is Harpacochampsacamfieldensis, a Miocene taxon from Aus-tralia. Because the rostrum is incompletelyknown in Harpacochampsa, we do notknow if it could be described as elongate.Its phylogenetic relationships are also un-clear—Megirian et al. (1991) and Salisburyand Willis (1996) agreed that it was a cro-codyloid of some sort, but their analysesdid not support a placement within Meko-suchinae, a clade including virtually all Ter-tiary crocodylians from Australia. Based ona brief examination of the specimen, I dis-agree with some of the character codings ofthese authors and suspect it belongs withinMekosuchinae, but more complete materialis required to comfortably fix its placementwithin the tree. Other putative slender-snouted crocodylians (or at least eusuchi-ans) of unknown affinity include Dolicho-champsa from the Cretaceous of SouthAmerica (Gasparini and Buffetaut, 1980),Aigialosuchus from the Cretaceous of Swe-den (Persson, 1960), Toyotamaphimaeafrom the Pliocene of Japan (Aoki, 1983),and Charactosuchus from the Tertiary ofSouth America (Langston, 1965; Langstonand Gasparini, 1997); this latter form is of-ten allied with tomistomines, but availablematerial is too incomplete to allow such aconclusion. So although the number of der-ivations on Figures 2 and 4 is six, the totalnumber of independent slender-snouted ac-quisitions could be as high as eleven withinthe crown group.


Blunt-snouted forms are especially com-mon among derived alligatoroids (Fig. 3).At least one diplocynodontine is blunt-snouted (Baryphracta), and the basalmostmembers of Globidonta are all blunt-snout-ed. Indeed, the ancestral alligatorid wasprobably a blunt-snouted form, which raisesinteresting questions about the polarity ofsnout shape evolution (see below).

Three crocodylids are categorized here asblunt-snouted (Fig. 4)—the modern Africandwarf crocodile (Osteolaemus) and theAustralian mekosuchines Mekosuchus andTrilophosuchus. The snouts of Mekosuchusand Trilophosuchus are incompletelyknown, but the morphology of the jaw, pal-ate, and skull table in both cases is consis-tent with the blunt-snouted morphotype(Balouet and Buffetaut, 1987; Willis, 1993,1997). The tree in Figure 4 suggests a min-imum of two transformations to a bluntsnout within Crocodylidae—one in Osteo-laemus and one in the last common ancestorof Mekosuchus and Trilophosuchus. Onecould also suppose that Mekosuchus andTrilophosuchus evolved a blunt-snoutedmorphology independently because theclosest relative of Mekosuchus is the zipho-dont Quinkana, but a single derivation ismore parsimonious.

Within Crocodylia, ziphodonts occur intwo distinct lineages—Pristichampsinae(based in the parsimony analysis on Pris-tichampsus, but also including Planocran-ia) and the mekosuchine Quinkana. Themekosuchine nature of Quinkana is sup-ported by phylogenetic analyses (Willis,1993; Willis et al., 1993; Willis and Mack-ness, 1996; Salisbury and Willis, 1996), butthis has been questioned by some authorson the basis of similarities between Quin-kana and Pristichampsus (Megirian, 1994;Rossmann, 1998). Until a phylogeneticanalysis unambiguously supports pristi-champsine monophyly with the inclusion ofQuinkana, I treat Quinkana as a mekosu-chine.

Only one crown-group lineage is duck-faced—the clade including Nettosuchidaeand Purussaurus. This morphology proba-bly arose at least twice among derived cro-codyliforms, as Stomatosuchus is probablynot a close relative, though this needs to be

tested by more rigorously determining therelationships of Stomatosuchus.

The distribution of snout shapes on Fig-ures 2 through 4 raises some interestingquestions about the polarity of snout mor-phology evolution. One might expect themore ‘‘specialized’’ snout morphs to havearisen from generalized ancestors, a corol-lary of the ‘‘law of the unspecialized’’(Cope, 1896) in which the expected direc-tion of morphological change is from thegeneralized (or unspecialized) to the spe-cialized. This is usually the case—the an-cestral snout morphology for Brevirostres,Crocodyloidea, and Alligatoroidea is gen-eralized. But it is not universally true.‘‘Generalized’’ forms unambiguouslyevolved from blunt-snouted ancestors with-in Alligatoridae at least twice—once in thederived caimans (Caiman, Melanosuchus)and once within Alligator. The number maybe higher, because depending on how oneoptimizes snout morphology at the lastcommon ancestor of Diplocynodontinaeand Globidonta, Diplocynodon may repre-sent another case of generalized morphol-ogy derived from blunt-snoutedness. It alsodepends on how one categorizes some spe-cies of Alligator; I here consider the Chi-nese alligator (Alligator sinensis) to be gen-eralized, but one could argue that it belongsin the blunt-snouted category, which wouldmake interpretation of snout evolution with-in Alligator more complex. Moreover, thereare fossil alligatoroids with more general-ized snout morphologies (such as Hispan-ochampsa; see Kalin, 1936) not includedhere that could complicate optimizations.Phylogenetic analyses of other organismssometimes find similar results, in which theobserved transformations do not match the‘‘law of the unspecialized’’ (e.g., Siddall etal., 1993; D’Haese, 2000), illustrating thepotential for phylogenetics to test some ofour strongest-held assumptions about evo-lution.

Resolution within Mekosuchinae mostparsimoniously suggests that the ziphodontQuinkana had a blunt-snouted ancestor.This may not be as counterintuitive as itfirst looks. Some modern blunt-snouted cro-codylians (e.g., the dwarf caimans, Paleo-suchus) have snouts that approach an orei-


FIG. 5. Stratigraphic distribution of basal eusuchians, based on the phylogeny shown in Figure 2. Taxa in grayhave not been analyzed phylogenetically and are placed in an approximate phylogenetic context on the basis ofexpected relationships from published descriptions or examination of fossil material.

nirostral condition, and many extinct blunt-snouted forms have rather deep, stoutskulls. The teeth of Mekosuchus and Tri-lophosuchus are not as enlarged as in blunt-snouted alligatorids, and so we are notfaced with the problem of deriving serrated,bladelike teeth from globular crushing den-tition. Nevertheless, as we continue to learnmore about mekosuchine morphology andrelationships, this scenario may change.

Optimization of snout morphology at theroot of Crocodylia is ambiguous. In thisanalysis, I consider Hylaeochampsa vecti-ana—the basalmost known eusuchian—tobe blunt-snouted. One of the outgroupsused in this analysis (the Glen Rose Form,see Langston, 1974) is blunt-snouted, andalthough the other (Bernissartia) is notblunt-snouted, it shares some features morecommonly seen in blunt-snouted forms,

such as enlarged posterior teeth. Clark andNorell (1992) tentatively suggested that thiskind of morphology might be ancestral forCrocodylia.

Historical biogeography of crocodyliansnout shape—are similar snout shapesgeographically restricted?

Very few clades consist entirely of non-generalized crocodylians. Notably, those re-stricted to specialized forms appear as fos-sils in the Late Cretaceous or Early Tertiary(Gavialoidea, Tomistominae, Pristichamp-sinae; Figs. 5 and 7). Globidonta is notstrictly restricted to specialized forms, asderived Alligator and jacarean caimans aregeneralized, but nearly all Cretaceous andEarly Tertiary globidontans are blunt-snout-ed (Fig. 6), and one might consider the


FIG. 6. Stratigraphic distribution of alligatoroid crocodylians, based on the phylogeny shown in Figure 3. Taxain gray have not been analyzed phylogenetically and are placed in an approximate phylogenetic context on thebasis of expected relationships from published descriptions or examination of fossil material.


FIG. 7. Stratigraphic distribution of crocodyloid crocodylians, based on the phylogeny shown in Figure 4. Taxain gray have not been analyzed phylogenetically (with the exception of Harpacochampsa) and are placed in anapproximate phylogenetic context on the basis of expected relationships from published descriptions or exami-nation of fossil material.

group to have been a specialized clade atthat time.

Specialized clades first appearing in theLate Cretaceous or Early Tertiary tend to

have broad geographic distributions. Gavi-aloids, first known in the Campanian, havebeen found on all continents except Austra-lia and Antarctica; the earliest-known taxa


are found in marginal marine deposits ofNorth America and Europe circumscribingthe Atlantic (Thoracosaurus and, probably,Thecachampsoides; see Troedssen, 1924;Troxell, 1925; Piveteau, 1927; Carpenter,1983; Schwimmer, 1986; Norell and Storrs,1986), and by the end of the Eocene theyare also known from Africa and mainlandAsia (Brochu, 2001). Some European Eo-cene fossils, such as Eosuchus (figured inSwinton, 1937), may represent gavialoids(Brochu, 2001); isolated teeth from the laterTertiary of Europe have been referred tothis group (Antunes, 1994), but I am skep-tical that teeth can be assigned to such aprecise taxonomic level. When they first ar-rived in South America is unknown, butthey are definitely present there from theOligocene (Gasparini, 1996; Langston andGasparini, 1997), and fragmentary remainsfrom the Solomon Islands suggest the pres-ence of a small gavialoid there in the laterTertiary (Aoki, 1988; Molnar, 1982).

Tomistomines have a similarly broad dis-tribution, first appearing in Europe in thelowermost Eocene (‘‘Crocodylus’’ spenceriand a variety of other European fossils thatmay represent the same taxon, such as Dol-losuchus; see de Zigno, 1880 and Swinton,1937) but also appearing in the WesternHemisphere (e.g., Auffenberg, 1954; Bram-ble and Hutchison, 1971; Erickson andSawyer, 1997; Brochu, 2001). The only Af-rican tomistomine reflected in the parsi-mony analyses is ‘‘Tomistoma’’ cairense,but other African fossils may representtomistomines as well (Jonet and Wouters,1977; Pickford, 1994; Brochu and Ginger-ich, 2000). Probable tomistomines areknown from mainland Asia from the Eo-cene (e.g., Tomistoma petrolica; Yeh, 1958;Li, 1975; Ferganosuchus, Efimov, 1993),and depending on the phylogenetic relation-ships of Charactosuchus and Toyotamaphi-maea, the group’s range may include SouthAmerica and Japan.

This broad pattern—that the slender-snouted clades diverging by the Eocene aregeographically widespread—holds whetherwe accept the morphological or molecularestimate for the relationships of Tomistomaand Gavialis. Either we have two morpho-logically-uniform, geographically wide-

spread clades in the Late Cretaceous andCenozoic, or we have one; molecular evi-dence agrees with morphology that slender-snouted Crocodylus are unrelated to eitherGavialis or Tomistoma (Poe, 1996; Brochuand Densmore, 2001) and are silent aboutthe placement of completely extinct slen-der-snouted taxa such as Euthecodon.

Pristichampsines are found broadlythroughout North America and Eurasiathroughout the Early Tertiary (Figs. 2, 5).The only taxa known from reasonably com-plete material are from the Paleocene andEocene (Kuhn, 1938; Berg, 1966; Lang-ston, 1975; Li, 1984; Efimov, 1993; Ross-mann, 1998), but less complete fragments(isolated flattened, serrated teeth) areknown from throughout the NorthernHemisphere during the Tertiary (Brambleand Hutchison, 1971; Buffetaut, 1978; Bar-tels, 1983; Busbey, 1986; Gingerich, 1989;Sah and Schleich, 1990; Hanson, 1996).This must be approached with care, as flat-tened, serrated teeth occur independently inother crocodyliform groups, including sev-eral outside Crocodylia in the Old WorldTertiary (Berg, 1966; Buffetaut, 1982c,1988; Vasse, 1995; Ortega et al., 1995).Moreover, we do not know if these repre-sent oreinirostral animals, as the snout itselfis unknown. Nevertheless, there is good ev-idence that Pristichampsinae was a wide-spread clade in the Northern Hemisphere atleast during the Paleocene and Eocene.

Alligatoroids are found all over NorthAmerica and Eurasia throughout the LateCretaceous and Tertiary. Taxon samplingreflected in Figures 3 and 6 is dominatedby New World forms, but several Europeanand Asian globidontans not included in theparsimony analysis are known, including‘‘Alligator’’ luicus, Eoalligator, and Acy-nodon (Young, 1964; Li and Wang, 1987;Buscalioni et al., 1997), that increase theknown diversity of the group in the OldWorld.

Conversely, specialized taxa appearinglater in the Tertiary tend to be more bio-geographically restricted, and they tend tobe parts of clades including multiple snoutmorphologies. Slender snouts arose at leastfour times independently after the Eocene(Figs. 4, 7)—Euthecodon from Africa and


three different species of Crocodylus fromAfrica (C. cataphractus), Australia (C.johnstoni), and South America (C. inter-medius). By itself, this observation meanslittle—individual gavialoid or tomistominespecies are no more widespread than these.But in a phylogenetic context, it suggeststhe presence of discrete adaptive radiationsin the Neogene.

That Euthecodon is distantly related toother slender-snouted crocodylians has beensuggested previously (Ginsburg and Buffe-taut, 1978), but the parsimony analysissummarized in Figures 4 and 7 is the firstto clarify its relationships. Euthecodon’sclosest relatives include generalized crocod-ylians (‘‘Crocodylus’’ lloidi and ‘‘C.’’ ro-bustus) as well as specialized blunt-snoutedtaxa (Osteolaemus), all known exclusivelyfrom Africa and Madagascar. This assem-blage is herein termed the ‘‘African Endem-ic Clade,’’ and its first appearance in thefossil record is in the Oligocene (Ginsbergand Buffetaut, 1978). Members of otherclades continued to exist in Africa duringthe Miocene and Pliocene, such as probabletomistomines and Crocodylus (Tchernov,1985; Pickford, 1994). Nevertheless, theAfrican Endemic Clade represents a geo-graphically-restricted group with a broaddiversity of snout morphology.

If Harpacochampsa is a mekosuchine,then it would be the only known mekosu-chine with a slender snout. Other membersare generalized (Baru, Pallimnarchus,Kambara, Australosuchus), blunt-snouted(Mekosuchus, Trilophosuchus), or zipho-dont (Quinkana). Mekosuchinae would thusresemble the African Endemic Clade inhaving a broad diversity of snout morphol-ogies within a geographically-restrictedclade.

Very few crocodylian faunas can be saidto be truly ‘‘endemic’’—that is, comprisedentirely of taxa whose closest relatives areall from the same geographic region. TheNorth American record during the Ceno-zoic, for example, is dominated by blunt-snouted alligatorids, but during the EarlyTertiary some of the generalized crocody-lians are crocodyloids (such as ‘‘Crocody-lus’’ affinis) more closely related to Euro-pean (Dormaal crocodyloid) and Asian

(Asiatosuchus grangeri) taxa. The NorthAmerican ziphodont during the Tertiary isa pristichampsine, more closely related toEuropean and Asian ziphodonts of the sameperiod of time, and while slender-snoutedforms are curiously absent from interior de-posits during the Tertiary, coastal slender-snouted taxa are dyrosaurids, gavialoids(Thoracosaurus), or tomistomines (NewWorld Gavialosuchus) related to Old Worldforms. The only endemic generalized formsappear late in the Cretaceous or very earlyin the Tertiary (Leidyosuchus, Borealosu-chus) or late in the Cenozoic (Alligator), bywhich point they are the only crocodyliansremaining in non-coastal regions.

South America is an interesting problem.The South American crocodyliform faunahas never been truly homogeneous; caimanshave been the most prominent component,but the ziphodont forms through most ofthe Cenozoic have been various ‘‘sebecos-uchians,’’ an assemblage of non-crocodyli-an crocodyliforms of uncertain monophyly(Gasparini, 1984, 1996; Gasparini et al.,1991; Clark, 1994), and with few possibleexceptions (Berg, 1966; Antunes, 1975),these are exclusively South American. Gav-ialoids are known from South America, butthese may represent an insular radiationwithin the group—the parsimony analysisreflected in Figure 2 only included one suchform (Gryposuchus), but future analysesshould include other South American gav-ialoids (e.g., Gurich, 1912; Sill, 1970; Boc-quetin and Buffetaut, 1981; Buffetaut,1982a; Langston and Gasparini, 1997;Kraus, 1998). The crocodyliform fauna hasthus been composite, but the componentclades are themselves generally restricted toSouth America.

An interesting exception is Orthogeny-suchus, a strange crocodylian from the Eo-cene of Wyoming (Mook, 1924) that someauthors considered to be a poorly-preservedpristichampsine (e.g., Rossmann, 1998).Parsimony analysis instead suggests thatOrthogenysuchus is a nettosuchid (Brochu,1999). It predates known South Americannettosuchids (Mourasuchus) by roughly 30million years and would be the only knownderived caiman in North America duringthe Tertiary. Other caimans are known from


North America during the Tertiary (Busbey,1989), but their relationships to SouthAmerican forms are unknown. If the phy-logeny in Figure 3 is correct, then Netto-suchidae is not a geographically-restrictedclade, though by the Miocene it was prob-ably restricted to South America.

Interestingly, although gavialoids are ab-sent from South America today, the slender-snouted morphotype has been replaced bya species of Crocodylus (C. intermedius)unrelated to other slender-snouted crocod-ylians. There is also a rare subspecies of thespectacled caiman (Caiman crocodilus apa-poriensis) with an extremely slender snout,at least compared with the snouts of all oth-er known alligatorids, living or extinct(Medem, 1955; Ayarzeguena, 1984). Wemay be seeing the replacement of one en-demic slender-snouted lineage (the SouthAmerican gavialoids, if they are monophy-letic) with one or two other endemic slen-der-snouted groups.

Crocodylus presents us with an interest-ing set of problems. Australian and SouthAmerican slender-snouted Crocodylus are,based on morphology, the sole slender-snouted members of clades restricted geo-graphically to the Indopacific basin andNew World (Brochu, 2000). The NewWorld and IndoPacific assemblages withinCrocodylus might be viewed as relativelyrecent insular radiations in their respectiveregions, with the IndoPacific radiation per-haps replacing the mekosuchines. But an-cestrally, Crocodylus is an African line-age—its closest relative in Figure 4 is theAfrican Endemic Clade, the basalmostmember of Crocodylus (C. cataphractus) isAfrican, and another basal crocodyline(‘‘C.’’ megarhinus) is African. Ancestrally,we might consider it to be a part of theAfrican Endemic Clade that radiated sub-sequent to its dispersal from Africa in theMiocene. The Nile crocodile (C. niloticus)is more closely related to non-African Cro-codylus and is not a close relative of C.cataphractus, suggesting that it may actu-ally be a more recent immigrant to Africa(Brochu, 2001). The relationships ex-pressed in Figures 4 and 7 are not robustlysupported by morphology, and several fos-sils from the African Miocene and Pliocene

were not included (Maccagno, 1948; Tcher-nov and Van Couvering, 1978; Tchernov,1985) that could influence these results. In-terestingly, several molecular analyses sup-port a closer relationship between C. cata-phractus and Osteolaemus (White andDensmore, 2001); this would imply that C.cataphractus is a slender-snouted memberof the African Endemic Clade, and thatCrocodylus is not ancestrally African.

DISCUSSION

Limitations of current study

The most significant weakness of thisanalysis is the arbitrary nature in whichtaxa have been assigned to shape classes.The categories used here reflect the mor-phospatial divisions that are at least appar-ent to this author, but morphometric anal-yses (e.g., Busbey, 1994) indicate muchmore variation in skull shape than impliedby the simplistic categories applied here,and future work should take this into con-sideration.

We are limited by ambiguities in the phy-logeny estimate. For example, I adopted thephylogeny for Crocodylus supported bymorphology, but molecular evidence some-times supports very different phylogeny es-timates (Densmore and Owen, 1989; Dens-more and White, 1991; Poe, 1996; Whiteand Densmore, 2001). These usually agreewith the basic point of this paper—that theslender-snouted members of Crocodylus ar-rived at their skull morphology indepen-dently—but the biogeographic details willdiffer. Several potentially-critical fossilshave not been analyzed phylogenetically;where shown in Figures 5 through 7, theyare shown in gray to indicate the tentativenature of their placement.

Another weakness is forced on us by thefossil record. When we regard the skull spe-cializations of mekosuchines and Africanendemics as a Late Tertiary radiation, weassume that the specialized morphs do notoccur prior to the Oligocene or Miocene,when they first occur in the fossil record.But the fossil record is incomplete, and wecannot know whether these specialized taxawere present prior to the Oligocene, butsimply have not been found. The patterns


described herein thus reflect predictionsbased on currently-available data on cur-rently-supported phylogenetic patterns; fu-ture discoveries could easily overturn anyof them.

Snout shape, biogeography, and theimportance of phylogeny

Crocodylian skulls have been phyloge-netically plastic. But the morphospatial re-gion within which they have varied is rathernarrow; a limited number of anatomical so-lutions may exist for a given ecologicalproblem, and because crocodylians tend tointeract with their surroundings with theirsnouts, similar snout morphologies seem tohave arisen multiple times in disparate lin-eages. This may be partially responsible forthe widespread view that crocodylians (andcrocodyliforms generally) are ‘‘living fos-sils’’ that have changed little since the Me-sozoic. When the group as a whole (livingand extinct) is viewed in a phylogeneticcontext, they no longer look so static.

As the Tertiary progresses, we seem tosee the origination of endemic crocodylianradiations that are craniologically diverse,whereas clades earlier in the Tertiary aremore morphologically uniform and geo-graphically widespread. We are limited bythe nature of the data, and the conclusionsof this paper should be viewed as prelimi-nary, but this highlights the power of a phy-logenetic perspective when approachingmorphology, the fossil record, and histori-cal biogeography—by addressing the phy-logenetic relationships within a group, wecan simultaneously test hypotheses aboutmorphological evolution and historical bio-geography.

Most crocodylian faunas remain compos-ite later in the Tertiary—that is, most ofthem include members whose closest con-temporary relatives are in a different geo-graphic region. But endemism is more char-acteristic of later Tertiary faunas. Endemicclades become more diverse during theMiocene, at a time when crocodylian di-versity appears to rise globally, at leastwhen taxa are counted without correctingfor phylogeny (Taplin, 1984; Markwick,1998). Is there a connection between thetwo patterns? This jump in diversity is usu-

ally correlated with climatic changes afterthe Oligocene (Hutchison, 1982, 1992;Markwick, 1998; Antunes and Cahuzac,1999), but land masses were more isolatedfrom each other in the Miocene than in theEocene, when there was another diversitypeak (Vasse and Hua, 1998). We shouldthus consider the possibility that tectonicsis also a factor in driving taxic diversity, asmore widely-spaced land masses may dis-courage dispersal and foster the generationof an adaptive radiation, much like thoseseen among African and Australian crocod-ylians. This issue should be addressed byre-calibrating diversity with phylogeny inmind, including ghost lineages.

ACKNOWLEDGMENTS

I thank two anonymous reviewers forhelpful comments on an earlier draft andDonald Swiderski for the opportunity toparticipate in the joint SVP/SICB sympo-sium leading to this paper. Funding for thisproject was provided by NSF DissertationImprovement Grant DEB-9423428 (to Tim-othy Rowe), the Roosevelt Fund of theAmerican Museum of Natural History, theUniversity of Texas Geology Foundation,The Paleontological Society, and the FieldMuseum.

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