Brochu, 2001

AMER. ZOOL., 41:564–585 (2001)

Crocodylian Snouts in Space and Time: Phylogenetic Approaches Toward Adaptive 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 us to compare the taxonomic, geographic, and temporal distributions of morphological features, such as snout shapes. A few basic snout morphotypes—generalized, blunt, slender, deep, and excessively broad (‘‘duck-faced’’)—occur multiple times in distantly-related lineages. Some clades—especially those found in the Northern Hemisphere or with minimum origination dates in the Cretaceous or lower Tertiary—are morphologically uniform, but geographically widespread; crocodylian faunas of the early Tertiary tend to be composite, with sympatric taxa being distantly related, and similar-looking taxa on different continents being close relatives.

In contrast, crocodylian faunas of the later Tertiary tend to be more endemic, with local adaptive radiations occurring in Africa and Australia containing members of most basic snout shapes. Endemic radiations in Africa and Australia have largely been replaced by Crocodylus, which can be divided into subclades that may individually represent endemic adaptive radiations.

INTRODUCTION Crocodyliforms are ubiquitous in continental deposits in much of the world throughout the latest Cretaceous and Cenozoic, and they occasionally occur in marginal marine deposits as well. Extinct crown-group crocodylians outnumber their living relatives by a wide margin, and there were times in the past where worldwide crocodyliform diversity clearly exceeded levels seen today, with only 23 living species (Taplin, 1984; Hutchison, 1982, 1992; Markwick, 1998; Vasse and Hua, 1998).

And yet, in any given period of time, individual crocodylian faunas were probably not much more diverse than today. There are few places now with more than two or three species occurring in the same general region (Thorbjarnarson, 1992), and most ‘‘sympatric’’ crocodylians segregate themselves ecologically (Magnusson, 1985; Magnusson et al., 1987; Webb et al., 1987; Kofron, 1992; Ouboter, 1996; Meshaka et 1 From the Symposium Beyond Reconstruction: Using Phylogenies to Test Hypotheses About Vertebrate Evolution presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 4–8 January 2000, at Atlanta, Georgia.

2 Present address: Department of Geoscience, University of Iowa, Iowa City, Iowa 52242; E-mail: christopher-brochu@uiowa.edu

al., 2000). Global crocodyliform diversity may have been higher in the past, largely because warmer climates allowed a broader geographic distribution for the group, but the number of crocodyliforms in a single deposit (and presumably living sympatrically or in close proximity) is usually five or less, with notable exceptions (e.g., Buckley et al., 2000).

Where they co-occur (now or in the past), crocodylians tend to differ morphologically (Fig. 1). The three crocodylians living in western Africa (Osteolaemus tetraspis, Crocodylus niloticus, Crocodylus cataphractus), for example, look radically different from each other—C. niloticus is a large-bodied stereotypical ‘‘crocodile,’’ with a flat toothy snout; Osteolaemus is small-bodied at adulthood (2 m total length), with a blunt snout and stout posterior teeth; and C. cataphractus has slender jaws and a tubular rostrum. Fossil faunas are similar in this regard—one usually finds one or two ‘‘generalist’’ crocodiles, one or two blunt-snouted forms, one or two taxa with long and slender jaws, and perhaps a representative of a morphotype not seen today. Not all morphotypes may be represented, but we do not see, for example, faunas with four or five generalist crocodiles.

These morphological divergences presum-

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FIG. 1. Representative examples of the snout shape categories used in this paper. A. Leidyosuchus canadensis, a ‘‘generalized’’ alligatoroid from the Late Cretaceous of North America. B. Thoracosaurus macrorhynchus, a slender-snouted gavialoid from the Paleocene of Europe. C. Alligator mcgrewi, a blunt-snouted alligatorid from the Miocene of North America (drawing adapted from Schmidt, 1941). D. Mourasuchus, a duck-faced caiman from the Miocene and Pliocene of South America (drawing adapted from Price, 1964 and Langston, 1965). E. Pristichampsus vorax, a ziphodont crocodylian from the Eocene of North America (drawing adapted from Langston, 1975). Drawings not to scale; A through D in dorsal view, E in right lateral view.

ably reflect ecological separation, though given how little we know about the ecology of many living crocodylian species, this cannot be stated with certainty. It is even more hazardous to infer ecological traits on extinct crocodylians, but the morphological differences between these crocodiles, which presumably lived sympatrically (or at least syntopically), are striking.

The focus of this paper is the interface between phylogenetic hypothesis, morphological evolution, and larger-scale temporal patterns. Are the crocodiles found in a single unit or region close relatives, with similar skull shapes arising independently in different places? Or did clades of crocodiles sharing a similar skull shape disperse widely, such that individual crocodile species are distantly related to geographic neighbors but closely related to species living elsewhere? In other words, have crocodylian lineages formed geographically-re-

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stricted adaptive radiations in the strictest sense 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 phenomenon uniformly understand that questions of adaptive radiation are inherently phylogenetic (e.g., Guyer and Slowinski, 1993; Losos and Miles, 1994; Larson and Losos, 1996). Thus, before we can answer this question, we must first obtain a phylogenetic hypothesis. Recent explorations of crocodylian phylogenetics, including a variety of data sets (both morphological and molecular) and considering both extant and extinct members of the group (Brochu and Densmore, 2001), give us a phylogenetic backbone for the study of adaptive radiation.

Phylogenetic systematics is only beginning to play the focal role it should adopt in the study of large-scale paleobiological patterns. A phylogenetic hypothesis gives us a map of the hierarchy of putative character state changes within the group. The chronicle of morphological evolution within a clade is revealed on this map. We can add geographic or temporal information, which lets us isolate adaptive radiations and, with enough information, discuss the beginnings and possible endings of these clades.

METHODS Nomenclature and hierarchy of relationships Crocodylia is a crown-group name based on the last common ancestor of Gavialis, Alligator, Paleosuchus, Caiman, Melanosuchus, Tomistoma, Osteolaemus, and Crocodylus, and all of its descendents (Clark, 1986, 1994; Benton and Clark, 1988; Brochu, 1997, 1999). I will apply the phylogenetic nomenclatural system for Crocodylia established by Norell et al. (1994) and expanded by subsequent authors (Salisbury and Willis, 1996; Brochu, 1997, 1999). This system no longer uses Linnean ranks. Within Crocodylia, there are three primary stembased group names established on the basis of living end members: Gavialoidea (Gav-

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

ialis gangeticus and all crocodylians closer to it than to Alligator mississippiensis or Crocodylus niloticus), Alligatoroidea (Alligator mississippiensis and all crocodylians closer to it than to Crocodylus niloticus or Gavialis gangeticus), and Crocodyloidea (Crocodylus niloticus and all crocodylians closer to it than to Gavialis gangeticus or Alligator mississippiensis). Within each of these, we may recognize a node-based crown group name on the basis of the last common ancestor of living members; Alligatoridae, for example, is the last common ancestor of Alligator, Caiman, Melanosuchus, and Paleosuchus, and all of its descendents.

Eusuchia is also a node-based group name, in this case referring to the group including the last common ancestor of Hylaeochampsa and Crocodylia and all of its

descendents (Brochu, 1999). These are effectively the advanced crocodyliforms with procoelous vertebrae and internal choanae completely surrounded by the pterygoids, although character state distributions within Crocodyliformes are somewhat more complex (Brochu, 1999). The only demonstrable non-crocodylian eusuchian in this analysis is Hylaeochampsa; Stomatosuchus may be a eusuchian, and several enigmatic crocodyliforms (e.g., Dolichochampsa, Aigialosuchus) probably belong to this group, though we do not know if they lie outside the crown group Crocodylia.

The phylogenetic hypothesis used in this study is reflected in Figures 2–4 and is based on several parsimony analyses of morphological characters for the crowngroup (Norell, 1989; Salisbury and Willis, 1996; Brochu, 1997, 1999, 2000). Gavialo-

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

idea today includes one species (the Indian gharial, Gavialis gangeticus), but also includes several extinct taxa from all over the world. Alligatoroids and crocodyloids form a clade named Brevirostres; sequential outgroups to Brevirostres include the deepsnouted pristichampsines (such as Pristichampsus) and more generalized Borealosuchus.

In this study, Tomistoma will be included

with the crocodylids. This is in contrast to several recent molecular analyses (Densmore, 1983; Densmore and Owen, 1989; Densmore and White, 1991; Gatesy and Amato, 1992; Gatesy et al., 1992; Hass et al., 1993; White and Densmore, 2001), in which Tomistoma and Gavialis are closest living relatives and Tomistoma is a gavialoid. The reasons for disagreement between various data sets, which otherwise

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

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agree with each other very strongly (Poe, 1996; Brochu, 1997; Brochu and Densmore, 2001), remain unknown. As will be discussed later, accepting the preferred molecular tree does not change the broad conclusions discussed herein.

Temporal calibration follows Salisbury and Willis (1996) for mekosuchine crocodyloids and Brochu (1997, 1999, 2000) for all other crown-group lineages. All three extant stem-based groups first appear as fossils in the Late Cretaceous. Crown-group alligatorids first appear with certainty in the Paleocene, although undescribed fragmentary fossils may extend the group’s known record into the Late Cretaceous (Brochu, 1999). Crown-group crocodylids first appear in the Early Eocene. Borealosuchus ranges from the Late Cretaceous to the Early Eocene. The first appearance of Pristichampsinae is in the Paleocene (Li, 1984; Gingerich, 1989), but based on the phylogeny in Figures 2–4, we would expect pristichampsine fossils to be discovered in the Late Cretaceous.

Snout shape categories It is widely believed that in crocodylians (and, more generally, crocodyliforms), most phylogenetic action is in the skull. The postcranium is viewed as relatively static with comparison to the phylogenetically plastic skull, which has morphologically fluctuated all over the evolutionary map.

This is an oversimplication—important changes can be documented within the group in most postcranial skeletal systems (e.g., Frey, 1988), and in recent phylogenetic analyses, postcranial information has proved pivotal in diagnosing several clades (Salisbury and Willis, 1996; Brochu, 1999, 2000). And yet, the interspecific differences in snout shape are much more striking; it is easy to see why crocodile systematists have been much more fascinated with the group’s craniology, given the relative uniformity of the limb skeleton against the obvious differences in the rostrum when we compare, for example, a living piscivore like Gavialis with an extinct crushing form such as Allognathosuchus.

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

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ylians. Some of these are common to any study applying fossil information. Because of the fossil record’s incompleteness, we are assessing the minimum age (and not absolute age) for a lineage, as the first appearance in the fossil record will postdate the lineage’s actual divergence time; the difference between first appearance and true age cannot be known. The relationships of some groups (living or extinct) are ambiguous, which further causes ambiguity in our interpretations of evolutionary patterns and forcing us to consider multiple equally-supported scenarios. Many fossils are incomplete or distorted, and characterizing snout shape can be difficult or impossible; and even if we accurately reconstruct phylogeny and accurately map snout shape on the tree, we cannot always be confident that snout shape (which can be preserved) and behavior (which is almost never preserved) are actually correlated—the number of living groups with derived snout morphologies is small, and our understanding of living crocodylian ecology is biased toward those with more generalized skulls.

Another important consideration is the continuous nature of snout variation between crocodylian species, especially when fossils are considered. It is easy to distinguish the snouts of a very derived gavialoid and a very derived alligatorid—one will be tubular, and the other will be very blunt.

But the line between a truly long and slender snout and that of a more generalized crocodylian that happens to be rather narrow is arbitrary. Furthermore, snout shape changes during ontogeny—snouts are generally shorter relative to skull length in hatchlings than in adults (Mook, 1921; Ka¨lin, 1933; Dodson, 1975; Webb and Messel, 1987; Hall and Portier, 1994; Busbey, 1994). For this reason, snout shape will be assessed on the basis of the snouts of mature individuals.

Busbey (1994) applied different sets of terms to describe cross-sectional shape and length in crocodyliform snouts. Snouts could be platyrostral (dorsoventrally compressed) or oreinirostral (mediolaterally compressed) in cross-section; and they could be short, medium, or long. Platyrostral snouts were further subdivided into

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broad, tubular, and narrow categories on the basis of the ratio of mediolateral and dorsoventral diameter—it approaches 1 in tubular snouts, and the mediolateral diameter is at least twice the dorsoventral in broad snouts. The shape terms applied here will combine the shape and length categories applied by Busbey (1994), but with some exceptions as discussed below.

Generalized crocodylians—Most crocodylians have a familiar dorsoventrally compressed snout that tapers gradually toward the narial region (Fig. 1a), and most living forms are dietary generalists as adults, eating any animal matter they can swallow, with dietary differences between taxa related to differences in available food supply (e.g., Reese, 1915; Cott, 1961; Gorzula, 1978; Magnusson et al., 1987; Banerjee et al., 1988; Pooley, 1989). ‘‘Generalized,’’ as used here, is a very heterogeneous assemblage of anything not having any of the specialized snout shapes discussed below. It is tempting to consider these forms as ‘‘unspecialized,’’ and use of the term ‘‘generalized’’ reflects the assumption that the other morphologies are derived specializations.

This may not be the case; in at least one group (the alligatorids), ‘‘generalized’’ forms and duck-faced forms may ultimately derive from blunt-snouted forms, and among modern crocodylians, sympatric generalized forms tend to segregate themselves ecologically (e.g., Magnusson, 1985; Kofrron, 1992; Herron, 1994; Ouboter, 1996).

Most of the animals in this category have broad platyrostral snouts of medium length sensu Busbey (1994). However, many of those he characterized as narrow-snouted (e.g., most extant Crocodylus) or longsnouted (e.g., some Borealosuchus) also fall within this category, as the snout shape is not markedly different from close relatives with more obviously broad snouts.

Most of these fell close to the dividing line between ‘‘medium’’ and ‘‘long’’ in Busbey’s scheme (1994: Fig. 10.2).

Longirostrine and slender-snouted crocodylians—Some crocodyliform snouts resemble a pair of toothed forceps (Fig. 1b).

The snout itself is very narrow and may be tubular, and the teeth are reduced in size.

These fall within the platyrostrate tubular category of Busbey (1994). In a few forms, the snout is not only narrow, but also elongate, and there may be an increased number of teeth—such taxa are not only slendersnouted, but longirostrine. Other morphological transformations have been correlated in the past with this morphology, including the evolution of large tubera on the basioccipital, enlargement of the supratemporal fenestrae and reduction of the palatal fenestrae, and homodonty (Langston, 1973; Busbey, 1994; Clark, 1994). This kind of morphology is usually thought to reflect piscivory (Pooley, 1989), but we lack good ecological data for many relevant living forms, and at least some living slender-snouted crocodylian populations are not strictly piscivorous (Webb et al., 1983).

The slender-snouted crocodylians have been a major thorn in the side of crocodile systematists for nearly two centuries. How many clades are there? Molecular analyses generally unite the two living crocodylians with the most derived snout morphologies (Gavialis gangeticus and Tomistoma schlegelii), but morphological analyses draw them apart, and though most data sets agree that the slender-snouted species of Crocodylus (C. cataphractus, C. johnstoni, and C. intermedius) are not particularly closely related to Gavialis or Tomistoma (Densmore and 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 relationships within Crocodylus.

Outside Crocodylia, the situation grows worse. Historically, long, slender snouts were thought to have evolved at least three times exclusive of crown-group forms: the thalattosuchians, pholidosaurids, and dyrosaurids. Some analyses continue to support that conclusion (Buffetaut, 1982b; Norell and Clark, 1990), but others recover a monophyletic group containing these exclusively (Clark, 1994). Are these analyses being misled by correlated characters related to the snout? Clark (1994) tried to eliminate snout-related characters and still recovered a monophyletic longirostrine clade, and morphological analyses of crown-group

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crocodylians do not generally recover clades including all of the long-snouted forms. Nevertheless, more work is needed to resolve this issue. Of these groups, the only one relevant to the present discussion is Dyrosauridae, which occurs in latest Cretaceous and Early Tertiary deposits, especially in the Tethyan region (Buffetaut, 1982b).

Blunt-snouted Crocodylians—The bluntsnouted morphotype (Fig. 1c) is represented today by the dwarf caimans (Paleosuchus) of South America and the African dwarf crocodile (Osteolaemus). These are small crocodylians, rarely exceeding two meters in length, and the rostrum is shortened relative to skull length. These have enlarged, compound palpebral ossifications and ontogenetic closure of the supratemporal fenestrae. The skull table is flattened and has abrupt margins. All of these taxa have broad platyrostrate snouts, and all would fall within the short or low-end medium length categories of Busbey (1994).

Characterizing this group is harder than it might seem on the surface, and more detailed quantitative approaches will doubtless refine the morphospatial assignments made here. Many extinct crocodylians are ‘‘blunt-snouted,’’ but are very different from Paleosuchus and Osteolaemus. This is most apparent in the basal members of Globidonta, such as Brachychampsa, Stangerochampsa, and most alligatorines; these forms do not close off the supratemporal fenestrae, and there is no evidence for enlarged palpebrals. Indeed, Paleosuchus and Osteolaemus are somewhat different from each other—the snout of Paleosuchus is deeper than that of Osteolaemus, and some large individual Paleosuchus approach the oreinirostral condition. Although the posterior teeth of Osteolaemus are relatively large in comparison with their counterparts in other crocodylids, they are not like the expanded, bulbous dentition seen in many extinct globidontans. These taxa often have a roughened region on the maxilla opposite the posterior dentary teeth, and some authors have suggested a specialization on turtles or mollusks (Abel, 1928; Weitzel, 1935; Carpenter and Lindsey, 1980; Aoki, 1989; Brinkmann, 1992).

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We cannot say that the blunt-snouted morphology, as defined here, is restricted to a common set of ecological factors. Paleosuchus and Osteolaemus tend to prefer forest-bound rivers and forest floors (Medem, 1958; Magnusson and Lima, 1991; Kofron, 1992; Ouboter, 1996), but this may not be true for most extinct forms. Certainly, the extremely bulbous cheek teeth seen in such taxa as Allognathosuchus are not found in any modern taxon. Hence, as used here, ‘‘blunt-snouted crocodylian’’ refers to a range of morphological similarity and not necessarily to a presumed ecological role.

In this analysis, I consider Hylaeochampsa vectiana—the basalmost known eusuchian—to be blunt-snouted. One of the outgroups used in this analysis (the Glen Rose Form, see Langston, 1974) is blunt-snouted, and although the other (Bernissartia) is categorized here as generalized, it shares some features more commonly seen in bluntsnouted forms, such as enlarged posterior teeth (Buffetaut and Ford, 1979). Clark and Norell (1992) tentatively suggested that this kind of morphology might be ancestral for Crocodylia.

Ziphodont crocodylians. The ziphodonts are characterized by deep, laterally compressed (oreinirostral) snouts (Fig. 1e).

Most have flattened, serrated teeth, and the term ‘‘ziphodont’’ refers to the teeth. These have been interpreted as terrestrial carnivores (Kuhn, 1938; Berg, 1966; Langston, 1975; Rauhe, 1995; Rossmann, 1998, 1999). As this morphotype does not exist today, we cannot refer to a modern example for functional studies.

Duck-faced crocodylians. There is little we can say about these animals, which are bizarre enough to belong in a Saturday morning cartoon (Fig. 1d). These taxa have long, very broad, platyrostrate snouts that resemble tombstones or kickboards in dorsal view. The skull table is reduced in size, and the teeth are small but very numerous.

We have no idea what they did, and there is nothing alive today resembling these forms.

One of these is Stomatosuchus, which was described from a large skull destroyed during the Second World War (Stromer, 1925). It is from the Late Cretaceous of

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Egypt, and its placement within Eusuchia is not certain. Its placement within Eusuchia is not certain, and it is tentatively placed at the root of this group in Figure 2. The lower jaw of this form may have been edentulous and supported a gular sac, like that of a giant pelican or baleen whale (Nopcsa, 1926).

I include Purussaurus as a duck-faced crocodylian, but this may not be entirely appropriate. Purussaurus is a close relative of Nettosuchidae (members of which are unambiguously duck-faced) and has a skull that is long, broad, but deep (Bocquetin et al., 1991). This is in contrast to the broad, but extremely flat, snout found in nettosuchids (Price, 1964; Langston, 1965; Bocquetin, 1984). Purussaurus is bizarre for other reasons—in particular, the external naris is greatly enlarged and in one form (P. brasiliensis) literally covers the snout.

No other crocodylian, living or extinct, closely resembles Purussaurus, although the external nares of a few broad-snouted crocodylids (such as Crocodylus palaeindicus) are enlarged.

RESULTS Evolution of snout shape in Crocodylia— are similar snout shapes phylogenetically restricted?

Mapping snout shape over phylogeny (Figs. 2–4) confirms the suspicions of nearly all previous workers—that snout shape has been very labile within Crocodylia, and similar snout morphologies have arisen multiple times (Ka¨ lin, 1955; Langston, 1973; Busbey, 1994; Russell and Wu, 1997). A few clades are dominated by one or two categories; most (but not all) include generalized forms, and other categories appear sporadically throughout the tree.

Within the crown group, a slender snout arose at least six times based on the phylogeny applied here—Gavialoidea, Tomistominae, Euthecodon, and three times within Crocodylus (C. cataphractus, C. johnstoni, and C. intermedius). Accepting the molecular hypothesis of phylogeny, it would have arisen only six times, as gavialoids and tomistomines would have been descended from a slender-snouted ancestor,

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

Actual longirostry—having a slender and elongate snout—arose only three times within the crown group. Gavialoids and Euthecodon both have extremely long rostra and an increased number of teeth; whereas most basal crocodylians have approximately 17 maxillary teeth, extant Gavialis has more than 20. Derived tomistomines (Tomistoma, Old World and New World Gavialosuchus) do not increase the number of teeth, but the spacing between alveoli is increased and the rostrum can be described as elongate.

There are several other possible independent derivations of slender-snouted morphology among crocodylians and close relatives. One of these is Harpacochampsa camfieldensis, a Miocene taxon from Australia. Because the rostrum is incompletely known in Harpacochampsa, we do not know if it could be described as elongate.

Its phylogenetic relationships are also unclear—Megirian et al. (1991) and Salisbury and Willis (1996) agreed that it was a crocodyloid of some sort, but their analyses did not support a placement within Mekosuchinae, a clade including virtually all Tertiary crocodylians from Australia. Based on a brief examination of the specimen, I disagree with some of the character codings of these authors and suspect it belongs within Mekosuchinae, but more complete material is required to comfortably fix its placement within the tree. Other putative slendersnouted crocodylians (or at least eusuchians) of unknown affinity include Dolichochampsa from the Cretaceous of South America (Gasparini and Buffetaut, 1980), Aigialosuchus from the Cretaceous of Sweden (Persson, 1960), Toyotamaphimaea from the Pliocene of Japan (Aoki, 1983), and Charactosuchus from the Tertiary of South America (Langston, 1965; Langston and Gasparini, 1997); this latter form is often allied with tomistomines, but available material is too incomplete to allow such a conclusion. So although the number of derivations on Figures 2 and 4 is six, the total number of independent slender-snouted acquisitions could be as high as eleven within the crown group.

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Blunt-snouted forms are especially common among derived alligatoroids (Fig. 3).

At least one diplocynodontine is bluntsnouted (Baryphracta), and the basalmost members of Globidonta are all blunt-snouted. Indeed, the ancestral alligatorid was probably a blunt-snouted form, which raises interesting questions about the polarity of snout shape evolution (see below).

Three crocodylids are categorized here as blunt-snouted (Fig. 4)—the modern African dwarf crocodile (Osteolaemus) and the Australian mekosuchines Mekosuchus and Trilophosuchus. The snouts of Mekosuchus and Trilophosuchus are incompletely known, but the morphology of the jaw, palate, and skull table in both cases is consistent with the blunt-snouted morphotype (Balouet and Buffetaut, 1987; Willis, 1993, 1997). The tree in Figure 4 suggests a minimum of two transformations to a blunt snout within Crocodylidae—one in Osteolaemus and one in the last common ancestor of Mekosuchus and Trilophosuchus. One could also suppose that Mekosuchus and Trilophosuchus evolved a blunt-snouted morphology independently because the closest relative of Mekosuchus is the ziphodont Quinkana, but a single derivation is more parsimonious.

Within Crocodylia, ziphodonts occur in two distinct lineages—Pristichampsinae (based in the parsimony analysis on Pristichampsus, but also including Planocrania) and the mekosuchine Quinkana. The mekosuchine nature of Quinkana is supported by phylogenetic analyses (Willis, 1993; Willis et al., 1993; Willis and Mackness, 1996; Salisbury and Willis, 1996), but this has been questioned by some authors on the basis of similarities between Quinkana and Pristichampsus (Megirian, 1994; Rossmann, 1998). Until a phylogenetic analysis unambiguously supports pristichampsine monophyly with the inclusion of Quinkana, I treat Quinkana as a mekosuchine.

Only one crown-group lineage is duckfaced—the clade including Nettosuchidae and Purussaurus. This morphology probably arose at least twice among derived crocodyliforms, as Stomatosuchus is probably not a close relative, though this needs to be

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tested by more rigorously determining the relationships of Stomatosuchus.

The distribution of snout shapes on Figures 2 through 4 raises some interesting questions about the polarity of snout morphology evolution. One might expect the more ‘‘specialized’’ snout morphs to have arisen from generalized ancestors, a corollary of the ‘‘law of the unspecialized’’ (Cope, 1896) in which the expected direction of morphological change is from the generalized (or unspecialized) to the specialized. This is usually the case—the ancestral snout morphology for Brevirostres, Crocodyloidea, and Alligatoroidea is generalized. But it is not universally true. ‘‘Generalized’’ forms unambiguously evolved from blunt-snouted ancestors within Alligatoridae at least twice—once in the derived caimans (Caiman, Melanosuchus) and once within Alligator. The number may be higher, because depending on how one optimizes snout morphology at the last common ancestor of Diplocynodontinae and Globidonta, Diplocynodon may represent another case of generalized morphology derived from blunt-snoutedness. It also depends on how one categorizes some species of Alligator; I here consider the Chinese alligator (Alligator sinensis) to be generalized, but one could argue that it belongs in the blunt-snouted category, which would make interpretation of snout evolution within Alligator more complex. Moreover, there are fossil alligatoroids with more generalized snout morphologies (such as Hispanochampsa; see Ka¨lin, 1936) not included here that could complicate optimizations.

Phylogenetic analyses of other organisms sometimes find similar results, in which the observed transformations do not match the ‘‘law of the unspecialized’’ (e.g., Siddall et al., 1993; D’Haese, 2000), illustrating the potential for phylogenetics to test some of our strongest-held assumptions about evolution.

Resolution within Mekosuchinae most parsimoniously suggests that the ziphodont Quinkana had a blunt-snouted ancestor.

This may not be as counterintuitive as it first looks. Some modern blunt-snouted crocodylians (e.g., the dwarf caimans, Paleosuchus) have snouts that approach an orei-

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

nirostral condition, and many extinct bluntsnouted forms have rather deep, stout skulls. The teeth of Mekosuchus and Trilophosuchus are not as enlarged as in bluntsnouted alligatorids, and so we are not faced with the problem of deriving serrated, bladelike teeth from globular crushing dentition. Nevertheless, as we continue to learn more about mekosuchine morphology and relationships, this scenario may change.

Optimization of snout morphology at the root of Crocodylia is ambiguous. In this analysis, I consider Hylaeochampsa vectiana—the basalmost known eusuchian—to be blunt-snouted. One of the outgroups used in this analysis (the Glen Rose Form, see Langston, 1974) is blunt-snouted, and although the other (Bernissartia) is not blunt-snouted, it shares some features more commonly seen in blunt-snouted forms,

such as enlarged posterior teeth. Clark and Norell (1992) tentatively suggested that this kind of morphology might be ancestral for Crocodylia.

Historical biogeography of crocodylian snout shape—are similar snout shapes geographically restricted?

Very few clades consist entirely of nongeneralized crocodylians. Notably, those restricted to specialized forms appear as fossils in the Late Cretaceous or Early Tertiary (Gavialoidea, Tomistominae, Pristichampsinae; Figs. 5 and 7). Globidonta is not strictly restricted to specialized forms, as derived Alligator and jacarean caimans are generalized, but nearly all Cretaceous and Early Tertiary globidontans are blunt-snouted (Fig. 6), and one might consider the

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

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

group to have been a specialized clade at that time.

Specialized clades first appearing in the Late Cretaceous or Early Tertiary tend to

have broad geographic distributions. Gavialoids, first known in the Campanian, have been found on all continents except Australia and Antarctica; the earliest-known taxa

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are found in marginal marine deposits of North America and Europe circumscribing the 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 they are also known from Africa and mainland Asia (Brochu, 2001). Some European Eocene fossils, such as Eosuchus (figured in Swinton, 1937), may represent gavialoids (Brochu, 2001); isolated teeth from the later Tertiary of Europe have been referred to this group (Antunes, 1994), but I am skeptical that teeth can be assigned to such a precise taxonomic level. When they first arrived in South America is unknown, but they are definitely present there from the Oligocene (Gasparini, 1996; Langston and Gasparini, 1997), and fragmentary remains from the Solomon Islands suggest the presence of a small gavialoid there in the later Tertiary (Aoki, 1988; Molnar, 1982).

Tomistomines have a similarly broad distribution, first appearing in Europe in the lowermost Eocene (‘‘Crocodylus’’ spenceri and a variety of other European fossils that may represent the same taxon, such as Dollosuchus; see de Zigno, 1880 and Swinton, 1937) but also appearing in the Western Hemisphere (e.g., Auffenberg, 1954; Bramble and Hutchison, 1971; Erickson and Sawyer, 1997; Brochu, 2001). The only African tomistomine reflected in the parsimony analyses is ‘‘Tomistoma’’ cairense, but other African fossils may represent tomistomines as well (Jonet and Wouters, 1977; Pickford, 1994; Brochu and Gingerich, 2000). Probable tomistomines are known from mainland Asia from the Eocene (e.g., Tomistoma petrolica; Yeh, 1958; Li, 1975; Ferganosuchus, Efimov, 1993), and depending on the phylogenetic relationships of Charactosuchus and Toyotamaphimaea, the group’s range may include South America and Japan.

This broad pattern—that the slendersnouted clades diverging by the Eocene are geographically widespread—holds whether we accept the morphological or molecular estimate for the relationships of Tomistoma and Gavialis. Either we have two morphologically-uniform, geographically wide-

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spread clades in the Late Cretaceous and Cenozoic, or we have one; molecular evidence agrees with morphology that slendersnouted Crocodylus are unrelated to either Gavialis or Tomistoma (Poe, 1996; Brochu and Densmore, 2001) and are silent about the placement of completely extinct slender-snouted taxa such as Euthecodon.

Pristichampsines are found broadly throughout North America and Eurasia throughout the Early Tertiary (Figs. 2, 5).

The only taxa known from reasonably complete material are from the Paleocene and Eocene (Kuhn, 1938; Berg, 1966; Langston, 1975; Li, 1984; Efimov, 1993; Rossmann, 1998), but less complete fragments (isolated flattened, serrated teeth) are known from throughout the Northern Hemisphere during the Tertiary (Bramble and Hutchison, 1971; Buffetaut, 1978; Bartels, 1983; Busbey, 1986; Gingerich, 1989; Sah and Schleich, 1990; Hanson, 1996).

This must be approached with care, as flattened, serrated teeth occur independently in other crocodyliform groups, including several outside Crocodylia in the Old World Tertiary (Berg, 1966; Buffetaut, 1982c, 1988; Vasse, 1995; Ortega et al., 1995).

Moreover, we do not know if these represent oreinirostral animals, as the snout itself is unknown. Nevertheless, there is good evidence that Pristichampsinae was a widespread clade in the Northern Hemisphere at least during the Paleocene and Eocene.

Alligatoroids are found all over North America and Eurasia throughout the Late Cretaceous and Tertiary. Taxon sampling reflected in Figures 3 and 6 is dominated

by New World forms, but several European and Asian globidontans not included in the parsimony analysis are known, including ‘‘Alligator’’ luicus, Eoalligator, and Acynodon (Young, 1964; Li and Wang, 1987; Buscalioni et al., 1997), that increase the known diversity of the group in the Old World.

Conversely, specialized taxa appearing later in the Tertiary tend to be more biogeographically restricted, and they tend to be parts of clades including multiple snout morphologies. Slender snouts arose at least four times independently after the Eocene (Figs. 4, 7)—Euthecodon from Africa and

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three different species of Crocodylus from Africa (C. cataphractus), Australia (C. johnstoni), and South America (C. intermedius). By itself, this observation means little—individual gavialoid or tomistomine species are no more widespread than these.

But in a phylogenetic context, it suggests the presence of discrete adaptive radiations in the Neogene.

That Euthecodon is distantly related to other slender-snouted crocodylians has been suggested previously (Ginsburg and Buffetaut, 1978), but the parsimony analysis summarized in Figures 4 and 7 is the first to clarify its relationships. Euthecodon’s closest relatives include generalized crocodylians (‘‘Crocodylus’’ lloidi and ‘‘C.’’ robustus) as well as specialized blunt-snouted taxa (Osteolaemus), all known exclusively from Africa and Madagascar. This assemblage is herein termed the ‘‘African Endemic Clade,’’ and its first appearance in the fossil record is in the Oligocene (Ginsberg and Buffetaut, 1978). Members of other clades continued to exist in Africa during the Miocene and Pliocene, such as probable tomistomines and Crocodylus (Tchernov, 1985; Pickford, 1994). Nevertheless, the African Endemic Clade represents a geographically-restricted group with a broad diversity of snout morphology.

If Harpacochampsa is a mekosuchine, then it would be the only known mekosuchine with a slender snout. Other members are generalized (Baru, Pallimnarchus, Kambara, Australosuchus), blunt-snouted (Mekosuchus, Trilophosuchus), or ziphodont (Quinkana). Mekosuchinae would thus resemble the African Endemic Clade in having a broad diversity of snout morphologies within a geographically-restricted clade.

Very few crocodylian faunas can be said to be truly ‘‘endemic’’—that is, comprised entirely of taxa whose closest relatives are all from the same geographic region. The North American record during the Cenozoic, for example, is dominated by bluntsnouted alligatorids, but during the Early Tertiary some of the generalized crocodylians are crocodyloids (such as ‘‘Crocodylus’’ affinis) more closely related to European (Dormaal crocodyloid) and Asian

(Asiatosuchus grangeri) taxa. The North American ziphodont during the Tertiary is a pristichampsine, more closely related to European and Asian ziphodonts of the same period of time, and while slender-snouted forms are curiously absent from interior deposits during the Tertiary, coastal slendersnouted taxa are dyrosaurids, gavialoids (Thoracosaurus), or tomistomines (New World Gavialosuchus) related to Old World forms. The only endemic generalized forms appear late in the Cretaceous or very early in the Tertiary (Leidyosuchus, Borealosuchus) or late in the Cenozoic (Alligator), by which point they are the only crocodylians remaining in non-coastal regions.

South America is an interesting problem.

The South American crocodyliform fauna has never been truly homogeneous; caimans have been the most prominent component, but the ziphodont forms through most of the Cenozoic have been various ‘‘sebecosuchians,’’ an assemblage of non-crocodylian crocodyliforms of uncertain monophyly (Gasparini, 1984, 1996; Gasparini et al., 1991; Clark, 1994), and with few possible exceptions (Berg, 1966; Antunes, 1975), these are exclusively South American. Gavialoids are known from South America, but these may represent an insular radiation within the group—the parsimony analysis reflected in Figure 2 only included one such form (Gryposuchus), but future analyses should include other South American gavialoids (e.g., Gu¨rich, 1912; Sill, 1970; Bocquetin and Buffetaut, 1981; Buffetaut, 1982a; Langston and Gasparini, 1997; Kraus, 1998). The crocodyliform fauna has thus been composite, but the component clades are themselves generally restricted to South America.

An interesting exception is Orthogenysuchus, a strange crocodylian from the Eocene of Wyoming (Mook, 1924) that some authors considered to be a poorly-preserved pristichampsine (e.g., Rossmann, 1998).

Parsimony analysis instead suggests that Orthogenysuchus is a nettosuchid (Brochu, 1999). It predates known South American nettosuchids (Mourasuchus) by roughly 30 million years and would be the only known derived caiman in North America during the Tertiary. Other caimans are known from

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North America during the Tertiary (Busbey, 1989), but their relationships to South American forms are unknown. If the phylogeny in Figure 3 is correct, then Nettosuchidae is not a geographically-restricted clade, though by the Miocene it was probably restricted to South America.

Interestingly, although gavialoids are absent from South America today, the slendersnouted morphotype has been replaced by a species of Crocodylus (C. intermedius) unrelated to other slender-snouted crocodylians. There is also a rare subspecies of the spectacled caiman (Caiman crocodilus apaporiensis) with an extremely slender snout, at least compared with the snouts of all other known alligatorids, living or extinct (Medem, 1955; Ayarzegu¨ena, 1984). We may be seeing the replacement of one endemic slender-snouted lineage (the South American gavialoids, if they are monophyletic) with one or two other endemic slender-snouted groups.

Crocodylus presents us with an interesting set of problems. Australian and South American slender-snouted Crocodylus are, based on morphology, the sole slendersnouted members of clades restricted geographically to the Indopacific basin and New World (Brochu, 2000). The New World and IndoPacific assemblages within Crocodylus might be viewed as relatively recent insular radiations in their respective regions, with the IndoPacific radiation perhaps replacing the mekosuchines. But ancestrally, Crocodylus is an African lineage—its closest relative in Figure 4 is the African Endemic Clade, the basalmost member of Crocodylus (C. cataphractus) is African, and another basal crocodyline (‘‘C.’’ megarhinus) is African. Ancestrally, we might consider it to be a part of the African Endemic Clade that radiated subsequent to its dispersal from Africa in the Miocene. The Nile crocodile (C. niloticus) is more closely related to non-African Crocodylus and is not a close relative of C. cataphractus, suggesting that it may actually be a more recent immigrant to Africa (Brochu, 2001). The relationships expressed in Figures 4 and 7 are not robustly supported by morphology, and several fossils from the African Miocene and Pliocene

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were not included (Maccagno, 1948; Tchernov and Van Couvering, 1978; Tchernov, 1985) that could influence these results. Interestingly, several molecular analyses support a closer relationship between C. cataphractus and Osteolaemus (White and Densmore, 2001); this would imply that C. cataphractus is a slender-snouted member of the African Endemic Clade, and that Crocodylus is not ancestrally African.

DISCUSSION Limitations of current study The most significant weakness of this analysis is the arbitrary nature in which taxa have been assigned to shape classes.

The categories used here reflect the morphospatial divisions that are at least apparent to this author, but morphometric analyses (e.g., Busbey, 1994) indicate much more variation in skull shape than implied by the simplistic categories applied here, and future work should take this into consideration.

We are limited by ambiguities in the phylogeny estimate. For example, I adopted the phylogeny for Crocodylus supported by morphology, but molecular evidence sometimes supports very different phylogeny estimates (Densmore and Owen, 1989; Densmore and White, 1991; Poe, 1996; White and Densmore, 2001). These usually agree with the basic point of this paper—that the slender-snouted members of Crocodylus arrived at their skull morphology independently—but the biogeographic details will differ. Several potentially-critical fossils have not been analyzed phylogenetically; where shown in Figures 5 through 7, they are shown in gray to indicate the tentative nature of their placement.

Another weakness is forced on us by the fossil record. When we regard the skull specializations of mekosuchines and African endemics as a Late Tertiary radiation, we assume that the specialized morphs do not occur prior to the Oligocene or Miocene, when they first occur in the fossil record.

But the fossil record is incomplete, and we cannot know whether these specialized taxa were present prior to the Oligocene, but simply have not been found. The patterns

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described herein thus reflect predictions based on currently-available data on currently-supported phylogenetic patterns; future discoveries could easily overturn any of them.

Snout shape, biogeography, and the importance of phylogeny Crocodylian skulls have been phylogenetically plastic. But the morphospatial region within which they have varied is rather narrow; a limited number of anatomical solutions may exist for a given ecological problem, and because crocodylians tend to interact with their surroundings with their snouts, similar snout morphologies seem to have arisen multiple times in disparate lineages. This may be partially responsible for the widespread view that crocodylians (and crocodyliforms generally) are ‘‘living fossils’’ that have changed little since the Mesozoic. When the group as a whole (living and extinct) is viewed in a phylogenetic context, they no longer look so static.

As the Tertiary progresses, we seem to see the origination of endemic crocodylian radiations that are craniologically diverse, whereas clades earlier in the Tertiary are more morphologically uniform and geographically widespread. We are limited by the nature of the data, and the conclusions of this paper should be viewed as preliminary, but this highlights the power of a phylogenetic perspective when approaching morphology, the fossil record, and historical biogeography—by addressing the phylogenetic relationships within a group, we can simultaneously test hypotheses about morphological evolution and historical biogeography.

Most crocodylian faunas remain composite later in the Tertiary—that is, most of them include members whose closest contemporary relatives are in a different geographic region. But endemism is more characteristic of later Tertiary faunas. Endemic clades become more diverse during the Miocene, at a time when crocodylian diversity appears to rise globally, at least when taxa are counted without correcting for phylogeny (Taplin, 1984; Markwick, 1998). Is there a connection between the two patterns? This jump in diversity is usu-

ally correlated with climatic changes after the Oligocene (Hutchison, 1982, 1992; Markwick, 1998; Antunes and Cahuzac, 1999), but land masses were more isolated from each other in the Miocene than in the Eocene, when there was another diversity peak (Vasse and Hua, 1998). We should thus consider the possibility that tectonics is also a factor in driving taxic diversity, as more widely-spaced land masses may discourage dispersal and foster the generation of an adaptive radiation, much like those seen among African and Australian crocodylians. This issue should be addressed by re-calibrating diversity with phylogeny in mind, including ghost lineages.

ACKNOWLEDGMENTS I thank two anonymous reviewers for helpful comments on an earlier draft and Donald Swiderski for the opportunity to participate in the joint SVP/SICB symposium leading to this paper. Funding for this project was provided by NSF Dissertation Improvement Grant DEB-9423428 (to Timothy Rowe), the Roosevelt Fund of the American Museum of Natural History, the University of Texas Geology Foundation, The Paleontological Society, and the Field Museum.

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