Paleobiology, 28(4), 2002, pp. 527±543 View metadata, citation and similar papers at core.ac.uk brought to you by CORE Dinosaurian and mammalian predators compared provided by ZENODO Blaire Van Valkenburgh and Ralph E. Molnar Abstract.ÐTheropod dinosaurs were, and mammalian carnivores are, the top predators within their respective communities. Beyond that, they seem distinct, differing markedly in body form and an- cestry. Nevertheless, some of the same processes that shape mammalian predators and their com- munities likely were important to dinosaurian predators as well. To explore this, we compared the predatory adaptations of theropod dinosaurs and mammalian carnivores, focusing primarily on aspects of their feeding morphology (skulls, jaws, and teeth). We also examined suites of sympatric species (i.e., ecological guilds) of predatory theropods and mammals, emphasizing species richness and the distribution of body sizes within guilds. The morphological comparisons indicate reduced trophic diversity among theropods relative to carnivorans, as most or all theropods with teeth ap- pear to have been hypercarnivorous. There are no clear analogs of felids, canids, and hyaenids among theropods. Interestingly, theropods parallel canids more so than felids in cranial propor- tions, and all theropods appear to have had weaker jaws than carnivorans. Given the apparent trophic similarity of theropods and their large body sizes, it was surprising to ®nd that species richness of theropod guilds was as great as or exceeded that observed among mammalian carnivore guilds. Separation by body size appears to be slightly greater among sympatric theropods than carnivorans, but the magnitude of size difference between species is not constant in either group. We suggest that, as in modern carnivoran guilds, smaller theropod species might have adapted to the threats posed by much larger species (e.g., tyrannosaurs) by hunting in groups, feeding rapidly, and avoiding encounters whenever possible. This would have favored improved hunting skills and associated adaptations such as agility, speed, intelligence, and increased sensory awareness. Blaire Van Valkenburgh. Department of Organismic Biology, Ecology, and Evolution, University of Cali- fornia, Los Angeles, California 90095-1606. E-mail: [email protected] Ralph E. Molnar. Museum of Northern Arizona, 3101 North Fort Valley Road, Flagstaff, Arizona 86001 Accepted: 4 June 2002 Introduction plained as adaptations to minimize competi- tion. Thus sympatric large carnivores usually Tyrannosaurus rex and the lion, Panthera leo, would seem to have little in common other differ in body size, dental morphology, or than both being the largest carnivores in their skeletal anatomy (see Eaton 1979; Van Valken- respective ecosystems. Among their many dif- burgh 1985, 1988; Dayan et al. 1990, 1992). It ferences, T. rex was a ®ve-ton biped with re- might be expected that sympatric theropods duced forelimbs and multiple serrated teeth; diverged in a similar fashion if they faced the other, a 100-kg quadruped with muscular, comparable pressures from interspeci®c com- clawed forelimbs and teeth specialized for petition. distinct functions, such as killing or slicing. In this paper, we ®rst review the relevant lit- Nevertheless, there is reason to believe that erature on mammalian carnivores to establish some of the same processes that shape mam- the background against which theropods will malian predators and their communities were be viewed. We then compare some of the pred- important to dinosaurian predators as well. atory adaptations of theropod dinosaurs and Interspeci®c competition, for example, ap- mammalian carnivores, focusing primarily on pears to play a signi®cant role in determining aspects of their feeding morphology (skulls, the abundance and distribution of many large jaws, and teeth). This is followed by an ex- extant carnivores. For instance, by choosing to amination of suites of sympatric species (i.e., hunt at a different place or time, coyotes avoid ecological guilds) of predatory theropods and wolves, cheetahs avoid lions, and leopards mammals, emphasizing species richness and avoid tigers (Van Valkenburgh 2001). Associ- the distribution of body sizes within guilds. ated with these behavioral differences are The results presented here are not meant to be morphological differences that can be ex- conclusive, as they are based on a limited sam- q 2002 The Paleontological Society. All rights reserved. 0094-8373/02/2804-0007/$1.00 528 BLAIRE VAN VALKENBURGH AND RALPH MOLNAR ple of mammals and theropods. Moreover, the ture on interspeci®c interactions among ex- gulf between carnivoran morphology and the- tant large carnivores found ample evidence of ropod morphology is large and this makes carcass theft, intraguild predation, and inter- quantitative comparisons dif®cult. For exam- speci®c avoidance in tropical and temperate ple, theropods differ from carnivorans in limb ecosystems (Palomares and Caro 1999; Van stance (biped vs. quadruped), masticatory Valkenburgh 2001). Four key points are sum- musculature, tooth replacement, and skull marized here. First, most interspeci®c inter- construction. Nevertheless, similar biome- actions between predators occur as contests chanical and ecological constraints are ex- for the possession of a kill and these contests pected to have been important to both kinds can be frequent. For example, in areas of high of predators. This paper is intended as no spotted hyena density, African wild dogs lost more than a preliminary exploration of tro- 60±86% of their kills to hyenas (Kruuk 1972; phic divergence among coexisting theropods, Fanshawe and Fitzgibbon 1993). As is true of and it is our hope that it will inspire further, carcass theft, the motivation for intraguild more-detailed studies. predation appears to be hunger in many in- stances. However, equally or more often, the The Guild of Large, Predatory Mammals victim is not eaten and the likely motivation is The ecological concept of guild was ®rst de- to remove a competitor who might also prey ®ned by Root (1967: p. 335) as ``a group of spe- on young. Second, body size is the usual de- cies that exploit the same class of environmen- terminant of rank within the guild; larger spe- tal resources in a similar way.'' As such, spe- cies tend to dominate smaller ones (e.g., lion cies within a guild are expected to compete over hyena in Africa or wolf over coyote in more intensely with each other than with North America). Third, the body size rule can those outside the guild. In theory, guilds are be overturned by the smaller species acting as not limited by taxonomic boundaries; an ex- a group (e.g., hyenas vs. lions; African wild tant predator guild could include reptiles, dogs vs. hyenas). Fourth, intraguild predation birds, and mammals, for example (see Jaksic and kleptoparasitism occur in both forested et al. 1981). However, in practice, they often and open environments. Although these types are so limited for various reasons, such as to of interaction have been observed less fre- maximize the potential role of competition or quently in forested environments, the behav- to facilitate the study (for review of the guild ior of some species strongly suggests that they concept, see Simberloff and Dayan 1991). have had a signi®cant impact. For example, Large, predatory mammals form a guild in leopards avoid areas where tigers are com- which competition is expected to be relatively mon (Seidensticker 1976; Seidensticker et al. intense. Their shared food resources, prey, are 1990), and do not cache their kills in trees often dif®cult and dangerous to kill, and con- where tigers and dholes are absent (Mucken- sequently carcass theft (kleptoparasitism) is a hirn and Eisenberg 1973). worthwhile alternative to hunting. Carcass Previous work on morphological diver- theft involves confrontation between individ- gence among sympatric large carnivores sub- uals, which can be dangerous given that the stantiates the importance of interspeci®c com- participants are armed with teeth and jaws, petition over evolutionary timescales. In a se- and in some cases, claws, designed to kill. ries of papers, Van Valkenburgh (1985, 1988, Moreover, given the species' predatory abili- 1991, 1994) explored morphological separa- ties, interspeci®c competition can be manifest tion among large carnivores in past and pre- as intraguild predation (Polis and Holt 1992). sent communities. Comparisons of dental and If competition among carnivores is as intense inferred dietary differences among sympatric as predicted, then adaptations to minimize predators from extinct and living communi- dangerous encounters and escape predation ties revealed repeated patterns of divergence are expected in less dominant members of the within guilds into meat specialists (e.g., felids, guild. extinct nimravids), bone crackers (e.g., hyaen- Two recent reviews of the published litera- ids, borophagine canids), and omnivores (e.g., DINOSAURIAN AND MAMMALIAN PREDATORS 529 coyotes, early amphicyonids). Within dietary theropods likely preyed on smaller ones and types, coexisting species were likely to differ may have even eaten juveniles of their own in body size or locomotor adaptations. Larger species. Consequently, we might expect to predators are capable of taking larger prey, ®nd predatory adaptations, as well as adap- thus effecting some dietary separation. Differ- tations to interspeci®c competition and intra- ences in locomotory abilities (such as climb- guild predation,