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Home , Allosaurus, Antrodemus, Ceratosaurus, Como Bluff, Dry Mesa Quarry, Marshosaurus

GAIA N'15, LlSBOAlLISBON, DEZEMBROIDECEMBER 1998, pp. 219-226 (ISSN: 0871-5424)

SKULL AND TOOTH MORPHOLOGY AS INDICATORS OF NICHE PARTITIONING IN SYMPATRIC THEROPODS

Donald M. HENDERSON Department of Earth Sciences, University of Bristol. Wills Memorial Building, Queens Road, Clifton. BRISTOL SS8 1RJ, . UK Department of Cell Biology and Anatomy, School of Medicine, Jonh Hopkins University. 725 North Wolfe Street, BALTIMORE MD 21205-2196. USA E-mail: dhende@jhmLedu

ABSTRACT: The Upper Morrison Formation of the western has pro­ duced a diverse assemblage of large, carnivorous . Analysis of the and teeth assigned to two of the best preserved of these sympatric Morrison Formation genera, MARSH and MARSH, reveals three distinct forms. These three and tooth morphologies are interpreted as evidence of feeding and behavioural niche par­ tioning among the top Morrison . One form, represented by some members ofthe Allosaurus, has a shortened face, and a tall, wide skull, with short, posteriorly di­ rected teeth. Other Allosaurus specimens have a long face, a low skull profile, and longer, more vertically oriented teeth. Ceratosaurus also has a long face, but is distinguished from the latter allosaurid by exaggeration of the depth of the skull and by having even longer and broader teeth. Distinct lacrimals and nasal ornaments seen in the three forms would aid in interspecific recognition, and reduce incidences of direct competitive interaction. The gen­ eral rarity of Ceratosaurus in the Morrison suggests competitive displacement of Cera to­ saurus by the long-toothed, long-skulled allosaurs.

RESUME: La Formation Morrison du Jurassique superieur de l'Ouest des Etats Unis a livre un ensemble varie de grands dinosaures carnivores. L'etude des cranes et des dents des espe­ ces les mieux preserves, Allosaurus MARSH et Ceratosaurus MARSH, revele trois ensembles distincts. Ces trois ensembles indiquent une division entre differentes niches alimentaires et comportementales. Quelques specimens du genre Allosaurus ont un museau tres court, un crane haut et large, et des dents courtes et dirigees en arriere. D'autre specimens d'Allo­ saurus ont un museau allonge, un crane bas et etroit, et les dents sont longues et verticales. Ceratosaurus a egalement un museau allonge, mais Ie crane est plus haut, et les dents de Ceratosaurus sont plus longues et plus larges que chez Ie deuxieme d'Allosaurus. Les lacrimaux et les ornements nasaux, qui sont distincts chez chaque forme, auraient pu aider a la reconnaissance interspecifique, et reduire ainsi la frequence des interactions directes. La rarete de Ceratosaurus dans la Morrison suggere une exclusion competitive par les allo­ saures avec des dents et un crane allonges.

INTRODUCTION rus, Allosaurus, , and (BRITT, 1991), and quarries have pro­ Many distinct genera of large, carnivorous dino­ duced associated Allosaurus, , and saurs are known from quarries in the Upper Jurassic other large carnivorous forms (BAKKER, 1996). Morrison Formation. Some individual quarries have . produced multiple genera of carnivorous forms: A review of the literature and of published skele­ Cleveland-Lloyd Quarry in has produced Cera­ tal illustrations and reconstructions suggests that if tosaurus MARSH, Allosaurus MARSH, Marshosaurus there are not two of Allosaurus from the Mor­ MADSEN and Stokesosaurus MADSEN (MADSEN, rison Formation, there is at least evidence for two 1976), Marsh, in has pro­ 'morphs'. PAUL (1988) argued for two, possibly duced Torvosaurus GALTON & JENSEN, Ceratosau- three, species. BRITT (1991) and CHURE (pers.

219 artigos/papers D.M. HENDERSON

comm., 1997) also claim at least two species of Allo­ study (DAYAN et al., 1990), small felids were also saurus. MADSEN (1976: pI. 2) produced a composite shown to fall into regularly, spaced categories with skull for Allosaurus based on material from the respect to mean canine diameter. PI MM & GITTLE­ Cleveland-Lloyd quarry. This restored skull shows MAN (1990), commenting on the work of DAYAN et al. marked differences between right and left examples (1989,1990), noted thatthese small mustelids and of the maxillae, lacrimals, pterygoids, and nasals felids form a natural guild with members of the guild suggesting that this Allosaurus is a chimera. MAD­ sharing the same prey. SEN (1976), commenting on the variation in the There are also examples of character displace­ number of tooth alveoli shown by his extensive sam­ ment in carnivores. WERDELIN (1996) found a ple of maxillae, states that the variation could not be regular partitioning of carassial total lengths and explained by different growth stages of the individu­ blade lengths in late and early als who possessed the elements. Gilmore's illustra­ hyaenids from Eurasia and Africa. The similarity of tions (GILMORE, 1920) of the reconstructed skull of the hyaenid partitioning to that seen in extant, sym~ "" LEIDY (currently regarded as a junior patric canids led WERDELIN (1996) to infer similar for Allosaurus) is very different from the re­ ecological roles for these early hyaenids who filled constructed skull presented by MADSEN (1976). the niches now filled by the true can ids. MASSARE The remains at the Cleveland-Lloyd, (1987), in a study of tooth variation in ma­ Dry Mesa, and Como Bluff quarries were all depos­ rine , found a range of tooth forms similar to ited within 3 million of each other (SMITH, that seen in modern carnivorous marine . 1996). With several different types of large, bipedal The material studied by MASSARE (1987) did not in the Morrison environment there would come from a single deposit or time plane, but her have been extensive niche overlaps with resultant study indicates that it is possible to identify feeding interspecific competition if they were living together. specializations and niche partitioning among car­ The competition between these sympatric carni­ nivorous diapsids. vores would have been heightened by the fact that The large theropods of the Morrison Formation they were all of roughly the same body length of 6-8 form a guild, one specialized for locating, killing, dis­ m. The morphological differences within the genus membering, and ingesting large prey. The skulls and Allosaurus cou ld be interpreted as showing the ef­ teeth of large theropods are their most variable fea­ fects of natural selection reducing intraspecific com­ tures, and the best indicators of how predatory petition by producing two different forms of niches were divided up. In their review of ­ Allosaurus. The present paper aims to interpret cra­ ian carnivore teeth, VAN VALKENBURGH & RUFF nial and dental morphology, as it relates to niche par­ (1987: 380) state that " ... canine teeth oflarge preda­ titioning and feeding strategy, for three, well known, tors are used to kill and dismember prey and to sympatric theropods - Ceratosaurus and two forms wound or threaten other individuals." and " ... differ­ of Allosaurus. For the purposes of this paper, one ences in bite power and behaviour should be re­ form of Allosaurus, based on the MADSEN (1976) re­ fiected by differences in canine shape if teeth are construction, wil l be referred to as Allosaurus. The designed to resist expected loads." It seems reason­ other Allosaurus, the one illustrated by GILMORE able to extend these observations to other taxa such (1920), will be refered to as "Antrodemus". as dinosaurs to infer how their teeth, and the associ­ ated skull bones and muscles, wou ld have func­ CARNIVORE ECOLOGY tioned. It is a common observation in ecology that the in­ visible hand of natural selection acts to reduce the METHODS physiological, morphological, and behavioural simi­ Published illustrations of Ceratosaurus and "An­ larities between competing organisms, thus lessen­ trodemus" (GILMORE, 1920) and Allosaurus (MAD­ ing the depression of fitness due to niche overlap. SEN, 1976) were used as the primary data sources. These reductions in similarities were termed "char­ Outline drawings of the three skulls are presented in acterdisplacements" by BROWN & WILSON (1956). Figure 1. Evidence for character displacement in sympat­ Skull lengths were measured from the posterior ric carnivores can be found in extant faunas. DAYAN end of the occipital condyle to the anterior margin of et al. (1989, 1990) presented separate instances the premaxillae from ventral views of the three taxa. that show niche partitioning among modern carnivo­ The postorbital heights were taken along lines that rous mammals. In one study (DAYAN et al., 1989), ran from the skull table, through the approximate three sympatric species of weasel (Muste/a sp.), center of the lower temporal fenestra, and inter­ from eight different localities across cepted perpendicularly with the ventral margins of were shown to have consistent, regularly spaced the juga Is. The antorbital heights were taken from groupings of mean canine diameter. In another

220 SKULL AND TOOTH MORPHOLOGY AS INDICA TORS OF NICHE PARTITIONING IN THEROPODS

Fig. 1 - Lateral and dorsal views of skulls and . Top - "Antrodemus va/ens" (LEIDY), redrawn from GILMORE (1920). Middle -Allosaurus (ragilis MARSH, redrawn from MADSEN (1976). Bottom - Ceratosaurus nasicomis MARSH, lat­ eral view redrawn from GILMORE (1920); dorsal view redrawn from PAUL (1988).

the suture lines between the nasal and frontal bones the supratemporal fenestra was taken along the and the ventral margins of maxillae. The antorbital midline of the fenestra from the left side ofthe skulls. widths were measured along lines that crossed the Tooth crown heights were taken along lines ori­ anteriormost rims of the antorbital fossae. The skull ented perpendicular to the ventral margins of maxil­ of the of Ceratosaurus described by Gil­ lae. The crown height of a tooth is the distance from more (GILMORE, 1920) is severely crushed laterally, the margin to the ventral most part of the 'and his restoration in dorsal view is too wide. Based tooth crown. Tooth width was measured as the fore­ on the interpretations of other skull features of Cera­ aft length of the visible portion of the crown. tosaurus (see discussion below), the dimensions of Tooth angle measurements represent the angle be­ the narrower skull restoration provided by PAUL tween the vertical length measuring line and a line (1988) were used for the calculation of the running from the anterior base of the tooth to the dis­ width/length ratios of Ceratosaurus. The length of tal most tip of the crown.

221 D.M HENDERSON

VARIATIONS IN SKULL CHARACTERS AND TOOTH FORE-AFT BASAL LENGTH/LENGTH FUNCTIONS "Antrodemus" has the broadest teeth in lateral TABLE I and Figure 2 summarize the morphomet­ view, when comparisons of the tooth fore-aft basal ric results. The most striking feature of the tabulated length/crown height ratios are made. This increased ratios is the closeness of the values between Cera­ length in ttie antero-posteriordirection would greatly tosaurus and Allosaurus in all but two cases. Only in increase the strength of the teeth about an axis ori­ relative maxillary tooth size and surangularform are ented transversely to the jaw margin (FARLOW et al., Allosaurus and "Antrodemus" similar. These find­ 1991). When combined with the marked posterior in­ ings suggest that the two 'morphs' of Allosaurus, clination of the teeth in "Antrodemus", it seems that each had very different feeding strategies and, pos­ the teeth have been selected to maximize their abil­ sibly, different behavioural patterns as well. There ity to withstand forces induced by the head pulling appears to be convergence between Allosaurus fra­ back while the teeth clamped onto the prey. A similar gilis MARSH and Ceratosaurus nasicornis MARSH to­ form of tooth is seen in the extant wards having a low, narrow skull, long teeth, and a (Varanus komodoensis) (AUFFENBERG, 1981). This wide gape. Ceratosaurus retains it's morphological large violently tugs it's head backwards with distance, and ecological uniqueness, by having the jaws securely clamped to the prey. longer teeth and more elaborate skull ornamenta­ tion. These observations, as well as functional inter­ SKULL HEIGHTS pretations, are expanded upon below. The postorbital and antorbital relative heights of the skulls of Allosaurus and Ceratosaurus are both TOOTH ORIENTATION low, while those of "Antrodemus" are high. A similar Ceratosaurus and Allosaurus have premaxillary pattern is seen when cat and dog skulls are com­ and maxillary teeth that are more vertically oriented pared . Dogs use their long, low skulls to deliver than are the teeth of "Antrodemus". The firsttwo ap­ quick, slashing bites to wound and harass prey. Cats pearto be converging on applying deep bites to their use their short, high skulls to deliver a deep, strong prey. If the teeth of theropods are viewed as general bite that holds onto the prey (VAN VALKENBURGH & purpose tools (FARLOW et al., 1991), then the elon­ RUFF , 1987). The longer skulls and jaws of canids gate teeth of Ceratosaurus and Allosaurus would have larger bending moments about the jaw articu­ correspond to multiple copies of the long, slashing lation resulting in increased stresses within the skull teeth of canids (VAN VALKENBURGH & RUFF, 1987). when a bite force is applied. The shorter, and there­ The associated elongation of the skull in these two fore stronger, skull of felids allows them to deliver a genera (see Skull Form below) further supports this much stronger bite, either to hold more tightly to prey slashing mode of attack as well. or drive the teeth deeper into the prey. By inference, the short face of "Antrodemus" suggests that it had a TOOTH SIZE stronger bite than that of the long faced Allosaurus. Comparisons of the relative tooth sizes reveal The relatively taller skull of Ceratosaurus, when niche partitioning among the three genera. Both compared with Allosaurus, could be explained in Ceratosaurus and Allosaurus, with their more verti­ terms of resistance to bending. The longer and cal teeth, have teeth that are a larger fraction of skull broader teeth of Ceratosaurus would present in­ length than "Antrodemus". Ceratosaurus is distin­ creased frictional resistance to penetration because guished from Allosaurus by having a crown of their increased surface area. More powerful jaw heighUskul1 length ratio 50% greater than Allosau­ adductor muscles, applying forces to propel the big­ rus. ger teeth, would increase the stresses experienced bythewhole skull. Increasing the depth of the skull in The exceptionally long teeth of Ceratosaurus Ceratosaurus would increase its flexural rigidity and have several possible interpretations. Ceratosaurus hence its strength. could be feeding on different prey than the long­ toothed Allosaurus. With large carcasses commonly An alternate interpretation could be thatthe taller available (DODSON et a/., 1980), it could be thatAllo­ skull of Ceratosaurus, its apparent height further en­ saurus and Ceratosauruswere scavenging different hanced by high lacrimals and a nasal , would aid portions of a large carcass, thus avoiding a potential in species recognition. In an environment hosting ecological overlap. The exaggerated form of the several similar carnivores, these skull ornamenta­ teeth in Ceratosaurus could be related to interspe­ tions can be viewed as acting to reduce niche over­ cific recognition (see skull heights discussion), or lap by permitting quick recognition of conspecifics could have importance in intraspecific behaviour, without the risk of direct contacUcompetition. A simi­ outside any function related to prey capture or inges­ lar case could be made for the two form of lacrimals tion (COOMBS, 1990; TANKE & CURRIE, 1995). seen in the allosaurids: low and rounded in Allosau-

222 SKULL AND TOOTH MORPHOLOGY AS INDICA TORS OF NICHE PARTITIONING IN THEROPODS

TABLE I Comparison of ratios for three Morrison Formation theropods.

Ceratosaurus Allosaurus "Antrodemus nasicornis fragi/is valens"

Maxillary Tooth Orientation sub-vertical sub-vertical backwards (Average Angle) (31 ") (31 ") (39") Maximum Maxillary Tooth Crown Height/Skull Length 0.127 0.081 0.072 Tooth Fore-Aft Basal Length/Crown Height Ratio 0.386 0.345 0.455 Skull Height/Length (Postorbital Region) 0.417 0.405 0.532 Skull Height/Length (Antorbital Region) 0.324 0.305 0.381 Skull Width/Length (Postorbital Region) 0.295 0.413 0.544 Skull Width/Length (Antorbital Region) 0.256 0.249 0.320 Supra Temporal Fenestra Length 0.107 0.136 /Skull Length Ratio 0.108 Anterior & Posterior Nasal Suture Amplitudes low low high

Surangular Dorsal Form low raised raised

Quadrate-Squamosal Orientation inclined inclined sub-vertical

rus; tall and pointed in "Antrodemus". An equivalent Both Allosaurus and Ceratosaurus share a more argument is made for the interpretation of the variety narrow snout when compared with "Antrodemus". of crests shown by the many sympatric species of This narrowness of the skull, when combined with hadrosaur from the Upper of western the low elevation of the skull, is similar to forms seen North America (HOPSON, 1975). in some extant varanids where the elongate, narrow skull is used for quick, darting movements (LOSOS & SKULL WIDTHS GREENE, 1988). The narrowness could also be inter­ In addition to a shorter, stronger face, "Antrode­ preted as "focussing" the bite force onto a smaller mus" has a wider postorbital skull than Allosaurus area, thus increasing the stress applied to region and Ceratosaurus nasicornis. Cats also have a rela­ where the teeth contact the prey. The longer teeth of tively broader post orbital region relative to dogs. Allosaurus and Ceratosaurus and the possible in­ This wide area for the expansion of jaw adductor creased frictional resistance caused by increased muscles gives felids a large jaw closing force. (Note: tooth surface area, or the need for a deep bite, may the wide zygomatic arches offelids, acting as origins have required a higher applied stress. for the masseter muscles, are also related to trans­ verse motion of the relative to the skull dur­ ANTERIOR AND POSTERIOR NASAL SUTURES ing feeding.) The very low postorbital width ratio of Other indicators of increased skull strength ex­ Ceratosaurus skull, the one with the most severe hibited by "Antrodemus" are the deeply interdigi­ crushing, may be a result of the restoration (PAUL, tated sutural contacts between the nasals, 1988) being too narrow. However, this low postorbi­ premaxillae, and frontals. These high-amplitude su­ tal width ratio is associated with low lateral profile tures would increase the rigidity of the sutural con­ and a narrow snout, and a similar association is seen tacts and increase the strength of the skull, in the narrow-skulled, long-toothed allosaur. important when applying a strong bite force and

223 D.M. HENDERSON

UTFLISL E3, A very strong bite. In contrast, the surangular in Cera­ • tosaurus is barely elevated above the dorsal margin SWAISL • E3 A of the dentary and has a much less rugose surface SWP/ SL GI • A (GILMORE, 1920: pI. 26). SHAISL ,E3 A . The' apparently weak development of the suran­ • gular and associated aponeurosis in Ceratosaurus 8 A SHP/SL • is at odds with its big teeth and deep, strong skull. TW/TL • B A Downward movement of the head while applying a TLiSL ., OJ bite has been observed in domestic cats (GORNIAK & '" , , , GANS, 1980) and this might have been important in 0 .0 0.2 0.4 0.6 0.8 1.0 Ceratosaurus as well. In Ceratosaurus the lower temporal fenestra is very large, suggesting a large Scaled Ratios muscle mass. A larger muscle mass inserting ont~a • Allosaurus fragilis t; "Antrodemus valens" larger area of the jaw might have allowed Cerato­ o Ceratosaurus nasicornis saurus to propel its large teeth. QUADRATE-SQUAMOSAL INCLINATION Fig. 2 - Skull ratio data rescaled to show the similarity between Allosaurus fragilis MARSH and Ceratosaurus na­ Allosaurus and Ceratosaurus both show an ante­ sicornis MARSH, and the differences between Allosaurus riorly inclined postero-Iateral margin to the skull, in fragi/is and "Antrademus valens" (LEIDY). The largest contrast to the almost vertical postero-Iateral margin value in ratio triple has been set to 1 and the smallest value shown by "Antrodemus". If bones are strongest of any triple set to O. The remaining intermediate value has when loaded along their long axis, the orientation of been rescaled proportional to its distance between the two extremes. Abbreviations: SHA- Skull HeightAnterior, SHP the quadrate-squamosal pair suggests differences - Skull Height Posterior, SKP - Skull Width Posterior, SL - in how jaw adductor muscle forces were applied by Skull Length, SWA -Skull Width Anterior, TL - Tooth the three genera. Allosaurus and Ceratosaurus Length, TW - Tooth Width , UTFL - Upper Temporal Fenes­ would appear to have applied maximum jaw force tra Length. when the lower jaw was wide open, and the jaw mus­ cles were parallel to the quadrate and perpendicular to the long axis of the mandible, similar to the con­ when pulling backwards to remove a portion offlesh figuration seen in . "Antrodemus", with a from a prey item. A similar form of strengthening is more vertically oriented quadrate-squamosal pair, seen in the skull roof of artiodactyls where intraspe­ would have applied a maximum jaw force when the cific combat with antlers is common (HILDEBRAND , jaw was almost closed . This ties in with the other 1982). The same sutural contacts in Allosaurus and skull features that indicate a strong "grip-and-pull" Ceratosaurus are much simpler. With only low- am­ style of jaw function. plitude contact between the nasals and frontals, and A more anteriorly inclined quadrate-squamosal much less total contact area between nasals and can also be viewed as posterior displacE'ment of thE' premaxillae, this would imply a potentially weaker, jaw , resulting in an increased gape. Sabre­ or more mobile (BAKKER, 1986), skull in these two toothed cats were capable of opening their jaws genera. very wide, and the large teeth of large theropods are similar to those of the sabre-toothed cat Smilodon SUPRATEMPORAL FENESTRA LENGTH RATIO (FARLOW et a/., 1991). The correlation between ad­ Allosaurus and Ceratosaurus have relatively aptations for a wide gape and long teeth exhibited by smaller dorsal temporal openings when compared Allosaurus and Ceratosaurusfurther distances their with "Antrodemus". The large upper temporal fenes­ feeding style from that of "Antrodemus". tra of "Antrodemus" correlates with the other fea­ tures that indicate enhanced development of jaw SPATIAL DISTRIBUTION adductor musculature in this . The type specimens for Ceratosaurus and "An­ trodemus" are both from the same quarry, Garden SURGANGULAR DORSAL FORM Park, Colorado (GILMORE , 1920). Ceratosaurus is The more elevated and complex dorsal surface also known from only four other quarries in the Morri­ of the surangular in "Antrodemus" and Allosaurus son (BRITI, 1991). From the Cleveland-Lloyd quarry suggests enhanced development of the aponeuro­ in Utah are the scattered remains of an single Cera­ ses associated with enhanced jaw adductor muscu­ tosaurus (MADSEN, 1976). The Cleveland-Lloyd lature. For "Antrodemus" this would tie in with the Ceratosaurus is vastly outnumbered by the remains other aspects of skull and tooth form that point to a of Allosaurus of many different sizes. The two quar-

224 SKULL AND TOOTH MORPHOLOGY AS INDICA TORS OF NICHE PARTITIONING IN THEROPODS

ries represent different environments - Garden Park over a large area to find sufficient food (DODSON et is interpreted as a channel deposit in a moderately al. , 1980). A large carnivore, with a lowered cost of drained floodplain, while Cleveland-Lloyd is a poorly locomotion, would have a selective advantage if it drained floodplain or delta (DODSON et al. , 1980). was able to follow these migrating, low abundance The Cleveland-Lloyd site is interpreted to have been herds. The closeness of the body sizes of the differ­ a predator trap with all the entombed individuals be­ ent Morrison theropods could be interpreted as con­ ing partof a single population (PAUL, 1988; MADSEN, vergence towards an optimal body size for tracking 1976). Ceratosaurus appears to have been a minor large, rare prey. player in the fauna of this locality. CONCLUSIONS With both Ceratosaurus and Allosaurus repre­ senting long, narrow-skulled, long-toothed preda­ The co-occurrence of many sympatric species of tors it is probable that the two species would have large carnivorous dinosaurs within the depositional been in direct competition with each other. The rarity environment represented by the Morrison Forma­ of Ceratosaurus atthe Cleveland-Lloyd site could be tion would have led to competition between these explained by its being displaced by the more abun­ top predators. Based on the adequate skull material dant Allosaurus. At the Garden Park site the short, from three of these species (Allosaurus, "Antrode­ wide-skulled, short-toothed form represented by mus", and Ceratosaurus), the distinct differences in "Antrodemus" would not be in a competitive situa­ skull and tooth form suggest three different feeding tion with the form represented by Ceratosaurus. A strategies andlor behavioural patterns. Conver­ similar situation exists at the Dry Mesa site, with the gence towards similar large body size by these sym­ remains of at least two Ceratosaurus in association patric carnivores might have been a mechanism to with allosaur remains that have the tall pointed lacri­ reduce competition by increasing the range of po­ mal of "Antrodemus" (BRITT, 1991). Although these tential prey. Differentiation in tooth and skull form groupings of genera pairs are little more than ran­ would lessen competition brought on by similar body dom associations, it is tempting to infer a situation of size and a conservative post-cranial . more "equal abundance" when it is two morphologi­ cally dissimilar carnivores, as seen at the Garden ACKNOWLEDGMENTS Park and Dry Mesa sites. I thank M.J. Benton and D.M. Unwin for critical readings and constructive criticisms of an early draft BODY SIZE EFFECTS of this paper. Thanks also to G. Cuny for improving Several trends have been identified with respect my French abstract. The ideas and interpretations in to carnivores, their prey, and their respective sizes. this paper are solely those of the author. VEZINA (1985) notes that log (prey mass) is posi­ tively correlated with log (predator mass). The simi­ REFERENCES lar sizes (6-8m) of the best known, large predatory AUFFENBERG, W. (1981) - The Behavioral Ecology of the Komodo dinosaurs of the Morrison Formation can be inter­ Monitor. Univ. of Florida Presses, Gainesville. 406 pp. preted as being a reflection of the large size of the BAKKER, R.T. (1986) - . William Morrow & potential prey - several genera of sauropods, stego­ Company, New York, 481 pp. saurs, and camptosaurs (FARLOW, 1976; DODSON BAKKER, R.T. (1996)- The Real Jurassic Park: Dinosaurs and Ha­ bitats at Como Bluff, , in MORALES, M. (Ed.). The et al., 1980), but this large predator size could also Continental Jurassic, Museum Northern Bull. , 60: 35- be viewed as a way of reducing competition. VEZINA 49. (1985) and GITTLEMAN (1985) both show that the BRITT 8.8. (1991) - Theropods of Dry Mesa quarry (Morrison For­ range in size of prey taken by a predator increases mation , ), Colorado, with emphasis on the osteo­ with increasing predator size. Being large would en­ logy of Torvosaurus tanneri. Brigham Young Univ. Geol. Studies, 37: 1-72. able a predator to take advantage of unexpected BROWN , W.l. & WILSON , E.O. (1956) - Character displacement. food sources while foraging at random. Increased Systematic Zool., 5: 49-64. choice of prey would increase potential prey bio­ COOMBS, W.P. (1990) - Behaviour Patterns of Dinosaurs, in mass, thus lessening competition. WEISHAMPEL, D.B.; DODSON , P . & OSMOLSKA, H . (Eds.), The Dinos8uria, Univ. California Press , Berkely and Los Angeles , GITTLEMAN (1985) notes that the abundances of pp. 210-224. large prey species decrease with increasing size. A DAYAN , T.; SIMBERLOFF, D. ; TCHNERNOV, E. & YOM-TOv, Y. large predator has increased cursorial ability and (1989) - Inter- and intraspecific character displacement in can forage over a larger area (MOLNAR & FARLOW, mustelids. Ecology, 70: 1526-1539. 1990). This increased foraging ability would enable DAYAN , T .; SIMBERLOFF. D. ; TCHNERNOV. E. & YOM-Tov. Y. a large predator to find more of its large and pre­ (1990) - Feline canines: community-wide character displace­ mentamong the small cats of Israel. Am. Naturalist, 36: 39-60. ferred, but rarer, prey items. The destruction and in­ DODSON, P. ; BEHRENSMEYER. A.K.; BAKKER , R.T. & MCINTOSH , gestion ofthe vegetation by many large dinosaurian J.S. (1980) - and of the dinosaur would require the herbivores to range

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