Dinosaur Faunas

Composition.—Dinosaurs began as minor components of Late Triassic large-tetrapod faunas (cf. Parrish, 1993; Rogers et al., 1993), but by the beginning of the Jurassic Period had become the dominant terrestrial large vertebrates. Over the remainder of the Mesozoic Era the taxonomic composition of herbivorous and carnivorous species in dinosaur faunas varied across time and space, but two particularly noteworthy faunal suites can be recognized. One of these has the herbivorous dinosaur component strongly influenced or even dominated by sauropods (e.g., the Morrison and Wessex Formations) (Table 1); this faunal type is characteristic of much of the world during the

Jurassic and Cretaceous Periods. Sauropods are absent or rare and ornithischians dominant in the second faunal type (e.g., the Dinosaur Park Formation), which occurred in the Late Cretaceous of western North America and eastern and central Asia (Table 1).

Neoceratosaurs, basal tetanurans, and carnosaurs (Holtz, 2000) are the dominant theropod groups in the first faunal type, and coelurosaurs in the second. Medium-sized and large theropods in the first faunal suite come from a variety of lineages, but all large-bodied taxa in the second type are tyrannosaurids. In both faunal types predatory dinosaurs individually are far less abundant than plant-eaters (Farlow, 1997)

Although we do not know which carnivorous dinosaur species specialized on which herbivorous species, the marked differences between the two kinds of faunas suggest the possibility of major differences in predator-prey interactions between them. For example, adult sauropods were considerably bigger than the largest theropods, while most big ornithischians were much closer to tyrannosaurids in body size. Even if tyrannosaurs preferred to attack immature individuals of prey species, it is easy to imagine a single tyrannosaur killing an adult hadrosaur or ceratopsian. It is much harder to visualize a single allosaur slaying an adult apatosaur or brachiosaur. Did large theropods in sauropod-dominated faunas attack only immature sauropods and ignore fully grown adults, or did they engage in group hunting to haul down big sauropods, or did they mainly scavenge sauropod carcasses?

Theropod species in the multi-taxon predator assemblages typical of sauropod-dominated faunas show interesting morphological differences from tyrannosaurids that suggest differences in the style of predation between carnivores in these communities. In the multi-taxon assemblages several large-bodied theropod taxa (basal tetanurans, spinosaurids, and carnosaurs) possessed very powerfully built forelimbs terminating in large talons. It is quite likely that these predators employed their forelimbs as weapons of prey acquisition. In contrast, tyrannosaurids are characterized by greatly reduced forelimbs, and so their style of prey acquisition would

TABLE 1—Comparison of the composition of several dinosaur faunas (X = Xiashaximiao Fm, China, Middle Jurassic; M = Morrison Fm, American West, Late Jurassic [LJ]; T = Tendaguru Grp, Tanzania, LJ; W = Wessex Fm, Isle of Wight, Early Cretaceous [EK]; Y = Yixian Fm, China, EK; C = Cloverly Fm, Wyoming and Montana, EK; B = Bahariya Fm, Egypt, Late Cretaceous [LK]; D = Dinosaur Park Fm, Alberta, LK; N = Nemegt Fm, Mongolia, LK; H = Hell Creek Fm, Montana amd Wyoming, LK).

TABLE 1—Comparison of the composition of several dinosaur faunas (X = Xiashaximiao Fm, China, Middle Jurassic; M = Morrison Fm, American West, Late Jurassic [LJ]; T = Tendaguru Grp, Tanzania, LJ; W = Wessex Fm, Isle of Wight, Early Cretaceous [EK]; Y = Yixian Fm, China, EK; C = Cloverly Fm, Wyoming and Montana, EK; B = Bahariya Fm, Egypt, Late Cretaceous [LK]; D = Dinosaur Park Fm, Alberta, LK; N = Nemegt Fm, Mongolia, LK; H = Hell Creek Fm, Montana amd Wyoming, LK).

Fauna:

X

M

T

W

Y

C

B

D

N

H

Herbivores: "+" indicates that the taxon is present and abundant; "r" indicates that the taxon is present but rare.

Sauropods

+

+

+

+

r

+

r

Ornithopods

r

r

+

+

r

+

+

+

+

Marginocephalians

r

+

+

+

+

Thyreophorans

r

+

+

+

r

+

+

+

+

Theropods: "*" indicates that one or all of the species in the taxon may not have been strictly carnivorous; "L" indicates that the taxon is present and includes the largest theropods in the assemblage; "p" indicates that the taxon is present; "?" indicates that the identification of this taxon in the assemblage is tentative at present.

Coelophysoids

p

p

Neoceratosaurs

p

L

Basal Tetanurans

p

p

p?

Spinosaurids

L

L

Carnosaurs

L

L

L?

p

L

L

Basal Coelurosaurs

p

p

p

p?

p

Tyrannosaurids

p

p

L

L

L

Ornithomimosaurs*

p

p?

p

p

p

Oviraptorosaurs*

p?

p?

p

p

p

p

p

Therizinosauroids*

L

p?

p

Troodontids*

p?

p

p

p

p

Dromaeosaurids

p

p

p

p?

p

Avialians*

p?

p

p

p

p

have relied on their powerful jaws alone.

Another difference between the faunal types is the overlap of theropod body sizes. In assemblages possessing multiple lineages of large-bodied theropods there is commonly great overlap in the size ofthe carnivores. For example, in the Morrison Formation the carnosaur Allosaurus, the basal tetanuran Torvosaurus, and the neoceratosaur Ceratosaurus would all include individuals of 1 tonne or greater body mass. Similarly, the Bahariya Formation's spinosaurid Spinosaurus, carnosaur Carcharodontosaurus, and basal coelurosaur Deltadromeus all exceeded 2 tonnes in mass (the first two by a considerable margin). The presence of comparable-sized predators suggests the possibility of competition among these taxa for food, perhaps mitigated by some form of morphologically mediated niche partitioning (Henderson, 2000). Similar size overlap occurs between the adults of medium-sized theropods in these assemblages, which would additionally have been in potential competition with immature individuals of the largest-bodied species.

In marked contrast, all the larger carnivorous dinosaurs in Late Cretaceous assemblages of western North America and eastern and central Asia are tyrannosaurids, and among these there is typically just one or two species present in potential sympatry. Furthermore, there is often a large discontinuity in adult sizes between the tyrannosaurids and the next largest unquestionably carnivorous dinosaurs in the fauna (generally dromaeosaurids), rather than the gradational distribution of adult sizes seen in non-tyrannosaurid-dominated faunas.

Macroecology of carnivorous dinosaurs.—The single most noteworthy feature of most dinosaurs, of course, is their large size. Body size affects or is correlated with numerous physiological and ecological features of animals (Brown, 1995; Brown and West, 2000). Large animals have bigger home ranges than do smaller species, and carnivores require more habitat space than herbivores (Kelt and Van Vuren, 2001). Farlow (2001) used published regressions of home range area against body mass in extant predatory mammals, birds, and lizards to speculate that the home range size of the 2500-kg carnosaur Acrocanthosaurus would have encompassed hundreds or thousands of square kilometers. Kelt and Van Vuren (2001), however, suggested that there may be some upper limit to home range area in mammals, regardless of body size and diet. If true, and if this upper limit holds for other terrestrial vertebrates, it raises the question of how gigantic predators like large theropods could have survived on relatively small (as compared with individual animal size) home ranges.

Because an individual animal's home range area becomes larger with increasing body size, population density (number of individuals / habitat area) must decrease (Damuth, 1987; Brown, 1995; Smallwood, 2001), which in turn mandates large geographic ranges if big-bodied species are to be represented by enough individuals for long-term viability (Calder, 2000). For trophodynamic reasons carnivores must have lower population densities than herbivores, and so the problem of sufficient habitat space should be particularly acute for enormous carnivores (Farlow, 1993; Burness et al., 2001). The huge sizes routinely achieved by carnivorous dinosaurs are therefore ecologically puzzling. Conceivably, theropod gigantism was facilitated by a combination of lower food requirements than expected for elephantine mammalian meat-eaters, along with elevated rates of biological productivity under the greenhouse conditions of the Mesozoic Era (Farlow, 1993; Farlow et al., 1995; Burness et al., 2001). In another scenario (Carrano and Janis, 1991) the greater reproductive capacity of herbivorous dinosaurs relative to placental mammals (due to oviparity of the former) would allow for more available "packages" of dinosaurian meat that could be consumed by theropods while still allowing for a viable sustainable population of prey.

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