The Spinosauridae was a specialised group of large Lower to Upper Cretaceous carnosaurs. The crocodile-like snout of Spinosaurus (Fig. 101d) from Africa suggests a connection with Baryonyx (Fig. 123) from England and possibly Africa, while Acrocanthosaurus (Fig. 101c) from the Lower Cretaceous of North America seems to have been allied to the Allosauridae.

The claws of Baryonyx (Fig. 123) reached a length of 30 cm but,because no attachment was found to the animal's skeleton, could have belonged to either the hind or the forelimbs. Since the latter were unusually thick and powerful in proportion to those of other theropods, it is usually assumed that the claws were carried on them, the animal probably bipedal. The hands bore three fingers with the claw presumably on the largest, the thumb. The neck was long and fairly straight while the skull was long and narrow like that of a crocodile, and Bary-

■ Fig. 123. Baryonyx (Spinosauridae; Lower Cretaceous; length ca. 6 m)

onyx had twice as many teeth as most theropods. It was almost certainly piscivorous as partly digested fish scales lay inside the fossilised rib cage. Car-charodontosaurus (Lower Cretaceous) and Spinosaurus (Upper Cretaceous) had a similar tooth and jaw shape (Ryan and Vickaryous 1997). Baryonyx probably hunted on all fours along riverbanks, hooking fish out of the water with its large claws in the manner of grizzly bears (Ursus spp.).

Some of the spinosaurids, such as Acrocanthosaurus, which was enormous, had backbones with low spinal crests while others, including Spinosaurus had extravagant sails on their backs. These, as we have seen, might have served in thermoregulation or agonistic display (Sect. 9.3.1) - quite possibly in both functions. Ouranosaurus (Fig. 101a; Iguanodontidae), a vegetarian contemporary of Spinosaurus from the same regions of Africa, also had a large sail on its back. This may well implicate the influence of some climatic or environmental factor in the evolution of these structures (Norman 1985; Palmer 1999). Spinosaurus was one of the largest carnivorous dinosaurs to evolve, so the sail, although it must have been vulnerable, was clearly not a disadvantage. The crocodile-like shape of its jaws and the straight, rather than curved teeth suggest that, like Baryonyx and the gigantic tyrannosaurid Carcharodontosaurus, Spinosaurus may actually have been piscivorous (Ryan and Vickaryous 1997). This suggestion is strengthened by the fact that its arms were larger than is usual among carnosaurs, and it might have spent much of the time on all-fours prowling the banks of rivers in search of large fishes and other inshore aquatic vertebrates.

11.4.4 Allosaurids

The largest carnosaurs of the Upper Jurassic were members of the family Allo-sauridae. Similar in build to the megalosaurids (Sect. 11.4.2) but even larger, they colonised all the land surface of the world. Allosaurus (Fig. 124) is the largest and best-known genus. About 12 m in length, it must have been at least 4.6 m high and weighed some 1-2 tonnes. There were strong bony prominences above the eyes, and a long, narrow ridge ran from between these down to the tip of the snout. The massive skull was lightened by the presence of several large fenestrae, and the bones were only articulated loosely, as in several other carnosaurs. These features would have conferred flexibility and added to the strength of the skull. The suggestion has been made that the teeth of Allo-saurus might have functioned like the canine teeth of sabre-tooth tigers, and that these dinosaurs slashed their prey and then waited for it to succumb to its wounds. Skull bones with movable joints could have allowed the jaws of Allosaurus first to gape open, then move apart to bite huge mouthfuls from its victims (Blount and Crowley 2001).

Some palaeontologists believe that Allosaurus would have been too heavy and clumsy to have caught living prey and that it probably fed on carrion (Sect. 11.4.1). Others are of the opinion that it was quite agile for its size and may even have hunted in packs, like prides of lions, to overwhelm the giant plant-eating dinosaurs of its time. These would have included Apatosaurus and

■ Fig. 124. Allosaurus (Allosauridae; Upper Jurassic - Lower Cretaceous; length ca. 12 m)

Diplodocus, as well as a number of smaller forms such as Camarasaurus, Cerato-saurus, Stegosaurus and Camptosaurus (Norman 1991). Bones of Apatosaurus have, indeed, been found in North America bearing the marks of teeth resembling those of Allosaurus. I believe that, like the larger mammalian carnivores today, Allosaurus would have been an opportunist, preying on anything it could catch and scavenging on any carrion it happened to find (Sect. 11.4.1).

The remains of at least 44 fossilised Allosaurus skeletons have been found in one bone bed in Utah. This accumulation of bones appears to have taken place over a relatively long period of time and provides no evidence of gregarious behaviour. It is more likely that the site was a 'predator trap'. In predator traps, herbivores become mired in quicksand,tar or mud. Their cries and struggles attract the attention of carnivores which themselves become trapped. Their carcasses then attract more predators and scavengers, which are also trapped, attracting still more carnivores to their doom. The final result is that far more predators are trapped than prey (Currie 1997b). Other explanations for the mainly monospecific occurrences of Allosaurus, as well as of Coelophysis (Fig. 117; Podoke-sauridae), in such bone beds have also been proposed. Farlow (1987) suggested that they might represent habitat preferences and that the animals died during a particular stage of their reproductive cycle. This could explain the presence of a variety of ontogenetic stages. Coombs (1990) thought that a catastrophic event might have driven the animals together. A group of two juveniles as well as a subadult and an adult Troodon (Sect. 11.2.3) may represent a family unit that perished together on the shores of a freshwater lake (Horner 1997).

Allosaurus may have been a ponderous predator, but the smaller Neovenator (ca. 9 m) from the Middle Cretaceous of England would easily have outrun its prey - probably Iguanodon, the remains of which have been found in the same site. Through modelling the mechanics of the skull of Allosaurus by computer, David Norman (1985) revealed that it was many times stronger than would have been necessary merely to remove flesh. He therefore concluded that these carnosaurs were lone ambush predators who rushed at their unsuspecting prey, took a huge bite and then withdrew until the unfortunate victim died from shock and loss of blood, as suggested above.

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