The Carnosauria is a group of mostly large theropods measuring 16 feet (5 m) long or more and characterized by massive heads; burly, powerful necks; relatively short arms with three fingers per hand; powerful hind limbs; and long, deep tails. Except for the spinosaurs and tyrannosaurs, the largest theropods come from the ranks of the carnosaurs. Carnosauria includes three subgroups: the Sinraptori-dae, the Allosauridae, and the Carcharodontosauridae.

The Sinraptoridae were large predators that measured up to 30 feet (9 m) long. Currently, the clade includes only two genera, both dating from the Late Jurassic of China: Sinraptor and Yangchuano-saurus. Sinraptor is the better known of these, with two species represented by nearly complete skulls and skeletons. Sinraptorids appear to be closely allied with allosaurids, with key differences being a longer and lower skull that is perforated with additional holes and a larger antorbital fenestra, or skull opening, in front of the eyes.

The clade Allosauridae was named after Allosaurus (Late Jurassic, western North America and Europe), one of the best known large theropods and one of only two taxa in this clade. Allosaurus was first discovered by fossil hunting crews working for Edward Drinker Cope in 1877, but Cope was delayed in examining the fossil, and the creature was subsequently named Allosaurus ("strange lizard") by Othniel Charles Marsh in the same year, based on a less impressive, fragmentary specimen. Many partial and complete skeletons and skulls of Allosaurus have since been found, making it one of the best-understood predatory dinosaurs.

Allosaurus was the most common large predator in North America during the Late Jurassic Epoch. Its large head, bladelike teeth, short arms with t hree-clawed hands, sturdy legs, and long tail characterize the basic traits of these carnosaurs. The Allosaurus

© Infobase Publishing An Allosaurus skull skull was also distinguished by a knobby horn on the brow above each eye. The largest allosaur specimens measure about 33 feet (10 m) long, big enough to threaten the well-being of the large herbivorous sauropods with which Allosaurus coexisted.

One of the most spectacular allosaur specimens is that of "Big Al," a subadult specimen discovered in Wyoming in 1991. This articulated skeleton was 95 percent complete and lay with its head twisted back in a convulsive death pose. Its excellent preservation is due to having been buried quickly by stream sediments that prevented the carcass from being scavenged by other animals.

Big Al was about 25 feet (7.5 m) long, making it nearly full grown. One remarkable aspect of the specimen is that the bones show evidence of a variety of life traumas and infections. At various points in its life, Big Al had broken several ribs and sustained injuries and infections to its hips, feet, and claws. Although the cause of these injuries cannot be known, it is not outside of the realm of possibility to think that this subadult had taken a beating while trying to attack a large sauropod and perhaps was brushed aside by a swipe of the herbivore's multi-ton tail. Whether these injuries led to the death of Big Al cannot be known, but the allosaur was certainly weaker and less able to defend itself while suffering from such maladies.

Recently, Allosaurus became one of the very few dinosaur genera known from more than one modern continent, with the discovery of Allosaurus europaeus in Portugal. Remains of this new European allosaurid include a nearly complete skull and some postcranial bones.

The only other taxon currently assigned to the Allosauridae is Saurophaganax (Late Jurassic, Oklahoma), based on a fragmentary specimen that may actually be a large specimen of Allosaurus. A few paleontologists also view Acrocanthosaurus (Early Cretaceous, Oklahoma and Texas) as an allosaurid, but most consider this taxon as part of the Carcharodontosauridae.

The third group of Carnosauria was the Carcharodontosauridae. During the Cretaceous, the Northern Hemisphere had Tyranno-saurus and its relatives; the Southern Hemisphere had the carcha-rodontosaurs. These extremely large-bodied predators arose in the Northern and Southern Hemispheres during the Early Cretaceous and were the dominant predators of their world until they diminished early in the Late Cretaceous. Three of the six largest known theropods were carcharodontosaurs (see the table "The Largest Predatory Dinosaurs," page 67).

Ernst Stromer first proposed the clade of theropods known as Carcharodontosauridae in 1931; he based his proposal on the first specimen of Carcharodontosaurus (Late Cretaceous, Egypt), which he was the first to describe. The original specimen of Carcharodontosaurus, or "shark tooth lizard," consisted of fragmentary bones and teeth. Unfortunately, like the remains of Spinosaurus, the original specimen of Carcharodontosaurus was destroyed in a bombing run on Munich during World War II.

In 1995, Paul Sereno discovered an isolated but remarkably complete skull of Carcharodontosaurus in Morocco, thereby confirming its affinities within the Theropoda. Sereno and colleagues created the clade Carcharodontosauridae to include this dinosaur and other, similar giant predatory dinosaurs that have been found in the Southern Hemisphere. The Carcharodontosauridae are defined as consisting of Carcharodontosaurus and all species closer to Carcha-rodontosaurus than to either Allosaurus, Sinraptor, or the common house sparrow.

Carcharodontosaurus itself is best known from its enormous skull measuring 5.5 to 6 feet (1.6 to 2 m) long. The size of that skull has only now been eclipsed by that of the more recently discovered Giganoto-saurus, a cousin from Argentina and also a carcharodontosaur.

Giganotosaurus (Late Cretaceous, Argentina), the "giant southern lizard," was described in 1995 by Argentinean paleontologists Rodolfo Coria and Leonardo Salgado and was immediately recognized as one of the largest known theropods, second only to the lost fossils of Spinosaurus. The original specimen was about 70 percent complete and represented a long, heavy-bodied predator measuring about 40 feet (12.2 m) long. In 1998, Jorge Orlando Calvo and Coria announced the discovery of a new specimen of Giganotosaurus with an isolated lower jaw, the size of which indicated an animal that was about 46 feet (13.7 m) long. Giganotosaurus had a large, long skull with a bony ridge running along the sides of the snout and short brow horns just before the eyes. Its jaw was lined with slender, bladelike teeth, the longest of which were about 6 inches (15 cm).

Unlike tyrannosaurs, whose jaw muscles ran further along the length of the jaw to provide increased bite force, the muscles of the Giganotosaurus jaw and those of other carcharodontosaurs were largely concentrated at the rear of the jaw to improve the speed with which the dinosaurs could snap their jaws onto their prey. Giganoto-saurus and other carcharodontosaurs probably used a grapple-and-bite killing tactic: They probably used their hand claws to hold or bring down the prey while their teeth did the killing. The narrow, bladelike teeth of carcharodontosaurs were well adapted for ripping ribbons of flesh from their victims.

Mapusaurus (Late Cretaceous, Argentina) is another carcharodontosaur of immense size like that of its close relative, Giganotosaurus.

The largest known specimen of Mapusaurus would have been about 41 feet (12.2 m) long. The skull of Mapusaurus differed significantly from that of Giganotosaurus by having a shorter snout; a narrower shape; a relatively large, triangular-shaped skull opening in front of the eyes; and a bumpy ridge extending on both sides of the skull from just behind the nostril, over the brow, and behind the eye.

Perhaps the most remarkable part of the Mapusaurus story is that the animal's discovery included the recovery of individuals of different ages from the same bone bed. The specimens ranged in size from 17 to 41 feet (5 to 12.2 m). The excavation and description of Mapusaurus was a joint effort by the Argentinean-Canadian Dinosaur Project led by Rodolfo Coria and Canadian Phil Currie. The job of excavating such a rich deposit of fossils required five field seasons between 1997 and 2001.

Having been a crew member for the 1999 expedition, I can attest to the enormous amount of work required simply to expose the bone bed so that the careful work of extracting bones could begin in earnest. The first week was spent chipping away with shovels, picks, chisels, and hammers at the massive amount of rock, called overburden, that covered most of the site. The crew removed an estimated 15 to 20 tons (14 to 18 tonnes) of rock over the course of a week—enough to fill a few small trucks. All of this was done using hand tools and plastic buckets to carry the rock to a waste pile. Removing the hundreds of bones required part of the 1999 field season plus two more.

The bone bed itself is interpreted as a freshwater deposit, probably formed by seasonally heavy rains in an arid or semiarid climate. Although the bone bed might represent a long-term accumulation of individual carcasses, the occurrence strongly suggests that a group of large theropods traveling together might have perished at the same time. This is unusual in the study of theropods because the remains of most predatory dinosaurs are found in isolation. It is generally assumed that they hunted alone.

Evidence that some of the largest of the theropods may have traveled in groups is extremely rare. A site in western Canada contains fossils of 22 individuals of the tyrannosaur Albertosaurus (Late

Cretaceous) that perished suddenly. The group consisted of juveniles and adults and is considered by paleontologist Phil Currie to have been a hunting pack. In imagining their behavior, Currie suggested that the swifter juveniles might have gone ahead of the pack to chase down and trouble a prey group, such as hadrosaurs. They would eventually single out individuals and distract them until the adults could arrive to make the kill. Currie later revised this view in 2006, saying that it was also feasible that the group was merely an accumulation of individuals rather than a pack, although evidence of group behavior in large theropods from other localities continues to fuel speculation about pack behavior in the biggest meat-eaters.

The Mapusaurus site also contains individuals of various sizes, from juvenile to adult, and suggests that they were traveling in a group when overcome by a violent rush of floodwater. If this group

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