Tetanurae

The group known as tetanurans includes theropods that share a more recent common ancestor with birds than with Ceratosaurus. This means that these theropods were more advanced in their anatomical adaptations than the primitive Ceratosauria. The majority of theropods belong to the group Tetanurae. The name Tetanurae means "stiff tails" and refers to the fact that the ends of the tails of these theropods were stiffened by interlocking connections on the vertebrae. Many of the most famous tetanurans lived during the Cretaceous Period, but a number of less derived, more basal members of the group lived during the Middle and Late Jurassic and are known as basal tetanurans. The basal tetanurans exhibit important trends in the evolution of meat-eating dinosaurs and are represented by spectacular predators such as Allosaurus (Late Jurassic, western United States) and also by tiny meat-eaters, including Compsogna-thus (Late Jurassic, Germany and France).

Tetanurans were more like birds than were members of the Ceratosauria and were the stock from which modern birds arose. Other anatomical features shared by most tetanurans included at least one small, accessory opening in between the antorbital fenestra and the nostril opening, further lightening the skull); a muscular ridge on the shoulder blade; the positioning of all of the teeth in front of the eye; the absence of fanglike teeth in the lower jaw; and modifications to the bones of the hands, the upper leg bone, and the knee joints. Basal tetanurans possessed all of these traits but in a less developed or derived form than later tetanurans.

Early tetanurans that lived during the Middle and Late Jurassic are divided in the two major divisions and associated theropod subgroups, the Spinosauroidea and the Avetheropoda.

Megalosaurus jaw

Spinosauroidea

Spinosauroidea is a group of medium- to large-bodied theropods that itself is divided into two subgroups, the Megalosauridae and Spinosauridae.

Megalosauridae. This clade is represented by six genera known from largely incomplete remains. The name of the clade is derived from Megalosaurus (Middle Jurassic, England), or "great lizard"—a poorly known yet famous theropod because of its prominent place in the history of dinosaur science. The first specimen was discovered in a slab of slate in England in the early 1820s. The specimen included only the right half of the lower jaw and teeth. It became the first dinosaur to be described scientifically when professor William Buckland (1784-1856) named and wrote about it in 1824. Still poorly known, the genus Megalosaurus was for many years a convenient category for categorizing other poorly known large theropods— fragmentary, medium-sized theropods that lacked enough material to adequately classify them were often just referred to "Megalosaurus" or as pertaining to "a megalosaur." Other taxa grouped with the Jurassic megalosaurs are somewhat better known but far from complete, including Torvosaurus (Late Jurassic, Utah, Colorado, and Wyoming) and Eustreptospondylus (Middle Jurassic, England).

Spinosauridae. Six genera that belong in this clade are currently known. Spinosaurs—including Baryonyx (Early Cretaceous, England and Spain); Suchomimus (Early Cretaceous, Niger); Irritator (Early Cretaceous, Brazil); and Spinosaurus (Late Cretaceous, Egypt and Morocco)—were among the largest predatory dinosaurs of the Cretaceous Period. Traits included a long, very narrow snout; a narrow skull; a long neck and long arms; and a ridge along the back that was sometimes highly pronounced and sail-like. Spinosaurs are generally interpreted as fish eaters. They are discussed in greater detail in Last of the Dinosaurs.

Avetheropoda

The clade known as the Avetheropoda includes some of the most remarkably complete remains of theropods. Curiously, it includes the largest theropods in one group, the Carnosauria, and the smallest predatory dinosaurs in a second group, the Coelurosauria. The name Avetheropod—"bird theropods" or "bird-beast foot"—refers to the birdlike feet of these predators. Traits uniting avetheropods include an extra fenestra in the upper jaw area and changes the roof of the mouth and back of the skull, as well as modifications to limbs and increased stiffening of the tail.

Carnosauria. All known members of this clade are large bodied. Only 5 of the 13 members of this clade date from the Middle to Late Jurassic. The best known carnosaur is Allosaurus (Late Jurassic, western United States), a large theropod known from several complete skulls and skeletons from more than 60 specimens collected since its discovery in 1877 by a fossil-collecting team working for Othniel Charles Marsh. At the time of its discovery, Allosaurus ("other lizard") was the largest known predatory dinosaur. Curiously, Marsh's archrival Edward Drinker Cope had found a more complete specimen of the same dinosaur prior to Marsh but had not had time to examine it before Marsh found, and named, his own specimen.

Although similar in basic body form to later theropods such as Tyrannosaurus, the carnosaurs actually were a separate line of theropods that were less closely related to birds than is Tyrannosaurus. Weighing up to three tons and measuring about 40 feet (12 m) long, Allosaurus was the most common large (and probably the top) predator of the Late Jurassic American West. It was one of the only predators capable of bringing down a large sauropod, although one might speculate that its most common prey victims were likely younger, smaller individuals or species that could pose less of a threat of attack. The skull of Allosaurus was adorned with prominent hornlets over the eyes, and its powerful jaws were lined with moderately large, bladelike teeth for slicing its prey.

Paleontologist Emily Rayfield of Cambridge University recently studied the bite and stress forces that probably took place in the skull of Allosaurus. Using noninvasive computed tomography (CT) scans to investigate the allosaur skull, Rayfield determined that the power of the bite was focused at the front of the jaw, resulting in a "slash and tear" approach to feeding that reduced stress on the teeth. Using this technique, an allosaur would grip its prey with its front teeth and swing its head from side to side to rip off meat.

Among the other known Jurassic carnosaurs are four genera from various parts of the world: Lourinhanosaurus (Late Jurassic, Portugal); Monolophosaurus (Middle Jurassic, China); Sinraptor (Middle Jurassic, China); and Yangchuanosaurus (Late Jurassic, China). Sinraptor ("China thief") is the best known of these and is represented by a nearly complete skull and skeleton first described in 1993 by a joint Canadian-Chinese paleontological team headed by Phil Currie (Royal Tyrrell Museum) and Dong Zhiming (Institute of Vertebrate Paleontology and Paleoanthropology). Sinraptor was a slender predator that measured about 25 feet (7.5 m) long.

The Carnosauria survived into the Cretaceous Period, with several famous, very-large-bodied carnosaur predators evolving during this time, including members of the Carcharodontosauridae, such as Carcharodontosaurus (Early Cretaceous, Egypt, Algeria, and Niger) and Giganotosaurus and Mapusaurus (Early Cretaceous, Argentina), both of which are described in more detail in Last of the Dinosaurs.

Coelurosauria. Most coelurosaurs were relatively small, although a few attained sizes that rivaled the giant spinosaurids and carcha-rodontosaurs. It is within this group that birds reside. Anatomical traits uniting the coelurosaurs included forelimbs that were more than half as long as the hind limbs, long second and third digits on the hand, enlargement of the fenestrae in front of the eye socket, and modifications of the hind foot that improved speed and agility.

While most coelurosaurs date from the Cretaceous Period, Compsognathus was a basal coelurosaurian from the Late Jurassic of Germany and France. About the size of a chicken, this small, lightweight predator is known from excellent specimens found in fine-grained limestone. What these fossils reveal is a delicate creature that was extraordinarily birdlike but lacked wings.

Several groups of coelurosaurs evolved during the Cretaceous, including the gigantic tyrannosaurs and smaller oviraptorosaurs and the dromaeosaurs, which are detailed in Last of the Dinosaurs. The relationship of Compsognathus to later coelurosaurs is uncertain.

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