Theropod Overview

Forelimb Morphology

Shortened forelimbs in relation to hind legs was a common trait among theropods, most notably in the abelisaurids (such as Carnotaurus) and the tyrannosaurids (such as Tyrannosaurus). This trait was, however, not universal: spinosaurids had well developed forelimbs, so also did many coelurosaurs. One genus, Xuanhanosaurus, has also been claimed to have been quadrupedal because of its comparatively robust forelimbs, but this is no longer thought to be likely.

The hands are also very different among the different groups. the most common is an appendage consisting of 3 fingers, the digits I, II and III (or possibly II, III and IV) with sharp claws. Some basal theropods (Herrerasaurus, Eoraptor) had 4 digits, and also a reduced metacarpal V. Ceratosaurians usually and 4 digits, while most tetanurans had 3.

The forelimbs' scope of use is also believed to have also been different among different families. The spinosaurids could have used their powerful forelimbs to hold fishes. Some small coelurosaurus (dromaeosaurids, scansoriopterygids) are believed to have used their forelimbs to climb in trees, and birds, which are believed to be descendants of theropods, use them for flight.

Forelimb Movement

Contrary to the way theropods have often been reconstructed in art and the popular media, the range of motion of theropod forelimbs was severely limited, especially compared with the forelimb dexterity of humans and other primates. Most notably, theropods and other bipedal saurischian dinosaurs (including the bipedal prosauropods) could not pronate their hands—that is, they could not rotate the forearm so that the palms faced the ground or backwards towards the legs. In humans, pronation is achieved by motion of the radius relative to the ulna (the two bones of the forearm). In saurischian dinosaurs, however, the end of the radius near the elbow was actually locked into a groove of the ulna, preventing any movement. Movement at the wrist was also limited in many species, forcing the entire forearm and hand to move as a single unit with little flexibility. In theropods and prosauropods, the only way for the palm to face the ground would have been by lateral splaying of the entire forelimb, as in a bird raising its wing.

In carnosaurs like Acrocanthosaurus, the hand itself retained a relatively high degree of flexibility, with mobile fingers. This was also true of more basal theropods such as herrerasaurs and dilophosaurs. Coelurosaurs showed a shift in the use of the forearm, with greater flexibility at the shoulder allowing the arm to be raised towards the horizontal plane, and to even greater degrees in flying birds. However, in coelurosaurs such as ornithomimosaurs and especially dromaeosaurs, the hand itself had lost most flexibility, with highly inflexible fingers. Dromaeosaurs and other maniraptorans also showed increased mobility at the wrist not seen in other theropods, thanks to the presence of a specialized half-moon shaped wrist bone (the semi-lunate carpal) that allowed the whole hand to fold backward towards the forearm in the manner of modern birds.

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Evolutionary History

During the late Triassic, a number of primitive proto-theropod and theropod dinosaurs existed and evolved alongside each other.

The earliest and most primitive of the carnivorous dinosaurs were Eoraptor of Argentina and the herrerasaurs. The herrerasaurs existed during the early late Triassic (Late Carnian to Early Norian). They were found in North America and South America and possibly also India and Southern Africa. The herrerasaurs were characterised by a mosaic of primitive and advanced features. Some paleontologists have in the past considered the herrerasaurians to be members of Theropoda, while other theorized the group to be basal saurischians, and may even have evolved prior to the saurischian-ornithischian split. Cladistic analysis following the discovery of Tawa, another Triassic dinosaur, suggests the herrerasaurs likely were early theropods.

The earliest and most primitive unambiguous theropods (or alternatively, "Eutheropoda" - 'True Theropods') are the Coelophysoidea. The Coelophysoidea were a group of widely distributed, lightly built and potentially gregarious animals. They included small hunters like Coelophysis and (possibly) larger predators like Dilophosaurus. These successful animals continued from the Late Carnian (early Late Triassic) through to the Toarcian (late Early Jurassic). Although in the early cladistic classifications they were included under the Ceratosauria and considered a side-branch of more advanced theropods, they may have been ancestral to all other theropods (which would make them a paraphyletic group).

The somewhat more advanced ceratosaurs (including Ceratosaurus and Carnotaurus) appeared during the Early Jurassic and continued through to the Late Jurassic in Laurasia. They competed alongside their more anatomically advanced tetanuran relatives and—in the form of the abelisaur lineage—lasted to the end of the Cretaceous in Gondwana.

The Tetanurae are more specialised again than the ceratosaurs. They are subdivided into the basal Megalosauroidea (alternately Spinosauroidea) and the more derived Avetheropoda. Megalosauridae were primarily Middle Jurassic to Early Cretaceous predators, and their spinosaurid relatives' remains are mostly from Early and Middle Cretaceous rocks. Avetheropoda, as their name indicates, were more closely related to birds and are again divided into the Allosauroidea (the diverse carcharodontosaurs) and the Coelurosauria (a very large and diverse dinosaur group including the birds).

Thus, during the late Jurassic, there were no fewer than four distinct lineages of theropods—ceratosaurs, megalosaurs, allosaurs, and coelurosaurs—preying on the abundance of small and large herbivorous dinosaurs. All four groups survived into the Cretaceous, and three of those—the ceratosaurs, coelurosaurs, and allosaurs—survived to end of the period, where they were geographically separate, the ceratosaurs and allosaurs in Gondwana, and the coelurosaurs in Laurasia.

Of all the theropod groups, the coelurosaurs were by far the most diverse. Some coelurosaur groups that flourished during the Cretaceous were the tyrannosaurids (including Tyrannosaurus) the dromaeosaurids (including Velociraptor and Deinonychus, which are remarkably similar in form to the oldest known bird, Archaeopteryx), the bird-like troodontids and oviraptorosaurs, the ornithomimosaurs (or "ostrich dinosaurs"), the strange giant-clawed herbivorous therizinosaurs, and the avialans, which include modern birds and is the only dinosaur lineage to survive the Cretaceous-Paleogene extinction event. While the roots of these various groups are found in the Middle Jurassic, they only became abundant during the Early Cretaceous. A few paleontologists, such as Gregory S. Paul, have suggested that some or all of these advanced theropods were actually descended from flying dinosaurs or proto-birds like Archaeopteryx that lost the ability to fly and returned to a terrestrial habitat.

Classification