Overview Of Cretaceous Theropods

Carnivorous dinosaurs were members of the saurischian clade known as Theropoda ("beast foot"), which included numerous distinct subgroups all sharing a common ancestor. About 40 percent of all known dinosaur taxa were theropods. Their remains have been found on every continent, and theropods ranged in size from the tiniest of dinosaurs to the largest terrestrial predators that ever existed. Another book in this series, Dawn of the Dinosaur Age, provides a detailed look at the classification of theropods and the anatomical adaptations that unite all of their taxa.

In evolutionary terms, the theropod group known as the Tet-anurae ("stiff tails") represented the last great wave of predatory dinosaurs, as well as birds. The tetanurans were the most birdlike, or derived, of theropods. The name Tetanurae refers to a stiffening of the tail because of interlocking projections on the tail vertebrae. Members of this clade also shared certain modifications, such as a ridge on the shoulder blade for the attachment of muscle, features of the hand, and changes to the leg and knee joint. As a group of related taxa, the tetanurans include modern birds and any theropods that share a more recent common ancestor with birds than with Cerato-saurus, which belonged to the earliest and most primitive branches of the evolutionary tree of carnivorous dinosaurs.

For the purposes of discussion, theropods may be defined as all of the descendants of the common ancestor of Coelophysis (Late Triassic, New Mexico) and Aves (birds). Accordingly, the following categories are used in this series, The Prehistoric Earth, to organize the discussion of theropods:

Ceratosauria (Late Triassic to Late Cretaceous Epochs). These are the most primitive theropods, including theropods more closely related to Ceratosaurus than to birds. The Ceratosauria is further divided into two subgroups: Coelophysoidea, consisting

primarily of small theropods that lived from the Late Triassic to Early Jurassic Epoch, and Neoceratosauria, composed of many medium- to large-bodied theropods, mostly from the Southern

Hemisphere, that lived throughout the Jurassic and Cretaceous and usually exhibited some form of skull ornamentation. Examples of Coelophysoidea and Neoceratosauria from the Late Triassic Epoch and the Jurassic Period are discussed in Dawn of the Dinosaur Age and Time of the Giants, two other books in this series. The discussion of neoceratosaurs below covers taxa that lived during the Cretaceous Period.

Tetanurae (Middle Jurassic to Late Cretaceous Epochs).

These were the most derived, nonceratosaurian theropods. Tet-anurans are defined as modern birds and any theropods sharing a more recent common ancestor with birds than with Ceratosaurus. The Tetanurae is further divided into two major subgroups: the Spinosauroidea, consisting of the most primitive, least-birdlike, basal tetanurans, and the more derived Avetheropoda. Among the Spinosauroidea were the largest of all theropods, the spinosau-rids from North Africa. The Avetheropoda were the most diverse theropod group and included many well-known genera that populated the Northern and Southern Hemispheres during the Cretaceous Period. These included giants such as Tyrannosaurus, Giganotosaurus, Mapusaurus, and Carcharodontosaurus; ostrichlike dinosaurs; the sickle-clawed dromaeosaurs popularly known as "raptors"; and others, including various taxa of feathered dinosaurs leading to modern birds.

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