Ievolution of the theropods

Recent cladistic analyses of theropods by leading paleontologists-including Jacques Gauthier (Yale University); Paul Sereno (University of Chicago); Thomas Holtz Jr. (University of Maryland); and Michael Benton (University of Bristol)—do not agree entirely on the placements of many genera but have led to some general agreement regarding the higher level of classification of these dinosaurs. Theropods may be defined as all of the descendants of the common ancestor of both Coe-lophysis (Late Triassic, New Mexico) and Aves (birds). Accordingly, the following categories are used here and in other books in the series The Prehistoric Earth to organize the discussion of theropods.

The categories used to organize the discussion of theropods are depicted in the figure Dinosaur Clades and Relationships: Theropoda.

Ceratosauria (Late Triassic to Late Cretaceous Epochs). These are the most primitive theropods. This group includes theropods more closely related to Ceratosaurus than to birds. This clade includes ^ Coelophysis, Dilophosaurus, Ceratosaurus, and others. ^

Dinosaur Clades and Relationships: Theropoda

Dinosaur Clades and Relationships: Theropoda

Tetanurae (Early Jurassic to Late Cretaceous Epochs). These are the most derived, nonceratosaurian theropods. Tetanurans are defined as modern birds and any theropods that share a more recent common ancestor with birds than with Ceratosaurus. The category of Tetanurae is further divided into two major subgroups. The most primitive, least derived, basal tetanurans are in the subgroup Spinosauroidea. The more derived tetanurans are in the subgroup Avetheropoda.

Among the Spinosauroidea were the largest of all theropods— the spinosaurs, from North Africa. The Avetheropoda made up the largest theropod group and included many well-known genera that populated the Northern and Southern Hemispheres during the Jurassic and Cretaceous Periods. These included giants such as Tyrannosaurus, Giganotosaurus, and Carcharodontosaurus; ostrichlike dinosaurs; the sickle-clawed dromaeosaurs popularly known as "raptors"; and others, including various taxa of feathered dinosaurs that led to modern birds.

This book, Time of the Giants, will explore some members of the first and second of these groups, the Ceratosauria and earlier, more basal members of the Tetanurae.

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