Tetanurae Avetheropoda and Its Numerous Clades

Tetanurae contains the theropods best known, such as Allosaurus, Tyrannosaurus, Velociraptor, Deinonychus, and Utahraptor. It also includes lesser-known genera, such as Oviraptor, Troodon, and Struthiomimus. The immediate ancestors of birds and their descendants are also placed within this clade, which has led most dinosaur paleontologists to state that birds are thero-pods (Chapter 15).

The overlapping of theropod traits with those of birds is the main reason why Tetanurae and its members are still the focus of much research and subject to revision with new scientific discoveries. As a result, their cladogram (see Fig. 9.1) has been revised since the first edition of this book and is only a consensus view as of this writing. It will likely displease some dinosaur paleontologists because it omits details of their phylogenetic classification or alternative cladograms. Moreover, it may be out of date by this book's publication. Indeed, the volatility of what paleontologists define as a clade for these theropods is emblematic of how dinosaur paleontology is a science that is constantly subjected to testing and modification (Chapter 2).

Some of the synapomorphies of tetanurans that unite them as a clade are:

■ Dentition in the maxilla only anterior to the orbital.

■ Antorbital and maxillary fenestrae, accompanied by increased pneumati-city of the skull.

■ Manus with digits I through to III, but digit III is reduced.

■ An expanded tibia that overlapped a reduced fibula.

■ Development of a large notch (obturator notch) on the ischium.

■ Well-developed stiffening of the caudal vertebrae by processes (zygopophyses) that extended anterior and posterior from the neural arches.

■ Pleurocoels in the dorsal vertebrae.

Although feathers are not a characteristic required for inclusion of a theropod as a tetanuran, one clade within Tetanurae (Coelurosauria) does have feathered representatives. Feathered coelurosaurs found so far include Beipiaosaurus, Caudipteryx, Microraptor, Protarchaeopteryx, Sinornithosaurus, and Sinosauropteryx, all of which have been found in Lower Cretaceous strata of northeastern China. Of these feathered coelurosaurs, Sinosauropteryx is probably the most primitive of the feathered dinosaur specimens. The others are probably more closely related to other theropod clades within Maniraptoriformes. Sinornithosaurus was at first considered the nearest possible non-avian theropod relative to birds, but now several other candidates may qualify instead (Chapter 15).

Prior to the advent of phylogenetic classifications, tetanurans and a few ceratosaurs were grouped together on the basis of size and divided into two simple categories: large meat-eaters and small meat-eaters. This sort of taxonomic lumping, first proposed by Friedrich von Huene in the 1920s (Chapter 3), eventually resulted in the assignment of genera as diverse as Albertosaurus, Allosaurus, Ceratosaurus, Carnotaurus, Dilophosaurus, and Tyrannosaurus to a group called Carnosauria for the large theropods. The smaller theropods were relegated to Coelurosauria. Recent detailed analyses of anatomical similarities and differences have revealed that Carnosauria is questionable as a taxon. Now it is regarded as a stem-based clade and only allosaurids and sinraptorids are placed in it.

As mentioned in the previous section, the large ceratosaurs were separated into their own clade, and tyrannosaurids were recognized as coelurosaurs (albeit large ones), a difference that was intimated earlier by von Huene. Convergent evolution, which is an appearance of a similar change in genotype in different lineages, that results in a similarly expressed phenotypic trait (Chapter 6), is a likely explanation of how large size was selected for at different times and places for prodigious carnivores, such as those within divergent lineages of Theropoda. Natural selection for gigan-tism in some theropods was exemplified by the allosaurids Carcharodontosaurus and Giganotosaurus (Fig. 9.6A), which were probably closely related to one another but lived on different continents (Africa and South America, respectively) during the Early

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