Definition and Unique Characteristics of Theropoda

Theropoda (thero = "beast" and poda = "foot") is a stem-based clade within Saurischia. Stem-based clades are those that have a shared common ancestor that is more recent than that of another group (Chapter 5). Theropoda is also a sister clade to Sauropodomorpha from the parent clade of Saurischia. A sister clade is a taxon that shares and splits from the same ancestral group as another taxon. In the case of theropods and sauropodomorphs, they had a common saurischian ancestor but then probably diverged early in their respective evolutionary histories. This clade is named after the Theropoda of O. C. Marsh (Chapters 2 and 3), who devised the term in 1881 to describe dinosaurs interpreted as meat-eaters. Marsh evidently made a mistake in creating such a name, because in the following year he authored a classification of dinosaurs wherein he named the Ornithopoda (= "bird foot": Chapter 11) on the basis of dinosaur feet that no longer resembled those of birds. Marsh's scientific reputation alone ensured that these names were cited, and they subsequently became entrenched in the paleontological literature.

Within Theropoda are two (maybe three) major stem-based clades, Ceratosauria and Tetanurae (Fig. 9.1). Recent re-evaluations of Herrerasauridae (Chapter 6), which was considered a clade within Theropoda, now places some doubt on that relationship. Regardless, it will be covered in this chapter because of its historical association with theropods. Each of these three clades includes 20 or more characters that distinguish them. Some of the more important traits

1ore than 20 synapomorphies define Theropoda.

of theropods include (Fig. 9.2):

■ A flexible jaw, indicated by an intramandibular joint.

■ An extra fenestra in the maxilla.

■ Lachrymal bone well exposed on dorsal surface of skull.

■ Minimum of five vertebrae in sacrum.

■ Manus with claws (unguals) and reduction or loss of digits IV and V.

■ Slightly curved femur, which is also more than twice as long as the humerus.

■ Pes with digits II through to IV; digit I reduced and separate, digit V reduced or absent.

■ Well-defined (long) processes on cervical and caudal vertebrae.

Thin, low-density (mostly cancellous) limb bones and vertebrae are also a common characteristic associated with theropods. This trait probably reflects an

Tyrannosauridae

Tyrannosauridae

Ornithomimosauridae

Abelisauridae

Ceratosauria

Neotheropoda

Ornithomimosauridae

Abelisauridae

Neoceratosauria

Ceratosauria o

Neotheropoda

Herrerasauridae

0 -1

Post a comment