Theropoda

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The *Theropod* (translates to "Beast-Foot" in Greek) dinosaurs are a diverse group of bipedal Saurischian **Dinosaurs**. They include the largest terrestrial carnivores ever to have made the earth tremble. What most people think of as theropods (such as T. rex, *Deinonychus*) are extinct today, but recent studies have conclusively shown that **Birds** are actually the descendants of small nonflying *Theropods,* thus when people say that dinosaurs are extinct, they are technically incorrect. Still it's not as exciting seeing a *Sparrow* at your bird-feeder as it would be to see a *Tyrannosaurus rex* there.

## Subgroups

The *Theropods* are divided into various different subgroups, including:

* '*Tyrannosauridae: Tyrannosaurinae* (Tyrannosaurus and more) and *Albertosaurinae* (Albertasaurus and more)
* **Carcharodontosauridae** (Dinosaur enthuasists refer to it as "Gigantosauridae"): Carcharodontosaurus, Giganotosaurus, Mapusaurus, Tyrannotitan and Acrocanthrasaurus

* **Allosauridae** (ancestors of the Carcharodontosauridae): Allosaurus and Saurophaganax (possibly a subspecies)
* **Ceratosauria**: Dilophosaurus and Ceratosaurus

## Anatomy

Our knowledge of the evolutionary history of the Theropoda is constantly under revision stimulated by new, exciting fossil finds every year or so such as "*Mononykus olecranus"*, a very bird-like theropod found recently in the **Mongolian desert**, or Giganotosaurus carolinii, a giant **Theropod** probably rivaling the size of T.rex, found recently in Argentina. In fact, the 1960's discovery and study of the remains of *Deinonychus antirrhopus* helped to revise *paleontology's* old vision of all **Dinosaurs** as slow, stupid reptiles, and was a key factor in the onset of the controversial hot-blooded/cold-blooded debate. Currently, there are two or three main groups of **Theropods**, depending on whom you ask; we have yet to fully understand their origin. Why is this so? The main reason is the lack of good specimens; theropod remains are fairly rare and more often than not, fragmentary — theropods have a poor fossil record compared to most of the ornithischian dinosaurs. Fossils of small theropods are especially rare, since small bones are harder to find and are weathered away easily. Without well-preserved, complete specimens, it is hard to tell who is most closely related to whom using **cladistics**.

### Characteristics

Several characters that typify a *Theropod*: hollow, thin-walled bones are diagnostic of *Theropod* **Dinosaurs**. A jumbled box containing theropod bones (from the UCMP collections) is shown at right. The hollow nature of the bones is certainly more obvious in 3D, but you should at least be able to make out the general tubular structure of the bones. Other **Theropod** characters include modifications of the hands and feet: three main fingers on the manus (hand); the fourth and fifth digits are reduced; and three main (weight-bearing) toes on the pes (foot); the first and fifth digits are reduced. Most *Theropods* had sharp, recurved teeth useful for eating flesh, and claws were present on the ends of all of the fingers and toes. Note that some of these characters are lost or changed later in **Theropod evolution**, depending on the group in question.

## Classification

The *Herrerasauridae* are an early group represented by *Herrerasaurus*, which was discovered in a wonderful middle-late **Triassic period** fossil locality (the famous **Ischigualasto Formation**) in Argentina in the 1970s. Another **Herrerasaur** is *Staurikosaurus*, which has been known since the 1960s from remains found in Brazil. More recently (in 1993), another herrerasaur-like fossil was found in the same general area and named *Eoraptor*, or "dawn thief." It appears to be closely related to the herrerasaurs, but smaller in size and slightly older. Both *Eoraptor* and the herrerasaurs seem to have been small to medium-sized carnivores. These curious animals have some basic theropod characteristics, but lack others; in fact, they lack some dinosaurian characteristics as well. The **Herrerasauridae** and *Eoraptor* may be the earliest group of theropods, or it is quite possible that they are not even theropods at all, but rather non-dinosaurs (**dinosauromorphs**) closely related to the ancestor of dinosaurs. The fact is, we don't know for sure. Experts in Dinosaur systematics are currently embroiled in a controversy over the exact relationships of the **Herrerasauridae** to *Theropods* and other dinosaurs.

A second group of theropods is the Ceratosauria, a more morphologically modified and diverse group which includes the UCMP's very own *Dilophosaurus*, one of the stars of the novel and movie *Jurassic Park*. *Segisaurus* is a small, mysterious theropod known from only one specimen, which is housed in the collections of the UCMP. Recent discoveries have revealed that ceratosaurs formed a more diverse group than was previously expected...

The last, and by far the largest group of **Theropods** is the Tetanurae, consisting of two major clades (sister taxa), the **Carnosauria** and the **Coelurosauria**. Some early tetanurines such as *Megalosaurus* fall outside of this dichotomy, but most are poorly known.

### Carnosaurs

The *Carnosaurs* were the huge, fierce predators such as *Allosaurus* (shown here chasing *Dryosaurus*, an **Ornithischian** dinosaur), and recent headline-makers like the gigantic *Carcharodontosaurus* and *Giganotosaurus*, both of which seemed to have reached or exceeded the size of *T. rex*, making them the largest terrestrial bipeds ever to have terrorized the terrestrial realm.

### Coelurosauria

The Coelurosauria were generally smaller in stature, but more diverse, including such famous creatures as *Velociraptor* (shown at right grasping *Protoceratops*, a ceratopsian dinosaur) and our feathered friends the birds. Recent studies have agreed that *T. rex* and the tyrannosaurs belong with the coelurosaurs, not with the carnosaurs as was originally believed. Such is the ever-changing nature of theropod phylogeny; new finds and analyses are frequently overturning old ideas. This area of dinosaur paleontology is in a major state of flux.