Tyrannosaurus rex bone map by Michelle Leveille

The name "theropod" means "beast-footed." Theropod feet were actually more like bird feet. Theropods walked upright on their rear feet which had three weight-bearing toes. Here is a photograph of an emu foot, which appeared similarly.

Emu foot by dosboof on flickr

Ancestral theropods were flesh-eating dinosaurs that reached their glory days during the Mesozoic. That's when Tyrannosaurus rex, probably the most well-known and feared predator of all time lived.

Dueling Tyrannosaurus rexes, illustrated by Michelle Leveille. Click on the image to see my blog post about its creation.

But not all theropods were big. Microraptor was only the size of a crow. Here is Velociraptor, a peacock-sized theropod:

Michelle Leveille's bone map of a Velociraptor. Click on the image to see our post with other Avian Dinosaur bone maps.

All theropods had hollow bones, like modern birds. Their fossils are not as durable as those of solid-boned dinosaurs, so more of the larger theropods have been discovered than the smaller ones.

Hollow theropod bones at the University of California Museum of Paleontology

Another attribute of theropods is that they were obligate bipeds. That means that they needed to walk on two legs, because their front legs were not equipped to support them. The front legs were used for gripping and tearing prey instead. The other group of saurichians, the sauropods, usually walked on four legs instead.

Ankylosaurus, an example of a sauropod dinosaur, walking on four legs. Bone map by Michelle Leveille

For years, it was believed that theropods walked almost completely upright, and dragged their heavy tails like alligators or kangaroos. Remember when dinosaurs were drawn like this?

The old-style tail-dragging allosaurus

Biomechanical research in the 1970s determined that the bone structure of these animals would not allow them to move that way, without fracturing their hips and crushing their spines. There was also the matter of the lack of fossil evidence of tail tracks to go with their footprints. Here is a current depiction of an allosaurus's posture:

Allosaurus bone map by Michelle Leveille

Another characteristic of ancestral theropods was their sharp, recurved teeth, typical of carnivores. Here is a diagram of the dentition of a Tyrannosaurus rex. The orange tooth is on display at the Los Angeles County Museum of Natural History, but of course it is not really orange.

Tyrannosaurus rex dental diagram by Michelle Leveille

Ancestral theropods also had claws on the digits of their modified "hands" (mani) and feet (pes). The theropod manus had three main fingers, with the first and fifth digit reduced. They walked on the three weight-bearing toes of their pes, again with digits I and V reduced.

Here is a bone map of Struthiomimus, showing the modified mani and pedi of typical theropods. Note the claws on the ends of the digits.

Oviraptor fossil showing Theropod dinosaur sitting on eggs, copyright Pearson Education, Inc.

Above is the fossil of a theropod with its eggs. Below is a bone map of that type of dinosaur, called Oviraptor.

Oviraptor bone map by Michelle Leveille

Many ancestral theropods left fossil evidence of primitive feathers. The feathers were actually modified scales, (as they are in modern birds), but they did not evolve into efficient thermoregulatory and aerodynamic structures until later.

Sinosauropteryx fossil. This theropod could not fly, yet it had downy proto-feathers along its back.

Even theropods which could not fly, such as T.rex, left impressions of downy coverings when they were young, which probably helped keep the juveniles protected and warm.

Baby Tyrannosaurus rex bone map by Michelle Leveille

Theropods are not extinct. Enough fossil evidence has been gathered, especially in Mongolia and China during the past two decades, to prove that the avian dinosaurs evolved into modern birds. Birds share most of the attributes listed above with ancestral theropods. Some birds, such as the hoatzin, spur-winged goose, crested screamer, torrent duck, emu, and spur-winged lapwing, even have vestigial claws on their wings.

Vestigial claws on a crested screamer

Birds also share another important trait - a bone called a furcula, or wishbone. This bone is formed by the fusion of the clavicles, which spreads the shoulders and enables flight.

Archaeopteryx, below, was long touted as the "missing link" between dinosaurs and birds. There is much more fossil evidence now of the commonalities between ancestral theropods and modern birds.

Archaeopteryx bone map by Michelle Leveille

The dinosaur-bird nexus is a topic that deserves much more discussion, but we are limiting our scope to ancestral theropods in this blog post.

Theropods are divided into two or three main groups, according to when they evolved and identifying traits. The first group is the Herrerasaurs.

Herrerosaurs such as Eoraptor and Herrerasaurus were small- to medium-sized predators. They had most of the attributes that define therapods. However, scientists debate whether herrerasaurs are indeed true therapods, because they lacked some of the characteristics and they even had some traits of non-dinosaurs.

Another group of therapods is the Ceratosauria. Therapods became more diverse at the time of the ceratosauria. One type of ceratosaur was Dilophosaurus, shown in the Jurassic Park movie with a fictitious frilled neck and a capacity for spitting poison.

Dilophosaurus bone map by Michelle Leveille

Dilophosaurus depiction in Jurassic Park

Here is another ceratosaur. This medium-sized carnivore was about half the weight of Allosaurus. Ceratosaurus lived in the Late Jurassic and it, like Dilophosaurus, had blade-like horns on its snout.

Ceratosaurus bone map by Michelle Leveille

Carnotaurus is distinguished by it's incredibly short forelimbs. Yet this ceratosaur was probably a fearsome predator.

Carnotaurus bone map by Michelle Leveille

Majungasaurus, another ceratosaurine theropod, is one of the few dinosaurs for which there has been direct evidence found of cannibalism.

Majungasaurus bone map by Michelle Leveille

The last group of theropods was the Tetanurae. Tetanurae included the sister taxa of the larger Carnosauria and the smaller Coelurosauria, and possibly Megalosaurus.

The Carnosauria were arguably the largest, fiercest predators ever to terrorize vegetarians. They included Allosaurus, Tyrannosaurus rex, Spinosaurus, and the even larger Gigantosaurus.  Carnosaurs had femurs that were longer than their tibias (thighs longer than shins), large eyes, and long narrow skulls.

Juvenile Tyrannosaurus rex bone map by Michelle Leveille

Sketch of fighting T. rexes by Michelle Leveille

Spinosaurus bone map by Michelle Leveille

Coelurosaurs were smaller tetanurine theropods such as Velociraptor, Ornithomimus, Compsognathus, and other predecessors of birds. Ancestral coelurosaurs had their heyday during the Late Jurassic. Qualifying characteristics included tibias that were longer than femurs, elongated sacrums, curved ulnas, and stiff tail-tips. Scientists are starting to believe that all coelurosaurs had feathers. The evolutionary persistence of the group has been attributed to the more recent adaptations of omnivory and flight.

Velociraptors were depicted in the Jurassic Park movies as clever girls. They differed from real velociraptors in that they were too large and they didn't have any feathers. Fossilized remains of velociraptors found in Mongolia showed that they were only about 28 inches tall and sported primitive feathers at least on their backs, legs, and arms. See our post about velociraptors here.

Click on the image to go to a post about its creation.

Caudipteryx was a diminutive coelurosaur which was smaller than a turkey. But it had teeth. It was one of the first-recognized feathered dinosaurs, discovered in China in 1998. It's remains have been found with intact gizzard stones, used for grinding food in its gut.