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baryonyx is a recently discovered carnivore from england with a peculiar crocodile-like snout. it may have been a fish-eater.

Late in the Cretaceous (the period from 144 to 65 million years ago), the scariest tetanurans of all came along, the tyrannosaurids. Where these giant predators came from isn't known. Maybe they came from the allosaur line of big predators, maybe from a common ancestor, along with the troodontids, a man-sized group of dinosaurs with many birdlike features. If you take all the features unique to tyrannosaurids, you can imagine a hypothetical ancestral tyrannosaurid. It would have been "small"—fifteen to eighteen feet long, with a long snout, long slender hind limbs, a pinched middle toe, and small forelimbs.

Most of us who work on dinosaurs think that the logical guess is that tyrannosaurs evolved from a more primitive meat-eating dinosaur. But which one we can't say yet.

Whatever it evolved from, Tyrannosaurus rex automatically by name belongs to that family of predators known as the tyrannosaurids, along with a few other kinds of meat eaters of similar anatomy. Each kind of

not all carnivorous dinosaurs are intim i dati ng. compsognathus was the size of a roasting chicken.

dinosaur we know represents a different genus. There are several genera (the plural of genus) of tyrannosaurs in the tyrannosaur family. Some we know far better than others. Though we have only eleven T. rex skeletons, T. rex is one of the best-known tyrannosaurs (see tyrannosaur chart).

Animals that are even more closely related than on the generic level we categorize as species. The usual definition of a species is a group of animals that look alike and can breed together. With living animals it's often easy to tell one species from another. The large cats belong to one genus (Panthera), but it's easy to see which species are lions and which are leopards.

You can't tell species so easily with fossil animals. You don't know anything about their interbreeding abilities, and often very little about their anatomy. Almost half of the six hundred or so species designations for dinosaurs are based on just a few teeth or bone scraps. So when we find something very reminiscent of another fossil we know already, but with differences we can't peg to a different sex or age, we create a new species for it. How closely related one fossil animal is to another is very much a matter of opinion. Some paleontologists not all carnivorous dinosaurs are intim i dati ng. compsognathus was the size of a roasting chicken.

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