Yrannosa Urus

didn't just arrive on earth one day like an alien monster. It evolved from earlier animals, just as we have. Most people think of evolution, wrongly, as simply the triumph of the fittest, and T. rex certainly looks like a winner.

But like 99 percent of all the animals that ever lived, T. rex is extinct. That doesn't make T. rex a loser. Evolution is change, but it isn't necessarily progress. As wonderful and varied as life is, living things are limited in how much and in what ways they can change. And the pace of change can vary, from slow and steady to quick shifts and long stops. We have hints of this in the fossil record, of rapid evolution of some animals, stasis or slow change in others.

T. rex was one of the last and most spectacular products of dinosaur evolution. It was an experiment that can't be repeated, but it was no more a fluke or freak of nature than any other creature.

T. rex evolved within what is to us a tangled bundle of meat-eating dinosaur lineages, among animals whose origins go back hundreds of millions of years to some of the earliest dinosaurs known. We could of course trace T. rex's ancestry back, very roughly, all the way to the dawn of animal life, in the oceans more than 600 million years ago. T. rex's distant nondinosaurian ancestors first crawled up on land about 350 million years ago.

Instead, let's take a quick walk through dinosaur time.

T. rex's line, the order of dinosaurs, begins "only" another 125 million years after the first animals that came up on land. And dinosaurs were the dominant creatures for most of the 160 million years that followed their initial appearance.

We don't seem to realize that. Instead, we think of dinosaurs as huge things that died out, and call them failures. For longevity, they don't compare with horseshoe crabs or cockroaches, but dinosaurs did last a hundred times longer than we've been around, so far.

Nor do we stop to think what exactiy it is that makes T. rex and its kin dinosaurs. It's not size. T. rex was horribly big, but most dinosaurs were smaller than bulls. And there were a lot of big animals that lived in dinosaur time that weren't dinosaurs. Other animals ruled the air—pterosaurs. And different animals, mosasaurs and plesiosaurs, were the giants of the sea.

Nor are dinosaurs necessarily dead. Many paleontologists think birds are the direct descendants of some meat-eating dinosaur cousins of T. rex, so you could easily argue that dinosaurs—small, flying ones—are alive today.

Some of the characteristics dinosaurs had in common they also shared with birds and other members of the groups we used to lump together as reptiles—scaly skin (look at a bird's legs*) and young hatched from eggs. But what distinguishes dinosaurs from other reptiles is that they moved on land, most up on their toes with their legs beneath them, not splayed out. Their hips and ankles were constructed differently from those of other reptiles, allowing them to walk and run efficiendy. There are other features of the skeleton that mark all dinosaurs, and some that separate one group of dinosaurs from another. But if an animal walked straight legged on land and had no fur, chances are it was a dinosaur.

The first dinosaurs evolved about 225 million years ago. The closest thing to a dinosaur ancestor that's been found yet is a house-cat-sized animal called Lagosuchus that lived in Argentina about 235 million years ago.

One of the oldest dinosaurs we know well also comes from Argentina. It wasn't a direct ancestor of T. rex and the rest of the dinosaurs, but it's the best we've got from that time. It's called Herrerasaurus, and it lived about 225 million years ago in the Triassic period (245 to 210 million years ago). Judging from a terrific skull University of Chicago paleontologist Paul Sereno found in Argentina in 1988, Herrerasaurus was a capable carnivore. It ran on two big hind legs and had huge teeth and a double-hinged jaw, just as T. rex did 160 million years later. But Herrerasaurus was more primitive in many ways, and a lot smaller than T. rex. It seems Herrerasaurus grew to "only" about ten feet long and five hundred pounds.

Over the next few million years two lines of dinosaurs arose—those with birdlike hips (ornithischians) and those with "lizard hips" or pelvises (saurischians). Con-fusingly, the latter, not ornithischians, are the group ancestral to birds. T. rex and the rest of the meat-eating dinosaurs, called theropods, and the giant "brontosaur" browsing dinosaurs, the sauropods, both belong to the so-called "lizard-hipped" line.

T. rex was one of the biggest of the theropods, but others were as small as chickens. All theropods were carnivores with many hollow bones. All walked on their back legs with three or fewer working toes on each foot. Some had flexible tails, while a more durable and equally varied group of theropods had stiff tails—the back

bones at the rear end of their tails interlocked in various ways. These are called tetanurans, and T. rex is one of them.

We once split meat-eating dinosaurs into big theropods called carnosaurs and litde ones called coelurosaurs. But since there was no real evolutionary connection between those in each group, we've stopped splitting them up that way. After all, you don't put elephants and giraffes into one group because both of them were huge. You do put elephants in the same group with little marmot-sized hyraxes because their skeletons show so many similarities that they must have evolved from a common ancestor. Lots of big dinosaurs had little cousins, too.

In the middle of dinosaur time, the Jurassic period (210 to 144 million years ago), the first tetanurans appeared. The most primitive we know of is the three-foot-long Compsognathus, one that walked over what is now the East Coast of the United States.

dinosaurs fall into two groups according to their hip structure.

top: among bird-hipped (ornithischian) dinosaurs, such as the duckbills, the pubic bone is tilted back horizontally, next to another bone of the pelvis. the ischium.

bottom: in l izard-hi pped (saurischian) dinosaurs, such as t. rex and other theropods and the giant browsing sauropods, the pubic bone is pointed downward and ends in

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