We know a lot more about T. rex's world than we do about most other dinosaur communities. Quite a few of the dinosaurs we know about come from the era of the tyrannosaurs, the last 15 million years of dinosaur life. That's less than 10 percent of dinosaur time. The
richest collections of dinosaurs, however, come from Montana and western Canada at the time of Albertosaurus, several million years before T. rex lived.
By most estimates (though not all), there were fewer kinds of dinosaurs by the time T. rex came along. Over the last several million years of dinosaur time, the weather was changing considerably. The shallow North American interior seaway, the Colorado Sea, appeared about 95 million years ago and receded and advanced several times in the next 30 million years. It was receding right at the end of T. rex's time, when the weather was becoming more continental, drier and cooler in winter, hotter in summer. The uplift of the Rocky Mountains was also creating new weather patterns. More variable weather meant, so it seems, fewer kinds of dinosaurs.
When you sample the dinosaur fossils up through different rock layers from the Late Cretaceous, you can see how the dinosaur community changed. You don't get just different kinds of dinosaurs—that always happens when you move up in time. The dinosaurs you see from T. rex's day were different in other ways. It appears that a lot of the other animals were dropping
out, such as the armored ankylosaurs, the dome-headed pachycephalosaurs, and some other kinds of dinosaurs that were more abundant earlier in the Cretaceous. Ether those animals were going extinct or they didn't like the particular environment in the Hell Creek Formadon where we find most T. rexes.
But T. rex wasn't limited to the Hell Creek Formation of Montana and the Dakotas. Just how wide its range was we can't be sure with so few fossils to go by. It is possible that T. rex could have ranged far to the north and south of where we have found it so far. We know duckbilled and horned dinosaurs like those we find with T. rex in Montana ranged as far north as the Alaskan Arctic in T. rex's time. The Arctic was almost as far north as it is today, and it got dark and cold in winter, even though Arctic weather was more like Seatde's than that of today's Alaskan winters. Browsing dinosaurs might have migrated north each spring for the rich summer plant growth, and T. rex might have followed the herds. We don't have any evidence of T. rex farther north than Alberta, but we're still looking.
The slirinking, but still substantial seaway across the middle of the continent may have kept T. rex from making it to the East Coast. Though we don't have evidence, T. rex could have ranged far south of Montana. In the Southwest, T. rex would have come across herds of not only horned and duckbilled dinosaurs, but huge four-legged brontosaurs called Alamosaurus. Relatives of these giants had ruled North America at the end of the Jurassic period (210 million to 144 million years ago). But in the Cretaceous (144 million to 65 million years ago), they were replaced in North America by the duckbilled dinosaurs. The brontosaurs kept thriving, however, in South America, and by T. rex's day, at least one kind had made its way back into what's now the southwest United States.
Dinosaurs were land creatures that couldn't have swum across an inland sea, but some may have spent a lot of their time around the water. We used to imagine the duckbills as spending much of their time swim ming. A lot of fossil evidence showed us that wasn't the case, but we may have gone overboard in taking all duckbills away from water. Some of them did spend a lot of time in watery places, and with their flat feet and flattened tails they could have been good swimmers. We've got footprints that suggest some of their ancestors were poling along river bottoms with their feet.
Paleontologist Don Baird imagines some hadrosaurs were like hippos in their lifestyle. "They'd breed and feed on land but spend lots of time in the water." As Don points out, elephants spend a lot of time in the water. It takes a load off their feet and cools them down—a very economical way to live.
Meat eaters go where the food is, and a water hole, seashore, or riverbank would have been a good place for a scavenging or hunting T. rex to hang out. By the sea it could have been rivaled by an even bigger predator, the giant crocodilian Deinosuchus. Ned Colbert, a dinosaur paleontologist who used to work at the American Museum of Natural History, has suggested that giant crocodilians could have eaten dinosaurs.
T. rex also lived among, and probably dined on, giant animals that lived inland from the seaway in the western United States. Here the plant eaters included not only duckbills and big horned dinosaurs, but Leptoceratops, a primitive-looking horned dinosaur only six feet long and weighing about 120 pounds. Like Triceratops and the other much bigger horned dinosaurs, Leptoceratops had a parrotlike beak. It had a short frill on the back of its head. But it had no horns, and it might have walked on two legs as likely as on four.
There were other, smaller animals scurrying around T. rex's habitat. Mammals were beginning to increase in size and diversity even before the fall of the dinosaurs gave them room to dominate the world. As the seaways shrank, newly exposed land connections allowed mammals to cross from Asia to North America, as well as from South America to North America, and vice versa. (Late Cretaceous dinosaurs might have done the same, explaining the close resemblance of
the last ice age notched these badlands, around the t. rex site in eastern montana, revealing layers of sediment laid down more than 65 million years ago.
T. rex to its Asian cousin, Tarbosaurus)
Though they still grew no bigger than bread boxes, the mammals of T. rex's world occupied many different habitats. In the trees, rodentlike mammals used their choppers to eat the fruits and leaves of flowering trees dinosaurs didn't get to. Marsupials such as the opossum came up from South America and made a good living in the trees and on the ground, eating most anything. The oldest-known primate, a little insect-eater, comes from the Hell Creek Formation. So does the earliest ancestor of the modern cow and other ungulates.
A few paleontologists have suggested that a wave of highly successful mammal immigrants from Asia ate the North American dinosaurs out of business, or that
mosasaurs were huge sea reptiles that grew to T. rex proportions and died out when T. rex and dinosaurs did, at the end of the cretaceous period.
animals crossing continents brought infectious diseases that killed the dinosaurs, or that the spread of flowering plants (flowers didn't evolve until the Cretaceous period) created new resources for little creatures. To me these are interesting guesses, but there is absolutely no proof behind any of them.
Up in the trees in T. rex's world were many more birds than we know from earlier dinosaur times. Ancestors of wading ducks with long limbs waddled in the water, and smaller birds patrolled the shoreline. Out on the water, six-foot long birds that could neither walk nor fly dove for fish. Some of the fish they couldn't have swallowed grew to thirteen feet long. And other water creatures, including the crocodilians, grew even larger. Sea turtles reached lengths of twelve feet, and whalelike mosasaurs (actually relatives of monitor lizards) exceeded forty feet long. Like T. rex, they might have eaten whatever they wanted. In the seas, that meant diving birds, sharks, and shelled animals.
But flying reptiles, the pterosaurs, still ruled the air. One was the biggest flying animal ever—Quetzalcoatlus. Its wingspan topped forty feet, as wide as a fighter plane.
On land, the dinosaurs remained very much in charge. In T. rex's neighborhood lived two-legged browsing dinosaurs no bigger than us, some with thick-skulled heads (probably for butting each other). The speediest dinosaur in T. rex's world was an Ornithomimus, an ostrichlike carnivore that might have been the fastest of all dinosaurs. Troodon, the smart little hunter with dextrous hands, keen eyesight, and a big brain, was around too. It might have hunted these animals or snatched eggs and the mousy little mammals that skittered through the underbrush.
But small dinosaurs were not very common in T. rex country, at least judging from the fossils we can find. We find many more of the huge dinosaurs (for speculations on why they got so big, see Chapter 8). The giants include some of the armored tanks of the dinosaur world, ankylosaurs. These squat plant-eaters grew
left: there was an animal bigger than and probably just as scary as t. rex.
deinosuchus, a crocodilian as much as fifty feet long. it may have snapped up dinosaurs along shores.
below: troodon, the smartest known dinosaur, no bigger than a human.
to thirty-five feet long. A far larger percentage of the dinosaur population of T. rex's world were herds of Edmontosaurus andAnatotitan, flat-headed duckbills that grew as long as T. rex, more than forty feet.
Eating the plant eaters were other huge carnivores, small only by comparison to T. rex. Nanotyrannus, loosely called the "pygmy" tyrannosaur was "only" fifteen feet long, and a compact model of Albertosaurus, which grew to about twenty to thirty feet long, was on the prowl for meat, too.
Look out over what was T. rex country. If you see any animals at all, it's cows, and maybe a coyote, deer, antelope, or badger. T. rexwould have seen more and larger creatures—most likely herds of horned dinosaurs, such as Triceratops and Torosaurus. Both of these animals were as big as or bigger than any land animal today, more than twenty-five feet long and six tons in weight. They had huge heads with thick frills and horns at least three feet long. Torosaurus's head was over eight feet long, the biggest skull of any animal that ever walked the earth.
Imagine a lush, warm lowland thick with these horned dinosaurs, herding by the thousands under the watchful eyes of a hungry T. rex. I wouldn't want to live there then, but it would be a heck of a place to visit.
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