a production of Black Hills Institute
T. rex's Territory
The fossil record has retained much more than bones. It tells loads of information about Territory, too. What was life like for T. rex - 65 million years ago?
The formations in which T. rex fossils have been found have different names - because they occur in different states or provinces - but they were deposited at roughly the same time: at the close of the Mesozoic Era, uppermost Cretaceous Period, upper Maestrictian stage. That translates to approximately 65 million years ago, during the last 1.5 to 2 million years of the age of dinosaurs - the last hour of the dinosaurs' 160 million-year reign.
The rocks and dinosaur bones found in the "pages of time" tell a story of an era that is long over, because they document much of what we know about terrestrial life at a critical time in the history of the earth. Even so, the record of the ancient ecosystem is missing chunks of time; ten feet of sediment may represent 100,000 years or one minute. The fossils embedded in these irregular chunks of Cretaceous sediments - which might seem like dry, crumbly, and unrelated details in the field - reveal the only clues we have, exquisite snapshots of the environment, climate, animals, and plants of the lost world of Tyrannosaurus rex. From these glimpses of geography, geology, and paleontology, we piece together a story of a world that came to an abrupt end 650 thousand centuries before Homo sapiens appeared.
Telling time: The earth is 4.5 billion years old. The first saber-tooth cat arrived around 35 million years ago. T. rex was alive 65 million years ago. Algae has existed for more than three billion years. How do we know all this?
To figure out how old the Earth is, scientists use volcanic rocks and chemistry. Volcanic ash in sedimentary rocks often contains a type of potassium that changes over time. By measuring how much the potassium has changed, scientists can calculate (with math!) how old it is and how long ago the ash was deposited.
To figure out how old a fossil is, scientists can see how close it is to the volcanic ash. If they already know when the ash was formed, and the bones are right below it, they must have been covered over at about the same time. They can also use other fossils to do the same thing! How? Most species exist for a million years or less. So if we find a Camarasaurus supremus just under some ash that we know is 145 million years old, then we can be pretty sure that the Camarasaurus is about the same, and that any others we find will be about the same age as the first one. And this means that we don't even need ash to figure out that bones from any other dinosaurs we find near a Camarasaurus will also be about 145 million years old.
Last, but not least, in some cases you can actually date bones directly. In a test similar to the potassium test, we look in the fossil for a rare form of carbon that changes over time. This only works for fossils less than 100,000 years old, so it can't be used with dinosaurs
What did it LOOK like 65 million years ago, and how do we know?
Specimens of Tyrannosaurus rex have been found in stream beds, on sand bars, or within the flood plains of meandering streams that flowed eastward on a broad, flat, forest covered plain. This plain, of which T. rex populated at least 500,000 square miles, was bounded on the west by the ancestral Rocky Mountains and on the east by the remnants of an inland sea which once bisected the North American continent.
DIRECT CLUES: To present an accurate picture of his landscape, we need the help of soil specialist Greg Retallack of the University of Oregon and K-T Boundary plant specialist Kirk Johnson of Denver's Museum of Natural History. Their research shows that a sparse forest was sprinkled with rare and relatively small trees that grew no more than one foot in diameter and 60 feet high. This forest joined a flat coastal plain with an annual rainfall of 35 to 45 inches. Cretaceous ground cover included mosses, ferns, ginkgoes, cycads, sequoias, palms - and an overwhelming abundance of modern flowering plants, including relatives of the bay laurels, sycamores, magnolias, palms, and berry-bearing shrubs. But absolutely no oaks, maples, willows or grasses grew around or under T. rex feet.
INDIRECT CLUES: Today's soft-shelled Trionyx turtle, for instance, is completely aquatic; its presence in T. rex country suggested the existence of permanent bodies of water in the Cretaceous, such as year-round running streams and rivers. Similarly, the presence of narrow-snouted 'crocodiles' and broad-snouted 'alligators' implied a tropical or subtropical climate.
The Little Guys
ON LAND: Aside from some dinosaurs, most animal life in the Cretaceous was relatively small. Half of the mammals scurrying through the brush were marsupials, relatives of the opossum. The other half was a combination of rat-sized creatures with unusual multiconed teeth, and tiny insect eaters the size of mice.
Thanks to the work of the late Richard Estes, a longtime researcher of the fossiliferous areas of Lance Creek, Wyoming, we also could provide an overview of Cretaceous lower vertebrates. He found thirteen different species of lizard, including one monitor; the oldest known North American snake, Coniophis precendens; and amphibians including salamanders, aquatic or semiaquatic frogs, and turtles.
UNDER WATER: In T. rex's neighborhood, scientists have found fourteen species of fish and four shark relatives; freshwater clams, snails, bivalves, and mollusks; and brackish-water oysters. Estes believed this particular array of underwater species indicated the presence of a delta, because it brought to mind the 'primitive fish fauna of the Mississippi River drainage, especially in its lower reaches near the Gulf coast.' Just off the delta, in neighboring sea waters, lived an amazing variety of ammonites, extinct relatives of the chambered nautilus, as well as plesiosaurs, mosasaurs, and giant marine turtles.
IN THE AIR: While ancient water teemed with life, in T. rex Country, either the air was very quiet or evidence of flight is elusive. Bones of pterosaurs, or 'flying reptiles,' have been found, but they are so rare that specimens have yet to be referred to genus or species. Bird bones also are rare; although we collected a number of specimens from at least three species at a duckbill quarry, only a few specimens of any kind have been referred to families. Recently-discovered bird tracks include that of a large, web-footed variety.
Of course the story of birds, and how they carried T. rex's banner out of the Cretaceous, has much more information than we find in just this area. The bird story is global, and current!
The Big Guys - or is it "the menu"?
VEGETARIANS: 'Ornithischian' translates to the scientific designation 'bird-hipped,' but in the list of main dishes, it means 'tastes like chicken.' These herbivores naturally made up the bulk of the Cretaceous dinosaur population, and by far the two most abundant forms were duckbilled hadrosaurs and horned ceratopsians.
Of Hell Creek duckbills, Edmontosaurus annectens led the charge at over thirty feet long and nine feet tall at the hips. The star of tremendous bone beds, this species must have traveled in huge herds, since we find its remains in quantities of perhaps as many as ten or twenty thousand individuals. More rarely we find evidence of several other species of duckbill, most of which await description.
Next on the abundance menu were two of T. rex's favorite dishes, the three-horned Triceratops horridus, and the rarer Torosaurus. While both these elephant-sized animals are known for their huge frills, the Torosaurus's perforated variety was even larger than Triceratops's solid model. We now also have evidence of two smaller ceratopsians, Leptoceratops and a brand new species, that may have served as midnight snacks.
Once their bony bits were stripped off, armored dinosaurs were also tasty. Ankylosaurus was a tank-bodied, club-tailed creature with armorlike scutes to protect it from predators. A nodosaur relative, Denversaurus, also sported armor, but was denied the use of the club.
Other dinosaurs that used bone as a weapon were the jester Pachycephalosaurus and its relative Stygimoloch. Fun to pronounce - pack-ee-sef-ah-low-SORE-us and sti-gee-MO-lock - these unusual creatures apparently bashed heads, like today's male bighorn sheep, with thick skulls covered by as much as six inches of solid bone. Scientists have postulated that the bony heads also could have been used to decline unwelcome dinner invitations.
MEAT-EATERS: If the fossil record is our guidebook, T. rex was not only the largest, but the most prevalent theropod alive in its territory. We find lots more, and more complete, specimens of it than anyone else. But still, T. rex had a little competition.
Other meat-eating dinosaurs are usually known only from fragments, single bones or teeth. One interesting creature, whose species is in debate, is presently called Nanotyrannus lancensis. For the last 60 years, this dinosaur was known only from isolated teeth and one skull. Now the Burpee Museum in Rockford, Illinois, has found most of a skeleton that will finally put the debate to rest.
Struthiomimus and Ornithomimus, 'ostrich-mimic' dinosaurs, were dynamic creatures. You'll remember their relative, Gallimimus, from a memorable scene in Jurassic Park, where T. rex chased an entire flock of them.
The movie's raptors were represented by Dromaeosaurus - in life armed with superb eyesight, large brains, and wicked claws on the hands and feet.
Finally, there are several Oviraptor relatives in T. rex Country, including Caenagnathus, the details of which are just now finally being understood. Plus, we've found a few theropods we know only from teeth or single bones, such as Avimimus, Paronychodon, and Sauronithoides - someday we hope to find the rest of them, too.
Evidence of Interaction
How do we know for sure who ate whom? Teeth marks, in the form of puncture holes and serration scrapes, have been found in Triceratops, Edmontosaurus, and T. rex. One duckbill seems to be the best evidence of "one that got away," with a healed bite at the base of its tail.
Another excellent indication: teeth. Shed teeth, broken off during eating (which was a brutal affair), litter excavation sites - the smoking gun. They also indicate something about behavior, because the presence of Nanotyrannus teeth, which are very common, reflect that these creatures were neat and tidy eaters (bones are usually whole and not very disturbed). When T. rex ate, they often left only fragments of bone behind.