The secret life of dinos: step back in time with the largest beasts ever to walk the Earth. (Earth/Life Science: Cretaceous Period * Dinosaurs * Fossils.

TIME: 67 million years ago

PLACE: The North American West (now the badlands of South Dakota)

A herd of hadrosaurs--hefty, duck-billed dinosaurs--wallow in a muddy river. They stir up sediment, sand and dirt carried by with their splayed flat feet. Here and there along the riverbank, palm trees rustle in the salty breeze that wafts from the inland sea.

Overhead, a giant reptile with a 12-meter (39-foot) wingspan son on an updraft. The pterodactyl's shadow flits across a lush glade dotted with stocky, three-horned dinos called triceratops. Lazily they munch succulent ferns and flowering herbs.

Suddenly, the ground rumbles. Triceratops scatter. An earthquake? No!

Plowing through the ferns--head down, tail straight out, muscular legs pumping fast and furious--comes colossal Tyrannosaurus rex. Across the glade an old hadrosaur plods away. But not fast enough.

The T. rex slams the duckbill to a cinnamon tree, using its short, two-clawed forelimbs (arms), and chomps the duckbill's back. The dagger-like teeth cleave flesh and bone. It's just another eat-or-be-eaten day in the Late Cretaceous Period.

Twenty years later: T. rex is old and diseased. Lumbering down to the riverbank, it collapses and dies. A week later, the river floods. A torrent of water rushes over the rotting carcass (dead body), and the current literally hurls the dino's pelvis (bone to which the legs attach) onto its head. In an instant, fine sand and mud bury the twisted heap of flesh and bone. This is one carcass that scavenging didelphodon, a pouched mammal the size of a badger, won't devour.

As hundreds, then thousands, then millions of years elapse, layers of sediment steadily pile atop the bones. The sediment presses down on the deposits below, compacting (squeezing) the silt and sand. Over eons sedimentary rock forms, layer upon layer (stratified). The weight forces water through tiny spaces in the T. rex bones; minerals collect in the spaces. Eventually the bones become rock-hard fossils.

FAST-FORWARD

TIME: August 12, 1990

PLACE: The Hell Creek Formation, western South Dakota

Welcome to Hell Creek. The temperature hovers between 110 [degrees] and 125 [degrees] F. There's no shade--nothing, in fact, but dust and rocks. For six weeks, fossil-hunter Susan Hendrickson and a crew from the Black Hills Institute of Geological Research have picked and hammered away at the weathered flat-topped hills of the Dakota badlands.

On the last morning in the field, Susan hikes off with her golden retriever toward a badlands outcrop she's never explored. Three miles out, at the foot of a 15-m (50-ft) cliff, she spies dark brown fragments of fossilized bone amid gray mudstone (rock made of mud or silt) and yellowish sandstone (rock made of sand).

Her gaze wanders up the cliff. Weathering out of the sandstone overhead are three huge articulated vertebrae (string of backbones). She climbs closer. The vertebrae feature a hollow, spongy construction unique to carnivorous dinos.

Later, Hendrickson exclaims, "I knew immediately! The bones were big. And T. rex was likely the only large carnivorous [flesh-eating] dinosaur that lived in the area in the Late Cretaceous Period." From then on, the fossilized dino is called Sue, in honor of its discoverer.

WHO WAS DINO SUE?

Amazingly, 67 million years after Sue's death it's still possible to see where muscles, tendons, and other tissues attached to Sue's bones. Some vertebrae are fused together, a sign of arthritis (inflamed joint disease). Some ribs broke and healed in her lifetime. Her diseased left fibula (lower leg bone) had a big pus pocket.

Between her neck and ribcage, Sue also has an enormous forked wish bone, or furcula: the first ever found in a T. rex--and key evidence linking the evolution of birds to carnivorous dinosaurs. Fossil details like these help scientists reconstruct how Sue might have looked and moved.

Is Sue really a she? Scientists don't know. But some venture that female T. rexes were bigger than males. Since Sue is the biggest T. rex ever found, "she's probably a female," says Hendrickson. "And I really hope she stays that way!"

Some paleontologists (fossil scientists) like Jack Homer, at the Museum of the Rockies in Bozeman, Montana, even think that T. rex wasn't the savage predator it's usually depicted to be. What was it then? "A scavenger," he says. Scavengers feed on already-dead animals. Homer's reasoning: "T. rex was too big and heavy to wrestle around with its prey, delivering bite after bite the way it does in movies."

Most paleontologists agree that T. rex scavenged, but they think it hunted as well. "Why would the largest-ever meat-eater be a scavenger?" asks Bill Simpson, who manages the Chicago Field Museum of Natural History's fossil collection. Two-thirds of each of T. rex's 30.5 centimeter (12 inch)-long teeth is embedded in jaw bone. "That's a pretty heroically anchored tooth," says Simpson. In other words, a tooth designed to hunt and tear into live flesh.

Also, says Simpson, "the sense of smell is critical for hunting, and T. rex smelled its way through life." X-rays show Sue's olfactory bulbs (smelling organs at the front end of the brain) to be as big as its football-size brain. Why do leading paleontologists have such different ideas about the same dinosaur? Because no one has ever seen one alive!

FOSSIL SCIENCE

When Hendrickson unearthed her find, she knew she was looking at a Late Cretaceous fossil. "You can tell a fossil's age by the layer of earth in which it's deposited. Give or take a million years," she says. (see diagram pp. 14-15).

Rock layers are windows into specific time periods. Poke around in 65-million-year-old sedimentary rock and you might find a T. rex. Fossils form only where sand or mud quickly buried carcasses: in deserts, floodplains, or debris-clogged riverbeds. When T. rex lived, the Hell Creek Formation was floodplain, webbed with rivers. But millions of years of erosion exposed Sue's bones.

Dinos may have roamed the entire Earth, but just as giraffes are indigenous (native) to Africa, scientists think T. rex evolved and lived only in what is now western North America.

REBUILDING SUE

For three weeks, Susan and the team move dirt and wrap bones in foil and plaster casts. "What's important is keeping every little piece," says Hendrickson.

As they move the massive but fragile bones encased in matrix (non-fossil rock that surrounds bone) into trailers for transport, the number of bones stuns them: with more than 250 bones, the skeleton is 90 percent complete--the most complete T. rex ever found!

In May 2000, all 14,000 pounds of beautiful, chocolate-colored bones are finally assembled on a custom iron frame, now on display at Chicago's Field Museum of Natural History. "I worried Sue wouldn't live up to expectations," Hendrickson says. "But she didn't let us down. I still get chills when I see her."

NOSE JOB The T. rex snouts you've seen in Jurassic Park III, The Lost World, or Disney's Dinosaur may be all wrong. New research suggests that dino nostrils belong down near the mouth. "It may appear dramatic in the movies," says biologist Lawrence Witmer of the DinoNose project at Ohio University in Athens, but he proposes a new T. rex nose design, with which many scientists agree.

Flesh isn't preserved in fossils, and a T. rex nostril could have been located anywhere along the skull's bony nasal openings. In dinos' closest living relatives, birds and crocodiles, the nostril sits at the front of the bony nostril. Also, blood vessels that feed nasal tissue leave distinct marks on the skull bones of modern crocodiles--Witmer found similar markings on dino skulls. That's nothing to sneeze at!

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Geological Time

Geologists know about ancient life from fossilized remains of creatures buried in layer upon layer of rocks or sediment. The geological time chart reveals the layers and the sequence of fossil history.

Precambrian Era: Earth formed. The first single-celled life forms appeared 3.6 billion years ago (bya). Continents took shape about 3 bya.

Paleozoic Era: Algae and other boneless life forms filled the oceans. Crustaceans and fishlike animals appeared. Simple plants grew on land, then giant ferns, and later vast conifer forests. By 290 million years ago (mya), reptiles evolved.

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Mesozoic Era: In the Jurassic Period dinosaurs and the first birds evolved. By the Cretaceous Period, small mammals appeared, and T. rex ruled North America. Dinosaurs died out at the end of the Mesozoic Era.

Cenozoic Era: Mammals diversified. Birds flourished. Continents took on their present forms. Modern humans appeared near the end of the Quaternary Period.

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Did You Know?

* When the last ice age thawed 10,000 years ago, melting glacier water cut deep gullies into sedimentary rock, revealing fossilized bones.

* Sue was the twelfth T. rex found. Since 1990, at least 15 more have been unearthed.

* Dinosaurs grew faster than modern reptiles. New studies estimate the giant Apatosaurus gained more than 30 pounds a day during adolescence.

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Directions: Read the article on dinosaurs, then write a brief paragraph using the provided vocabulary words.

1. Imagine you're the T. rex Sue. Describe a day in your life. (Cretaceous period, forelimbs, carcass)

2. You're a paleontologist and have just found what could be a T. rex fossil. Write field notes describing what you see. (mudstone, articulated vertebrae, matrix)

Cross-Curricular Connection

History: Barnum Brown discovered the grist Tyrannosaurus rex fossil in 1902. Research the details of Brown's amazing find.

National Science Education Standards 

Grades 5-8: Earth's history * structure of the Earth's system * diversity and adaptations of organisms

Grades 9-12: the origin and evolution of the Earth system * biological evolution

Resources

Hunt for the Past: My Life as an Explorer by Sue Hendrickson, Scholastic, 2001

The Complete T. Rex by John R. Homer and Don Lessem, Simon & Schuster, 1993

See Sue at www.fmnh.org/sue