Image: Guanlong wucaii
Zhongda Zhang  /  IVPP
The tyrannosaur known as Guanlong wucaii, shown in this artwork, had a strange narrow crest on its head and long, three-fingered arms that were unlike T. rex's two-fingered stubby arms. It lived 160 million years ago and ranks as one of the earliest ancestors of T. rex ever found.
By
updated 9/16/2010 2:47:00 PM ET 2010-09-16T18:47:00

This isn't your daddy's Tyrannosaurus Rex. The king of the dinosaurs evolved from ancestors that spent most of their evolutionary history skulking in the shadows of other giant predators, and may have even sported hair-like feathers.

T. rex made a late evolutionary arrival on the dinosaur scene, according to the latest paleontological evidence. Earlier tyrannosaurs remained the size of humans until about 80 million years ago, said Stephen Brusatte, a vertebrate paleontologist and evolutionary biologist at the American Museum of Natural History in New York City.

"Up until about ten years ago we only knew about T. rex and a handful of its closest relatives — all colossal, apex predators from the end Cretaceous in North America," Brusatte explained. "Now we know of about 20 tyrannosaur species that span a time period of 100 million years, most of which are very small."

Multi-ton late-Cretaceous specimens had defined scientific understanding of the tyrannosaurs since the first discovery 105 years ago. But the last decade saw an explosion of tyrannosaur discoveries that doubled the family's diversity, including six new species in the last year alone.

Image: Body silhouettes
M. Donnelly/The Field Museum
Reconstructed body silhouettes of three tyrannosaurs, showing where Xiongguanlong falls in the spectrum of body sizes in this lineage. Dilong on the left is 125 million years old and the smallest known tyrannosaur. Xiongguanlong, shown in grey, is much larger, but is still dwarfed by T. rex, shown on the right.

Now Brusatte and colleagues have created the first comprehensive family tree for tyrannosaur cousins large and small, as detailed in the Sept. 17 issue of the journal Science.

Supersize me
The family tree shows how tyrannosaursfirst originated about 165 million years ago during the Middle Jurassic. They remained mostly small, fast-footed dinosaurs just one hundredth the size of T. rex.

In fact the late-Cretaceous tyrannosaur giants looked more like their nimbler ancestors as young dinosaurs, capable of more active hunting. But then they grew rapidly to reach full maturity at the age of 20, putting on more than 4 pounds (2 kg) per day, and losing much of their fancy footwork.

Such growth spurts allowed the late tyrannosaur adults to weigh in at 11,000 to 17,600 pounds (5 to 8 metric tons), but also limited their running speeds to between 16 and 36 feet per second (5 and 11 meters per second). By comparison, a racehorse running full-tilt can hit speeds of almost 66 feet per second (20 mps).

Hungry, hungry dino
T. rex made up for its slow movement with keen hearing and smell, as well as a bone-crushing bite that might have topped 3,000 pounds of force (13,400 newtons). Bite marks on a wide variety of dinosaur fossils reveal that it was no picky eater, and probably represented both a hunter and scavenger.

Researchers still don't know why tyrannosaurs got a shot at quickly becoming top predators during the twilight years of the dinosaurs. But the fossil record shows that many other giant predators had gone extinct by about 80 million years ago, and tyrannosaur body size exploded soon after.

"We hypothesize that the giant size of tyrannosaurs was only possible once other large predators went extinct, giving tyrannosaurs the space and freedom to flower and become apex predators," Brusatte told LiveScience in an e-mail.

Image: Tarbosaurus
Matt van Rooijen
An artist's depiction on how the tyrannosaur known as Tarbosaurus might have scavenged on the upper arm bone of a duck-billed dinosaur.

Many other questions about tyrannosaurs also remain, despite the fact that the group represents perhaps the best-studied among the dinosaurs.

Riddles upon riddles
For instance, speculation about T. rex having hair-like feathers comes from knowledge that one species of tyrannosaur, Dilong paradoxus from the Early Cretaceous period of China, had such features. Earlier ancestors also carried the feathery plumage.

"Because tyrannosaurs clearly weren't flying, it is likely that their feathers were used for display or insulation," Brusatte said.

Figuring out the sex of each tyrannosaur from a specimen has also proved baffling. Researchers have recently hit upon a "100-percent foolproof" method of looking for  medullary bone tissue — a calcium phosphate used in creating eggshells within female dinosaurs. But that relies upon finding a fossilized female that had been ovulating.

One of the most exciting areas of research involves the possible discovery of soft tissues, such as cells, blood vessels and collagen, in one tyrannosaur specimen. Although controversial, the findings could overturn ideas about what body parts can survive fossilization. They might even lead to the possibility of preserved dinosaur DNA.

"We have come a long way in our understanding of dinosaur biology, and of dinosaur genealogy,” Brusatte said, “ but one of the joys of paleontology is that each new fossil has the potential to tell us something new, and even to overturn ideas we once thought were bulletproof."

© 2012 LiveScience.com. All rights reserved.

Interactive: Are dinosaurs alive?

Explainer: The world's seven deadliest dinosaurs

  • Copyright 1985 Mark Hallett, "Awakening of Hunger"

    Yeah, it's cliche to say Tyrannosaurus rex was deadly. But the tyrant king was likely true to the billing. Its bone-crushing jaws could splinter prey like toothpicks, after all. And the beast was big, up to 40 feet long, 20 feet tall, and may have topped the scales at nearly 16,000 pounds.

    The king walked on two legs over a vast territory in western North America at the end of the Cretaceous, about 65 million years ago. Scientists wonder whether T. rex was more a lumbering scavenger or a quick and agile predator, but dead or alive, its meals were big, meaty and bloody.

    T. rex wasn't the only deadly dino, however. The globe was filled vicious killers. Click the arrows above to see more.

    — John Roach, msnbc.com contributor

  • Family diner

    Demetrios M. Vital

    Family meals for Majungasaurus crenatissimus were often bloody affairs of the grimmest sort: Kin were the main course. The evidence of its cannibalistic ways comes from telltale tooth marks on Majungasaurus bones that match up perfectly with the size and spacing of teeth in its jaws.

    The 20-foot-long dinosaur stalked the plains of Madagascar about 70 million years ago. At that time, the crime-scene investigators said, pickings were often slim. Their data indicate Majungasaurus fed on dried out "dino jerky" from its plant-eating compatriots as well as members of its own species. Cannibalism was likely just as common among dinosaurs as it is among living animals, but the evidence is rare.

  • Brow beater

    Todd Marshall

    Eocarcharia dinops' brow was swollen into a massive band of bone. The menacing head piece may have been used as a battering ram against rivals and to attract potential mates. Its blade-shape teeth were reserved for disabling live prey and severing their body parts.

    The 110-million-year-old beast and its snout-nosed, gut-and-carcass-scavenging relative Kryptops palaios were discovered in Africa's Sahara Desert. The approximately 7-foot-tall and 25-foot-long duo likely teamed up with a third carnivore and feasted on the long-necked plant-eater Nigersaurus.

  • Vicious rocker

    Parsons

    The toothy Masiakasaurus knopfleri likely speared prey with its forward projecting front teeth and then sliced and tore the captives into chewy chunks with its bladelike rears. This type of tooth arrangement is otherwise unknown in predatory dinosaurs.

    Scientists believe the German Shepherd sized beast feasted on fish, lizards, and other critters on the southern supercontinent Gondwana in the Late Cretaceous period, about 65 to 70 million years ago.

    The first part of this dinosaur's name means "vicious lizard" and the second part is derived from the Dire Straits singer and guitarist Mark Knopfler, whose music inspired the discoverers as they toiled on the African island nation of Madagascar.

  • Biggest carnivore?

    Prof. Rodolfo Coria / Ap

    At about 40 feet long and weighing an estimated 6 tons, Mapusaurus roseae was one of the biggest meat-eating dinosaurs to ever stalk the Earth. Bones of several individuals were discovered in one place, suggesting the giants may have hunted in packs that could have toppled perhaps the largest dinosaur that ever lived — a 100-foot-long plant eater called Argentinosaurus.

    Mapusaurus lived about 100 million years ago and was bigger than well known Tyrannosaurus rex and possibly larger than its older cousin, Gigantosaurus. Its teeth were narrow and blade like, made for slicing its prey. The bones were discovered in the Patagonia region of Argentina.

  • Going green

    MWS

    Falcarius utahensis is the living image of a vicious meat eater trying to go vegetarian. Scientists aren't sure if the bird-like relative of Velociraptor had fully kicked its taste for flesh, but its meat-cutting teeth had shrunk to leaf-cutting size and its gut had expanded sufficiently to ferment plants.

    Falcarius walked on two legs and stood about 4.5 feet tall. Head to tail, it was about 13 feet long and wielded sharp, curved, four-inch long claws — perhaps to grab a bite when it fell off the wagon? The dinosaur lived during the Early Cretaceous, about 125 million years ago, in what is today Utah.

  • Plant ripper

    utah.edu

    No plants were safe from Gryposaurus monumentensis, a big boned, duck-billed dinosaur that could have eaten any vegetation it stumbled across. Its massive skull packed more than 300 teeth for slicing up fibrous greens. Hundreds more replacement teeth rested in its jawbone for the call-up to action.

    The dinosaur lived in the Late Cretaceous, about 75 million years ago on the western side of a giant, shallow ocean that split North America at that time. The plant-munching beast may have reached 30 feet long as an adult and had a 3-foot-long head.

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