New Tyrannosaur Discoveries Reveal Details about T. rex [Slide Show]

A flood of new tyrannosaur finds is helping to shed light on how their gargantuan successor developed

By Katherine Harmon

September 17, 2010

Tyrannosaurs emerged about 165 million years ago, during the middle Jurassic, about 100 million years before T. rex lived. The earliest tyrannosaurs were quite a bit smaller than the tremendous Tyrannosaurus rex, including many that were close to human size. Guanlong (seen here, an illustration of the Guanlong wucaii) lived some 160 million years ago and measured about three meters long. This unusual-looking dinosaur was in fact relatively typical for an early tyrannosaur in that its head was crowned by a large crest.

By examining many of the newly described species, scientists have learned that early tyrannosaurs were far from central figures in their ecosystems. Instead these dinosaurs inhabited fringe roles. Also, the comparison revealed that T. rex "is quite abnormal when compared with other members of the group," Stephen Brusatte, a graduate researcher with the American Museum of Natural History and co-author of the new Science paper that assessed a host of new discoveries about tyrannosaurs, said in a prepared statement.

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Guanlong

Tyrannosaurs emerged about 165 million years ago, during the middle Jurassic, about 100 million years before T. rex lived. The earliest tyrannosaurs were quite a bit smaller than the tremendous Tyrannosaurus rex, including many that were close to human size. Guanlong (seen here, an illustration of the Guanlong wucaii) lived some 160 million years ago and measured about three meters long. This unusual-looking dinosaur was in fact relatively typical for an early tyrannosaur in that its head was crowned by a large crest.

By examining many of the newly described species, scientists have learned that early tyrannosaurs were far from central figures in their ecosystems. Instead these dinosaurs inhabited fringe roles. Also, the comparison revealed that T. rex "is quite abnormal when compared with other members of the group," Stephen Brusatte, a graduate researcher with the American Museum of Natural History and co-author of the new Science paper that assessed a host of new discoveries about tyrannosaurs, said in a prepared statement.

Tyrannosaurs are theropod dinosaurs, a group which also includes dromaeosaurs, oviraptorosaurs and avialans (or modern birds). Dilong (shown here), which lived some 130 million years ago, seems to already have been covered with protofeathers. Although the dinosaur was not built for flight (researchers speculate that the early feathers helped with warmth), these primitive structures appear in more advanced forms later in theropod evolution.

High-tech tools have also helped to tease out how these dinosaurs differed from modern birds. Computerized tomography (CT) scans have allowed researchers to peer inside the fossilized skulls of many specimens, providing a new look at how big their brains were and how they were likely structured. "The work on tyrannosaurs underscores how much can be done using modern techniques to understand the biology of fossil organisms," Mark Norell, a curator at the American Museum of Natural History and co-author of the new paper, said in a prepared statement. Tyrannosaurs had larger brains (relative to body size) than earlier theropods but ones that were likely smaller than those of modern birds. Studies have shown that many tyrannosaurs had large olfactory regions and well-developed ear canals, suggesting that the carnivores had excellent senses of smell and hearing.

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Dilong

Tyrannosaurs are theropod dinosaurs, a group which also includes dromaeosaurs, oviraptorosaurs and avialans (or modern birds). Dilong (shown here), which lived some 130 million years ago, seems to already have been covered with protofeathers. Although the dinosaur was not built for flight (researchers speculate that the early feathers helped with warmth), these primitive structures appear in more advanced forms later in theropod evolution.

High-tech tools have also helped to tease out how these dinosaurs differed from modern birds. Computerized tomography (CT) scans have allowed researchers to peer inside the fossilized skulls of many specimens, providing a new look at how big their brains were and how they were likely structured. "The work on tyrannosaurs underscores how much can be done using modern techniques to understand the biology of fossil organisms," Mark Norell, a curator at the American Museum of Natural History and co-author of the new paper, said in a prepared statement. Tyrannosaurs had larger brains (relative to body size) than earlier theropods but ones that were likely smaller than those of modern birds. Studies have shown that many tyrannosaurs had large olfactory regions and well-developed ear canals, suggesting that the carnivores had excellent senses of smell and hearing.

Although a predecessor to the line that would lead to T. rex, Raptorex already exhibited many of the interesting characteristics of these diverse dinosaurs, including a big head and teeth, long legs and short arms. It was even smaller than Guanlong, growing to about three meters long. "Until recently, the prevailing notion was that tyrannosaur body size gradually and progressively increased over time," the researchers noted in their paper. And with about 100 million years separating the first known tyrannosaurs and T. rex, such a theory stands to reason.

But as more discoveries have filled in the tyrannosaur family tree, scientists have found that "for most of their evolutionary history, tyrannosaurs were small and living in the shadow of the other giant predators," Brusatte said. "They stayed small until the end of the Cretaceous—the final 20 million years of dinosaur history."

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Raptorex

Although a predecessor to the line that would lead to T. rex, Raptorex already exhibited many of the interesting characteristics of these diverse dinosaurs, including a big head and teeth, long legs and short arms. It was even smaller than Guanlong, growing to about three meters long. "Until recently, the prevailing notion was that tyrannosaur body size gradually and progressively increased over time," the researchers noted in their paper. And with about 100 million years separating the first known tyrannosaurs and T. rex, such a theory stands to reason.

But as more discoveries have filled in the tyrannosaur family tree, scientists have found that "for most of their evolutionary history, tyrannosaurs were small and living in the shadow of the other giant predators," Brusatte said. "They stayed small until the end of the Cretaceous—the final 20 million years of dinosaur history."

Alioramus, which lived some 70 million to 65 million years ago, measured in at about five to six meters long and was larger than many of the more basal tyrannosaurs. Tyrannosaurs, especially the larger ones, spent much of their lives growing to full size. A T. rex that might live for 25 years did not reach full physical maturity until about 20, the researchers noted. "Its maximum growth rate may have exceeded 767 kilograms per year, equivalent to adding a remarkable two kilograms per day," according to the paper.

Along with this size increase went many changes in physical attributes. Lankier juveniles developed sturdier legs and shorter arms. Many of these changes had confounded paleontologists of the past. "The differences between juvenile and adult tyrannosaurids are so great that different growth stages have often been mistaken for different species," the researchers noted.

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Alioramus

Alioramus, which lived some 70 million to 65 million years ago, measured in at about five to six meters long and was larger than many of the more basal tyrannosaurs. Tyrannosaurs, especially the larger ones, spent much of their lives growing to full size. A T. rex that might live for 25 years did not reach full physical maturity until about 20, the researchers noted. "Its maximum growth rate may have exceeded 767 kilograms per year, equivalent to adding a remarkable two kilograms per day," according to the paper.

Along with this size increase went many changes in physical attributes. Lankier juveniles developed sturdier legs and shorter arms. Many of these changes had confounded paleontologists of the past. "The differences between juvenile and adult tyrannosaurids are so great that different growth stages have often been mistaken for different species," the researchers noted.

A close relative of Alioramus and T. rex, Tarbosaurus is the second-largest known tyrannosaur, with a 1.3-meter-long skull and 12-meter-long body.

The probable biomechanics of these extinct animals suggest that they were likely not very fast, even though "the long legs and large pelvic limb muscles…intuitively seem to indicate fast running capacity," the researchers wrote in the new study. These attributes, however, were likely "inherited from small, presumably fast-running ancestors."

And although the classic image of the tyrannosaur is as the ultimate hunting machine, scientists are now finding evidence that, like many modern carnivores, these dinosaurs likely fed on both stalked and scavenged food. Scientists have found tyrannosaur bite marks on fossilized bones from herbivores that died en masse and had presumably been found already dead or dying by the hungry carnivores. Other fossils have shown partially healed bite marks, suggesting a failed tyrannosaur attack on a live animal.

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Tarbosaurus

A close relative of Alioramus and T. rex, Tarbosaurus is the second-largest known tyrannosaur, with a 1.3-meter-long skull and 12-meter-long body.

The probable biomechanics of these extinct animals suggest that they were likely not very fast, even though "the long legs and large pelvic limb muscles…intuitively seem to indicate fast running capacity," the researchers wrote in the new study. These attributes, however, were likely "inherited from small, presumably fast-running ancestors."

And although the classic image of the tyrannosaur is as the ultimate hunting machine, scientists are now finding evidence that, like many modern carnivores, these dinosaurs likely fed on both stalked and scavenged food. Scientists have found tyrannosaur bite marks on fossilized bones from herbivores that died en masse and had presumably been found already dead or dying by the hungry carnivores. Other fossils have shown partially healed bite marks, suggesting a failed tyrannosaur attack on a live animal.

The so-called king of dinosaurs, Tyrannosaurus rex, has been shown, by its more humble forerunners to be a complex, nuanced creature, the product of more than a hundred million years of tyrannosaur evolution.

And these formidable giants were efficient killers. Researchers have performed analysis of bite marks preserved in fossilized bone to learn more about tyrannosaur striking patterns. These dinosaurs "characteristically bit deeply into carcasses, often through bones, and then pulled back, creating long cuts," the researchers described in their paper. Scientists have even found bone fragments in fossilized dinosaur droppings, which suggests that "bone was fractured, ingested, and used for sustenance." And analysis of tyrannosaur skulls has shown that they were "optimized to endure strong bites" as "the bite forces needed to crunch through bone would have been enormous."

Researchers are still attempting to assemble a better understanding of how these massive carnivores lived, grew and fed. And more tyrannosaur discoveries in the future should help scientists glean even more details about this sharp-sensed, bone-crushing paragon.

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Tyrannosaurus rex

The so-called king of dinosaurs, Tyrannosaurus rex, has been shown, by its more humble forerunners to be a complex, nuanced creature, the product of more than a hundred million years of tyrannosaur evolution.

And these formidable giants were efficient killers. Researchers have performed analysis of bite marks preserved in fossilized bone to learn more about tyrannosaur striking patterns. These dinosaurs "characteristically bit deeply into carcasses, often through bones, and then pulled back, creating long cuts," the researchers described in their paper. Scientists have even found bone fragments in fossilized dinosaur droppings, which suggests that "bone was fractured, ingested, and used for sustenance." And analysis of tyrannosaur skulls has shown that they were "optimized to endure strong bites" as "the bite forces needed to crunch through bone would have been enormous."

Researchers are still attempting to assemble a better understanding of how these massive carnivores lived, grew and fed. And more tyrannosaur discoveries in the future should help scientists glean even more details about this sharp-sensed, bone-crushing paragon.

Fonte: http://www.scientificamerican.com