Likely Tyrannosaurids

Alectrosaurus olseni was named by Charles Gilmore in 1933 from bones found in the Gobi Desert of Inner Mongolia. Gilmore may have combined two different animals. Ken Carpenter thinks Alectrosaurus is not a tyrannosaurid, but Phil Currie believes it was a primitive tyrannosaurid. It was about sixteen feet long and less than half a ton in weight, with skinny shoulders and legs.

Alioramus remotus was named by Sergei Kurzanov of the USSR in 1976 from a skull and a few bones from Mongolia. Alioramus was a mysterious creature about twenty feet long. The back of the skul I looks like a tyrannosaurid, but Alioramus had no holes in its cheek and no eyebrow bones like other tyrannosaurids. It also had lots of little horns on its muzzle, something not seen in any other tyrannosaurid. Ken Carpenter excluded Aliosaurus from tyrannosaurids in his reclassification of the family. It may be related to Alectrosaurus. Once again, Phil Currie says it definitely was a tyrannosaurid, most closely related, by braincase comparison, to Tarbosaurus.

Chingkankousaurus fragilis was named by C. C. Yang in China in 1958. A big theropod, it is known only from the shoulder blade and other fragments and may be a Tarbosaurus.

MAYBE TYRANNOSAURS, SOMEDAY

A new genus of tyrannosaurid, or at least a new species of Daspletosaurus, may be described soon from the Horseshoe Canyon Formation of Alberta, Canada, by Bob Bakker, Mike Williams, and Phil Currie.

A new species of tyrannosaurid may be described by Bakker and Currie from the tyrannosaurid on display at the Field Museum in Chicago now labeled Albertosaurus libratus.

A new species of tyrannosaurid, an intermediate between Daspletosaurus and Tyrannosaurus rex, will be described by Dave Varricchio from a complete skull with articulated leg found at a Museum of the Rockies' site in western Montana.

Daspletosaurus

t. rex couldn't have visited new york if it had wanted to. a shifting inland sea separated eastern and western north america throughout much of late cretaceous time, including the reign of t. rex.

Tarbosaurus bataar, a big predator with a striking resemblance to T. rex. Skeletons of T. bataarhave been found several times in Mongolia's Gobi Desert, the first immediately after World War Q*. A Russian paleontologist suggested that Tarbosaurus was intermediate between Albertosaurus and Tyrannosaurus, but the truth is Tarbosaurus is a lot closer to Tyrannosaurus than anything else. You have to look very hard to find the differences between Tarbosaurus bataar and Tyannosaurus rex:, so hard that Tarbosaurus is considered by some an invalid genus. Instead, the Mongolian giants could be grouped as Tyrannosaurus bataar, in the same genus as their American cousin, T. rex.

Ken Carpenter, who did that regrouping, gives a telling example of how minor the differences are between T. bataar and T. rex. The back of the upper jawbone in T. rex ends before the front rim of the eye socket. In Tarbosaurus, the bone ends farther back, under the front part of the eye socket. For another, T.

bataar's tiny arms are even smaller than T. rex's.

Ken thinks the minor nature of differences between T. rex and Tarbosaurus suggests they are closely related species. And he thinks Tarbosaurus's features are more primitive than those of T. rex So he thinks Tarbosaurus was T. rex's ancestor. Perhaps some Tarbosaurus crossed over the link that had emerged between Asia and North America and developed into T. rex in this continent at the very end of the Cretaceous period. (A few teeth and jaws found in China suggest Tarbosaurus lived there also, though the Chinese have named three other species of Tyrannosaurus from these fragments.)

Not everyone thinks Tarbosaurus was T. rex's older sister species. Some other differences between the animals are more substantial—for instance, T. bataar was more lightly built. Now some studies are underway that suggest those differences are big enough after all for the Mongolian tyrannosaurs to rate their own genus. Then they would have been older cousins of T. rex. In that case Tarbosaurus would turn out to be a valid genus after all.

These name changes are confusing for all of us. But that's how it often goes with classifying dinosaurs. We know so litde about them that when we take a closer look or get a bit of new evidence, the categories we've squeezed them into don't seem to fit anymore.

Another member of the tyrannosaurid family has only recently been identified, though it sat gathering dust in a museum basement for decades. Inspired by T. rex discoverer Barnum Brown, paleontologists from the Cleveland Museum of Natural History went to T. rex country to get a dinosaur in 1942. They found a skull of a small tyrannosaurid. In 1946Charles Whitney Gilmore of the Smithsonian identified it as a new kind of gorgosaur, Gorgosaurus lancensis. In 1970, Canadian dinosaur paleontologist Dale Russell decided it was a dwarf species of Albertosaurus.

Paleontologist Bob Bakker came across the skull on a visit to the Cleveland Museum in 1988. Bob and paleon cat scans of the skull of nanotyrannus show the delicately spiraling turbinal bones of the nose.

tologist Mike Williams of the museum had their doubts about whether a skull that small, narrow, and otherwise peculiar could belong to Gorgosaurus. The more they examined the skull, the more they became convinced that this was not a juvenile Gorgosaurus, but an adult of something else. Their suspicions were confirmed when Mike Williams chipped away the horns of the skull and found they were pureplaster. An overeagerfossilworker had added a couple of plaster horns to the skull to make it look more like the Gorgosaurus known elsewhere.

Bob, Mike Williams, and Phil Currie renamed the animal Nanotyrannus ("littie tyrant"). Nanotyrannus had things in common with T. rex and no other dinosaur. The skull had a narrow, doglike face—it was far wider in the back than across the snout. It had no horns over its eyes, and the eye sockets were pointed so far forward that they may have seen with binocular vision.

T. rex and Nanotyrannus lived at the same time. They're found in the same rock formation. If T. rex and Nanotyrannus are not from the same genus, then they may have evolved their matching characteristics inde-pendendy from entirely different stock. Bob Bakker thinks the latter—that they evolved from very different ancestors.

cat scans of the skull of nanotyrannus show the delicately spiraling turbinal bones of the nose.

Juvenile Tyrannosaurus

Other dinosaurs, much less completely known have been pigeonholed into the tyrannosaur family (see sidebar). We may yet find more of these animals, enough to tell whether they were something new or something familiar after all. Certainly there are a lot more carnivorous dinosaurs out there. As they are found, they will answer more of our questions about how many tyrannosaurs there were, where they lived, and how they evolved.

We may not have to wait all that long for some answers. We dug up a beauty of a juvenile Allosaurus in Wyoming in 1991. Bones ofyoung allosaurs have been found before—by the truckload at the Cleveland-Lloyd quarry near Price, Utah—but never before in a single skeleton so well preserved. This one should tell us a lot about how Allosaurus looked as it was growing up.

Phil Currie and his colleagues from the Royal Tyrrell Museum of Palaeontology in Alberta have found a half-dozen Albertosaurus skeletons in the decade since the museum opened. Another Albertosaurus was recendy found in Alabama. That discovery shows that tyrannosaurids lived east of the big seaway that ran north-south through the heart of North America at the end of dinosaur time.

And I've been finding lots of interesting dinosaurs, including big carnivores, throughout the Two Medicine Formation of western Montana. The Two Medicine Formation is more than two thousand feet thick there, full of fossils from 6 million years of late dinosaur times. And we've been able to link particular kinds of dinosaurs to the shifting pattern of the inland sea that stretched across Montana at that time. As the seaway grew, or trangressed, dinosaurs (and other land animals) were pushed up into isolated upland habitats, such as mountain valleys. When the seaway shrank, dinosaurs flourished in delta environments. (I describe this geology more in my book Digging Dinosaurs.)

Not only did the kinds of dinosaurs vary with the changing habitats, but the way in which they evolved changed according to the environment. It was a stress-

fill time when the seaway expanded, encroaching on the dinosaurs' turf, and as you might expect, it was the less specialized species that survived in those conditions. They were more adaptable to changing environments. There was less diversity and smaller populations of dinosaurs when the seaway moved in. But new species still evolved. It was straight-line evolution, however, one species evolving into just one other, instead of radiating into several. We call this straight-line evolution anagenesis.

When the North American seaway closed in on dinosaur habitats in the Two Medicine Formation of Montana where I've been exploring, new species evolved, in a straight-line path. Sometimes they just got bigger. But one kind of duckbill led to another, one horned dinosaur to another.

When the seaway receded, new territories opened up. Dinosaurs radiated into new species to meet the opportunities to fill these new niches, an example of cladogenesis. One population of animals enters into an environment with many niches and evolves daspletosaurus was a close relative and possible ancestor of t. rex.

Daspletosaurus

daspletosaurus was a close relative and possible ancestor of t. rex.

into many species to fill them.

We see these two patterns of change, straight-line and branching, primarily with the dinosaurs we have the greatest samples of, duckbills and horned dinosaurs. Right after the end of a major transgression, or inland movement of the sea, ends, we see the greatest diversity in dinosaurs. In the last few years we've collected four thousand specimens from these few million years in dinosaur time in one area. One hundred of those dinosaurs can be identified to the species level. That's more of a record than anyone's ever had for dinosaurs. This is a dramatic confirmation of the theory of evolution itself. To me it's about the most important finding of all my work on dinosaurs.

What does all this have to do with T. rex's evolution? Well, it was during that stressful time recorded in the top of the Two Medicine that Daspletosaurus was replaced by a tyrannosaur forerunner of Tyrannosau-rusrex. We know that because we've found a predator intermediate in appearance between those two animals, with no features peculiar to it alone. That shows anagenesis was going on in tyrannosaurid evolution at that time. Intermediates, like this new tyrannosaurid, coelophysis was one of the earliest carnivorous dinosaurs known. not much bigger than i, it lived in what is now arizona in the first dinosaur period. the triassic, some 2 2 0 million years ago.

archaeopteryx is one of the few known m e tat ax a, a true intermediate animal, in this case between birds and dinosaurs.

in straight-line evolution between animals, are rare. Before I wrote about the intermediates (metataxa) we found among duckbills, horned dinosaurs, and tryrannosaurids, there were only two dinosaur metataxa known—Coelophysis, an early predator, and Archae-opteryx, the earliest bird.

From this new discovery it now looks as though T. rex wasn't a particularly sturdy branch on the spreading bush of big predators. More likely it was the one descendant of a big theropod, Daspletosaurus, nearly as big as T. rex itself. At least that's what the littie evidence we've got now says. We'll know even more about the relationship between the two big predators when my doctoral student Dave Varricchio finishes his thesis research on Daspletosaurus.

But there are still many mysteries to be solved about T. rex's evolution. I think it's only a matter of time before we find more tyrannosaurids, enough to tell us more about where T. rex came from. But unlike the other tyrannosaurids, T. rex didn't lead anywhere—it never evolved into anything else. There never was another tyrannosaurid after T. rex. I think that's a shame.

chapter 7

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