are "splitters." They see most every difference between skeletons as reflecting different species. Other paleontologists are "lumpers"—they tend to ascribe the differences between specimens to different sexes, ages, or other variations within a species.
Most of the time when we find an unfamiliar dinosaur, it looks so different from anything we've seen before that it rates a new genus (written in italics, first letter capitalized—e.g., Tyrannosaurus). We also give it a second or species name (written in lowercase italics— e.g., rex). We use Latin or Greek words a lot, by agreement, but you can name a dinosaur after anything. Many dinosaurs have been named for dead paleontologists and places where the fossils come from, but some have even been named for companies that lend money or equipment. With paleontologist Don Baird's advice, I named the first "feminine" dinosaur—the duckbill Maiasaura ("good mother reptile")—using the feminine sufix -saura instead of the masculine -saurus because the dinosaur was found with nests and babies. Tom Rich in Australia named the second feminine dinosaur, LeaeUynasaura, for his daughter, Leaellyn.
Some of the dinosaurs that scientists have named as new species have turned out to be babies or juveniles of a species already named. All of us, scientists and dinosaur lovers alike, pay a lot more attention to the genus names than to the species names with dinosaurs. T. rex is the only dinosaur I know of that we commonly call by both its species and genus name, or by the first letter of its genus name.
But Tyrannosaurus is just one of several tyrannosaurid genera. So what do tyrannosaurids have in common? They are big and scary, but then so were many other dinosaurs. Tyrannosaurids were among the last dinosaur carnivores. They all lived toward the end of dinosaur time, during the Late Cretaceous period, 83 million to 65 million years ago.
As it turns out, all the tyrannosaurids had so much in common that Canadian paleontologist Phil Currie, who knows them as well as anyone, says they were as similar
tyrannosaurus rex (top left) looks imposing even when compared to other tyrannosaurids — tarbosaurus, maleevosaurus, albertosaurus (two species) and daspletosa urus.
as "breeds of dogs, or models of cars." They shared innovations not seen before on big meat-eating dinosaurs—only two fingers on tiny front arms, and a heavily reinforced skull. Tyrannosaurids all had huge long teeth, curved and serrated on the edges. Their necks were short and thick, their chests broad and deep. Compared with tyrannosaurids' huge size, their tails were short and skinny, their hips narrow. Their legs were huge, with thick drumstick shins.
A lot of the other anatomical distinctions of tyrannosaurids may be too technical to be of much interest to anyone but us paleontologists, from the size and shape of backbones to the size and shape of teeth (all tyrannosaurid incisors are D-shaped when you cut across them) and skull openings in the cheek and eyebrow bones. These subde distinctions are important because they show us how and how much the tyrannosaurid body plan changed from that of earlier tyrannosaurus rex (top left) looks imposing even when compared to other tyrannosaurids — tarbosaurus, maleevosaurus, albertosaurus (two species) and daspletosa urus.
carnivores. In contrast to earlier big meat-eaters, a tyrannosaurid had narrower neck bones and skinnier shoulder bones, but a much bigger skull and much larger pubic bones.
The common ancestor of all the tyrannosaurs was probably an animal a few million years older than Albertosaurus, which lived about 75 million years ago. George Olshevsky, a San Diego computer whiz, former comic book collector, and chief keeper of the dinosaur family tree, thinks the first tyrannosaurid may have looked \ikcAlectrosaurus, a questionable tyrannosaurid that lived in Mongolia about 80 million years ago. We don't know Alectrosaurus well, and its bones were once mixed up with those of another dinosaur that had large front limbs with big claws. But if you take away those puzzling front limbs from Alectrosaurus (as paleontologists Bryn Mader and Robert Bradley did lately in redescribing the animal) and fill in some missing parts with guesswork, you can get an animal close to that make-believe tyrannosaurid ancestor I described earlier—a svelte, eighteen-foot-long hunter with a long low head and long skinny legs. It isn't such a big evolutionary step from this recreated Alectrosaurus to Albertosaurus, and on to a variety of other tyrannosaurids, ending in Tyrannosaurus rex.
Which was T. rex's closest relative? Probably
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