So Big, So Cosmopolitan

By Carl Zimmer|Wednesday, January 01, 1997
RELATED TAGS: DINOSAURS
Tyrannosaurus rex was the undisputed giant of all land predators for 90 years, and in that time it developed the aura of a dinosaurian Muhammad Ali. It was, truly, the greatest. But in 1995 paleontologists in Argentina discovered a new dinosaur, called Giganotosaurus, which was probably heavier and certainly as long as the 40-foot, 70-million-year-old T. rex. And this past year, like the second blow of a one-two punch, came the report of another predator the size of a T. rex, this one from Africa.

For dinosaur paleontologists today, Africa is like the old Wild West, where spectacular fossils like T. rex lay waiting for those who could get to them. In 1993, Paul Sereno of the University of Chicago began organizing expeditions to the Sahara, and despite grueling conditions--You have to really enjoy thinking about where the next oasis will be, he says- -he uncovered interesting specimens in Niger. Two years later, while working near the border between Morocco and Algeria, he came across a knobby fragment of bone that fits on the back of the skull of a carnivorous dinosaur. Searching further, he found ten-inch-long teeth nearby, like giant spearheads lodged in rock. In all, he unearthed more than half the animal’s skull and three vertebrae.

Back in Chicago he put these chunks and scraps together into a 90-million-year-old, T. rex-size monster--and realized that it was an obscure species called Carcharodontosaurus, which paleontologists had known for decades chiefly from oddly wrinkled teeth and a few enigmatic bone fragments. When reporters thronged a press conference in May--to which Sereno had brought a frightening reconstruction of the beast’s skull--he knew what their main question would be: Was it the biggest? Sereno can’t say for sure. I don’t want to get into that argument--it’s ridiculous for a couple reasons. We don’t have the whole animal, for example, and what do you mean by biggest--the longest, or the largest? Its 62-inch-long skull is about 3 inches longer than the biggest T. rex’s and about the same as that of Giganotosaurus, but Sereno doesn’t have any leg or tail bones from his Moroccan specimen.

Yet this three-way tie for the biggest meat-eating dinosaur has interesting implications. It speaks potentially of a maximum size, says Sereno. We now have three animals getting up to 45 or 50 feet long, we’ve never seen anything larger, and we see this size approached in independent lineages. It seems as though this might be the natural limit for these things.

Despite superficial appearances, Carcharodontosaurus was not closely related to T. rex. The stocky, conical-toothed, scrawny-armed T. rex belongs to a select group called maniraptors (still with us today in the form of birds); whereas with its long arms, narrow head, and steak- knife teeth, Carcharodontosaurus was descended from another branch of bipedal marauders that included the smaller Allosaurus. On the other hand, Sereno found it was very closely related to a lesser-known North American dinosaur, the huge Acrocanthosaurus--and also to a South American one: none other than last year’s behemoth, Giganotosaurus.

To find close dinosaur cousins living on three separate continents 90 million years ago was surprising. The continents had begun drifting apart at least 60 million years before that, when the supercontinent Pangaea broke into northern and southern landmasses, called Laurasia and Gondwana, and those in turn broke into North America and Asia, South America and Africa. Paleontologists used to think that these breaks permanently isolated the dinosaurs from one another, forcing them to evolve apart. Sereno now thinks Laurasia and Gondwana remained loosely linked for tens of millions of years, perhaps by short-lived island chains in Central America or Western Europe. That way, the immediate ancestors of Carcharodontosaurus and its close relatives could have wandered between the southern and northern landmasses. The geologists are very good at creating maps of where the continents were, but they freely admit that they don’t have an absolute shoreline, Sereno points out.

Only after about 90 million years ago, the new African find suggests to Sereno, did the continents become so isolated that they developed fauna not found in other parts of the world. Only then, for example, did a once-unique animal called Tyrannosaurus rex begin to roam the American West.
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