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Fossils of 2 newly discovered elephant-sized dinosaurs glimpse into evolutionary trends

Scientists hope discoveries will help bridge evolutionary and migratory gaps

By Jeremy Manier | TRIBUNE REPORTER
February 14, 2008

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A team of Chicago fossil hunters has discovered two massive meat-eating dinosaurs that prowled the ancient African landscape and may have picked over the same prey, much as modern-day lions and hyenas do.

One of the new specimens taken from rock beds in the Sahara was a fearsome killer with bladelike teeth, and the other seems to have scavenged scraps of meat predators left behind.

The new finds offer a glimpse of the way dinosaurs evolved as the African continent split off from South America about 100 million years ago, said University of Chicago paleontologist Paul Sereno, whose team dug up the fossils and published a study on them Wednesday.

Those creatures inhabited a very different Earth, where land bridges connected Africa to India and even Antarctica, which was then a temperate home to dinosaurs. But later Africa became isolated and its dinosaurs followed unique evolutionary paths that scientists have just begun to uncover.

"It's like a treasure hunt in that anything you find is going to be new," Sereno said in an interview in his lab, which is full of raw African fossils his team has not yet unveiled.

Both of the new specimens were about the size of elephants, and they appear related to other species that lived about 10 million years later. One of the new finds, called Eocarcharia dinops or "fierce-eyed dawn shark," was a forerunner to an even larger predator, Carcharodontosaurus, that was as big and nasty-looking as Tyrannosaurus rex.

"This is an important slice in geological time, and we don't yet fully comprehend how dinosaurs on the southern continents were evolving then," said Peter Makovicky, curator of dinosaurs at the Field Museum, who was not part of the U. of C. team.

He called the new discoveries "an important data point toward a deeper understanding of what happened."

The "fierce eyes" of Eocarcharia refer to its bony eyebrows, which may have protected the beasts' eyes as they butted their brows together, perhaps as part of mating contests. All of the specimen's primary teeth had eroded away, so Sereno's team drilled into the upper jaw to reveal teeth that had not yet emerged. The large, sharp teeth were well-suited to grabbing live prey and tearing it to shreds.

"This guy would have been slicing and ripping off limbs," Sereno said.

The other new species, called Kryptops palaios, had relatively small teeth that were probably better-suited to gnawing dead animals than killing live ones, Sereno said. It also had a hard covering over its snout, which could have helped it dig into carrion.

In addition to studying the individual species, Sereno wants to figure out where they fit within the broader African ecosystem at the time. Fossilized plants in that part of the Sahara suggest that the dinosaurs lived in a lush flood plain.

They would have had to share the region with several other imposing predators Sereno's group has studied, including 40-foot-long crocodiles and a huge fish-eating dinosaur with a crocodile-like head. Each of those animals carved out its own evolutionary niche of diet and feeding strategies, much as lions, cheetahs and hyenas do today.

For example, Eocarcharia would have been too slow to run down the speediest prey, but it may have lain in wait and ambushed other dinosaurs -- the same strategy modern lions use.

The progression of smaller dinosaurs like Eocarcharia to the gargantuan Carcharodontosaurus suggests a steady evolutionary trend that resulted in larger predators, similar to the separate path that led to T. rex in the region that is now the American West.

Species that evolve successful body plans can grow larger until they hit a natural limit, Sereno said. Tyrannosaurus and Carcharodontosaurus were the largest predators of their day, but since the dinosaurs went extinct 65 million years ago, mammalian predators have never gotten much bigger than polar bears. No one knows the exact reason dinosaurs and mammals differ in that way.

Sereno's group found both new species during an especially productive expedition in 2000 to the desert of Niger. His basement storage and preparation rooms are stacked with large rocks containing fossils his group has not yet extracted, and half-finished preparations of some new and strange-looking species not yet ready for official release. "We have not released even half of all that we found there," Sereno said, chuckling.

Ultimately paleontologists hope that collecting more dinosaur fossils from a range of places will offer clues about the sequence in which the southern continents broke off from one another. But some of the puzzle may remain hidden for centuries, locked in Antarctic fossils that are buried beneath a mile of ice.

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jmanier@tribune.com

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