Teeth left behind by extinct Dimetrodon give important clues

Researchers have uncovered the earliest fossil evidence yet for serrated teeth in vertebrates. They jut from the jaws of a top-of-the-food-chain predator whose genus vanished some 40 million years before the first dinosaurs appeared.

Known as Dimetrodon, these creatures and the evolution of their teeth are opening a window on the environment they inhabited during the Permian period, which opened about 290 million years ago and ended with the largest mass extinction in Earth's history some 38 million years later.

Dimetrodons appeared early in the Permian, when the first modern-looking terrestrial ecosystems were taking hold, explains Kirstin Brink, a graduate student and researcher at the University of Toronto who is the lead author of a paper describing the results. The paper was published Friday in the online journal Nature Communication.

With a large head and rounded snout, Dimetrodon species ranged from three feet to more than 10 feet long and weighed up to 660 pounds. They carried a rounded sail on their backs supported by a series of vertical spines. Despite the range of sizes, Dimetrodon species’ consistently massive heads suggest that their teeth evolved in response to changes in the food available, rather than evolving in response to changes in the skull's shape.

"By looking at the variety of tooth shapes, we're actually able to pick out differences in the ecology and the roles different species of Dimetrodon played" over the millions of years that passed as the creatures and their ecosystem evolved, Ms. Brink says.

She and University of Toronto paleontologist Robert Reisz analyzed Dimetrodon fossil specimens spanning much of the genus's 25 million years on Earth.

During this time, Dimetrodons' teeth gradually changed from smooth to serrated. This came as a surprise to the duo, Brink says. The older species had smooth teeth, while younger ones sported the serrations.

The appearance of serrated teeth coincide with the arrival of large-body plant-eating animals in the fossil record – creatures that in some cases were larger than the predators that hunted them.

Over time, the plant-eaters grew in number and diversity. The tooth serrations allowed Dimetrodons to puncture food effectively, while the curve of the teeth suggests they bit and pulled as they ate, rather than crunched. Evidence from the skull and jaws points to large facial muscles, which would have been needed to bite and hold on to animals larger than the Dimetrodons.

If such feasts were unavailable, Dimetrodons were inclined to dig for dinner. Last fall Robert Bakker, a Houston Museum of Natural Science paleontologist, described finding eight fossil remains of a Permian amphibian known as Diplocaulus, a creature whose flattened, horizontal skull took the shape of a broad crescent.

The research team he represented last October at the Geological Society of America's annual meeting in Denver came across a large number of these fossils at a location known as the Craddock Bone Bed in Baylor County, Texas. The eight Diplocaulus were buried in what at the time would have been a burrow. And they were in various states of disrepair. Some were missing tails, others ribs. One had a puncture wound in its skull, which also was missing its rounded snout.

The burrow also contained shed teeth from Dimetrodons.

For Brink and Dr. Reisz, finding the serrated teeth was only one of three surprises they uncovered as they analyzed the Dimetrodon fossils.

This genus also hosts the earliest cusps on teeth, which also would have helped to cut up food as the animals chewed.

"This isn't dominant in teeth until we see the first true mammals," Brink says. Mammals emerged sometime between 225 million and 167 million years ago. Dimetrodon is not a direct ancestor to today's mammals, she adds, but is a very distant cousin.

Finally, by today's standards, the food chain was topsy-turvy.

The fossil evidence shows that a number of meat-eaters shared the same turf as Dimetrodon. Indeed, meat-eaters outnumbered plant-eaters at the time, she says – a stark contrast to ecosystems today, where herbivores are more numerous than carnivores.

Differences in teeth among different groups of carnivores, as well as differences among species of Dimetrodon, indicated that they ate different creatures, so they could share the same habitat, Brink explains. This also runs counter to the patterns ecologists see today, where relatively few species of top predators occupy a region where they would have to share the same prey.