My, what sharp teeth

December 15, 2020

Gorgonopsians were the apex predators of their day 250 million years ago. These forerunners to mammals evolved from cat-sized animals to bear-sized hunters during the middle and late Permian Period. But what made them truly formidable was the make-up of their iconic teeth.

Gorgonopsians were the first saber-toothed animals. Their canines extended up to 13 centimeters and had serrations to tear into prey the way a person uses a steak-knife to cut meat. That imposing dental feature helped make them dominant carnivores before they went extinct. A new Harvard-led study bolsters that fearsome reputation by taking a closer look at those saber-like teeth.

The work published Wednesday in Biology Letters.

It shows that the serrations on the teeth of gorgonopsians were made of a complex arrangement of tissues called enamel and dentine. These tissues folded along the cutting edges of the blade-like teeth to form a series of tightly packed serrations.

This type of complex tooth serration made of enamel and dentine was thought to be unique to the masters of carnivory -- the therapod dinosaurs, which includes the revered tyrannosaurus rex. This new analysis, however, shows dinosaurs and gorgonopsians developed this specific cutting tooth independently. Not only that, the study shows gorgonopsians, a lineage more related to humans than dinosaurs, actually did it first.

"Our finding shows that it happened more on the mammal line as opposed to the reptile line and that it happened much earlier than we previously thought since these animals are much older than the theropod dinosaurs," said Megan Whitney, a postdoctoral researcher in the FAS Department of Organismic and Evolutionary Biology. "What we're seeing is this really cool instance of convergent evolution."

The finding is significant because of the timing and complexity involved. The fact that this happened in gorgonopsians means it occurred early in in amniote evolution. Amniotes include reptiles and mammal lineages.

"It's a very complicated way to build a serration because it involves two different tissues making really complicated folds," Whitney said. "Typically, we associate that with time, right? The more time you have for evolution to take place, the more we think there's the ability to evolve complicated traits. What we're finding here is that actually these animals early in amniote evolution were able to evolve a very complicated and specialized structure."

Whitney worked with Aaron LeBlanc of King's College in London, Ashley Reynolds from the University of Toronto, and Kirstin Brink from the University of Manitoba in Canada on the study.

The project started because of a mistake. The group was cutting up fossilized teeth to study the tissues at the microscopic level. Working on one of the gorgonopsian canines, Whitney accidently cut through a serration. One of the co-authors examined it on a whim and realized it looked like a theropod tooth. The group thought they were onto something and decided to dive deeper.

Teeth with serrations are known as ziphodont dentitions. The team compared the serrations in gorgonopsians to carnivores who also had these dentitions. The comparison group included a theropod dinosaur and other synapsids. Synapsids are a group of animals that include both modern mammals and their predecessors like the gorgonopsians.

The gorgonopsian teeth appeared similar to other synapsid teeth, but under a microscope looked more dinosaur-like. For instance, one of the comparison synapsids was Smilodon fatalis, better known as the saber-toothed tiger. While both Smilodon and gorgonopsians had saber-tooths with serrations, those serrations were different and so were the way they used them. Smilodon's serrations were made entirely of a thick enamel. This helped led to different feeding strategies. Gorgonopsians tore flesh with their long canines, Smilodon mainly used them to dispatch prey.

Comparing gorgonopsians with the therapod dinosaur, the scientists saw they had the same interdental folds made of enamel and dentine. This setup helps strengthen the teeth and last longer in the mouth, making the animal a more efficient eater. It's reflected in their puncture and pull feeding style to cut through prey more easily.

"These animals have all evolved serrations, but the microscopic ways in which they do so may differ," said Reynolds, a graduate student at the University of Toronto. Ultimately, it shows "there's more than one way to skin a cat [or in this case] a cat's prey."

The researchers have spent the past decade been working on trying to understand how complex features of teeth have evolved in ancient species. While findings like this are satisfying, they believe there is a lot more to be discovered.

"It shows us that there's a lot of complicated traits that have evolved earlier than we thought," Whitney said. "Broadly, the group is really interested in looking at these hidden features and trying to shed light on a more complicated evolutionary history in the evolution of teeth than I think we've previously recognized."

Harvard University

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