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What big teeth that plant-eater had!

Juan Cisneros

An illustration by Brazilian paleontologist Juan Carlos Cisneros depicts the mammal-like creature known as Tiarajudens eccentricus displaying its teeth to scare off a carnivorous dinocephalian.

Back in 260 million B.C., the mammal-like creature known as Tiarajudens eccentricus looked as fearsome as any predator —possessing rows of teeth that went all the way back into its palate, with two saber teeth sticking out in the front. But paleontologists say this dog-sized monster lived on a strictly vegetarian diet. So why did it have all those menacing choppers?

In this week's issue of the journal Science, researchers from Brazil and South Africa examine the strange case of Tiarajudens, a newly identified type of distant mammalian relative known as a therapsid, and they go on to suggest possible solutions to the dental dilemma.

First, about those saber teeth: Although they weren't used for spearing prey, they could have been brandished to keep predators away. Or perhaps the teeth helped chop up the Paleozoic salad fixings before Tiarajudens chewed it up.

Juan Cisneros

A reconstruction shows the head of Tiarajudens eccentricus.

Another possibility is that the teeth were actually used by one male against another in the competition for mates. The researchers noted that musk deer use their own saber teeth for just that purpose. Other types of Paleozoic animals, such as dinocephalians, apparently butted heads to fight over mates. Tiarajudens' teeth may have enabled an alternate form of ritualized combat.

"Some other Paleozoic animals also had enlarged canines, but they were all carnivores. This is the first case of a saber-toothed herbivore at that time," research team leader Juan Carlos Cisneros, a paleontologist at Brazil's Federal University of Piaui, told me in an email. "Other herbivores with long canines appeared much later in the Cenozoic, including fossil deer. ... Some researchers have proposed that these fossil deer (which did not possess antlers) used their canines for male-male combat, like modern saber-toothed deer do."

Cisneros said he and his colleagues from the Federal University of Rio Grande do Sul found the Tiarajudens skull in March 2009  during a field campaign in southern Brazil. During the two years that followed, the research team cleaned the fossilized pieces, glued them together and analyzed how they were put together. The creature's name comes from "Tiaraju," the place in Brazil where the fossil was found; "dens," the Latin word for teeth; and "eccentricus" ... well, you can figure that one out yourself.

In addition to the saber teeth, the bunches of teeth in the palate make Tiarajudens eccentricus "extremely unusual," Cisneros told me.

"I would compare it with modern mammals, which have one row of enlarged teeth for mastication — except that no mammal has teeth in the palate, but the animal is so bizarre that no comparison is perfect!" he said.

Why so many teeth? "Tiarajudens, as far as we know, was the earliest therapsid capable of actual chewing," Cisneros said. "Its teeth are an answer to make possible the digestion of abundant but poorly nutritious food (fiber plants)."

Those molars in the palate provided extra chewing power, plus replacements as the teeth wore down.

Present-day ruminants, such as cows and sheep, also have to do a lot of chewing to digest their similarly high-cellulose fare. In fact, they've developed a complex set of stomach compartments to break down their food for multiple chewings. There's no sign that Tiarajudens had a similar digestive system, however.

Did Tiarajudens pass down any of its dazzling dentition to present-day species? Almost certainly not. "They are dead ends," Cisneros said. But the saber teeth and long rows of molars show how some of the same solutions arise over and over again in the course of evolution.

Juan Cisneros

A fossil from southern Brazil shows the skull of Tiarajudens eccentricus, including the remains of its saber teeth and palatal teeth.

"Both the saber teeth and the enlarged molar-like teeth in the palate represent convergence with other similar animals," Cisneros said. "These traits appeared and disappeared many times in therapsids."

The discovery by Cisneros and his colleagues "provides novel insights into early tooth differentiation" among therapsids, as well as the evolution of plant-eating "and its accompanying complex social interactions," Jörg Fröbisch, a paleontologist at the Humboldt Museum in Berlin, wrote in a Science commentary.

Therapsids have often been called "mammal-like reptiles," but Fröbisch told me that term isn't quite correct. "These animals are more closely related to mammals than to reptiles," he said. So maybe we have more in common with the toothsome Tiarajudens than we might think.

In addition to Cisneros, the authors of "Dental Occlusion in a 260-Million-Year-Old Therapsid With Saber Canines From the Permian of Brazil" include Fernando Abdala, Bruce S. Rubidge, Paula Camboim Dentzien-Dias and Ana de Oliveira Bueno.

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