This ancient plant-eater had a twisted jaw and sideways-facing teeth

(Press-News.org) In a dry riverbed in Brazil, in a dense forest near the Amazon, a team of paleontologists found a fossilized jawbone from an ancient animal. Over the course of their fieldwork, they found eight similar bones, each around six inches long—but no other bones that they could confidently use to complete a skeleton for one of these mystery animals. However, the jawbones alone were enough to reveal that they belonged to a species that would have been a “living fossil” for its time, 275 million years ago when it lived. What’s more, the jawbones were oddly twisted, with some teeth pointed out and to the sides, and numerous smaller teeth lining the inside of the jaws—a sign that these strange creatures were among the first of their kind to grind up plants for food.

In a new paper published in the journal Proceedings of the Royal Society B, the researchers described the new species, giving it the name Tanyka amnicola. Tanyka is from the local Indigenous Guaraní language, meaning “jaw,” and amnicola means “living by the river.”

“Tanyka is from an ancient lineage that we didn’t know survived to this time, and it’s also just a really strange animal. The jaw has this weird twist that drove us crazy trying to figure it out. We were scratching our heads over this for years, wondering if it was some kind of deformation,” says Jason Pardo, the study’s lead author, who worked on this project as part of his post-doctoral fellowship at the Field Museum in Chicago. “But at this point, we’ve got nine jaws from this animal, and they all have this twist, including the really, really well-preserved ones. So it’s not a deformation, it’s just the way the animal was made.”

Tanyka is part of a much larger group of animals called tetrapods. Tetrapods are four-legged animals with backbones; modern tetrapods include reptiles, birds, mammals, and amphibians. The oldest tetrapod lineage, called the stem tetrapods, eventually split into two groups—ones that laid eggs outside of water, and ones that laid their eggs in the water. Today’s reptiles, birds, and mammals are all descendants of the branch that laid watertight eggs on land; modern amphibians like frogs and salamanders are the relatives of the tetrapods whose eggs needed to remain moist.

But even after the tetrapod family split into these new groups, some of the stem tetrapods remained. Tanyka was one of them.

New members of an older lineage can evolve alongside their more “advanced cousins”—the mammal family is a good example. The first mammals laid eggs, but some mammals evolved the ability to give birth to live young. While most of the mammals alive today, whether marsupials like kangaroos or placental mammals like humans, reproduce this way, there are still a couple mammals, like platypuses, that are members of the older, egg-laying lineage.

“In the sense that Tanyka was a remaining member of the stem tetrapod lineage, even after newer, more modern tetrapods evolved, Tanyka is a little like a platypus. It was a a living fossil in its time,” says Pardo, who is now a research associate at the Field Museum while working on a postdoctoral fellowship through the University of Vilnius in Lithuania.

There’s a lot about Tanyka that remains a mystery: namely, its body. “We found these jaws in isolation, and they're really weird, and they're very distinctive. But until we find one of those jaws attached to a skull or other bones that are definitively associated with the jaw, we can't say for sure that the other bones we find near it belong to Tanyka,” says Ken Angielczyk, a curator of paleomammalogy at the Field Museum in Chicago, who served as Pardo’s advisor during his post-doctoral fellowship there, and a co-author of the paper.

“We can say, by comparison with close relatives, that Tanyka might have looked kind of like a salamander with a slightly longer snout,” says Pardo. The researchers aren’t sure how big Tanyka would have been, but they estimate that it might have been up to three feet long, and it probably lived in lakes, based on the kind of rocks in which the fossils are found

But Tanyka’s jawbone alone was enough to show scientists what an unusual animal it was.

Run your tongue over the teeth on your lower jaw. Feel how the tops of your teeth are facing up, towards the roof of your mouth? In Tanyka, the lower jaw was twisted, so that instead of facing up, the teeth pointed out to the sides. Meanwhile, the part of its jawbone that, in us, faces the tongue, in Tanyka is facing up towards the roof of its mouth. This surface of Tanyka’s jawbone is covered in a series of smaller teeth called denticles, which form a grinding surface sort of like a cheese grater.

Scientists have yet to find the bones that would make up Tanyka’s upper jaw, but they imagine its top teeth and denticles were oriented similarly to the ones on the lower jaw. “We expect the denticles on the lower jaw were rubbing up against similar teeth on the upper side of the mouth. The teeth would have been rasping against each other, in a way that’s going to create a relatively unique way of feeding,” says Pardo.

In general, teeth that are able to grind against each other are used for crushing up plant material. “Based on its teeth, we think that Tanyka was a herbivore, and that it ate plants at least some of the time,” says Juan Carlos Cisneros, an author of the paper at the Federal University of Piauí (UFPI) in Brazil. The researchers say that it’s surprising that a stem tetrapod like Tanyka would have evolved to eat plants, since most of its fellow stem tetrapods only ate meat.

Finding out that a stem tetrapod like Tanyka was living when and where it did helps scientists fill in big gaps in the paleontological record. When Tanyka was alive, 275 million years ago, the area that’s now Brazil was part of a supercontinent called Gondwana. It included much of modern South America, Africa, Australia, and Antarctica, but compared with areas in the Global North, very few of its fossil animals from this time period have been discovered in Gondwana.

“The Pedra de Fogo Formation in Brazil is one of the only windows we have into Gondwana’s animals during the early Permian Period of Earth history, and Tanyka is telling us about how this community actually worked, how it was structured, and who was eating what,” says Angielczyk.

The following scientists co-authored this study: Jason Pardo (Field Museum, University of Vilnius), Claudia Marsicano (Universidad de Buenos Aires, CONICET), Roger Smith (Iziko South African Museum, University of the Witwatersrand Johannesburg), Ken Angielczyk (Field Museum), Jörg Fröbisch (Museum fur Naturkunde - Leibniz-Institut fur Evolutions- und Biodiversitatsforschung), Christian Kammerer (North Carolina Museum of Natural Sciences), and Martha Richter (Natural History Museum, London).

 

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