Football-sized fossil creature may have been one of the first land animals to eat its veggies

(Press-News.org) Life on Earth started in the oceans. Sometime around 475 million years ago, plants began making their way from the water onto the land, and it took another 100 million years for the first animals with backbones to join them. But for tens of millions of years, these early land-dwelling creatures only ate their fellow animals, rather than grazing on greenery. In a new paper in the journal Nature Ecology and Evolution, scientists describe the 307-million-year-old fossil of one of the earliest known land vertebrates that evolved the ability to eat plants.

“This is one of the oldest known four-legged animals to eat its veggies,” says Arjan Mann, assistant curator of fossil fishes and early tetrapods at the Field Museum in Chicago and co-lead author of the study. “It shows that experimentation with herbivory goes all the way back to the earliest terrestrial tetrapods—the ancient relatives of all land vertebrates, including us.”

“The specimen is the first of its group to receive a detailed 3D reconstruction, which allowed us to look inside its skull and reveal its specialized teeth, helping us to trace the origin of terrestrial herbivory,” says Zifang Xiong, a PhD student at the University of Toronto and co-lead author of the paper.

The researchers named the new species Tyrannoroter heberti, meaning Hebert’s tyrant digger, in honor of its discoverer, Brian Hebert. The animal’s skull is the only part that scientists have found, but based on the size of its head and the more complete skeletons of its relatives, Tyrannoroter was probably a stocky four-legged creature about a foot long. “It was roughly the size and shape of an American football,” says Mann. By modern standards, that’s not terribly large, but it was one of the largest land-dwelling animals of its time. Tyrannoroter probably looked a little like a lizard, but it lived before the ancestors of reptiles and mammals split off from each other, so it technically wasn’t a reptile.

The team found Tyrannoroter on Cape Breton Island, Nova Scotia, under harsh fieldwork conditions. “Nova Scotia has the highest tides in the world—when we’re working there, we’re racing against the tide, when the ocean comes back in,” says Mann. “It’s very rocky, and the fossils are in cliffs on the shore. Paleontologists hate excavating in cliffs, because the cliff could come down on you.”

Brian Hebert, an avocational paleontologist from Nova Scotia, discovered the small skull in a fossilized tree stump during a field season led by Hillary Maddin, a professor of paleontology at Carleton University. “The skull was wide and heart-shaped, really narrow at the snout but really wide at the back,” says Mann. “Within five seconds of looking at it, I was like, ‘Oh, that’s a pantylid microsaur.’”

The pantylids are a fairly early chapter in the story of vertebrate animals living on land. When lobe-finned fish first evolved limbs that let them scoot onto the land, they still depended largely on their watery homes. “The pantylids are from the second phase of terrestriality, when animals became permanently adapted to life on dry land,” says Mann. They're what scientists call stem amniotes—animals closely related to the group of tetrapods that evolved eggs that could stay dry outside of water. In later years, these stem amniotes would split into reptiles and the early ancestors of mammals.

Mann prepared the specimen by carefully chipping away rock from the fossilized bone, but the skull had fossilized with its mouth closed, and internal structures like its brain case remained hidden. To see inside the skull, the researchers CT scanned it, producing a series of stackable X-ray images to generate a 3D picture.

“We were most excited to see what was hidden inside the mouth of this animal once it was scanned—a mouth jam-packed with a whole additional set of teeth for crushing and grinding food, like plants,” says Maddin, the study’s senior author. These teeth, including ones on the roof of its mouth, hint that our stem amniote tetrapod relatives were eating plants sooner than scientists had previously thought.

“Tyrannoroter heberti is of great interest because it was long thought that herbivory was restricted to amniotes. It is a stem amniote but has a specialized dentition that could be used for processing plant fodder,” says Hans Sues, senior research geologist and curator of vertebrate paleontology at the Smithsonian National Museum of Natural History and co-author of the study.

That’s not to say that Tyrannoroter ate only plants. “When Hans Sues was my advisor during my post-doctoral fellowship at the Smithsonian, he would always say that just about all herbivores alive today consume at least some animal protein, and that herbivory is best seen as a gradient,” says Mann. Tyrannoroter probably ate smaller animals, including insects, in addition to vegetation, and the insect exoskeletons in early tetrapods’ diets may have paved the way for stem amniotes like Tyrannoroter to be able to crush and process tough plant materials. What’s more, digesting the bodies of plant-eating insects may have given early tetrapods the gut flora and microbes they would need to process plants.

In addition to shedding light on the origins of herbivory, the research could also provide insights into what happens when plant-eating animals are faced with the destruction of those plants. Tyrannoroter lived near the end of the Carboniferous Period, when the planet underwent a period of climate change, the last icehouse-to-greenhouse transition since the one we’re currently in. “At the end of the Carboniferous, the rainforest ecosystems collapsed, and we had a period of global warming,” says Mann. “The lineage of animals that Tyrannoroter belongs to didn’t do very well. This could be a data point in the bigger picture of what happens to plant-eating animals when climate change rapidly alters their ecosystems and the plants that can grow there.”

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