307-Million-Year-Old *Tyrannoroter heberti* Fossil Reveals Early Terrestrial Herbivory

For tens of millions of years following animals’ first steps onto land, almost every vertebrate was a predator. Plant food, including leaves and stems, was plentiful, yet early terrestrial animals mostly left it untouched.

When, then, did the first animals with backbones start to eat plants? A recently described Nova Scotia fossil is shedding light on this long-standing mystery.

Researchers have identified a 307-million-year-old skull from one of the earliest known land vertebrates able to process plant matter. The find moves the origins of terrestrial herbivory further into the past.

It indicates that plant-based diets started to evolve far earlier than scientists had previously thought, providing an unusual view of a pivotal change in the history of life on land.

Early life leaves oceans

Life originated in the sea. Plants gradually began colonising land about 475 million years ago, and vertebrate animals followed roughly 100 million years later. These early four-limbed animals are known as tetrapods.

Initially, nearly all of them ate meat. Very few showed evidence of consuming plants for tens of millions of years.

Scientists had thought that herbivory, or plant-eating, developed later among animals that laid their eggs on land. The newly discovered fossil calls that interpretation into question.

“This is one of the oldest known four-legged animals to eat its veggies,” said Arjan Mann, an assistant curator 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.”

Meet Tyrannoroter heberti

The team gave the species the name Tyrannoroter heberti, meaning Hebert’s tyrant digger. It honours Brian Hebert, who found the fossil.

Only the skull has been recovered. Judging by the size of its head and the skeletons of related animals, Tyrannoroter was probably around 30 cm long. “It was roughly the size and shape of an American football,” said Mann.

Although that seems small by modern standards, it was among the biggest land animals of its era. It likely resembled a lizard to some extent. However, it existed before reptiles and mammals diverged into separate groups, meaning it was not actually a reptile.

Tyrannoroter was a member of the pantylids, a group from an early phase of life on land. Pantylids appeared during the second stage of terrestrial adaptation, when animals became permanently suited to living on dry land.

Researchers describe animals of this kind as stem amniotes. Stem amniotes later separated into reptiles and the ancestors of early mammals.

Finding Tyrannoroter heberti

The skull was recovered on Cape Breton Island, Nova Scotia, where fieldwork can be hazardous.

“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,” said 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.”

During a field season, Brian Hebert, an avocational palaeontologist from Nova Scotia, discovered the small skull inside a fossilised tree stump.

“The skull was wide and heart-shaped, really narrow at the snout but really wide at the back. Within five seconds of looking at it, I was like, ‘Oh, that’s a pantylid microsaur,'” said Hebert.

Scanning the ancient skull

Once the fossil had been found, Mann carefully removed the surrounding rock from the bone. Because the mouth was still closed, its internal features could not be seen.

The researchers addressed this by using CT scanning, a technique that produces detailed X-ray images to create a three-dimensional model.

This is the first specimen from its group to receive a detailed 3D reconstruction. It enabled the researchers to inspect the skull’s interior, uncover its specialised teeth and investigate the origins of terrestrial herbivory.

Plant-crushing dental tools

“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,” said study senior author Hillary Maddin, a professor of paleontology at Carleton University.

Some of its teeth even grew from the roof of its mouth. These features would have helped it break down tough plant material.

Tyrannoroter heberti is especially important because herbivory had long been considered limited to amniotes. While it was a stem amniote, it had specialised teeth capable of processing plant matter.

Tyrannoroter heberti also ate insects

Tyrannoroter was probably not dependent entirely on plants. Studies suggest that most herbivores eat at least a small amount of animal protein, so herbivory is more accurately viewed as a dietary gradient than a rigid category.

Alongside vegetation, this animal probably consumed insects and other small creatures.

Breaking insect shells may also have assisted early tetrapods in evolving stronger teeth and the gut microbes required to digest plants more effectively.

Herbivores face climate stress

Tyrannoroter lived towards the close of the Carboniferous Period, when Earth experienced substantial climatic change. Rainforests collapsed before a period of global warming followed.

“At the end of the Carboniferous, the rainforest ecosystems collapsed, and we had a period of global warming,” said 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.”

The fossil reveals more than an ancient diet. It also provides indications of how plant-eating animals respond when the climate changes. Examining ancient life gives scientists insight into the pressures faced by modern species today.