Over 300 million years ago, before dinosaurs roamed the Earth, a small, four-legged creature was foraging for plants in what we now know as Nova Scotia. This newly identified species, Tyrannoroter heberti, is believed to be one of the earliest tetrapods to adopt a plant-based diet, altering our understanding of how terrestrial life transitioned from carnivorous to herbivorous.
Arjan Mann from the Field Museum in Chicago, a co-lead author of the research, stated, "This is one of the oldest known four-legged animals to eat its veggies. It indicates that the exploration of herbivory dates back to the dawn of terrestrial tetrapods, the ancient ancestors of all land vertebrates, including humans."
The findings, published in Nature Ecology & Evolution, are based on a remarkably preserved skull discovered in a fossilized tree stump on Cape Breton Island.
A Skull that Changed the Menu
While plants first colonized land approximately 475 million years ago, animals took much longer to adapt. For nearly a century, early land animals primarily consumed meat and exhibited limited diversity.
Previously, scientists believed that the transition to a plant-based diet occurred much later. However, Tyrannoroter challenges this notion. This creature lived towards the end of the Carboniferous Period and bore a resemblance to modern lizards, predating the divergence between reptiles and mammals. Its name translates to "tyrant ploughman."
Measuring about 10 centimeters in length, the skull of Tyrannoroter is roughly one-third of its total body size. Despite its lizard-like appearance, it existed before the separation of reptiles and mammals. Mann remarked, "The skull was wide and heart-shaped, narrow at the snout but broad at the back. Within moments of examining it, I recognized it as a pantylid microsaur."
A What?
Pantylids are part of an early microsaur lineage. Contrary to their name, microsaurs are not tiny dinosaurs.
These small, four-legged pioneers, resembling lizards, belong to a side branch of the family tree distinct from the lineage leading to modern reptiles, birds, and mammals. They developed robust skin and sturdy bodies to thrive in arid inland forests.
Researchers utilized high-resolution CT scans to investigate the skull's interior. The scans revealed unexpected features: dense clusters of teeth arranged in opposing "dental batteries" capable of shearing and grinding tough plant material.
"We were thrilled to uncover what was concealed within this animal's mouth after scanning--it was filled with an additional set of teeth designed for crushing and grinding food, particularly plants," explained Hillary Maddin from Carleton University, the study's senior author.
Rise of Herbivory
The fossil provides crucial insights into vertebrate evolution. The skull's structure indicates that Tyrannoroter could process high-fiber vegetation. Similar features have been identified in related fossils dating back as far as 318 million years, suggesting that herbivory spread rapidly among early tetrapods.
"Tyrannoroter is the earliest and most complete vertebrate land herbivore exhibiting adaptations for processing high-fiber plant material," Mann stated.
However, this creature was likely not a strict herbivore. Like many modern plant-eaters, it probably supplemented its diet with insects or other small prey. Crushing insect exoskeletons may have even set the evolutionary stage for plant consumption, while plant-eating insects could have introduced gut microbes capable of digesting cellulose.
The ecological implications are significant. Herbivory played a vital role in establishing the layered food webs that characterize modern terrestrial ecosystems. Discovering such behavior earlier in the fossil record suggests that these ecosystems began forming sooner than previously thought.
Mann concluded, "At the end of the Carboniferous, rainforest ecosystems collapsed during a period of global warming. The lineage to which Tyrannoroter belongs faced challenges. This could provide valuable insights into how plant-eating animals respond to rapid climate changes and the impact on the vegetation they rely on."