Scientists have unearthed in Canada's province of Nova Scotia the skull of a creature dating to about 307 million years ago that is one of the oldest-known plant-eating land vertebrates, representing a pivotal juncture in animal evolution.
The creature, named Tyrannoroter heberti, possessed a somewhat triangle-shaped skull that facilitated large cheek muscles for eating tough plant material, with a mouth full of specialized teeth to crush, shred and grind vegetation.
Tyrannoroter looked like a reptile but is not actually included in that class of animals, instead categorized as part of a group called microsaurs. While the researchers have discovered only its skull, they estimate based on the anatomy of related animals that Tyrannoroter was about 12 inches (30.5 cm) long, with a stocky build akin to today's blue-tongued skink.
It lived during the Carboniferous Period and was among the early members of a broad lineage of four-legged land animals called tetrapods that were forerunners to today's amphibians, reptiles, mammals and birds.
These pioneering tetrapods evolved from fish with fleshy limb-like fins that became the first vertebrates - animals with backbones - to emerge onto land. The oldest tetrapod skeletal fossils date to roughly 375 million years ago.
The earliest tetrapods were meat-eaters. Over time, some apparently began eating insects and then, as Tyrannoroter illustrates, turned to plants.
"This is highly important because it means that the essential components of the terrestrial ecosystems we recognize today - as herbivore-dominated - have been around and maintained since the Carboniferous Period," said paleontologist Arjan Mann of the Field Museum in Chicago, co-lead author of the research published on Tuesday in the journal Nature Ecology and Evolution.
An animal called Melanedaphodon that lived around the same time, with fossils found in Ohio, has been interpreted by scientists as consuming softer plants alongside other foods like insects. Tyrannoroter also may have eaten insects, the researchers said, but its skull was better adapted than that of Melanedaphodon for processing tough plants.
"Tyrannoroter is the earliest and most complete vertebrate land herbivore to show adaptations that could process high-fiber plant material," Mann said.
Extensive and lush forests, the fossilized remains of which now account for most of the world's coal, flourished during the Carboniferous. Eventually, the tetrapods decided to have a salad for their meal, and evolved teeth and cheek muscles like those of Tyrannoroter to make it happen.
"Basically, 'How can I get energy from different types of food?' And the abundant plant matter in the mangrove-like forests of the Carboniferous Period was a good opportunistic solution," Mann said.
Tyrannoroter's skull was about 4 inches (10 cm) long.
"The skull is very robust," said paleontologist and study senior author Hillary Maddin of Carleton University in Ottawa.
"The traits that indicate herbivory include its downturned snout, angled optimally for snipping at low-lying plants, large chambers for housing powerful muscles for processing plants, and most importantly, it has a mouth full of opposing dental fields - one on the palate (roof of the mouth) that occludes (fits together) with another on the lower jaw," Maddin said. "These dental batteries are seen in other herbivorous animals."
To view the inside of the skull, the researchers examined it with a CT scan, and discovered the dozens of conical teeth on the roof of its mouth.
The genus name Tyrannoroter means "tyrant digger," reflecting its relatively large size for its time and the fact it is believed to have been a burrowing animal. Its species name heberti honors research collaborator Brian Hebert, who found the skull in a rocky cliff on Cape Breton Island along Nova Scotia's Atlantic coast.
It long was thought that the first true vertebrate herbivores did not appear until the twilight of the Carboniferous, which ended about 299 million years ago.
"This discovery reveals vertebrate animals radiated into modern-like niches, including herbivory, much more quickly than we thought," Maddin said.
Insect-eating may have been a key step before plant-eating.
"This paper furthers the idea that insectivory was likely a preadaptation for herbivory, and by eating early herbivorous insects, tetrapods secondarily acquired the gut flora needed to process plant material," Mann said.
