Scientists in the Canadian province of Nova Scotia have discovered the skull of a creature about 307 million years old, which is among the oldest known terrestrial herbivorous vertebrates and represents a key moment in animal evolution.
The animal, named Tyrannoroter heberti, had a somewhat triangular skull that allowed for powerful cheek muscles for chewing tough plant material, as well as a mouth full of specialized teeth for crushing, splitting, and grinding vegetation.
Tyrannoroter looked like a reptile, but it actually didn't belong to that group of animals - it was classified as a group known as microsaurs. Although the researchers found only a skull, they estimate, based on the anatomy of related species, that Tyrannoroter was about 30,5 centimeters long, with a stocky build, similar to today's blue-tongued skink, Reuters reports.
It lived during the Carboniferous period and was among the early members of a broad lineage of four-legged land animals, the so-called tetrapods, that preceded today's amphibians, reptiles, mammals, and birds.
These pioneering tetrapods evolved from fish with fleshy, limb-like fins, which became the first vertebrates, animals with a backbone, to emerge onto land. The oldest tetrapod skeletal fossils date back to about 375 million years ago.
The earliest tetrapods were carnivorous. Over time, some apparently began to eat insects, and then, as Tyrannoroter shows, turned to plants.
"This is extremely important because it means that the key components of terrestrial ecosystems as we know them today - dominated by herbivores - have existed and been maintained since the Carboniferous period," said paleontologist Arjan Mann of the Field Museum in Chicago, co-author of the study published in the journal Nature Ecology and Evolution.
An animal called Melanedaphodon, which lived around the same time and whose fossils were found in Ohio, is interpreted as a species that ate softer plants along with other foods, such as insects. Tyrannoroter may also have eaten insects, the researchers said, but its skull was better adapted than Melanedaphodon's for processing tough plants, Reuters reports.
"Tyrannoroter is the earliest and most complete terrestrial herbivore among vertebrates to show adaptations for processing fiber-rich plant material," Mann said.
During the Carboniferous period, vast and lush forests flourished, the fossilized remains of which now make up most of the world's coal deposits. Eventually, tetrapods, figuratively speaking, decided to make "salad" their meal - and developed teeth and cheek muscles like those of Tyrannoroter.
"Basically, the question was, 'How do I get energy from different types of food?' And the abundant plant mass in mangrove-like forests in the Carboniferous period was a good opportunistic solution," Mann said.
The skull of Tyrannorother was about 10 centimeters long.
"The skull is very robust," said paleontologist and senior author of the study, Hilary Madin of Carleton University in Ottawa.
"Characteristics that suggest herbivory include a curved, downward-pointing snout, optimally positioned for plucking low vegetation, large cavities to house powerful muscles that process plants, and, most importantly, a mouth full of opposing tooth arrays - one on the palate (the roof of the mouth) that fits with the other on the lower jaw. Such 'tooth batteries' are also seen in other herbivores," Madin explained. She added that, thanks to CT scans, they revealed dozens of conical teeth on the roof of the mouth.
The genus name Tyrannoroter means "tyrant-digger", referring to its relatively large size for the time and the belief that it was a burrowing and burrowing animal. The species name heberti is given in honor of research associate Brian Hebert, who found the skull in a rock on Cape Breton Island, along the Atlantic coast of Nova Scotia.
It was long thought that the first true herbivores among vertebrates did not appear until the very end of the Carboniferous period, which ended around 299 million years ago.
"This discovery shows that vertebrates branched out into 'modern' ecological niches, including herbivory, much more quickly than we thought," Madin said.
Insect feeding may have been an important intermediate step towards herbivory.
"This work further supports the idea that insectivory was likely a preadaptation for herbivory, and that tetrapods, by eating early herbivorous insects, secondarily acquired the gut flora necessary for processing plant material," Mann said.
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