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Early Land Vertebrates Chewed Their Way to the Top

herbivore
Image: Courtesy of ROBERT REISZ/University of Toronto

To take over the land, our vertebrate ancestors first had to master a task most of us take for granted: chewing food before swallowing. The earliest land-dwelling plant eaters, which appeared some 290 million years ago, simply ripped leaves off plants and gulped them down whole. But at some point herbivores evolved a more efficient way of dining, using their teeth and jaws instead of their guts to process plant matter¿a development that enabled them to include tough, high-fiber plants in their diets. Now new research indicates that this key innovation had emerged by 260 million years ago. According to a report published today in the journal Nature, the remains of a small mammal-like reptile from Central Russia represent the earliest evidence for the anatomy necessary for chewing.

Unearthed in 1990 from deposits dated to the Upper Permian period, the fossil¿dubbed Suminia getmanovi (right)¿is not itself a new discovery. Rather, the findings come from detailed analyses of its jaws and teeth. Specifically, wear patterns on Suminia's jaw joint indicate that the creature employed a so-called power stroke during eating, and electron microscope imaging shows that the teeth bear the horizontal scratches caused by shearing motion. Together, these observations reveal that Suminia had an oral apparatus made for grinding up tough plants.

Intriguingly, chewing may have set the stage for other important innovations. "Chewing is particularly important because if an animal can more efficiently chew its food, it can digest more quickly and increase its rate of food intake," University of Toronto paleontologist Robert Reisz a co-author of the study, explains. "Such increased intake could have supported an elevated metabolism, similar to mammals." Indeed, Reisz notes that among living animals, it is mainly mammals that chew their food extensively, particularly when it comes to plants.

"There is a link between the time when land-dwelling herbivores started processing food in the mouth and a great increase in animal diversity," Reisz remarks. "So you can say that the evolution of the moden terrestrial ecosystem with lots of herbivores supporting a few top predators is based on animals efficiently eating the greenery on land."

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