Chewing on Evolution

Researchers have discovered that a small mammal-like reptile that lived 260 million years ago is the first known efficient land vertebrate chewer—able to use a shearing chewing action to break down tough vegetation.

The finding, the scientists say, provides evidence that the seemingly modest ability to orally process food efficiently allowed animals to digest a wider range of vegetation, sparking the evolution of a diversity of herbivores. This diversity enabled the evolution of the modern terrestrial animal ecosystem, in which abundant herbivores serve as food for a small number of carnivores. Before this evolution, the scientists say, the ecosystem was quite different, with herbivores being very rare and most vertebrates eating either invertebrates or other vertebrates that fed on invertebrates.

In an article in a June issue of Nature, Duke graduate student Natalia Rybczynski and Robert Reisz, professor of zoology at the University of Toronto at Mississauga, report microscopic studies of the teeth of the foot-long reptile Suminia, whose distant relatives eventually evolved into mammals. Suminia predated the dinosaurs (which branched from the vertebrate tree millions of years later), some of whose plant-eating species evolved similar chewing mechanisms.

The scientists’ studies of the teeth of the gangly, big-eyed, large-toothed Suminia found telltale horizontal scratches whose structure revealed that the animal brought its posterior teeth together and created an upward and backward shearing motion, called a power stroke, to shred plant material efficiently.

“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,” Rybczynski says. “Such increased intake could have supported an elevated metabolism, similar to mammals.”

Rybczynski says herbivores without such chewing ability tend to eat the more tender leaves, flowers, or buds of plants. Minimal oral processing of vegetation is also associated with a slower digestion rate. An example is the iguana, which, like other animals that do not chew extensively, simply swallows vegetation and allows it to digest for a long time. “So, Suminia is the best example we have from such an early era of an animal that is adapted to high-fiber herbivory. It was clearly more specialized to eat coarse, fibrous food than anything else of the time.”

The scientists were prompted to study Suminia in part because of anatomical evidence that a close relative, called dicynodonts—the first successful terrestrial plant-eating vertebrate—was also probably efficient at “oral processing” of food. Rybczynksi notes that most dicynodonts could not be said to “chew,” since that term applies only to animals with teeth.

“What is immediately striking about this animal is that it has really large teeth and they occlude, or meet. This is unlike iguanas, crocodiles, and most other non-mammalian vertebrates, whose teeth don’t even touch. Since the teeth occluded, we knew that Suminia had some sort of specialized chewing mechanism,” Rybcynski says.

The scientists performed electron-microscopic studies that revealed the details of striations resulting from shearing of food. Besides the horizontal nature of those striations, the scientists observed that one end of the worn surface showed a deeper “divot” at the junction between the hard surface enamel and the softer dentine than the other. The shape of this dentine surface indicated where sand and other particles had been jammed against the enamel, proving that the motion was backward.

The scientists obtained their specimens from the Paleontological Institute of the National Academy of Sciences in Russia, which receives support from the University of Toronto. Their study was supported by the National Geographic Society and the National Sciences and Engineering Research Council of Canada.