Ancient reptile is efficient chewer

DURHAM, N.C. -- 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.

This finding, the scientists said, 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, said the scientists, 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 the June 7 issue of Nature, Duke University 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 said. "Such increased intake could have supported an elevated metabolism, similar to mammals." By contrast, Rybczynski said, 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, she said. An example is the iguana, which like other animals that do not chew extensively, simply swallow vegetation and allow it to digest for a long period of time, she said. "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," said Rybczynski.

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

Anatomically, both Suminia and the dicynodonts had jaw muscles and jaw joints that supported a sliding motion. However, since the keratinous beaks of dicynodonts did not survive in the fossil record, there was no way to firmly establish efficient oral processing in those animals. Suminia did have teeth that are similar in shape to those of some plant-eating reptiles and dinosaurs, suggesting that Suminia also fed on plants, Rybczynski said.

"What is immediately striking about this animal is that it has really large teeth and they occlude, or meet," she said. "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."

The scientists performed electron microscopic studies that revealed the details of striations resulting from shearing of food. Besides the horizontal nature of those striations, Rybczynski noted, 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. The scientists' study was supported by the National Geographic Society and the National Sciences and Engineering Research Council of Canada.

Note to editors:
Natalia Rybczynski may be reached at (919) 660-7384. E-mail: nr3@duke.edu. Images to accompany the story are available at http://photo1.dukenews.duke.edu/pages/Duke_News_Service/ in the directory "chewing." Note that the images shows Natalia Rybczynski with a model of a Suminia skull that is four times actual size.