Ancient "Chewing" Reptile Linked With Leap in Animal Diversity

National Geographic News
June 7, 2001
Researchers have discovered the first known example of a land-based vertebrate that had the ability to fully chew and digest plants for food. This trait is important because it enables animals to break down and efficiently process many different kinds of vegetation.

The development of a sophisticated chewing ability paved the way to the emergence of a wide variety of plant-eating animals, say the researchers, who reported their findings in the June 7 issue of the journal Nature.

The evidence came from the fossilized skull of what the scientists said was a gangly, big-eyed, large-toothed land-dwelling reptile, called Suminia getmanovi. It lived 260 million years ago—about 50 million years before dinosaurs.

"The real boost in the success of vertebrates on land started with the ability to process plant material efficiently," said Robert Reisz, a professor at the University of Toronto in Mississauga. His collaborator and co-author of the paper in Nature was Natalia Rybczynski, who is now at Duke University.

The research was funded by the National Geographic Society and the Natural Sciences and Engineering Research Council of Canada.

Changing Ecosystem

The advent of chewing as seen in Suminia paved the way to the first great burst of diversity among terrestrial herbivores, according to the researchers. This diversity is mirrored today in the ecosystem of land-based animals that evolved over time.

The modern-day animal kingdom has many herbivores, which serve as food for a much smaller number of carnivores. Plant-eating species such as gazelles and antelope are abundant, for example, while carnivores such as lions and leopards are relatively much more scarce.

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

Before this evolution of diversity, the scientists explained, the ecosystem was very different.

According to Reisz, the first terrestrial herbivore appeared on land about 290 million years ago. But herbivores at that time had a more rudimentary style of eating. They tore the leaves off the plant and swallowed them whole, and the leaves were then processed in the animals' guts.

Suminia, however, developed a much more advanced way of eating. Microscopic analysis of the reptile's teeth revealed marks indicating that Suminia used its back teeth to chew with a shearing motion that enabled it to shred plant material more effectively.

The researchers said the skull, which was discovered in 1990 in central Russia, looks like that of a monkey or a rodent, with huge eyes and large, distinctive teeth. Dating techniques indicate the animal lived in the Paleozoic era, about 260 million years ago.

"Suminia is the best example we have from such an early era of an animal that is adapted to high-fiber herbivory," said Rybczynski. "It was clearly more specialized to eat coarse, fibrous food than anything else of the time."

Common Trait of Mammals

Developing a more advanced system of chewing enabled ancient herbivores and their descendants to increase their intake of food and process it more efficiently. By shredding leaves into small bits before swallowing, they can absorb the maximum amount of energy and nutrients in the plants they eat.

A class of reptiles related to Suminia, called dicynodonts, were the first successful land-dwelling and plant-eating vertebrates, which means they were probably also efficient in processing their food, the researchers said. But dicynodonts had beaks rather than teeth, so they technically were not able to chew.

In contrast, Suminia's teeth were highly pronounced and similar to the teeth of some plant-eating reptiles and dinosaurs.

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

Today, mammals are the animals that chew their food, particularly plants, most intensively. Suminia was distantly related to a class of reptiles that eventually evolved into mammals.

The researchers speculate that an advanced chewing ability is tied to the "warm-blooded" nature of mammals (and perhaps dinosaurs).

To maintain a high body temperature and high metabolism, mammals need an efficient way to digest and absorb nutrients from food. In contrast, "cold-blooded" animals, such as modern-day lizards and many of their ancient reptile ancestors, do not have the same energy requirements.

Herbivores that lack a well-developed chewing ability tend to eat the more tender leaves, flowers, and buds of plants, Rybczynski said. Minimal oral processing of vegetation is associated with a slower rate of digestion. The iguana, for example, swallows vegetation and allows it to digest for a long time.

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