Dung Fossils Suggest Dinosaurs Ate Grass

Nicholas Bakalar
for National Geographic News

November 18, 2005

Ancient pieces of plant minerals have offered up the first evidence that dinosaurs ate grass, a new study says.

The proof was found in what might seem an unlikely location: fossilized dung left by titanosaurs.

The coprolites—the technical, and polite, term for dung fossils—were found in India and date to about 65 million years ago.

Evidence of ancient plants is often found in fossils that contain outlines of easily visible leaves and stems. Such fossils of grasses have been dated to about 55 million years ago but no older.

The plant evidence found in coprolites, however, is based on microscopic bits of minerals that form in plants. When plants are eaten or decay, the mineral bits are released and pass through an animal's digestive system.

Researchers were able to examine and date minerals from ancient grasses found in the fossilized dinosaur dung. The scientists describe the fossils in tomorrow's issue of the journal Science.

Their work "is the first unambiguous evidence that [grasses] originated and had already diversified during the Cretaceous," Dolores Piperno and Hans-Dieter Sues of the Smithsonian Institution's National Museum of Natural History wrote in a review accompanying the journal paper.

The Cretaceous period extends from 145.5 to 65.5 million years ago.

Grassy Dino Diets?

The fossils containing the plant minerals were found close to Pisdura in central India and date within the late Cretaceous.

Coprolites are very common in the area and are often found in rocks that have been worn down by weather. Based on their common association with titanosaur bones, many of the dung fossils probably come from the massive plant-eating reptiles.

The finding is the first indication that grasses evolved before the dinosaurs went extinct.

Until now "it has been assumed that dinosaurs lived in virtually grass-free ecosystems," said Caroline A.E. Strömberg.

Strömberg is a coauthor of the paper and a researcher at the Swedish Museum of Natural History in Stockholm.

Grasses exist today on every continent except Antarctica, and many animals—including humans—depend on them for food.

Scientists have long believed that the now ubiquitous plants first began to spread and diversify some 70 to 60 million years ago.

Fossil evidence had suggested that grasses evolved along with early plant-eating mammals. Hoofed animals with high-crowned teeth suitable for chewing grass first began to appear about 25 million years ago.

But the grass minerals in the Indian coprolites were much older than the hoofed mammals and were already diverse. Five different species were evident, which means that grasses likely diversified substantially before the end of the late Cretaceous.

The researchers believe that various species of grass had spread before India became geographically isolated from other continents about 125 million years ago.

Tooth Evolution

Along with the grass minerals, the coprolites contained evidence of other plants, including broad-leaved flowering plants, palms, and conifers.

The grasses formed a relatively low proportion of the total plant material found in the coprolites, which indicates that they did not form the major part of the titanosaurs' diet.

Still, the inclusion of grass in the dinos' diets might mean that widespread occurance of grasses contributed to the titanosaurs' success, said Kenneth Carpenter, a curator at the Denver Museum of Nature and Science.

"Cropping low vegetation, such as grass, may explain why some of these titanosaurs have very wide mouths reminiscent of the wide mouth of the grass-feeding hippo and white rhino," he said.

Mammals called gondwanatherians also lived during the late Cretaceous and might have been more dependent on grasses for food.

The early mammals had high-crowned teeth, which puzzled paleontologists, because it was thought that grasses were quite rare at this time.

Some researchers felt the teeth must have evolved for digging or gnawing on wood.

"Our study shows that grasses existed in India simultaneously with the … gondwanatherians," Strömberg said.

"These remarkable results will force reconsideration of many long-standing assumptions about grass evolution, dinosaurian ecology, and early plant-herbivore interactions," Piperno and Sues wrote in their review.

But, she added, "I think it is fair to say that the classic hypothesis is still that [hoofed mammals] evolved [high-crowned teeth] during the Tertiary, primarily in response to the spread of grasslands."

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