Clues To Horse Extinctions Point To Gritty Grass, Climate Change

Like the protagonist in an evolutionary detective thriller,Stanley pursued a hunch that apparently had never occurred toother scientists. His long shot hit a bull's-eye, enablingStanley to learn how shifting climate and changing vegetationlikely altered the fate of horses in North America millions ofyears ago.

Stanley, a professor in the Johns Hopkins Department of Earth andPlanetary Sciences, pieced together the findings of otherscientists and connected those data in a way no otherresearchers had done previously. When taken together, the datapaint a picture of how Earth's changing climate and vegetationmay have been directly involved in the dramatic evolutionarytrends of horses and other animals.

The time was marked by the largest extinction rate of NorthAmerican mammals in the last 30 million years; about 60 genera,containing numerous species, perished.

Some scientists believe that changing atmospheric concentrations of carbondioxide could have helped bring about the climatic shifts that Stanleybelieves were directly linked to changes in the vegetation thriving in NorthAmerica. Because the extinctions apparently were brought about by thosechanges in vegetation, the lesson is that carbon dioxide-linked global warming-- if it is occuring again today as many scientists believe -- could have aprofound impact on future extinction rates.

Stanley will touch on his theory during a 1:35 p.m. talk onSunday, Oct. 19, at the start of the annual meeting of theGeological Society of America. The talk will be in the SaltPalace Convention Center, in Salt Lake City, Utah.

Contrary to the popular belief that horses were foreign to theNew World until they were brought here by the Spaniards, theanimals actually evolved in North America, spreading to Europe bycrossing the Bering land bridge that once connected Alaska andSiberia. But they later died out in North America near the end ofthe Ice Age.

Well before their disappearance, however, their life history tookan abrupt turn that killed off all but those horses with thelongest teeth. In fact, numerous other mammals, including camelsand rhinos, suffered the same fate in North America.

Scientists have known that the extinctions were somehow relatedto expanding grasslands and shrinking forests. Grasses possess agritty compound called silica, which is contained in sand and isused to make glass. As animals chew grass, the silica wears downtheir teeth. Therefore, animals with longer teeth live longerbecause their teeth don't wear down as fast, and they cancontinue to feed.

For tens of millions of years, as the Earth's climate becamecooler and dryer, the trend toward expanding grasslands andreceding forests continued in North America. About 13 millionyears ago, the 15 or so species of horses in North America weresplit between those with long teeth and those with shorter teeth.Also at that time, a few new species emerged that had very longteeth.

As grasslands expanded, the horses with long teeth lived longerbecause they were best adapted to eating grasses instead ofleaves. Living longer enabled them to produce enough offspring toguarantee survival of their species and the evolution of newspecies.

By 11 million years ago, only the horses especially adapted toeating grasses -- those with the long and very long teeth -- wereliving in North America.

"Then, there is this sudden event, 6 million years ago, more orless, and what you see is a big extinction pulse, a big drop intotal diversity, and the survivors are all the ones with verylong teeth," Stanley said.

The conventional wisdom has suggested that the long-toothedhorses disappeared because of expanding grasses. But that justdidn't make sense, Stanley said, because the horses with longteeth were especially adapted to eating grasses.

"So, why would more grass be a problem for them?" Stanley asked.

Somehow, something about the grasses must have changed, hereasoned.

Meanwhile, other scientists had discovered that, as the climatebecame dryer and cooler, a different type of grasses began todominate North America. Those grasses, known as C-4 grasses,which thrive in dryer climates, replaced many of the previouslydominant grasses, known as C-3 grasses.

"I thought, well, this seems like a long shot, but I wonder ifthere are on average more silica bodies in the C-4 grasses thanC-3 grasses," Stanley said.

His hunch proved correct. Stanley found that, on average, C-4grasses contain about three times as many of the silica particlesas do C-3 grasses.

"Think about a species that was doing all right eating C-3grasses. Maybe it lived 10 years on average and produced enoughcolts to reproduce the species. Well, what happens if that horseis suddenly only living seven years, or six years? It may notproduce enough colts to perpetuate its species.

"I think that's what happened. I think there was a big grinddown."

The title of his talk is, "Geobiology: Studying the deep historyof the earth-life system."