200 million-year-old self-segregation

BROWN (US) — Just like buying a house today, climate and location were major factors in where animals lived 200 million years ago.

Even without geographic barriers like mountains and ice caps, animals living on Pangaea, a continent with the landmass equivalent of Earth, confined themselves to specific regions.

Studying a transect of Pangaea stretching from about three degrees south to 26 degrees north (a long swath in the center of the continent covering tropical and semiarid temperate zones), scientists determined that reptiles, represented by a species called procolophonids, lived in one area, while mammals, represented by a precursor species called traversodont cynodonts, lived in another. Though similar in many ways, their paths evidently did not cross.

“We’re answering a question that goes back to Darwin’s time,” says Jessica Whiteside, assistant professor of geological sciences at Brown University. “What controls where organisms live? The two main constraints are geography and climate.”

The frequency of rainfall along lines of latitude directly influenced where animals lived, according to a new study, published online in the journal Proceedings of the National Academy of Sciences.

In the tropical zone where the mammal-relative traversodont cynodonts lived, monsoon-like rains fell twice a year. But farther north, in the temperate regions where the procolophonids predominated, major rains occurred only once a year. It was the difference in the precipitation that sorted the mammals’ range from that of the reptiles.

The scientists focused on an important physiological difference between the two: how they excrete. Mammals lose water when they excrete and need to replenish what they lose. Reptiles (and birds) get rid of bodily waste in the form of uric acid in a solid or semisolid form that contains very little water.

On Pangaea, the mammals needed a water-rich area, so the availability of water played a decisive role in determining where they lived. “It’s interesting that something as basic as how the body deals with waste can restrict the movement of an entire group,” Whiteside says.

In water-limited areas, the reptiles had a competitive advantage over mammals. The reptiles didn’t migrate into the equatorial regions because they already had found their niche.

Using samples collected from lakes and ancient rift basins stretching from modern-day Georgia to Nova Scotia, researchers compiled a climate record for Pangaea during the late Triassic period, from 234 million years ago to 209 million years ago, when it was something of a hothouse.

Temperatures were about 20 degrees Celsius hotter in the summer, and atmospheric carbon dioxide was five to 20 times greater than today. Yet there were regional differences, including rainfall amounts.

The rainfall gap is based on variations in the Earth’s precession, or the wobble on its axis, coupled with the eccentricity cycle, based on the Earth’s orbital position to the sun.

The skull of the procolophonid Hypsognathus was found in Fundy basin, Nova Scotia, which was hotter and drier when it was part of Pangaea. Mammals, needing more water, chose to live elsewhere. (Credit: Brown U.)

Together, these Milankovitch cycles influence how much sunlight, or energy, reaches different areas of the planet. During the late Triassic, the equatorial regions received more sunlight, thus more energy to generate more frequent rainfall. The higher latitudes, with less total sunlight, experienced less rain.

The findings are important, Whitesaid says, because climate change projections show areas that would receive less precipitation, which could put mammals there under stress.

“There is evidence that climate change over the last 100 years has already changed the distribution of mammal species,” says Danielle Grogan, a graduate student in Whiteside’s research group. “Our study can help us predict negative climate effects on mammals in the future.”

Researchers from Rutgers and Columbia University contributed to the study, that was funded by the National Science Foundation and the Richard Salomon Foundation.

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