Bruce MacFadden, a Columbia-educated paleobiologist, is the curator of mammal fossils at the Florida Museum of Natural History and teaches zoology at the University of Florida in Gainesville. He spoke at the Smithsonian Tropical Research Institute’s Tupper Auditorium on February 3, about why we know so little about the extinct large animals of the isthmus, what we do know about the subject, and what we can find out about area’s vegetation from the fauna’s fossilized remains.
From about 65 million years ago until Panama moved in to plug the gap between continents about 3.5 million years ago, MacFadden explained, South America was a biological island, isolated from other continents and home to an endemic set of animals. South America had different horse-like and giraffe-like mammal orders unknown elsewhere, and marsupials that bore superficial resemblances to distinct North American animals.
From fossils found at the Cucaracha Formation, a Middle Miocene (15-19 million years old) outcropping on the Culebra Cut, paleontologists have since the 1950s been finding the remains of extinct rodents, dogs, bear-dogs, horse, rhinos and deer-like things. All of these fossils are related to North American species, none to the South American animals.
With the opening of the Pliocene Land Bridge 3.5 million years ago, Panama became a crossroads for North American animals moving south and South American animals moving north. It seems that the northerners did better colonizing the south than vice versa, and there are various hypotheses about how and why that happened.
This review of Panama’s fossil record, however, is not the cutting edge of what MacFadden does.
In 1999 the paleobiologist published an article in Science magazine, describing how he had analyzed the radioactive carbon isotopes in the fossilized tooth enamel of six species of five-million-year-old extinct horses dug up at a phosphate mine in Lakeland, Florida. Scientists in his field had long looked at the sizes, shapes and wear marks of fossil animals’ teeth to estimate the species’ diets, but MacFadden’s method held the promise of preserved radiocarbon “signatures” of the sorts of foods the animals ate.
This, then, can take us past the bounds of zoology, into botany, ecology and other fields of inquiry.
For example, there is not just one type of photosynthesis. (Given the DNA work that Dr. J. Craig Venter is doing on microscopic marine life discussed in an article in the last issue’s Science section and all the photosensing genes he and his colleagues have been encountering, it’s likely to turn out that there aren’t even just a few kinds of photosynthesis.) As far as the diets of grazing land animals is concerned, there are two important types of the photosynthesis processes by which light and nutrients taken from the air, soil and rain are transformed into food and plant bulk: C3 and C4.
The C4 plants evolved later than those that use C3 photosynthesis, generally among the dicots, generally on equatorial plains, between five and 30 million years ago. These plants stand up better to hot and dry conditions because their photosynthesis takes place in inner cells near leaf veins rather than cells on the leaf surface and gets its carbon from the soil rather than the air, which factors combine to reduce plant respiration and water loss. The C3 plants, on the other hand, depend on carbon dioxide absorbed through leaf pores, and breathe both in an out.
There arose around the planet great C4 grasslands, and mammals specially adapted to graze on them. But there are also C3 grasses, and woodlands are generally composed of C3 plants.
Now one might guess by a fossil mammal’s skeletal structure and by looking at its teeth whether it reached up into the trees or down to the ground to graze. But because C3 and C4 use different chemical processes to obtain carbon, plants that grow by these processes leave different radiocarbon “signatures” --- which in turn can be picked up in the remains of the animals that fed on them.
So 15 million years ago, when Panama had risen from the Caribbean Sea but hadn’t yet linked up with South America, were we covered with C4 grasslands, or did our extinct mammals live in a canopy forest, which would have been composed of C3 plants?
Dr. MacFadden thinks he may know this shortly, as he’s testing the radiocarbon signatures of fossil teeth found in these parts at the University of Florida’s Stable Isotope Lab.
