Fossil Discoveries in Madagascar Fill in the Holes in Some Long-Held Evolutionary Theories Robert Vosper While digging through layers of sandy sediment in two rift basins in western Madagascar, an international team of paleontologists led by John Flynn, MacArthur Curator of Fossil Mammals, uncovered two fossil sites teeming with the remains of the long-extinct animals that once ruled this island nation. These animals, some of which have been entombed in the sediment for 230 million years, are helping the scientists gain new insight into the vast array of life forms that inhabited Madagascar during the Mesozoic Era (65 million years to 245 million years ago). And with each new animal they exhume, the scientists are filling in the holes in some long-held theories about the evolutionary history of dinosaurs and mammals. In the fall of 1999, the team — which includes Field Museum paleontologist William Simpson and research associates André Wyss and J. Michael Parrish — published the results of their most significant discoveries in articles in the journals Nature (Sept. 2, 1999 ) and Science (Oct. 22, 1999). In Nature, they described their discovery of a new species of mammal about the same size as a shrew; and in Science, they reported on their discovery of a collection of cynodonts (“mammal-like reptiles”) and the jawbones of two plant-eating dinosaurs. These jawbones, the team argues, might be the oldest-known dinosaur bones ever unearthed. A Triassic Fauna from Madagascar Toward the end of their first of four field seasons in Madagascar in 1996, the team discovered a promising fossil bed in the Morondava Basin, just east of the town of Sakaraha in southwestern Madagascar. Although they didn’t have time to examine the site in detail, Flynn knew immediately that they had stumbled upon something significant. “While we were looking around on the first day, we found the skull of a cynodont sitting on the surface with its eye socket staring up at us,” he explains. “The skull was deeply weathered but still in great condition, so we immediately knew that it was a great site at that point. ” The following year, the team returned to the area and unearthed the 230-million-year-old remains of a variety of cynodonts and true reptiles in the white sand and red silt of an ancient river channel and flood plain. According to Flynn, the remains of these early vertebrates, which included some partial and complete skeletons, are shedding new light on the origins of true mammals. About 330 million years ago, he explains, primitive land vertebrates (amniotes) split into two evolutionary branches: the Reptilia (reptile line) and the Synapsida (the mammal line). During this split, some of the early synapsids were the cynodonts, a sort of physiological hybrid between warmblooded mammals and coldblooded primitive land vertebrates. “These cynodont fossils will help us complete the picture of that evolutionary transformation,” Flynn says. “The fossils are exquisitely preserved, showing a level of detail far superior to anything else from that time.” Only a few hundred yards away from the first discovery, the scientists uncovered the 3-inch-long and 5-inch-long bleached-white jawbones of two previously unknown species of plant-eating dinosaurs. These dinosaurs, the team calculates, are about two million years older than Herrerasaurus, a 228-million-year-old flesh-eating dinosaur from Argentina that for the past decade has held the distinction of being the oldest dinosaur ever found. Unlike Herrerasaurus, which was a ruthless 13-foot-long predator, the two dinosaurs from Madagascar were both prosauropods, gentle kangaroo-sized herbivores that had small heads, long necks and strong hindquarters that allowed them to amble on either two or four legs. Most paleontologists believe that these early dinosaurs either shared a common ancestor with, or were themselves the ancestors to, the mighty sauropod dinosaurs like Apatosaurus that evolved much later. Dinosaurs are divided into two major groups: the Saurischia (the “lizard-hipped dinosaurs” like Brachiosaurus and T. rex) and the Ornithischia (the “bird-hipped dinosaurs” like Triceratops and Stegosaurus). Within Saurischia are the two evolutionary branches known as the theropods, or the meat eaters, and the Sauropodomorphs, or the plant eaters. The prosauropods found by Flynn’s team are early members of the latter group. Although the two dinosaurs from Madagascar are fairly primitive in form, Flynn is quick to point out that they are by no means the ancestral species of all dinosaurs. “Dinosaurs have to be older than this simply because of the fact that you have prosauropods, which tells you that the major dinosaur branches had already split apart. So, the fact that you have a representative of some of the sub-branches of the tree tells you that the root of the tree is deeper in time. Now, how much deeper it goes is hard to tell. But, I think we are getting close.” A Middle Jurassic Mammal from Madagascar A few weeks before discovering the fossil site in southern Madagascar, the team found a much younger fossil bed in the Mahajanga Basin in northwestern Madagascar. After prospecting the site, which is about 450 miles north of Sakaraha, the team scooped up a few hundred pounds of sediment and shipped it back to Chicago for analysis. Over the next few years, Field Museum volunteers Dennis Kinzig, Ross Chisholm and Warren Valsa sifted through the sediment using high-powered microscopes. In the summer of 1998, they struck pay dirt when they uncovered a jawbone about half the size of a piece of rice, complete with three tiny teeth, each no larger than the head of a pin. The scientists have determined that this jawbone belonged to a previously unknown species of a shrew-sized mammal that lived in Madagascar 165 million years ago. In Nature, the team argues that this mammal, which they have named Ambondro mahabo, doubles the age of the oldest-known mammals from the island and shatters the widely held theory that the subgroup of mammals that encompasses most living forms (marsupials and placentals) arose first in the northern hemisphere. “This jaw is the first mammal fossil of any kind found from the southern continents during this time interval,” Flynn says. “And it is much older than any advanced mammal from the north, even though the Jurassic fossil record is much better known from the northern continents.” According to Flynn, Ambondro mahabo represents a group of mammals known as the Tribosphenida that had an advanced set of molars that are characteristic of most modern mammals, including humans. The more primitive forms of mammals, he explains, basically had a bunch of cusps (elevations on the chewing surface of the tooth) on their molars that formed an elongated oval pattern. As mammals evolved, however, the cusps on their upper molars formed a more triangular pattern that, in combination with basin-like platforms that developed on the back of their lower molars, allowed the animals to use a more effective “slice and grind” method of chewing. “While the Jurassic dinosaurian giants grab most of the attention, major evolutionary advances were occurring in our mammalian ancestors during that period, but their tiny size has up until now made it hard to find and study them,” Flynn says. Once they have finished studying the fossils, the scientists will construct cast replicas of the bones, returning a portion of the original material to scientists at Madagascar’s Université d’Antananarivo. For the past 10 years, Museum scientists have been collaborating with researchers from this university on a number of different projects. “Our Madagascar project illustrates that modern science is a truly international and collaborative endeavor,” Flynn says. “Expeditions in remote and little-explored regions, with Malagasy and U.S. scientists working side by side, have yielded extremely important paleontological discoveries. Our teams of students, professionals and volunteers are making key contributions to understanding the evolution of two groups near and dear to humans: dinosaurs and mammals.”

Research Feature Follow-up • Geology at The Field Museum • John Flynn discovers fossil monky skull in Chile • John Flynn's CV • Bill Simpson's CV