Fossils are clues to kangaroo mystery

Press, 24 April 1986, Page 13

Fossils are clues to kangaroo mystery

By

BORIS WEINTRAUB,

National

Geographic News Service

Scientists know little about the early evolution of marsupials, the pouched mammals whose modem representatives include opossums and kangaroos. Now,, thanks to recent discoveries made in the badlands of south-central Bolivia by a French-American-Bolivian team, some of the mystery may be stripped away and some answers provided to the puzzling questions of where marsupials originated, where they went and how they got there, and how they evolved. The discoveries were made by a team leaded by Larry G. Marshall, assistant director of the Berkeley Geochronology Centre at the Institute of Human Origins, and Christian de Muizon, a researcher with the French National Museum of Natural Histfij, Paris, in association with a

technological institute at Santa Cruz. Their work, 96 kilometres southeast of the city of Cochabamba, was supported by the National Geographic Society and the French Museum. Where marsupials, especially the Australian varieties, originated and how they evolved are major points of controversy. In recent years, many scientists have suggested that they originated in the southern hemisphere and perhaps moved from South America to Australia via Antarctica. In past geologic eras, those continents were connected, and early marsupials may have

reached Australia either through a direct connection, over a land bridge, or by island-hopping. Among the scientists’ finds were at least four complete and five partial fossilised skulls of small marsupials. Marshall and de Muizon believe the animals lived between 66 million and 70 million years ago, towards the end of the Cretaceous period, a time when dinosaurs still were dominant but on the verge of extinction. The discovery represents the first skulls and associated skeletons of late Cretaceous mammals discovered in South America. A site in Mongolia is the only other place

where late Cretaceous mammal skulls and associated skeletons have been reported. “What’s interesting is that, with only a few exceptions, what we know about the evolution of marsupials in the late Cretaceous is based on dental charaters, the structure of the teeth,” Marshall says. “Here we have complete skulls, so we can look at other characteristics. Parts of the ear

region and the basal parts of the skull are very important in establishing the relationships of early marsupials. And since we have these partial skeletons, we can look at structures in the ankle bones'and the pelvis that are very important for identifying affinities with other marsupial groups.” The skulls are less than two inches long and come from at least two species of what Mar-

shall calls “generallised opossum types.” In 1982, a team headed by Marshall and de Muizon found the remains of seven specimens fron three marsupial species in the same region. They now have about 20 species of mammals, including 12 varieties of marsupial. “Now we can start addressing some major questions,” de Muizon says. “It’s possible we have the common ancestor of many later South American, and possibly all Australian, marsupials here. What did they show affinities with? What are the relationships with similar-age marsupials in North America? What are the

relationships with other South American forms, as well as with those forms in Antarctica and Australia?” In addition to finding the marsupial remains, the team also found remains of placental mammals, those whose unborn young develop in the womb. Among those findings, primarily of teeth and jaws, were the remains of two groups that had never before been found in the late Cretaceous of South America. One of these groups, known as pantodonts, is common in Asia and North America, but usually doesn’t begin to show up in the fossil record of those continents

until about 62 million years ago. Finding their remains in a deposit thought to be at least 66 million years old makes this the earliest record of pantodonts anywhere, Marshall says. Pantodonts were lumbering, plant-eating animals, about the size of a cat, that diversified into a variety of forms before dying out about 50 million years ago. The other new placentals whose remains were found in Bolivia were proteutherians, small insectivores known in similarly aged fossil beds in North America and Asia but previously unknown in South America. “Their first appearence in South America means that we have to reconsider the biogeographic histories of these groups on a world-wide basis,” says Marshall.