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Reverse course

Paleontologists seek evidence that marsupials arose in Antarctica

 

Ever see the logo for the Sherwin-Williams Company? It shows a bucket of paint tipped over above the planet. The paint covers the Northern Hemisphere, oozing and dripping south.

Ross MacPhee thinks it’s a perfect metaphor for the disrespect the Southern Hemisphere receives from some of his colleagues regarding the evolution and distribution of mammals tens of millions of years ago when Antarctica was part of a larger continent called Gondwana.

This austral summer he’ll make a fourth attempt to find mammalian fossils that at least prove some kinds of land animals were present on the continent long before Antarctica finally separated from other land masses and started to go into a deep freeze 40 million years ago.

Before then, Antarctica was sandwiched between South America and Australia, all remnants of the former supercontinent Gondwana. MacPhee is particularly interested in the history of Australian marsupials, believing that their ancestors emerged in South America and traveled across Antarctica to Australia sometime in the neighborhood of 80 million years ago.

“It was relatively easy, not only for the small guys like the marsupials, but also bigger ones like some of the southern ungulates [hoofed animals] to get across the land bridge connecting Antarctica and South America and settle down until Antarctic winter came in permanently about 40 million years ago,” MacPhee said during an interview at the American Museum of Natural History, where he is a curator in the mammalogy department.

The problem is that fossil record from Antarctica is spotty at best, especially considering most of the continent is entombed in ice. “That leaves you with a few little windows of seasonally bare ground on the Antarctic Peninsula and nearby islands,” MacPhee noted.

One of the better windows is on Seymour Island off the tip of the Antarctic Peninsula. Almost 30 years ago, U.S. scientists in the journal Science described the first fossil land mammal found in Antarctica, belonging to the extinct marsupial family Polydolopidae. The fossils were recovered from rocks about 45 million years old.

The find strengthened the theory that Australian marsupials originated from South American species that dispersed across Antarctica prior to about 65 million years ago, MacPhee said. Marsupials are mammals that are distinguished by the pouches many of them use to carry their young — the most well-known examples being kangaroos.

More recently, a study published last summer in the online journal PLoS Biology uses DNA data to suggest that Australian marsupials evolved from a common South American ancestor well before the end of the Cretaceous period 65 million years ago when the dinosaurs famously met their end.

MacPhee, “wildly” but enthusiastically speculating on what he might find during his upcoming expedition, said the possible discovery of marsupial fossils on Seymour or neighboring islands as primitive as those in South America could indicate that the divergence (or origin) of Australian marsupials actually began in Antarctica rather than South America proper.

“It would be damn interesting. It implies that Antarctica has been more than a superhighway for species to occasionally traverse,” he said. “It means that during times like the late Cretaceous, when it was much warmer than today, important episodes in mammal evolution might have happened there.”

Antarctica of 80 million years ago would hardly resemble the frozen wasteland of today. Some regions would have sported highly diverse vegetation, based on paleobotanical evidence. That means a temperate climate, which favors mammals.

“Since there was a great variety of plants, that implies there would have been a commensurately great variety of arthropods and insects, which means there would have been all kinds of available niches for mammals — small mammals, in particular — that eat insects,” MacPhee said.

The idea of Antarctica as a cradle for species isn’t without precedent.

Julia Clarke, as associate professor in the Department of Geological Sciences at the University of Texas at Austin, led a team five years ago that reported that close relatives of at least one order of modern birds co-existed with dinosaurs.

The new species, Vegavis iaai, was collected in 1992 by scientists from Argentina on Vega Island off the Antarctic Peninsula. Clarke and her U.S. and Argentine team re-examined the material more than a decade later.

It offered some of the best fossil evidence to date that linked modern bird divergence, the spread of today’s species, before the K-T boundary — when geologic time turned the page from the Cretaceous to the Tertiary period after the mass extinction of the dinosaurs.

MacPhee said such evidence about the Vega waterfowl helps turn the Sherwin-Williams model upside down. “It does suggest the southern end of the world is not as biogeographically irrelevant as it’s been previously — inadvertently perhaps — thought to be.”

Clarke will join MacPhee and several other paleontologists, including dinosaur fossil experts, on this latest expedition to the islands near the Antarctic Peninsula in February 2011.

“I’m really excited to have the opportunity after looking at all of the Antarctic material to go down there,” Clarke said. “I’ve never been to Antarctica, and I’m very excited to see what we’ll find.”

MacPhee hopes the fourth time is the charm. His first try in 2007 was largely frustrated by heavy snow on the ground that made finding the small fossils he would expect to find nearly impossible. The ancient marsupials would be about the size of modern shrews, up to 100 grams in weight — something that could easily fit in the palm of your hand.

A freak storm in 2008 wrecked the team’s field camp, forcing an early end to their season. Last year, sea ice conditions prevented the research vessel carrying the scientists to access the islands except for two days.

Despite the setbacks, MacPhee is willing to commit another two months to his search for mammalian fossils.

“I feel it’s important. People have to be willing to put up with some pain and suffering if they have any expectations of being successful,” he said philosophically. “Antarctica is not going to be kind. We already know that with regard to how hard it has been to collect mammalian or indeed any vertebrate fossils.”

The team hopes to begin its search on Vega Island, where Clarke’s avian fossils were found.

Other likely expedition sites include nearby Seymour and James Ross islands, where others have found fossils from younger geological periods. (Argentinean scientists recently uncovered ancient turtle fossils on Seymour Island dating from roughly 45 million years ago that don’t belong to any species known to live in the region during that time period.)

The scientists expect to spend long hours on their hands and knees, crawling along the ground hoping to spy interesting-looking bits.

“I’m somewhat sanguine, although you have to recognize the realities. That next rock that you needed to kick over to find that tooth that was going to explain all of this might be the very rock that you ignore,” he said.

“For my purposes it’s worth doing, because I think the Antarctic part of mammalian divergence and diversification story in the latter part of the Cretaceous is the part that we know least about. Virtually any evidence would suggest new possibilities.”

NSF-funded research in this story: Ross MacPhee, American Museum of Natural History, Award No. 0636639.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs