Tiny Fossils Reveal the Rise of Mammals on Madagascar

Life on Madagascar is unlike life anywhere else in the world. The vast majority of the island’s creatures are only found within its borders, from the lemurs hopping through the trees to the colorful reptiles that clamber through the undergrowth and over stretches of desert.

Islands are often hot spots of biodiversity because isolation is usually an essential ingredient for evolution. Organisms that wind up on islands, separated from their mainland haunts, can become adapted to different habitats than those of their ancestors, much like the many varieties of Darwin’s finches. If the celebrated naturalist had visited Madagascar instead of the Galápagos, “Darwin’s lemurs” might instead be the textbook standard.

But Madagascar has long presented a mystery: The island's fossil record has been nearly blank between about 66 million years ago and 26,000 years ago, leaving biologists to ponder how today's fantastic display of biodiversity came to be after the end of the Age of Dinosaurs.

“Madagascar has some of the most endemic, endangered and bizarre plants and animals on the planet,” says paleontologist Karen Samonds of Northern Illinois University. “Yet we know very little about how they arrived.” Now, thanks to years of backbreaking work and careful sifting of tiny fossils, Samonds and her colleagues are starting to piece together Madagascar’s missing evolutionary tale.

Paleontologists and geologists had previously determined that proto-Madagascar, at the time attached to ancient India, split off from mainland Africa about 135 million years ago. About 88 million years ago, Madagascar and India parted ways, leaving the dinosaurs, mammals, and other creatures there to spin off into strange new forms.

Recent discoveries from rocks dating to about 70 to 66 million years ago include the predatory, knobby-headed Majungasaurus, the armored sauropod Rapetosaurus and the gopher-like early mammal Vintana. After that, the fossil trail picks up again around 26,000 years ago, when enormous lemurs, elephant birds, dwarfed hippos and other now-extinct forms called the island home. But what happened in the long interval in-between? That mystery is what drew Samonds to the island.

“I love the challenge of exploration to remote areas,” Samonds says. “It was clear that finding this ‘missing piece’ in the Cenozoic fossil record had huge potential to answer many different research questions.”

Her team's persistence has been paying off. In 2009, Samonds and her colleagues announced the discovery of a 40-million-year-old sea cow they named Eotheroides lambondrano. This was the first good mammal fossil found in the gap between the reign of the dinosaurs and the late Pleistocene.

The sea cow was uncovered near the little village of Ampazony on the northwestern coast of Madagascar. Not very far away, across the mouth of the Betsiboka River, sits another fossil site that holds even more potential. Its name is Nosy Makamby, and it is just a little spit of land off the coast of the main island.

Previous paleontologists had found fragments of sea cow there at the beginning of the 20th century, but at between 23 and 5 million years old, these were geologically younger than the beast Samonds and her team named. Nosy Makamby looked to hold additional pieces of the Cenozoic puzzle.

During a decade of fieldwork on Nosy Makamby, “the biggest challenge we have is dealing with the ocean tides,” Samonds says. “We camp on the beach, and some of our sites are underwater during certain times of the day.” The team has to carefully coordinate when the island’s fossil-bearing rocks are above the waves.

“A few times we have stubbornly tried to push our window of opportunity and have gotten really stuck”, Samonds says. For instance, after recent cyclones washed away a significant part of the beach, an especially high tide nearly washed out their camp, leading to a waterlogged night. Yet the draw of discovering remnants from an unknown time period keeps the paleontologists going back year after year.

Some of the fossils discovered by the team can be seen with the naked eye. These big bones are prepared for study back in the lab, using tools called airscribes that delicately chip away stone from bone. Not a scrap goes to waste, though. Tiny fossils hide in the mix, and so the matrix chipped off the big bones is left to dissolve in acetic acid and screened through a small sieve. This reveals some of the smaller bones that would have otherwise been missed.

From the fossils recovered so far, it seems that Nosy Makamby was a near-shore marine habitat back in the Miocene, too.

“The most common fossils we discover are animals that live in the ocean near the shore like snails, stingrays, sharks, fish, crocodiles and turtles,” Samonds says. Just last year, the team found more sea cow material, including a lower jaw and possibly another piece of skull. But to Samonds, “the most exciting recent finds are tiny terrestrial animal fossils” that include the teeth and bones of animals such as bats and rodents.

“For each group we find, they fill a gap of knowledge,” Samonds says. Prior to the discovery of Eotheroides, she notes, sea cows were thought to have evolved in the Northern Hemisphere and spread south. But the sea cow from Madagascar in the Southern Hemisphere is so archaic that “it has really turned our perception of sea cow evolution upside down.”

The team has also found fossils of roundleaf bats in the island's Miocene rock, which is not entirely surprising because the animals are found in strata of the same age in many parts of the world. Still, their presence in Madagascar “represents a range expansion, and since they are found in Madagascar today, it helps us bracket their arrival time.”

Each new expedition brings back more fossils and the potential to add a few pieces to the story of how life on Madagascar became so beautiful and strange.

“Since I work in a time period we know virtually nothing about what lived on the island, pretty much everything we find is surprising in some way,” Samonds says. These not only include the beginnings of lineages still alive today, but perhaps even groups of animals that made it to Madagascar but went extinct long before humans arrived.

Samonds is optimistic that she and her team will uncover more of these lost worlds: “We could have some pretty interesting surprises ahead in the fossil record.”

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.