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A fossil mistaken for a bat may shake up lemurs’ evolutionary history

Aye-aye ancestors may have reached Madagascar on their own

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11:00am, August 21, 2018
aye-ayes

ANCIENT REACH  Aye-ayes, a type of lemur found on Madagascar, gouge holes in trees and extract grubs with narrow, elongated middle fingers.

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In one published swoop, an ancient fossil fruit bat has turned into a lemur. If that transformation holds, it suggests that lemur ancestors made two tricky sea crossings from Africa to Madagascar, not one as researchers have often assumed.

A new fossil analysis finds that the ancient species Propotto leakeyi, which lived in East Africa between 23 million and 16 million years ago, was not a bat, as scientists thought, but a primate closely related to modern aye-ayes. These strange-looking lemurs are found only on Madagascar along with another closely related lemur lineage.

What’s more, Propotto teeth and jaws display key similarities with fossils of a roughly 34-million-year-old primate, Plesiopithecus teras, previously found in Egypt, researchers say. Plesiopithecus, previously suspected to have been a primate, was an ancestor of Propotto and of modern aye-ayes, they conclude. Together, the findings, published August 21 in Nature Communications, may help rewrite lemurs’ evolutionary history. 

The research challenges a long-standing view that all Madagascar lemurs, including aye-ayes, evolved from a single population of African ancestors that somehow reached the island at least 54 million years ago. That estimate rests largely on genetic studies of modern lemurs and other primates. Destruction of ancient lemurs’ African habitats by global cooling around 34 million years ago left their kind isolated on Madagascar, according to this scenario.

But the survival of aye-aye ancestors in Africa millions of years after that, as suggested in the new study, raises the possibility that Propotto reached Madagascar on its own — separate from the other lemur lineage found on the island — and gave rise to present-day aye-ayes. No Propotto fossils have been found on Madagascar.

“Our identification of both Propotto and Plesiopithecus as African relatives of the aye-aye implies that [these] lemurs weren’t present on Madagascar until 30 million years or more later than previously thought,” says study coauthor paleontologist Erik Seiffert of the University of Southern California in Los Angeles.

Ancestors of the modern lemurs other than aye-ayes traveled to Madagascar sometime between around 41 million and 20 million years ago, the researchers estimate. During that period, ancestors of the only other mammal groups now inhabiting Madagascar — rodents, Malagasy mongooses and insect-eating creatures called tenrecs — also reached the island from Africa. Previous computer simulations indicated that ocean currents at that time could have carried animals stranded on storm-uprooted trees and vegetation mats from East Africa to Madagascar.

The team, which included Duke University’s Gregg Gunnell (who died in 2017), created digital reconstructions of Plesiopithecus and Propotto fossils for comparison with fossil and living primates, including aye-ayes (Daubentonia), and to closely related mammals called colugos. Evolutionary trees based on tooth and jaw analyses and available DNA data pointed to a link between the two ancient species and aye-ayes.

Plesiopithecus and Propotto might have used enlarged teeth projecting from the front of their mouths to gouge holes in trees and expose grubs’ nests, as modern aye-ayes do. Aye-ayes also poke through tree holes with long, skinny middle fingers to extract grub. But no hand fossils from either ancient creature have been found, so it’s a mystery whether they shared aye-ayes’ taste for finger food. 

The discoverer of three Propotto tooth-bearing lower jaws in Kenya originally reported in 1967 that the finds belonged to a new primate species, possibly an ancestor of primate relatives of lemurs called lorises. But within the next two years, the same scientist accepted another researcher’s proposal that Propotto’s jaws and teeth more closely resembled those of a fruit bat. A 1984 report describing several more Propotto teeth unearthed in Kenya also concluded that they came from a fruit bat.

The new identification of a line of ancient African lemurs that ran from Plesiopithecus through Propotto “is an interesting discovery,” says paleoanthropologist Marc Godinot of the National Museum of Natural History in Paris. “I have thought for years that Propotto was more likely a primate than a fruit bat.”

Godinot also argued in a 2006 study that the shape and positioning of teeth at the front of Plesiopithecus’ mouth pegged it as a relative of aye-ayes, consistent with a double colonization of Madagascar by lemur ancestors.

That possibility “merits serious consideration,” but a single African origin for lemurs on Madagascar remains the simplest, most likely scenario, says evolutionary biologist Anne Yoder of Duke University. Most African mammals couldn’t manage even one colonization of the island, so attributing two of these “highly improbable” events to lemur ancestors alone demands more evidence, Yoder says.

Still, it can’t be discounted that several ancient African lines of primates might have evolved in the aye-aye lineage but only one made it to Madagascar on a sea crossing that occurred independently of other African lemurs, Yoder says. Or, in line with her own view, Madagascar may have been colonized by one group of ancient lemurs that gave rise to multiple lines of creatures, one of which was a direct ancestor of modern aye-ayes. Only further fossil discoveries can resolve this mystery, Yoder says.   

Citations

G. Gunnell et al. Fossil lemurs from Egypt and Kenya suggest an African origin for Madagascar’s aye-aye. Nature Communications. Published online August 21, 2018. doi:10.1038/s41467-018-05648-w.

Further Reading

B. Bower. Fine toothcomb: New fossils add to primate-origins debate. Science News. Vol. 163, March 29, 2003, p. 198.

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