Looking inside lemurs’ mouths to find their ancient relatives

Endemic to Madagascar, the aye-aye (Daubentonia madagascariensis) is the only primate with the skills of a woodpecker: it uses its large rodent-like anterior teeth to find wood-dwelling grubs. Scientists compared the morphology of its dentition with the fossilized bones of the extinct primate Plesiopithecus teras from Egypt, and three 20 million-year-old partial lower jaws of Propotto leakeyi from Kenya. 

“Mammal fossil teeth are like fingerprints. They can tell you so much about animal affinities,” says Hesham Sallam, a member of the team working at Mansoura University Vertebrate Paleontology Center in Egypt.

Although the quest is complicated by the scarcity of lemur fossils, Plesiopithecus’ large pointed anterior tooth, as well as Propotto’s cheek teeth with intermediate features between Plesiopithecus and the aye-aye could be explained by a shared evolutionary pathway. Thus, although Propotto was labelled as a fruit bat in the late 60s, several lines of evidence led the team to reclassify it as a primate. 

According to the research team’s morphological and molecular analyses, the last common ancestor of the aye-aye and Propotto was still living on mainland Africa around 28 Ma (million years ago). This challenges current estimates that lemurs crossed the sea between Mozambique and Madagascar between 50 and 60 Ma and then rapidly split into two evolutionary lineages: a group leading to aye-ayes (chiromyiform), and another containing all the other lemurs (lemuriform). 

The study presents an alternative scenario where the two lineages reached Madagascar later and independently. Lemuriforms could have arrived on the island between 41 and 20 Ma, and chiromyiforms at any point after 28 Ma. 

“This new interpretation of the Propotto fossils, which were found 50 years ago, is amazing and highlights the complexity of primate evolution and dispersal,” explains Christian Roos from the German Primate Centre, who was not involved in the study. “That lemur ancestors crossed the Mozambique Channel twice is unexpected and in contrast to earlier thinking, as almost all earlier studies suggested just a single colonisation event. However, this study relies on limited material, so other fossilised body parts of Propotto would certainly shed more light on the whole story.”

“Palaeontologists now need to carefully look through the fossil collections, because other Propotto fossils might have been misidentified,” says integrative anatomical scientist Erik R. Seiffert of the University of Southern California and corresponding author of this study.  “Any additional evidence from Propotto will allow us to further test the hypothesis that it is related to the aye-aye,” he adds. 

While lemurs had been regarded as the first placental mammals to colonize Madagascar, this study estimates that they reached the island during the same timeframe of ancestors of other Malagasy endemic terrestrial mammals, like fossas, rodents and tenrecs. 

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