By combining genetic and anatomical data of extinct and living lemurs, a Stony Brook University team of evolutionary biologists has developed the most complete evolutionary tree of lemurs yet. With extinct and living lemurs in a single evolutionary tree, scientists can better understand the evolution of these primates and what their future may be on earth. Their findings are published in the journal Systematic Biology.
"With all living and extinct lemurs together in the same tree, we can begin to understand how evolution and extinction have worked in the past, and project risks to these primate species into the future," emphasized the study authors, James Herrera PhD, of the American Museum of Natural History and a 2015 doctoral graduate of the Stony Brook Anthropological Sciences program, and Liliana M. Dávalos, PhD, Professor in the Department of Ecology and Evolution at Stony Brook University.
Indigenous to Madagascar, lemurs form a unique branch of the primate family tree. Evolutionary biologists have grappled with one important obstacle to understanding where lemurs came from and how they evolved- there are no ancient fossil lemurs. This void makes it difficult to track stages of evolution often observed in ancient fossil forms. However, there are remains of lemurs extinct about two thousand years ago. Those lemurs were radically different from the ones alive today. Many of the extinct lemurs were giants, with some weighing as much as gorillas. Placing them in the tree with living species has been difficult because their unique features drastically differ from those of lemurs that did not go extinct.
To bridge that evolutionary gap within lemurs and better understand the factors that may place species at risk of extinction, it is important to compare the traits of extinct lemur species to those of living lemurs. That is what Drs. Herrera, and Dávalos set out to do with their new study of lemurs.
For their Systematic Biology paper, titled "Phylogeny and divergence times of lemurs inferred with recent and ancient fossils in the tree," the researchers collected new morphological data on the living and extinct lemurs of Madagascar. The data collection and analysis were completed at Stony Brook University.
They combined these data with published DNA sequences from lemurs and used powerful statistical techniques to estimate evolutionary trees. Including extinct and living species into evolutionary trees with dates has been difficult because the morphological traits and molecular sequences evolve at very different rates. Additionally, ecological pressures and other constraints may shape traits in ways that violate a key assumption of statistical methods: independence between observations. Using recently developed techniques to estimate the rates of evolution of traits, as well methods to ensure statistical independence between traits pioneered by Dávalos, the team was able to recover evolutionary trees including all known lemurs.
The results of the analysis reveal many well-supported relationships among living and extinct lemurs, highlighting the unique nature of many now-extinct lemur lineages.
Surprisingly, the new study found some of the subfossil species were unique branches with no living close relatives:
- The giant, extinct koala lemur (Megaladapis) had short limbs and long finger and toe bones, which may have allowed it to cling to branches, much as living koalas do now.
- Megaladapis also had an extremely long snout with no upper front teeth, and may have even had long dexterous lips for manipulating food.
- The koala lemur was thought to be closely related to either of two living groups: the sportive lemurs (Lepilemur), or 'true' lemurs, like the ring-tailed lemur (Lemur). Because of the unique features with few similarities to living species, placing them in the lemur tree based on skeletal traits has been difficult. Fragments of ancient DNA had suggested a close link between koala and true lemurs. The new methods find the koala lemur was a completely extinct, unique branch of the lemur tree.
Some lemur fossil groups were closely related to living species, but they were giant compared to living species. While the largest living lemur is about 22 pounds, the largest and closely related extinct lemurs may have weighed more than 300 pounds. "Our approach also included the full complement of extinct lemurs and lemur relatives," added Dr. Herrera, explaining that 95 percent of living lemurs are threatened with extinction. "Large body size is one important component of extinction risk, and so by understanding the evolution of body size, we can better understand the factors threatening lemurs today," explained Dr. Dávalos.
The findings shake up ideas on how geographic changes help species split off. "By using these methods, we confirmed previous molecular studies that estimated lemurs first evolved 50 to 60 million years ago," said Dr. Herrera. Lemurs first evolving 50 to 60 million years ago means the ancestor of lemurs likely arrived on Madagascar by dispersal -- perhaps washed from Africa to Madagascar floating on vegetation, or with some small stepping-stone islands to help along the way.
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