The team's work was published earlier this month in the Proceedings of the National Academy of Sciences.
Redrawing the Primate Family Tree
Most animals from the period when Dryomomys szalayi lived are known only by their teeth. But the team's finds produced skulls and skeletons of the two new species to paint a more complete picture of the way the ancient animals looked and lived.
"We're really getting into their paleobiology, where we can start to reconstruct what they ate, how they moved around, and how their features evolved over time," Sargis said.
"Both of these [fossil animals] had been proposed to be closely related to flying lemurs and had been proposed to be gliders.
"Now that we've analyzed the skeletons, we find no evidence that they were gliders. They were probably just typical tree-living primates."
In addition to detailing the fossil finds, the study compared 173 skeletal characteristics of plesiadapiforms, primates, tree shrews, and flying lemurs, in hopes of unveiling their evolutionary links.
Comparisons of 85 living and extinct species suggest that all plesiadapiforms are actually primitive primates, Bloch said.
"The skeletons allowed us to assess, are they really the ancestors to primates and if so what are they like?" Bloch explained.
"We've determined that they are the ancestors of modern primates. And we're taking back ten million years of primate evolution where we see a gradual evolution of the traits that we recognize as primate-like."
Modern primates debuted some 55 million years ago, about ten million years after the disappearance of the dinosaurs. The study suggests that early primates evolved the features associated with modern primates—such as grasping hands and feet, and nails instead of claws—during that ten-million-year period.
"The results of this study suggest that plesiadapiforms are the critical [group] to study in understanding the earliest phases of human evolution," said co-author Mary Silcox, an anthropologist at the University of Winnipeg.
The study also changes ideas about why and how the individual features of modern primates evolved.
The authors theorize that the early primates co-evolved with flowering plants and trees, because the animals depended on vegetation as well as the insects attracted to it.
The team's finding is likely to spark debate, some scientists note, because Dryomomys szalayi doesn't have all of the traits traditionally associated with modern primates.
"The idea [of plesiadapiforms as primates] will be controversial to people who want the earliest primates to resemble modern primates," said Susan Cachel, an anthropologist at Rutgers University who is unaffiliated with the study.
"This is unrealistic given that we don't expect this to be true for other orders of mammals.
"Being a primate back at the beginning of the Age of Mammals [when mammals first became the dominant land animals, about 65 million years ago] is not the same as being a primate nowthe primate adaptive zone has evolved."
Bloch notes that the Yellowstone skeletons may be only the first specimens to yield new clues about the early history of primates.
His laboratory currently houses a block of Wyoming limestone that holds an estimated 150 additional skeletons inside, he said.
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