A 1 million-year-old tibia, or shin bone, of a monkey has been found fossilized in the Dominican Republic. It was in an underwater cave, and researchers say it is from a now-extinct monkey species known as Antillothrix bernensis.

According to a news release, scientists were able to verify that the fossil belonged to this species by using three-dimensional geometric morphometrics, meaning they analyzed the shape by using Cartesian geometric coordinates. Their findings were recently published in the Journal of Human Evolution.

"The presence of endemic new world monkeys on the Caribbean islands is one the great questions of bio-geography and our work on these fossils shows Antillothrix existed on Hispaniola (the name of the island that contains the Dominican Repubic and Haiti) relatively morphologically unchanged for over a million years. By establishing the age of these fossils we have changed the understanding of primate evolution in this region," Dr. Helen Green, lead researcher from the University of Melbourne's School of Earth Sciences, said in a news release.

The Antillothrix bernensis species is believed to have been a tree-dwelling monkey about the size of a small cat, with a diet that consisted primarily of fruit and leaves. Researchers have been interested in fossil remains from endemic mammals, such as Antillothrix, in order to better understand how they adapted to unique island environments, according to the release.

 "Very little was known about the native monkey from this island," Dr. Siobhán Cooke, assistant professor from Northeastern Illinois University, said in the release. "Prior to our discoveries in Altagracia we knew almost nothing even though this species was first described by Renato Rímoli back in 1977."

Researchers from Grand Valley State University, in Michigan, used 3D models of the monkey's leg bone to help reconstruct how this small animal would have moved in its environment. This allowed the researchers to compare different fossil samples.

Dr. Helen Green and Dr. Robyn Pickering from the University of Melbourne were then able to calculate the age of these newly found fossils. By measuring the levels of uranium, thorium and lead present in the limestone rocks in which the fossils were found, they estimated them to be roughly one million years old, according to the release. 

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