July 15 (UPI) -- Paleontologists have recovered the remains of the smallest extinct Old World monkey species in Kenya. Researchers estimate the species, Nanopithecus browni, which lived 4.2 million years ago, weighed two to three pounds and was roughly the size of a bunny rabbit.
Today, the species' relatives are scattered throughout Africa. The tiny monkey was about the same size as a modern talapoin monkey, the smallest living Old World monkey. Talapoins -- of which there are two species -- are members of a larger group of monkeys called guenons.
Despite their ubiquity across Africa, the evolutionary history of guenons is poorly understood. Most scientists estimate the breakup and convergence of different forest ecosystems, the result of tectonic and climate shifts, was the primary driver of guenon diversity over the last few million years.
Unlike their larger relatives, which are scattered across the continent, talapoins are found only in the tropical forests of West Central Africa. Researchers have previously suggested the monkeys shrank as they adapted to life in wet, woody habitats.
Nanopithecus browni, however, was found on the other side of Africa, the eastern side, in a region of Kenya known as Kanapoi. During the species' heyday, Kanapoi was dry and blanketed by grasslands.
Some of the oldest remains of early humans, Australopithecus anamensis, have been recovered from Kanapoi sites, suggesting the world's smallest Old World monkey lived alongside early hominins.
Only one other guenon fossil predates Nanopithecus browni. It was found on the Arabian peninsula just ten years ago. The two fossils, each found far from their modern relatives in West Central Africa, suggests the evolution of guenon monkeys was more complex than previously thought.
"The discovery of Nanopithecus browni reaffirms Kenya's contribution to understanding the evolution and diversity of Pliocene fauna and the environmental contexts in which they lived," Fredrick Kyalo Manthi, researcher with the National Museums of Kenya, said in a news release. "Environmental changes during the Plio-Pleistocene may have influenced the present-day distribution of guenons."
Researchers detailed their discovery of Nanopithecus browni this week in the Journal of Human Evolution.