Knowledge of the environment inhabited by the early ape Proconsul can help in understanding and interpreting the connection between habitat preferences and that diversification, they said.
Proconsul and a primate relative, Dendropithecus, inhabited "a widespread, dense, multistoried, closed canopy" forest on Rusinga Island in Kenya, the researchers report in the journal Nature Communications.
A fossil of a single individual of Proconsul, which lived 18 to 20 million years ago, was found among geological deposits that also contained tree stump casts, calcified roots and fossil leaves, they said.
The discovery underscores the importance of forested environments in the evolution of early apes, anthropologist Holly Dunsworth of the University of Rhode Island said.
"To have the vegetation of a habitat preserved right along with the fossil primates themselves isn't a regular occurrence in primate paleontology," she said. "It's especially rare to have so many exquisite plant fossils preserved at ancient ape sites.
"It's probably the best evidence linking ape to habitat that we could ask for."
Proconsul probably had a body position somewhat similar to modern monkeys but details of its anatomy suggest some more ape-like climbing and clambering abilities, the researchers said.
Work at the Kenyan site is ongoing, Dunsworth said.
"We don't know exactly what we're going to find, but without a doubt, if we keep searching, we're going to find knowledge about early ape evolution, which was, of course, a significant chapter in our own history," she said.
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