Their research, which was published today in the journal Nature Communications, demonstrates that Proconsul and its primate relative Dendropithecus inhabited “a widespread, dense, multistoried, closed canopy” forest.
Holly Dunsworth, URI assistant professor of anthropology, said that the research team found fossils of a single individual of Proconsul, which lived 18 to 20 million years ago, among geological deposits that also contained tree stump casts, calcified roots and fossil leaves. The discovery underscores the importance of forested environments in the evolution of early apes.
“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.”
Rusinga has been known since the 1980s for preserving a fossil ape and other creatures in a hollowed out, fossilized tree trunk. But it wasn’t until the research team’s discovery of additional tree trunks and fossil primates preserved in the same ancient soil that there was a strong link between the ape and its habitat at the site.
“It’s probably the best evidence linking ape to habitat that we could ask for,” Dunsworth said. “Combined with analyses of the roots, trunks and even beautifully preserved fossil leaves, it’s possible to say that the forest was a closed canopy one, meaning the arboreal animals, like Proconsul, could easily move from tree-to-tree without coming to the ground. This environmental evidence jibes with our behavioral interpretations of Proconsul anatomy–as being adapted for a life of climbing in the trees–and with present-day monkey and ape ecology.”
Additional evidence from the excavation site has shown that the landscape was stable for many years while the forest grew.
According to co-author Daniel Peppe of Baylor University, evidence from the forest soil suggests “the precipitation was seasonal with a distinct wet and dry period. During the dry season, there was probably relatively little rainfall,” he said. “Additionally, by studying fossil leaves at the site, we were able to estimate that there was about 55 to 100 inches of rainfall a year and the average annual temperature was between 73 and 94 degrees Fahrenheit.”
Research on Rusinga Island has been ongoing for more than 80 years and has resulted in the collection of thousands of mammal fossils, including many well-preserved specimens of Proconsul and other primates. Evidence from these fossils indicate that Proconsul probably had a body position somewhat similar to modern monkeys, but that details of its anatomy suggest some more ape-like climbing and clambering abilities. Since 2011, the research team’s work at the fossil forest site has resulted in the collection of several additional new primate fossils.
Dunsworth said that her work at the site is continuing.
“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.
The early ape Proconsul (center) and the primate Dendropithecus (top) inhabited a warm and relatively wet, closed canopy tropical seasonal forest 18 million years ago in equatorial eastern Africa. (Illustration by Jason Brougham.)