DEKALB, Ill. — A team of researchers led by Northern Illinois University paleontologist Dan Gebo has discovered the fossils of 45 million-year-old, thumb-length primates.
Recovered from the fissure-filled sediments of a limestone quarry in China in 1996, the fossils easily represent the smallest known primates, with one species estimated to have weighed only 10 grams. These distant relatives of monkeys, apes and humans were once the prey of owls, the researchers say.
The discovery of the smallest primates may have widespread implications as scientists plot out the evolutionary family tree leading from lower to higher primates. Living lower primates, also known as prosimians, include lemurs and tarsiers. Living higher primates include monkeys, apes and humans.
"Few would have predicted such a diminutive monkey-like creature at such a key branch of evolution," said Gebo, a professor of anthropology at NIU. "These are the smallest primates ever discovered, alive or extinct. Some of these fossils are one-third the size of the living mouse lemur from Madagascar, which at one ounce (31 grams) is the smallest known primate."
Gebo’s research team includes Marian Dagosto of Northwestern University Medical School in Chicago; K. Christopher Beard of Carnegie Museum of Natural History in Pittsburgh; and Qi Tao of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
Writing in the April edition of the Journal of Human Evolution (London), the team reports on two of the tiny primate fossils. Estimated to have weighed 15 grams, the larger species is believed to have been a higher primate belonging to the extinct family Eosimiidae.
"Both of the fossils are related to a branch of primate evolution that eventually leads to humans," Gebo said.
Primates are mammals, characterized by having bigger brains, grasping hands and feet, nails instead of claws and eyes located in the front of the skull. Hundreds of animal fossils, including those of at least three minute primate species, have been culled from a commercial limestone quarry 100 miles west of Shanghai.
"The limestone itself is of Triassic age—from the very beginning of the Age of Dinosaurs some 220 million years ago," researcher Christopher Beard said.
"The chemical composition of limestone makes this rock dissolve easily in rainwater, leading to the creation of numerous small fissures or limestone caverns that are often much younger than the original limestone itself," Beard added. "As chance would have it, the Triassic limestone in the quarry near Shanghai is crisscrossed by fissures dating to the middle Eocene (some 45 million years ago), in the middle of the interval in which a poorly known lineage of higher primates must have existed."
During the middle Eocene, a rainforest occupied the site. However, unlike other prehistoric forests across the globe that had a mix of large and small primates, the Shanghuang rainforest’s fossil record is nearly absent of the larger creatures.
"There is no place that mirrors what we’re finding in China," Gebo said. "Other forests in the past have had a few smaller species but they also had lots of big creatures."
According to Beard, "The fact that we are sampling a whole radiation of tiny primates at these Chinese sites indicates for the first time that we are glimpsing a part of the evolutionary tree of primates that previously eluded paleontologists."
The researchers have no complete skeletons but as many as 50 foot bones belonging to primates weighing less than 100 grams. Markings on the fossils have led the researchers to believe the primates were once the prey of owls. Researcher Marian Dagosto discovered the heel bone of the smallest primate as she sifted through a cardboard box filled with matrix retrieved from the site and processed at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
Both Gebo and Dagosto, who in addition to being research partners are husband and wife, are experts in identifying foot bones.
"We had literally thousands of bones from all sorts of animals," Dagosto said. "It’s usually a matter of going through them one by one. I found the primate heel bone in the corner of the box, almost under the paper lining. Once I realized just how tiny it was, it did strike me as really odd. I remember calling Dan over right away."
Dagosto said the researchers used a statistical technique called regression, comparing the bone size with living animals, to estimate the primate’s size and weight.
The researchers say the tiny primates were tree dwellers that relied on a steady diet of insects, fruit and nectar to fuel their high metabolisms. Unlike contemporary higher primates, the tiny primates likely were nocturnal and solitary creatures.
"The implication is staggering," Gebo said. "You would think that early higher primates would have a lot of characteristics of later higher primates, which were social creatures that occupied a daytime niche. It probably means we’re getting close to the transition between higher and lower primates."
Calling the China site a treasure trove of fossils, the researchers added that 90 percent of the recovered specimens have not yet been analyzed.
Note to Reporters/Editors: A primate press kit, with links to illustrations and other resources, can be found at http://www.niu.edu/pubaffairs/RELEASES/2000/MAR/primate/
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