Earliest primate had tree-climber ankles

A creature known only from fossils of its teeth gets some more parts

RALEIGH, N.C. — Finally, the earliest known primate has more than a mouth. Several 65-million-year-old fossil ankle bones found among unsorted bits from excavations in Montana belong to a tree-dwelling species of Purgatorius, Stephen Chester of Yale University announced October 19 at the annual meeting of the Society of Vertebrate Paleontology.

EARLY DAYS Skeletal fossils of the North American ancient primate Dryomomys szalayi indicate its shape (reconstruction shown) and suggest what its older relative Purgatorius may have looked like. Doug Boyer, Duke Univ.

The first signs that Purgatorius existed came from fossil teeth unearthed nearly half a century ago at an excavation site in northeastern Montana not far from where the first Tyrannosaurus rex was found. Paleontologists named two species of what they called Purgatorius. William Clemens of the University California Museum of Paleontology and his colleagues described more teeth and some bits of mouth bones since then, but nothing had turned up from the rest of the body.

“The teeth were so primitive we didn’t know much,” said Ken Rose of Johns Hopkins University School of Medicine in Baltimore, who heard the presentation at the meeting. Several lineages diverged early in the history of primates, and sorting out their relationships has been tricky.

On the off chance that some Purgatorius bit had been overlooked during Clemens’ long-running excavations in Montana, Chester and paleontologist Jonathan Bloch of the Florida Museum of Natural History started looking through the vast stored trove of fossils that has yet to be classified. After about two days of looking, they found an ankle bone. Its form shares details with known fossils of the primate lineage, but Bloch recalled, they recognized what it was as soon as they saw it. “You know a Picasso when you see one,” he said.

No other animal known from the excavation could have had an ankle like that first find or several later specimens, Chester said. Also the size calculations put the creature in the same range as the size predicted by the tooth analysis. The shapes of the two components indicate that the ankle would have moved easily in many directions, perfect for scampering through the complex angles and knobs of trees.

The identification convinced anthropologist Daniel Gebo of Northern Illinois University in DeKalb. “A type of slam dunk,” he said of linking the ankle bone to what are called plesiadapiforms, very ancient primatelike creatures. They didn’t have all the traits of primates known today, though, so he doesn’t call Purgatorius a true primate.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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