324-Million-Year-Old Fossil With 18 Extra Legs Reveals How Insects Lost Their Extra Limbs

A 324-million-year-old fossil dug up from a shale bed in West Texas has 18 extra legs along its belly, a body plan unlike any living insect, making it one of the oddest insects ever found, researchers report August 26 in the journal Nature. The find "straddles a big gap in the fossil record," offering a glimpse of the transition from many-legged, shrimplike ancestors to today's six-legged insects, says paleoentomologist Erik Tihelka of the University of Cambridge, who led the study.

Tihelka's team named the fossil Chosha praecursor, combining the Navajo word for "insect," ch'osh, with the Latin for "predecessor," inspired by a Navajo creation myth about insectlike people who populated the Earth. Collected north of Big Bend National Park in Texas in 1985, the specimen was mistaken at the time for a juvenile crustacean, Tesnusocaris goldichi, and sat unstudied in a San Diego museum drawer for decades.

Tihelka rediscovered it after spotting a line drawing in a secondhand Russian paleontology textbook, then examined the fossil under polarized light at the San Diego Museum of Natural History. He identified a three-segmented thorax, an 11-section abdomen, whiplike antennae and three pairs of walking legs, all pointing to an insect, but also nine extra pairs of short legs on the abdomen, the rearmost ending in flattened, grooved paddles that may have helped it swim through muddy shallow seas.

Comparing Chosha with known insects and crustaceans, including a 405-million-year-old fossil from Scotland and undescribed specimens from Illinois' Mazon Creek beds, Tihelka found they all sit just outside the group containing every living insect. The picture: many-legged, semi-aquatic protoinsects, one of which moved onto land and shed its extra legs until only six remained. "We call insects 'hexapods,' meaning six legs," Tihelka said. "But it turns out that the original ones had many more."

"This fossil is a major milestone in a puzzling period when everything was changing from mostly aquatic to terrestrial, and we had almost no fossil evidence from that time," said paleoentomologist Dany Azar of Lebanese University in Fanar.