Insects are famous for their six-legged body plan, but that wasn’t always the rule. A 324-million-year-old fossil dug up from a shale bed in West Texas had a markedly different setup, sporting a whopping 18 extra legs along its belly — a body plan unlike that of any living insect.
It is one of the oddest insects ever found, and possibly the most consequential, researchers report August 26 in Nature.
The discovery “straddles a big gap in the fossil record,” says paleoentomologist Erik Tihelka of the University of Cambridge. It offers a glimpse of the transition from many-legged, shrimplike ancestors to the six-legged insects of today. “It shows that there were these transitional forms along the way,” he says, before insects shed their extra legs and settled on six.
Tihelka and his team named the fossil Chosha praecursor. It is inspired by the Navajo creation myth, in which a primordial world was inhabited by insectlike people whose descendants went on to populate Earth, combining the Navajo word for “insect” (ch’osh) with the Latin for “predecessor.”
For decades, however, the fossil bore a different name. Collected from a rocky outcrop north of Big Bend National Park in 1985, the specimen was initially mistaken for a juvenile of a crustacean called Tesnusocaris goldichi, found in the same dark shale. Researchers saw little significance in the find, and it languished in a museum drawer in San Diego for years.
Then Tihelka spotted a line drawing of the fossil in a Russian paleontology textbook he had bought secondhand. He tracked down the original papers, but the specimen’s supposed identity as a crustacean didn’t sit right with him. He traveled to the San Diego Museum of Natural History to see it for himself.
At first glance, the unassuming fossil resembled little more than a “black smudge,” Tihelka recalls. But by peering at it through polarized light filters, which cut the glare and sharpened contrast, he could make out a series of anatomical details that all pointed toward insects: a three-segmented thorax; an extended abdomen with 11 sections; whiplike antennae; a long central tail flanked by two shorter ones; a stiff tube for laying eggs; and, of course, three pairs of walking legs.
There was just one, tiny hangup. The Chosha fossil had nine extra pairs of short, stubby legs on its abdomen, the rear ones ending in flattened, grooved paddles. These paddles may have helped the insects swim or scuttle through muddy shallow seas.
What struck Tihelka was that he had seen other fossils like it, including a 405-million-year-old specimen from Scotland and an undescribed pair of multi-legged, insectlike fossils from the Mazon Creek beds of Illinois.
Comparing the bodies of these ancient oddballs with known insects and crustaceans, both living and extinct, more closely, Tihelka could see that Chosha and the other fossils all landed same place. They all sat just outside the group containing every insect alive today, with crustaceans a distant branch away.
The picture that emerges, then, is of Chosha and its many-legged kin, with their extra limbs and paddlelike appendages, living semi-aquatic lifestyles. Somewhere along the way, one such protoinsect presumably made the leap onto land full-time, shedding the surplus legs until only six remained. That terrestrial ancestor then gave rise to the millions of insect species on Earth today.
“It’s a funny thing,” Tihelka says. “We call insects ‘hexapods,’ meaning six legs. But it turns out that the original ones had many more.”
For all its strangeness, though, Chosha may offer a surprisingly straightforward explanation for how insects made the leap from water to land. “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,” says Dany Azar, a paleoentomologist at Lebanese University in Fanar and the Nanjing Institute of Geology and Palaeontology in China.
“It opens a lot of doors to the study of insect evolution,” says William Shear, a retired paleobiologist from Hampden-Sydney College in Virginia. “And one can only hope that more fossils of this particular organism will turn up upon further searching.”