For nearly 40 years, one of the strangest insects ever found was hiding in plain sight, mislabeled as a baby crustacean. The flattened fossil from West Texas looked like little more than a dark smudge on dark shale.
Then Erik Tihelka, a paleoentomologist at the University of Cambridge, came across an old line drawing of the specimen in a secondhand Russian textbook and became suspicious of what he was told he was seeing. He eventually obtained access to the fossil and, under cross-polarized light, the smudge resolved into something much stranger: an insectlike animal with the familiar six legs on its thorax — plus 18 more along its abdomen.
In other words, this was an insect relative with 24 legs, built partly for land and perhaps partly for water. Scientists have never seen something like this before.
A new study in Nature identifies the 324-million-year-old animal as Chosha praecursor, a probable early-diverging insect. Along with several other strange fossils, Chosha offers a rare glimpse of insects before their modern body plan became fixed. And it may capture a stage in the extraordinary evolutionary transition that turned aquatic, crustacean-like ancestors into the six-legged animals that now dominate life on land.
The insect hiding in a crustacean fossil
Chosha was collected in 1985 near Rough Creek Ranch in Brewster County, north of Big Bend National Park. Because it came from the same fossil bed as Tesnusocaris goldichi, researchers initially interpreted it as a juvenile of that many-segmented crustacean.
The new team instead found a distinctly insectlike body: a three-part thorax, an 11-segment abdomen, one pair of long antennae, a central tail filament and an ovipositor, the egg-laying structure that indicates the specimen was probably an adult female. Nine abdominal segments carried pairs of jointed legs, making 24 legs in all. The rear appendages ended in flattened, grooved paddles. The body itself was about 3.2 centimeters long and nearly 5 centimeters including its tail filaments.
The fossil “straddles a big gap in the fossil record,” Tihelka told Science News.
Before insects became six-legged
Chosha‘s most striking feature is its peculiar leg count, which reflects an ancient body plan combining features now associated with crustaceans and insects.
The researchers compared Chosha with 22 other living and fossil arthropod taxa, scoring 58 anatomical traits. Their best-supported evolutionary tree placed Chosha just outside the group containing living insects. It’s not a crustacean as previously assumed, though they could not rule out a slightly deeper position at the base of all hexapods. The fossil is informative, but it is still a single, incomplete specimen, so there’s still some uncertainty on where exactly it falls on the tree of life.
Besides Chosha, the analysis also recast three other enigmatic fossils. Most consequentially, it places the roughly 405-million-year-old Scottish species Leverhulmia mariae on the insect stem, making it the oldest insect recognized in the study. Two many-legged fossils from Illinois’s Mazon Creek, about 306 million years old, join the same broader group.
Together, they help bridge the roughly 80-million-year “hexapod gap” between early Devonian hexapods and the diverse insects that appear later in the Carboniferous.
Previously, a 2017 Current Biology phylogenomic study concluded that insects are nested within crustaceans and estimated that hexapods began moving onto land in the late Cambrian to early Ordovician — much earlier than their clear body-fossil record.
The new fossils may help narrow down the timeline. And beyond genetic studies, the new study offers an actual body showing how some early insects were put together.
A shoreline route onto land
The paddle-shaped abdominal legs are the most tantalizing clue about how Chosha might have lived. They resemble, in position and shape, the gill-like structures of some living mayflies. The authors suggest Chosha may have used them for breathing, swimming or pushing through mud in shallow water. At the same time, its thoracic legs carried structures suited to walking on land, and its ovipositor could have helped place eggs in protected crevices.
This mosaic of features supports a gradual shoreline transition rather than a clean evolutionary leap from sea to soil. Similar thinking has emerged elsewhere in arthropod evolution. A 2020 PNAS study of aquatic stem-group myriapods showed how amphibious forms can help reconcile molecular dates with a sparse terrestrial fossil record.
Still, the habitat remains uncertain. When Chosha lived, the Tesnus Formation is thought to have been a shallow marine, deltaic or littoral environment. And the fossil preserves no spiracles, the openings of an insect tracheal system. The abdominal paddles may have been gills, swimming devices or both.
Early-insect fossils also have a history of being overinterpreted. A Devonian fossil named Strudiella devonica was hailed in 2012 as a complete early insect. Then a 2013 Nature reassessment argued that the identification was untenable. Chosha is more persuasive because its anatomy was tested across multiple phylogenetic analyses, but the larger story will strengthen only if more fossils appear.
“It opens a lot of doors to the study of insect evolution,” paleobiologist William Shear told Science News. For a lineage now defined by six legs, one of those doors leads back to a time when six was not yet the rule.
