Ancient 24-Legged Creature Reveals Insects’ Possible Path From Water to Land

A small fossil found in Texas is providing scientists with a rare glimpse of the transition between aquatic ancestors and the six-legged insects living today. The creature, which lived about 324 million years ago, had 24 legs and appears to have combined traits associated with both crustaceans and insects. The fossil was discovered in 1985 and was initially identified as a juvenile specimen of the crustacean Tesnusocaris goldichi. Erik Tihelka, a paleoentomologist at the University of Cambridge, reconsidered that identification after seeing an illustration of the specimen in a Russian paleontology textbook. He later examined it at the San Diego Natural History Museum, where it had remained in a drawer for decades. Using cross-polarized light imaging, Tihelka brought out details that were difficult to see in ordinary light. The fossil had whip-like antennae, a long central tail flanked by two shorter ones, an egg-laying structure, an 11-section abdomen and a three-segment thorax. It also had three pairs of walking legs, features associated with insects. But the animal possessed nine additional pairs of shorter abdominal legs, bringing the total to 24. Its rear legs widened into flat, grooved paddles, suggesting adaptations useful in water. The researchers named the previously undescribed genus and species Chosha praecursor, combining a Navajo word for “insect” with a Latin term meaning “predecessor.” The study, published in Nature, places the animal within an approximately 80-million-year gap in the fossil record. The researchers propose that early insects may not have moved directly from water to permanently dry land. Instead, they may have spent a period living in amphibious environments while retaining extra appendages that helped them function in water. The team also reexamined three other multi-legged fossils, including one from Scotland that lived about 405 million years ago and two from Illinois dating to roughly 306 million years ago. Together, the fossils support the possibility of a gradual transition to land. According to the researchers, losing abdominal appendages may have been an important step in adapting to fully terrestrial life. Modern insects are known as hexapods because they have six legs, but this fossil suggests their early relatives did not begin that way.