324-Million-Year-Old Fossil Shows Insect With 24 Limbs

Under cross-polarized light, a Texas fossil changed identity. Researchers from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, working with colleagues in the UK, the US and Spain, now identify Chosha praecursor as a 324-million-year-old insect rather than an ancient crustacean.

The new view exposed three distinct thoracic segments and three pairs of legs, the basic insect body plan still seen today. It also revealed nine pairs of abdominal appendages—18 additional limbs—that appear suited to use in water. The animal therefore combined six insect legs with structures adapted to an aquatic environment.

That combination matters because the fossil came from a place that was once an interface between water and land. The researchers describe it as an evolutionary bridge: around 320 million years ago, ancient insects may have been developing body structures for breathing and moving on land while retaining equipment for life in water. Shifts in water temperature and oxygen, scarce resources and access to new habitats are among the possible pressures discussed around this transition.

So what changes, concretely? For paleontologists, the specimen widens the map of how life colonized land. The transition was not necessarily a single march led by a few familiar groups; insects and other invertebrates may have explored it through several different body plans. Fossil insects also give researchers a way to study the history of life on Earth, a field known as paleoentomology.

The specimen appears to be an adult female, measuring 3.2 cm—around 5 cm with its tail extended. Its anatomy supports a transitional interpretation, but it remains one fossil from a distant ecosystem, not a complete record of how the appendages changed over generations. The study, published in Nature, connects that narrow glimpse to the later success of insects: more than a million described species belong to the insect class, while around 5,000 species belong to the other hexapod group, Entognatha.