324-Million-Year-Old, 24-Legged Fossil Insect Sheds Light on Life's Move to Land

A newly analyzed 324-million-year-old fossil insect with 24 legs is offering scientists fresh evidence for how ancient animals made the transition from water to land, according to a study published in the journal Nature.

The fossil, originally unearthed in Texas and named Chosha praecursor, had previously been classified as a type of ancient crustacean, because features such as its segmented abdominal appendages matched crustacean anatomy. An international team led by researchers at the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, working with colleagues in the UK, the US, and Spain, re-examined the fossil under cross-polarized light, which revealed more of its structure.

Under the new light, the researchers could see features specific to modern insect body plans: three distinct thoracic segments and three pairs of legs. But the fossil's abdomen also sprouted nine more pairs of appendages, which appear to have been built for use in water rather than for walking.

"This discovery serves as an evolutionary bridge between aquatic and terrestrial life forms," said Cai Chenyang of the Nanjing Institute. "Around 320 million years ago, ancient insects at the water-land interface were progressively evolving new body structures suited to breathing and moving on land."

The specimen is an adult female measuring 3.2 centimeters (1.3 inches) in length, or about 5 centimeters (2 inches) with its tail extended. It was found in an area that would once have been where water and land met, a habitat the researchers say played a key role in its evolution.

According to the researchers, once this lineage of insects was able to live out of water, its extra 18 water-adapted appendages were lost over generations, leaving the six-legged body plan that modern insects still carry today.