Urokodia fossil reveals chelicerae and book gill origins

The fossil Urokodia aequalis, discovered in China's Chengjiang biota, has provided new anatomical details that shed light on the evolutionary origins of chelicerae and book gills in chelicerates. Published online on July 1, 2026, in the journal Nature, the research utilized microtomography reconstructions and phylogenetic analyses to re-interpret these crucial anatomical structures.

These findings offer an updated understanding of how these defining features of the chelicerate subphylum, which includes arachnids like spiders and scorpions, as well as horseshoe crabs, first emerged. The Chengjiang biota, known for its exceptionally preserved Cambrian fossils, provided a unique window into early animal life, and Urokodia aequalis is now contributing significantly to our knowledge of arthropod evolution.

The detailed microtomography scans allowed researchers to visualize the internal and external morphology of Urokodia aequalis with unprecedented clarity. This enabled a more precise comparison with extant chelicerate species and their fossil relatives, facilitating a robust phylogenetic placement and a clearer picture of the evolutionary trajectory of chelicerae and book gills. The study's implications extend to understanding the broader diversification of arthropods during the Cambrian explosion.