Fossils reveal the nervous system of a 508-million-year-old chelicerate

January 24, 2022

A new study in Nature Communications describes the exceptionally preserved nervous system of the soft-bodied euarthropod Mollisonia symmetrica from the mid-Cambrian Burgess Shale in British Columbia.

Mollisonia symmetrica is a member of a relatively inconspicuous group of euarthropods knows as mollisoniids, which have a simple dorsal exoskeleton consisting of a head shield, multisegmented trunk, and a posterior shield. Although recent studies suggest that mollisoniids are related to chelicerates, whose extant representatives include horseshoe crabs and arachnids, little was known about their internal anatomy.

Fossilized central nervous system and morphological reconstruction of Mollisonia symmetricaThe study led by Professor Javier Ortega-Hernández and members of his lab, examined specimens with exceptionally preserved internal organs, such as the digestive system and neurological remains including the optic nerves, the ganglionated ventral nerve cord, and even branching segmental nerves. 

"Altogether, the results suggest that Mollisonia's central nervous system is an ancestral relative to that of extant chelicerates," Ortega-Hernández said, "which can help us understand the early history of the group, but also suggest a complex pattern of mosaic evolution in which the internal and external anatomy are giving off different signals."

The researchers examined specimens from Invertebrate Paleontology collections at the Museum of Comparative Zoology (MCZ) and the Smithsonian Institution. The study was funded in part by the MCZ Wetmore Colles Grant.

The study was featured in the Nature Portfolio Ecology & Evolution Blog.


Images: 1) Mollisonia symmetrica from the 508-million-year-old Burgess Shale in British Columbia. MCZ 1811 is preserved in dorsal view and shows the overlap bewteen the digestive and nervous systems, and also well-developed eyes. Images by Javier Ortega-Hernández.  2) Fossilized central nervous system and morphological reconstruction of Mollisonia symmetrica (USNM 305093). Note that the nervous system (purple in the diagram) is expressed as a continuous reflective film connected with the eyes, and which runs throughout the entire body. The digestive system (green in the diagram) is located above the nervous system, as observed in modern euarthropods. Images by Javier Ortega-Hernández.