Walcott Rust fossils reveal secrets of trilobite enrolment

How did trilobites curl up into a protective ball? The great diversity of trilobite fossils has answered this question thanks to their durable biomineralized dorsal exoskeleton, but what happens underneath has been a mystery for over a hundred years! thanks to new research into the Ordovician Walcott-Rust Quarry led by Sarah Losso, in collaboration with lab members Pauline Affatato (MEME student) and Karma Nanglu (postdoctoral researcher), we now have an unprecedented view of the trilobite's "dark side". LIke other arthropods, trilobites have ventral exoskeletal plates called sternites, which articulate with each other. Sarah et al. found that the pattern of sternite articulation in trilobites is extremely similar to that of extant arthropods that also enrol in a protective ball, including isopod crustaceans and pill millipedes. This shows that despite being separated by hundreds of millions of years of evolutionary history, all these arthropod species basically reinvented the same mechanics for curling up effectively for protection. The results are published Proceedings of the Royal Society B, and are also featured in the New York Times, and the Harvard Gazette.

Partially curled specimen of the trilobite Flexycalymene senaria from the Walcott-Rust Quarry in sagittal cross section showing exceptional preservation of how the protopodite bases fit relative to each other during enrolment	Morphological and functional comparison of ventral adaptations for enrolment in an isopod (a) and a glomerid millipede (d).

This work was supported by a Research Grant from HFSP (Ref.-No: RGY0056/2022)