Cyrolites carinatus
Upper Ordovician
Kope Formation
Campbell County, Kentucky, USA

34mm monoplacophoran

Cyrolites is an early mollusc (Class Monoplacophora) with a cap-like shell known primarily from the Late Cambrian to Early Ordovician of North America with occurrences in South Korea and Antarctica.  It lived during a time when monoplacophorans and other molluscs were diversifying.

Monoplacophorans disappeared from the fossil record by the end of the Ordovician and were therefore thought to have gone extinct so it was quite a surprise when scientists discovered modern specimens from deepwater environments in 1952.