Proplina cornutaformis
Other Mollusc
Lower Ordovician
Gasconade Dolomite
Ozarks, Sullivan, Missouri, USA

Two monoplacophorans, 20mm and 15mm on 42mm matrix.

Proplina 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.

Here’s a nice matrix piece with two of them on it.