Shell microstructure of Cornulites semiapertus Opik, 1930 and other early cornulitids from the Ordovician of North Estonia

摘要

The shell microstructure of Ordovician Cornulites species is in general similar to Silurian species. The earliest cornulitid from Baltica, C. semiapertus Opik, 1930 (Darriwilian), has vesicles,lamellar shell structure, but no pseudopunctae. The earliest Cornulites with pseudopunctae appears in the Katian. The initial shell structure of cornulitids may have been microlamellar, but without pseudopunctae.