Cryptic fauna in nautiloid shells from the Ordovician of Estonia

Abstract

The Middle Ordovician cryptic encrusting community was numerically dominated by bryozoans (66.7%), followed by cornulitids (26.7%) and graptolites (6.6%). Bryozoans also occupied the largest encrustation area. Similarly, the Late Ordovician cryptic encrusting community was dominated by bryozoans (60%), while cornulitids (30%), brachiopods (8%), and graptolites (2%) formed a minor part of the community. In the studied samples, the disparity of Upper Ordovician cryptic encrusting fauna (including Bryozoa, Brachiopoda, Tentaculita, and Hemichordata) was greater than that of the Middle Ordovician, which consisted of Bryozoa, Hemichordata, and Tentaculita. There appears to be an increase in the maximum encrustation density within nautiloid shell interiors from the Darriwilian to the Katian. Additionally, a statistically significant increase in the percentage of encrusted nautiloid interiors from the Darriwilian to the Katian suggests a growing prevalence of cryptic encrusters over time. The increase was likely driven more by the evolutionary diversification of encrusting taxa than by the warming climate during the Katian in Baltica.