The oldest heliolitids from the early Katian of the East Baltic region

Abstract

The earliest heliolitids appeared more or less simultaneously in the Late Ordovician shallow seas of different palaeocontinents. In this paper the early Katian heliolitids, represented by the genus Protaraea, are described from the East Baltic region (North Estonia and north-west Russia) and discussed in detail for the first time. A new species Protaraea procella n. sp. Mõtus is established and a neotype for Protaraea diffluens (Eichwald) is designated. The new species shows a wide intraspecific variability including variably oriented septal trabeculae, which results in corallites commonly being poorly defined or not apparent in transverse sections of coralla. P. procella has much larger coralla than P. diffluens, with predominantly laminar growth forms, whereas the latter species is usually encrusting, suggesting that the two species were adapted to different palaeoenvironmental conditions. The new data indicate that these heliolitids, together with other tabulate corals, were already geographically dispersed in the early Katian and thereby contribute to a better understanding of coral diversification patterns in Baltica.