Sclerobionts and some representatives of the endobenthos of soft substrates of the Lower Frasnian of the East of the Main Devonian Field

Abstract

The article considers sclerobionts on the shells of the Frasnian bivalves and brachiopods of the east of the Main Devonian field, as well as common traces of bioerosion of biogenic and abiogenic substrate. In contrast to the averaged data on the Frasnian brachiopod sclerobionts, which demonstrate a clear dominance of auloporids, craniform brachiopods, bryozoas and microconchids, microconchids predominate among Frasnian bivalves sclerobionts and brachiopods of the Main Devonian field and among the eroding sclerobionts — the ichnogenus Arachnostega Bertling. Next in the degree of frequency of occurrence are cornulitids, then auloporids and eroding sclerobionts of the ichnogenus Trypanites Mägdefrau. The ichnogenus Arachnostega Bertling was first identified in the Frasnian rocks of this region. Some Frasnian sclerobionts of the east of the Main Devonian field were inhabitants of a fairly wide range of conditions. For example, microconchida Palaeoconchus omphalodes (Goldf.) are found both on shells burrowing bivalves of inhabitants of soft muddy bottom, and on shells of various representatives of epifauna that lived in conditions of both firmground and hardground. Representatives of three different ethological groups have been identified in cornulitids — fully cemented to the substrate; with a raised anterior part and free-living, attached only at the juvenile stage. The maximum morphological and ethological diversity of cornulitids was recorded on the bivalves of the Main Devonian field from the Dubnik beds corresponding to the maximum of the Frasnian regression. The active development of cornulitids in the Dubnik time was probably favored by the lack of competition with other groups of organisms of similar ethology. A fairly wide range of morphological types of cornulitids isolated within one stratigraphic level indirectly confirms O. Vinn’s hypothesis about cornulitids as a group of biological opportunists. The author proved for the first time the presence of two morphologically similar ichnogenera Trypanites Mägdefrau and Palaeosabella Clarke in the Frasnian rocks of the Main Devonian field.