The distribution of worm borings in brachiopod shells from the Caradoc Oil Shale of Estonia

Abundant worm borings were found in some brachiopod shells (Clitambonites, Estlandia, Nicolella) from the Ordovician (Caradoc) oil shale in North Estonia. 9 of 21 brachiopod genera (43 %) have been bored. Excluding the size and thickness of valves, no common morphological feature discriminates the brachiopods with borings from those without them. The Trypanites are host-specific, and the frequency of bored valves varies from 6.5 % in Bekkerina to 51 % in Estlandia. The worm larvae preferred hosts with thick lamellose shells, such as those of the clitambonitids, especially Clitambonites schmidti. The boring organisms were size-selective; they preferred large adult specimens. The majority of the borings are oriented, and living hosts were preferred to dead shells. Some clitambonitidine brachiopods, like C. schmidti tolerated a large number of the borer's shafts in their valves. Few bored valves have the blister-like shell-repair structures in their interior.