The Berriasian–Albian Speeton Clay Formation is renowned for its rich and diverse belemnite fauna. Borings occur in a considerable proportion of the belemnite guards that can be found in the type locality south of Filey, but have never been described. This preliminary survey of the endolithic biota colonizing Speeton belemnites reveals the presence of at least 15 ichnotaxa, including traces made by grazing echinoids (Gnathichnus pentax), brachiopod pedicles (Podichnus centrifugalis), and a variety of macro- and microendoliths. The commonest macroendolith borings belong to two ichnospecies of Trypanites and were probably excavated by polychaete worms. Examples of borings made by other suspension feeding animals, including bivalves (Gastrochaenolites lapidicus), acrothoracican barnacles (Rogerella mathieui) and sponges (Entobia), are less abundant. Among the microendoliths, Orthogonum ispp. and Saccomorpha clava, both of which are very common, were probably made by fungi, whereas Semidendrina pulchra is thought to be the trace of a boring foraminifer. The tentative identification of the microboring Rhopalia catenata, which is made by autotrophic chlorophyte algae, suggests that at least some of the Speeton Clay Formation was deposited within the photic zone. Endoliths in the Speeton belemnite guards have future potential for developing a better understanding of the changing environments on the seabed during the long period of deposition of the Speeton Clay.

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