Borings, bodies and ghosts: spicules of the endolithic sponge Aka akis sp. nov. within the boring Entobia cretacea, Cretaceous, England

The fossilised borings of endolithic sponges are generally abundant in marine sediments since the Jurassic, and sparse occurrences date back to the Lower Palaeozoic. However, the zoological identity of the boring sponge is not revealed by the morphology of the boring alone. The preservation potential of the boring is far greater than that of the spicular skeleton of the endolithic sponge. The spicules are opaline silica, which is readily soluble in seawater. Thus, while the borings are abundant and diverse, we have almost no knowledge of the bioeroding sponges that produced them. In situations where early diagenetic cementation of the sediment occurs before the dissolution of the silica, the spicules may be preserved as casts. Such a case exists in association with hardground formation. A hardground in the Upper Cretaceous (Turonian) Chalk Rock of Buckinghamshire, SE England, yielded an intraclast containing a sponge boring referable to Entobia cretacea Portlock, 1843. Many of the chambers contain sparite-filled casts of sponge spicules. Only megascleres are present, comprising smooth oxeas about 2 mm in length and 74 μm in width, indicating an extinct species of the phloeodictyid genus Aka. No part of the original skeleton of the sponge is preserved, only very accurate external moulds of the spicules, representing little more than the ghost of the animal. But this is not unusual in palaeontology and, as it appears that this body fossil is new to science, it is herein named Aka akis sp. nov. This is only the fifth account of fossilised Aka spp. where spicule morphology and erosion traces can be observed. It is hoped that further search for spicules preserved within Entobia will allow an investigation of the endolithic sponge communities of the Mesozoic and Tertiary seas, about which almost nothing is known at present.