A new ichnogenus for etchings made by cheilostome bryozoans into calcareous substrates

Some encrusting cheilostome bryozoans etch a pattern of small pits into hard calcareous substrates, especially calcitic and aragonitic shells of molluscs. These patterns, herein described as Leptichnus ichnogen. nov., comprise pits which are sub-circular to elongate in cross section and are found in either uniserial (L. dromeus isp. nov.) or multiserial arrangements (L. peristroma isp. nov., the type species). Each pit corresponds to the location of a single zooid in the bryozoan colony. The oldest known Leptichnus is Late Cretaceous (Maastrichtian), the trace fossil first becomes common in the Cenozoic, and at least nine modern cheilostome genera produce incipient Leptichnus. Leptichnus can be the only evidence remaining of encrusting cheilostomes following taphonomic or diagenetic loss of their calcareous skeletons. The mechanism by which bryozoans etch into their calcareous substrates is unknown but is almost certain to be chemical and necessitates having windows in the basal walls of the zooids which permit contact with the substratum beneath. Etching may result in better adherence to the substrate, giving protection from abrasion and bioerosion.