Some encrusting: cheilostome bryozoans etch a pattern of small pits into ha
rd calcareous substrates, especially calcitic and aragonitic shells of moll
uscs. These patterns, herein described as Leptichnus ichnogen. nov., pits w
hich 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 si
ngle zooid in the bryozoan colony. The oldest known Leptichnus is Late Cret
aceous (Maastrichtian), the trace fossil first becomes common in the Cenozo
ic, and at least nine modem 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 unkno
wn but is almost certain to be chemical and necessitates having windows in
the basal walls of the zooids which permit contact with the substratum bene
ath. Etching may result in better adherence to the substrate, giving protec
tion from abrasion and bioerosion.