Several kinds of microborings were observed on oysters' shells, belemnites' rostra and other fossil fragments sampled in the «Marnes à Ostrea acuminata formation (upper Bajocian) from Burgundy. These microborings were identified as the result of endolithic fungi (Ostracoblabe and Dodgella like structures) and endolithic chlorophyceae (very close to recent Codiolum activities). Their qualitative and quantitative distribution shows a close relationship with the gradation of the 6 biosedimentary facies recognized by one of us.

Microborings appear to be very helpfull indicators of bathymetry, and specially those of the Codiolum type of which recent representatives are restricted to the upper part of the photic zone (from 0 m to 50 m depth). Such an example appears to be a new case-history of the usefullness of microtrace fossils in paleoenvironmental studies