A morphologically distinct type of trace fossil left by boring organisms in hard biogenic substrates. Oichnus Bromley 1981 is a formal taxonomic name with the rank of ichnogenus (a trace fossil genus). Oichnus is a Latin-derived term that literally means a trace (ichnus) shaped like the letter “O”; it denotes the small circular to subcircular holes (borings) found primarily in fossilized skeletal remains of invertebrate animals, such as mollusk shells, ostracode valves, or echinoid tests.