Pholad borings

[1] Pholads are unusual bivalves capable of boring into various kinds of rock, firm clay and peat, and they live permanently in the burrows they create.

These clams use a set of ridges or "teeth" on the outer anterior surfaces of their shells to grind into suitable substrate, creating a non-branching burrow.

[2] Ancient, sand-filled pholad borings are a distinctive type of trace fossil known as Gastrochaenolites that are found in some limestones, dolomites and lithified mudstones.

They are assigned to the Trypanites ichnofacies,[3] and evidence submarine hardgrounds that formed soon after deposition, typically during episodes of non-sedimentation, with subsequent infilling of the burrow and burial when sedimentation resumed.

[4][5] Pholad burrow trace fossils are not to be confused with specimens of the Trypanites ichnogenus, which are toothpick-sized masses of holes that characterize some hardgrounds.