Pholad boreholes

In the first pictures of my last blog, I showed boreholes made by pholad clams (bivalves) in relatively soft sediment. I decided to expand somewhat on the topic and show pholad boreholes made in harder substrate. Pholad boreholes are trace fossils made by highly specialized clams belonging to the family Pholadidae (pholad, for short) that bore into hard substrate consisting of mud, rock (including cement-covered pilings), coral, shell, and wood. 

The pholads shown below all bored into hard rock and are representative of pholad boreholes commonly found along rocky shorelines of ancient and modern oceans.

Boring is done by the rough file-like anterior of the shell, which is twisted back and forth by the clam, which, as a juvenile, enters a small hole in the rock. As the clam grows larger, it enlarges the borehole to accommodate its growing shell. The borehole is periodically cleaned of debris by jets of water emanating from inside the clam.

These pholad boreholes were made in solid, hard rock (8 cm in width). The middle borehole has both valves of the actual clam, which is in its living position.

A transported clast (eroded rock material) of pholad boreholes in an upper Pliocene, shoreline deposit, Simi Valley, southern California.  Other fossils are fragments of pectinid clams. The blue-colored part of the rock handle is 18 cm long.

Rocky shoreline large cobble containing several empty pholad boreholes. The large cobble has a maximum width of 15 cm.

Sand fillings (casts) of a cluster of pholad boreholes (the rock that once surrounded these boreholes is missing). The rock is 5 cm in width.