Sunday, 12 February 2017

SCAPHOPODA: TUSK SHELLS

The Oligocene Lincoln Creek Formation has produced several dozen different species of infaunal molluscs, burrowing worms and is well-known for crabs.

This specimen is from the massive, tuffaceous siltstone and sandstone that runs through the town of Porter on the east side of the road.

The fossil-rich bedding planes are well-exposed with concretionary beds throughout.

Collecting was possible in the late 1990’s and early 2000’s but may be forbidden (or heavily discouraged today) as the site is on a busy roadway. I made multiple trips to Porter back in the day with folk from Vancouver and Washington State. The primary focus in the early 2000’s were the crabs. They were just in the early stages of being written-up and much excitement surrounded them.

But to each his own -- as it happens, this wee tusk shell is one of my favourite fossils from the site as my trips to Porter were focused mainly on the molluscs.

Tusk shells, are members of a class of shelled marine mollusc with a global distribution. Shells of species within this class range from about 0.5 to 15 cm in length. This fellow is 8 cm end to end, so near smack dab in the centre of his cohort.

The Scaphopoda get their nickname "tusk shells" because their shells are conical and slightly curved to the dorsal side, making the shells look like tiny tusks (picture a walrus or mammoth tusk in your mind’s eye). The scientific name Scaphopoda means "shovel foot," a term that refers to the "head" of the animal, which lacks eyes and is used for burrowing in marine sediments.

The most distinctive feature of scaphopods, however, and one that differentiates them from most molluscs, is the duo openings on their tubular shells. Most molluscs are open at just one end.

We could call scaphopods the great deniers. They live their adult lives with their heads literally buried in the sand. A tiny bit of their posterior end sticks up into the seawater for water exchange. Water is circulated around the mantle cavity by the action of numerous cilia.

When the available dissolved oxygen runs low for this fellow he ejects water from the yop end of his shell by contraction of his "foot."