Diving into the Oligocene ocean

A few weeks back we headed out for a hike along the shoreline near the West Twin River, on a beautiful night just before Washington's stay at home order kicked in.  As we were poking along, we came upon some of the usual bivalve fossils that are quite common in the area:

but also ran into some quite nice examples of fossils of a type I'd seen before, but less frequently...and I'd never quite been able to figure out what I was looking at.  To me they look sort of kelpy:

But as I understand it kelp fossils are pretty rare.  Also, if you look closely, many of these fossils have a lot of internal structure going on, in a way that kelp don't:

So I reached out to Liz Nesbitt at the University of Washington, who I had the good fortune to meet doing some field work at Discovery Bay a few years ago.  Liz has to be the foremost expert on the paleontology of Washington's coast, and so I was very delighted that she replied to my email quickly, and not at all surprised that she could explain what I had seen.  

Turns out that these are teredolites (here is a nice example of some similar fossils from Wyoming) or fossilized wood that has been bored by clams of the genus Teredinidae, commonly known as shipworms.  

Liz Nesbitt is also an expert on the chronology of the strata that these fossils are associated with, and she places these in the Oligocene, a time in which what are now the Olympic Mountains were just emerging from the ocean fringing the North American continent.  Where I found these fossils was presumably a warm (the Oligocene was substantially warmer than today's world) fringing sea.  In fact, one of the oldest known whale fossils emerged from the rock near where we found these teredolites.