To review, here's the fossil as we found it on the beach:
Amazingly, we had trouble even identifying what type of organism this might be. For example, suggestions ranged from a walrus tooth, to a rudist (an extinct bivalve), to a barnacle, to a plant.
When we looked under a microscope, we noticed channels on the outer wall:
These eroded channels seemed reminiscent of a barnacle. For comparison, here's a close-up of a wall plate from a local barnacle, Balanus nubilus, that lives in the rocky intertidal zone:
You can see why some people thought this fossil might be a barnacle. But we were confused because we couldn't see any sutures, or divisions between wall plates, which would help confirm the identification as a barnacle. [Many barnacles have 4-6 wall plates that fit together in a circle to form the familiar shell that is shaped like a little volcano.]
We sent the photos to Bill Newman at Scripps, one of the world's foremost barnacle experts. Bill suggested that we had discovered the basis (basal plate) of a fossil barnacle! Since it was only the basis, that would explain why we didn't see individual wall plates.
Here's a local barnacle (see below). Many (but not all) barnacle species have a calcareous basal plate where they are attached to a rock (or other surface). The wall plates sit on top of that base. Most of the time, the basal plate is flat. But note that sometimes the basal plate is elongated upwards and forms a shallow cup. The blue arrow is pointing to the division between the lower basal plate and the upper wall plates.
Now imagine if that basal plate could be elongated a lot more, forming a conical tube (like our fossil!). It's an unusual shape for a barnacle, but it's typical of an extinct barnacle called Tamiosoma gregaria (formerly Balanus gregarius).
We found some older illustrations of Tamiosoma fossils, so here's a comparative view from above — the fossil we found is on the right:
Left-hand illustration from Pilsbry, H. A. 1916. The sessile barnacles (Cirripedia) contained in the collections of the U.S. National Museum, including a monograph of the American species. Bulletin of the United States National Museum 93.
Tamiosoma has been found in fossil deposits from central California to Baja. It's thought to have lived in shallow embayments between the early Miocene (16-23 million years ago) through the Pleistocene (2.6 million to 11,700 years ago).
Previous studies suggested that Tamiosoma's very elongate base might have been an adaptation to living in mud where sediment accumulated rapidly. [Fun fact: Tamiosoma means "distributed body" and might refer to its unusual elongate form.]
Although we don't know what Tamiosoma looked like when alive, Eric thought it would be helpful to imagine how this amazing barnacle might have been positioned in the mud. He scanned the fossil basis that we found, and then added wall plates (from a drawing of a Tamiosoma fossil) and cirri (feeding appendages). The barnacles probably attached to small hard objects (e.g., a piece of shell) on the surface of the mudflat, and then continued to grow upward as sediment accumulated around them over time. Sometimes multiple individuals were clustered together. Check out Eric's excellent illustration:
P.S. Many thanks to everyone who contributed to this story, especially Bill Newman, folks on the Fossil Forum, and Karen Whittlesey's thesis on this intriguing barnacle!
P.P.S. The next question — where did the fossil we found come from? Do you know of fossil deposits in the Bodega Bay area?