One of the characters I discussed in my SEAVP talk last Thursday was the shape of the periotic bone (or petrosal). The periotic is one of the earbones, and is fantastically complex; it houses the cochlea, and several nerves run through it. I specifically talked about the shape of the posterior process of the periotic. This is not the most “character-rich” part of the periotic, but it does provide some useful information.
The periotic shown at the top is from a modern specimen of Balaenoptera. The posterior process is the long projection on the left. In Balaenoptera the process is typically very long and flat.
Here is the Carmel Church specimen of Diorocetus hiatus; again, the posterior process is on the left:
Note that the process is relatively short, and that it’s largest near the tip (this is even more obvious from other angles). In the Calvert Formation these short, fat posterior processes are found in Diorocetus and Parietobalaena (Parietobalaena has not yet been identified from Carmel Church, but it’s the most common baleen whale in the rest of the Calvert Formation).
Now, here is Eobalaenoptera harrisoni (this time the posterior process is on the right):
Eobalaenoptera has a very long posterior process, like Balaenoptera. However, unlike the flat, blade-like process in Balaenoptera, E. harrisoni’s process is a massive cylindrical structure that is widest near its midpoint. Among the Calvert whales, this is also seen in Aglaocetus patulus (also known from Carmel Church), although it’s not so massive as in Eobalaenoptera:
There are two other posterior process morphologies seen in the Calvert. I don’t have a picture of Pelocetus, which has a long skinny process with an expanded tip. The last type occurs in the poorly known genus Metopocetus (here the periotic is outlined in green, with the posterior process at the bottom):
Metopocetus is generally considered to belong to a group of whales that includes Herpetocetus. One of the important characters of the group is the presence of a hugely swollen posterior process that reaches all the way to the lateral surface of the skull (as big as the posterior process is in Eobalaenoptera, it’s not that big).
In the Calvert, Metopocetus is “officially” only known from a single skull fragment, but we have several skulls in the VMNH collection with this type of posterior process, and they all come from Calvert Bed 14 (just like the holotype Metopocetus). These skulls also have certain similarities to each other in the vertebral column, especially the atlas vertebra.
Last week we opened another Carmel Church whale skull (below). We’ve already prepared the atlas vertebra that goes with this skull, and it’s very similar to the other atlases that are associated with Metopocetus-type periotics. So I’m going to go out on a limb and make a prediction that this skull will also have a Metopocetus-like periotic.