Periotic bone in baleen whales

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 BalaenopteraE. 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.

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This entry was posted in "Caroline", "Picasso", "Sinistra", Carmel Church mysticetes, Carmel Church Quarry, Chesapeake Group and tagged . Bookmark the permalink.

6 Responses to Periotic bone in baleen whales

  1. Brian Beatty says:

    As you know, I am convinced you have something there, particularly of importance to understanding the relationships of Aglaocetus with other Miocene mysticetes.
    What I wonder is, how many phylogenies have paid attention to the posterior process of the periotic? I am not familiar enough with the characters used in these phylogenies, but it seems like something of an elusive character, mainly because the more complete the specimen, the less visible it is.

  2. Alton Dooley says:

    I don’t think it’s been used a great deal, in part because there is a fairly large amount of individual variation that make it difficult to quantify. Even so, I think the general trends (long and blade-like, short and fat, etc.) work well.

    There has been some usage, however. In the Eobalaenoptera description we used the shape of the posterior process as an apomorphy of Eobalaenoptera (at the time we weren’t aware of its similarity to Aglaocetus). The exposure of the process on the lateral side of the skull was used by Whitmore and Barnes as a synapomorphy of the Herpetocetinae (no cladistic analysis, but that’s still what it amounted to), and Bouetel and Muizon used it as a synapomorphy of Cetotheriidae sensu stricto (probably equivalent to Herpetocetinae).

  3. Boesse says:

    There is some interesting variation within the Cetotheriidae ss. that likely may provide some phylogenetic info – for example, Herpetocetus, Nannocetus, and Piscobalaena all share a posterior process that is far shorter (along the mediolateral axis) than Metopocetus and most other mysticetes. In Herpetocetus (and possibly Nannocetus), there is also a dorsal extension of the posterior process, so that it looks antero-posteriorly compressed as well.

    Thought I’d mention – after adding several characters, and including more species of Herpetocetus into Bouetel and Muizon’s phylogeny, the phylogeny I presented at SVP supported the lateral exposure of the PP as a synapomorphy; then again, I still need to add characters, taxa, and my H. bramblei specimen into the phylogenies of Bisconti and Demere and Berta (and Steeman) to see where they all plot out.

    The anterior process of that periotic is very interesting… I don’t have a good photo of the type periotic on me, but lets just say that looks very similar to Mio-Pliocene Nanno- and Herpetocetus – and that goes for the squamosal as well. Beautiful specimen!

  4. Alton Dooley says:

    I’m not 100% positive that the specimens we have at VMNH at Metopocetus; the posterior process appears to be much larger in the type Metopocetus, although it’s difficult to say for sure. The type skull is just a fragment of the cranium. But at this point Metopocetus is the only named herpetocetine in the Calvert Fm.

    There is also some variation in the shape of the process among the three skull that we have. Even so, I think they’re likely the same taxon.

    Unfortunately, none of the VMNH skulls include a preserved posterior portion of the dentary, and the vertex is damaged or missing in all the specimens (although I might be able to reconstruct it in two of them).

    Bobby, I was trying to post on your blog awhile back, but it’s set up so that you need a Google login to post. Any way to change it so that anyone can post?

  5. Boesse says:

    I’ll try poking around with that tommorrow. I thought you could post without one, but I’ll check – if I find out how to without a login, or change it, I’ll let you know. I’ve been pretty patchy with communications – I’ve been out of cell service and internet (and relying on the radio and word of mouth for news, weather, etc.) out in southwestern MT (TA’ing field camp).

    Keep up the whale posts! I really wanted to go to SEAVP this year; will it be at VMNH next year? If so, I’ll make time for it.

  6. Alton Dooley says:

    I leave for Wyoming in two days, so everything will be shifting to Mesozoic stuff for a few weeks.

    SEAVP will be meeting again next year, but not at VMNH. The official announcement of the location will hopefully be available soon.

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