Eobalaenoptera specimens

As we move the Diorocetus specimen out of the lab, new projects are moving in. Here is my next top priority – a massive lower jaw from Carmel Church. This specimen had been way down on my “to-do” list, but jumped to the top when I decided to do my SEAVP presentation on it.

I think this is the lower jaw from Eobalaenoptera, the first whale ever found at Carmel Church. In fact, we provided this jaw to RCI to use in making the Eobalaenoptera cast hanging in the museum’s Great Hall:

Here’s the problem: the type specimen of Eobalaenoptera includes cranial fragments, including the ear bones, flippers, ribs, and 28 vertebrae, but no dentaries. There is a skull at the Smithsonian which I believe is a second Eobalaenoptera specimen, but it also lacks the lower jaws. Therefore, I can’t be positive the jaw really is Eobalaenoptera. Here’s the case for it:
  1. Using the skull fragments from the type specimen and the Smithsonian skull, we calculated about how long the lower jaw should be, and this specimen is exactly the predicted length (and we really did it in that order). Moreover, Eobalaenoptera is the largest described whale from the Calvert Formation; only Pelocetus approaches it, but it’s still not as large. There may also be another large, unnamed mysticete from the Calvert that approaches this size, known from one specimen, but it’s still smaller.
  2. The specimen is from Carmel Church, which is the type locality for Eobalaenoptera. It was actually found in 1991 when we were excavating the holotype, about 200 feet away (no way they’re from the same individual, though). Here’s the dentary during excavation:
  3. Although we excavated the dentary, we didn’t do any large-scale follow-up excavation at the same spot. We did find some other heavily-weathered bones in about the same area (within about 2 meters) over the years, though. These included a few large vertebrae, consistent with Eobalaenoptera, an enormous vomer (not preserved in the other specimens), and a tympanic bulla. The bulla is similar to the type Eobalaenoptera bulla. It’s also similar to the much smaller Aglaocetus patulus (an interesting feature of Eobalaenoptera is that it has tiny tympanic bullae for the size of the animal).

So, is this dentary really from Eobalaenoptera? It’s certainly not a slam-dunk, but it looks pretty good, although I can’t definitely rule out Pelocetus or some unnamed large mysticete at this stage.

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10 Responses to Eobalaenoptera specimens

  1. Boesse says:

    Perhaps once the dentary is assembled, it will have a different curvature than Pelocetus in dorsal aspect, or maybe a different cross section.

    I don’t have Kellogg’s monograph handy right now; does Pelocetus have the anteriordentary twist so that the medial surface is sorta dorsomedially oriented? Demere (1986) considered that a feature of Balaenopterids (although it definitely occurs in Herpetocetus and Piscobalaena).

    The first complete dentary I collected (Herpetocetus, naturally) had the posterior end tangentially oriented with respect to the cliff face, and then curved in, requiring a 3′ hole straight into the cliff. What’s worse, the locality was ‘cordoned’ off by two points, so it was only accessible at very low tides (which, at the time of discovery, very early in the morning).

  2. Alton Dooley says:

    Looking at Kellogg’s drawings of Pelocetus this morning, it doesn’t appear to be very similar to the Carmel Church specimen. The dentary in Pelocetus is strongly bowed, almost like a right whale. In addition, in Pelocetus the condyle is massive, almost circular in posterior view, and there’s a pronounced angle that projects well below the posterior margin of the rest of the dentary. In the Carmel Church specimen the angle is not so pronounced, and the condyle is transversely very narrow, only about half as wide as it is tall.

  3. Doug says:

    Well you won’t know until you put it back together. I don’t know, tyrannosaurs have taught me quite a bit about naming fossils. On one hand, you have Appalachiosaurus, who was since the 80’s listed as a species of Albertosaurus, a species already well known. Then just four years ago it was found to be a new genus and species! On the other hand, we have Nanotyrannus. It was thought to be Grogosaurus. Then it it became it’s own genus and species. Now we think it’s a juvenile of a species already known (T. rex).

    Anyway, your post here helps me understand that’s there’s more diagnostic features to a whale other than the skull (as the Diorocetus ribs did also).

  4. boesse says:

    Alton,

    I’m sure you’ve read this, but I remember Demere (1986) mentioning that of all the Calvert Fm. “cetotheres” (s.l. of course), Pelocetus had the most bowed dentary, and likewise also the broadest rostrum, a relationship similar in different modern species of Balaenoptera. I guess it makes sense that if you have a larger cross sectional area of the mouth, a wider rostrum might be necessary to generate a greater normal force to the palate during opening of the mouth, to keep the entire animal from doing a nose dive during this process.

    When you go out and collect your own fossil material to study yourself, you often (not always, but often) collect material that is of slightly less quality than the material already existing in museum collections. So, often if someone wants to identify or study most of what they collect, they must rely on features not just associated with 100% preserved and articulated skulls (although those are very nice when found, especially when you find them yourself). This is primarily due to the fact that many historical collections in museums are made from fossil sites that have been prospected for decades, and sometimes over a century (as is the case with the Chesapeake Group in Virginia/Maryland). Unfortunately, there are currently a lot of graduate students in paleontology (and marine mammal paleontology is not immune to this) who do no fieldwork and rely upon currently established museum collections, which is a somewhat disturbing trend. Alton told me at SVP that there is no reason to not publish on fragmentary or incomplete material, and as another marine mammal researcher who digs up his own stuff, I wholeheartedly agree.

    Anyway… ya, mysticete jaws are neat!

  5. Alton Dooley says:

    That’s an interesting idea, Bobby, concerning the wide rostrum. I’m not sure it works behaviorally, though. I think most lunge feeders actually roll to one side in feeding (i.e. they’re handed), as was talked about a bit on TetZoo (coincidentally, Brian Beatty and I have been looking at mysticete handedness over the last few weeks). Moreover, even if a whale were feeding horizontally, the cervical series is so inflexible in “cetotheres” and balaenopterids that I think the rostrum would always have to be held parallel to the direction of motion, so any forces on it normal to the direction of travel would be close to zero, no matter its surface area.

    Interesting also that the mysticetes with the most bowed mandibles, the right whales, have the narrowest rostra. Maybe the bowing in the jaws is related to swimming speed and volume of water processed. In other words, you can process the same amount of water by swimming fast with narrow jaws, or swimming slow with wide jaws.

    Or maybe bowed jaws have completely different functional significance in balaenopterids/Pelocetus than in balaenids; I’m making this up as I go (can you tell?).

  6. Boesse says:

    Ya, I was making it up as I went as well, sort of semi-intellectual verbal diarrhea, if you will…

    Yes, balaenids would be a departure from the wide rostrum/bowed mandible phenomenon; however I think you might be right that since balaenids are skim-feeders, the relationship might not be completely comparable, thus potentially explaining the discrepancy.

    In any event, Demere 1986 used that to predict that Balaenoptera davidsonii would have a narrow rostrum, and then Demere 2005 referred a narrow-rostrumed skull of the right length to B. davidsonii, using that prediction.

    What I was referring to with regards to the normal force on the palate is during the opening of the mouth, Goldbogen et al. (2007) mention a force exerted on the mouth/mandibles. This causes a normal force to also be exerted on the palate; I guess this is what you were talking about, anyway. In any event, this may be the reason for the interdigitation of rostral/cranial elements at least in balaenopterids (and possibly as well as in cetotheriids, as you mentioned on tet zoo). Anyway, just some speculation on my part.

  7. Alton Dooley says:

    Ha, “semi-intellectual”, better than “pseudo-intellectual” 🙂

    As you pointed out, by saying that there are no normal forces on the skull, I directly contradicted what I said on TetZoo a few days ago! Is that science or what?

    So maybe there IS a normal force. Perhaps when the throat is inflated, it changes the animal’s trim such that the cranium/vertebral column are not parallel to the direction of movement (not as unlikely as it sounds; DC-10s famously flew most efficiently with a 4 degree positive trim).

    That brings us back to Pelocetus, which does not have the interdigitating rostral bones, so presumably was not encountering high rostral forces even with its broad rostrum.

  8. Brian Beatty says:

    What is the rostrum of Diorocetus like, in terms of interdigitation? I don’t recall that it is, which is interesting if it was a benthic and/or ram-feeding “skimmer” like gray whales and balaenids.
    Regarding the arching of balaenids, I believe Alex Werth pretty conclusively determined its role in the flow regime during feeding, and as I recall, some other workers had measured the filtering area in balaenids and found that the arch made a huge difference in increasing the area as compared to balaenopterids in proportion to body size.
    Could the arching found in some balaenopterids simply be there to maximize the total area of the mouth when open? For an engulfment feeder, the gape is so wide and the mandible rotated so much along its longitudinal axis, that it would make sense to curve the mandible to increase that area when open.
    What do you think?
    Brian

  9. Boesse says:

    Alton,

    Expounding a little further on this normal force – the way I understood it from the Goldbogen et al. paper is that once the mouth is opened, the force by the water on the inside of the mouth will ‘pull’ (so to speak) the head ventrally; this ventral rotation causes another normal force in a dorsal direction by the water onto the ventral surface of the rostrum, which I believe Bouetel (2005) argued why some mysticete crania are so interdigitated.

    Now, one thing that I’ve been wondering about is how exactly cetotheres sensu lato were feeding. I suspect that it is safe to say they were not skim feeding; perhaps they were engaging in some form of less-efficient, less-evolved form of lunge feeding (i.e. not opening the mouth to the full 90 degrees).

    As far as I can reason, lunge feeding or something similar sounds like the primitive feeding behavior, especially when one considers another part of Goldbogen et al. 2007, about ‘biting little pieces off of a superorganism’; effectively it is a modification of raptorial predation, which (thanks to aetiocetids) appears to be the feeding method prior to the acquisition of baleen. In that context, lunge feeding seems less of a departure from raptorial predation than skim feeding or ‘benthic’ filter feeding. Again, just some ideas I’ve been playing with recently while thinking about Herpetocetus…

  10. Alton Dooley says:

    After several hours of birthday partying, I have to think seriously about hydrodynamics? Oh my…

    Diorocetus essentially doesn’t have any interdigitation of the rostral bones; it’s like Pelocetus and Parietobalaena in that respect. Aglaocetus patulus and Eobalaenoptera are essentially similar, with apparently only a slight posterior extension of the maxillae.

    My starting assumption has always been that the arched rostrum is just to increase mouth volume, and I certainly think that’s true for balaenids. In balaenopterids I’m not so sure, though. The increase in volume due to arching seems so insignificant compared to the huge throat volume when the pleats are expanded. I wonder if the rostral arch has some hydrodynamic significance in non-feeding swimming. Perhaps it evens out turbulent water flow over the skull. Or maybe it’s related to maximizing baleen plate length. When the mouth is closed (non-feeding), the mouth is so shallow that maybe the arched rostrum allows maximum baleen plate length with the minimum increase in cross-sectional area (this is 3 bottles and a lot of birthday cake speculation!).

    I don’t think we should assume that lunge feeding is ancestral. It might be, but it is possible to make a case for other ancestral feeding modes. However, I agree that cetothere feeding modes might have been different from balaenopterids, and in fact there may have been a great diversity of cetothere feeding styles. Certainly modern mysticetes have shown that behaviorally there are many different possible feeding modes–lunge, skimming, benthic, engulfment from below, lateral head sweeps–and Megaptera alone does all of those, I believe.

    I’m going to be driving to Carmel Church tomorrow, so I won’t be online much, but by all means continue the discussion. Lots of good ideas floating around in these comments!

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