Monday, February 15, 2010

Benthic feeding in basal mysticetes, part 2: Mammalodon, the freak

Sorry about the "short" delay; the last week or so has been pretty rough, between my Paleobiology course, getting sick, and administering/grading my first exam. But, all of that's over now, so I've been able to allot some time to this again.

My last post concerned paleopathology and histology of the "cetothere" Diorocetus hiatus, published by Brian Beatty and Alton Dooley last summer in Jeffersoniana. In it they posited that Diorocetus may have been a benthic feeder based on peculiar pathologies as well as an interesting pattern of rib osteosclerosis.

Comparison of the size of Mammalodon and a blue whale. From

There's been some buzz in the science blogosphere recently about the truly bizarre Australian toothed mysticete Mammalodon, the subject of a new monograph by colleague Erich Fitzgerald. I'll admit I've been anticipating this paper since Erich's talk at SVP in 2008 (the same session as my first ever SVP talk, you know, the one where no one could see any of my text). Some other blogs have covered Mammalodon already, but mostly just some of the stuff you already hear in the news; ya, it was a mysticete with teeth (all toothed mysticetes look pretty weird), and it likely a benthic suction feeder, so what? There's far more interesting 'meat' in the paper, if you will, that someone who wasn't a mysticete worker and therefore didn't have time or interest to read 110 pages might not pick up on. Okay, I didn't read the description, but I've read the in depth discussion.

The skull and dentary of Mammalodon. From

Mammalodon colliveri was originally described in 1939 from the Oligocene Jan Juc Formation of Australia, although it was not identified as a toothed mysticete until the 1980's when Ewan Fordyce (U. Otago, New Zealand) undertook additional preparation of the fossil. In 1966, Doug Emlong (an extremely prolific and gifted amateur collector) described a peculiar toothed cetacean he discovered in the Oligocene of Oregon: Aetiocetus cotylalveus. Emlong thought it was an archaeocete, but Leigh Van Valen soon after recognized several mysticete features. Toothed mysticetes may seem odd at first, but when you think about it - 1) teeth are primitive for cetaceans and indeed mammals, and 2) they had to eat with something before baleen evolved. Mammalodon shares many similarities with Janjucetus, including a short rostrum, short triangular supraoccipital, and large orbits; however, Mammalodon is distinct in having laterally oriented teeth, and a relatively flat rostrum.

Comparison of Mammalodon (left) and Janjucetus (right). From Fitzgerald (2010).

Mammalodon has some strange features - a short, flat snout, large, upward facing orbits, large foramina on the maxilla, upper teeth that project laterally, and peculiar wear facets on the lower dentition that look like they were filed down at the same time (so to speak) - i.e. they share wear facets oriented on one single plane. Some authors have interpreted the denticulate teeth of basal mysticetes like Llanocetus and Mammalodon to have had a role in filter feeding, much like the teeth of the extant crabeater seal (Lobodon carcinophagus). Fitzgerald (2010) rejected this interpretation, as he did earlier (2006) for the tooth morphology of Janjucetus. Based on the probable presence of hyaline cartilage in the jaw joint of Mammalodon (as opposed to fibrocartilage in lunge feeding balaenopterids), Fitzgerald also rules out lunge filter feeding. Bony correlates of baleen, nutrient foramina, are absent on the palate of Mammalodon, suggesting that it did not possess baleen (such as later diverging aetiocetids - I'll talk more about those next time). Taken in full, few features of Mammalodon suggest filter feeding, rather than raptorial/pierce feeding (aka, biting stuff). Aside from this, some of the features unique to Mammalodon such as the very short rostrum, large maxillary and mandibular foramina, and extreme tooth wear suggest an alternative behavior. Typically, large foramina indicate improved blood flow and innervation to a region; for example the tactile whiskers of pinnipeds are correlated with a greatly enlarged infraorbital foramen (a pinniped synapomorphy), relative to "fissipeds". In this case, the short rostrum, probable highly innervated oral region, and wear most likely due to substrate interaction, all point towards benthic suction feeding. Additionally, the up-and-forward facing orbits would have conferred some degree of binocular vision, similar to the walrus and the bizarrer walrus convergent odontocete, Odobenocetops.

Various archaeocetes (top row), mammalodontids (second row) and aetiocetids (bottom two rows). From Fitzgerald (2010).

Most interestingly are the implications of the phylogenetic hypothesis of this study on the evolution of feeding in the mysticetes, relative to other recent hypotheses. Raptorial feeding is primitive for cetacea as a whole, including basal mysticetes. Interestingly, Llanocetus, Janjucetus, and Mammalodon all form a southern toothed mysticete clade, and Fitzgerald (2010) lumped Janjucetus and Mammalodon into the family Mammalodontidae, scrapping the family Janjucetidae he proposed earlier (2006). Llanocetus exhibits no adaptations for suction feeding, although Janjucetus does, suggesting that suction feeding is derived for this clade, but not a typical feature of basal mysticetes in general. Thus, this suggests that whatever feeding mode the mammalodontids employed was likely not representative of early mysticetes, contra Fitzgerald (2006).
Feeding evolution in baleen whales. From Fitzgerald (2010).

What the hell is Llanocetus, anyway? Llanocetus is perhaps the most interesting and strange described/undescribed cetacean out there. Well, what does that mean? Llanocetus was collected from the late Eocene La Meseta Formation of Antarctica. Yes, the Eocene of Antarctica, my friends. Why is that particularly awesome? The Eocene is dominated by archaeocetes, and elsewhere on earth during the late Eocene fantastic critters like Basilosaurus and Dorudon were kicking ass and taking names. And Llanocetus is the earliest known mysticete, let alone record of the Neoceti. The original description of Llanocetus denticrenatus by Ed Mitchell (1989) includes a maxillary fragment, mandible fragment, and an endocast - overall, marginally crappy. Later on, Ewan Fordyce revisted the site and collected what he initially thought was an archaeocete, and it even included (apparently) vestigial hindlimb elements (plural), similar to the condition in basilosaurid archaeocetes (which are not too phylogenetically divorced from Llanocetus, after all). This turned out not only to be a toothed mysticete, but the remainder of the holotype skeleton. My guess is that whoever originally collected the holotype scraps was freezing their ass off and said "to hell with it" and left. However, even though a cast of the skull is now on display at the USNM, the rest of the holotype is not yet described, and the dotted outline provided by Fitzgerald (2010) is the most informative figure ever published of this rather interesting beast. Anyway, I'll be pretty excited whenever the description gets published (much of the above information was presented by Fitzgerald, 2010, in his discussion of taxa used for the cladistic analysis).

The next post will (hopefully) be less-rambling, and will detail the transition from teeth to baleen in archaic mysticetes.


Fitzgerald, E.M.G. 2006. A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proceedings of the Royal Society B: Biological Sciences
273: 2955–2963.

Fitzgerald, E.M.G. 2010. The morphology and systematics of Mammalodon colliveri (Cetacea:Mysticeti), a toothed mysticete from the Oligocene of Australia. Zoological Journal of the Linnean Society. 110p.


J. Velez-Juarbe said...

Thanks for summarizing some of the important point from that paper. Mammalodon is indeed kind of weird! Something I really like about the paper (in addition to the science) are the figures showing the anatomical features. I don't know about you, but I am often disappointed by the lack of such figures in some publications (which I think is often directly related to who the author(s) is).
As for Llanocetus I was really surprised when I read here that it is still not properly described, it is a pity, sounds like a very important specimen. And where is it displayed in the USNM, the new ocean hall? I must have completely missed it last time I walked through there.

Alton Dooley said...

The presence of large maxillary foramina in Mammalodon is interesting, because it's not a widely distributed character in the group. I find it especially interesting because Eschrichtius also has a large maxillary foramen, and in Diorocetus it is absolutely gigantic (in some individuals it almost divides the maxilla completely in two.)

Boesse said...

I wish I was able to write further, as one of the things I really enjoyed about this paper is the extensive discussion of character transformations in the mysticeti. That, and I agree with Jorge - it had a refreshing number of figures - I always like papers that really figure material to death. Which is great, because now I practically wouldn't have to fly to Australia to look at Mammalodon.

Llanocetus is near the Basilosaurus in the new Ocean hall - I haven't seen it, but I've seen pictures on Flickr.

I went to the library today to look at Kellogg's monograph, and the maxillary foramina on Diorocetus ARE huge! Holy cow. I take it that's something you're going to wait and put in the Diorocetus monograph you're going to write, right???

J. Velez-Juarbe said...

I was at the USNM yesterday and went upstairs to look at the exhibit, yes, a cast of Llanocetus is there. Unfortunately it is so high that you only get to see ventral and ventrolateral views of it. It is a big critter with a skull, I estimate, nearly 2 meters long.

Alton Dooley said...

It was tempting to talk about the enlarged maxillary foramina in the paper where we were talking about benthic feeding (the Carmel Church specimen does have them, although they're imperfectly preserved). However, we don't actually know why grey whales and Diorocetus have the enlarged foramina. It makes sense in things like walruses that are searching for individual prey items, but how is it useful to a benthic filter feeder taking up a whole mouthful of sediment?

In the end we decided it was better to let our suggestion stand on just the injury and the ribs, but it is something we considered at length. I probably will revisit it in that monograph...

J. Velez-Juarbe said...

Enlarged (sometimes multiple) maxillary foramina are also found in animals with big movable lips, like sirenians. Their presence in some cetaceans is indeed intriguing.

Brian Lee Beatty said...

Part of the reason we left out the foramina size portion in the paper was that very little data exists on the contents of those foramina in modern mysticetes. At the time I did not have a dead mysticete lying around waiting to be dissected, but it would be nice to do a comparative study of the contents (and percentage of cross sectional area) of tissue types in the cranial foramina of cetaceans in general. It has been fairly clearly demonstrated in mice that the infraorbital foramen relates well to the thickness of the infraorbital nerve, but cetaceans do so much crazy stuff with their cranial foramina and adipose tissues, that it would be very speculative to assume that all foramina represent nerve thickness uniformly.
Regarding Llanocetus, it certainly is unfortunate that the best material of it remains undescribed, but one should note that the outline drawing includes parts that were reconstructed for it. The original specimen lacks much of the distal end of the rostrum, and the narrowed end is reconstructed material. I would not be surprised if the end did not narrow, but was broad like that in Mammalodon or Janjucetus. I could be wrong, and certainly Ewan should correct me, but that is what seems to be the case from the photos I have seen. I guess we'll just have to wait until the description....

J. Velez-Juarbe said...

The infraorbital nerve correlates with the size of the infraorbital canal not only in mice but also in other terrestrial mammals. Muchlinski (2008) measured the infraorbital nerve in a series of mammals and found that the size of the nerve accounts for about 85% of the size of the foramen. Unfortunately that study looked only at land mammals, it will be very interesting to see how it looks across marine mammals!

Muchlinski, M. N. 2008. The relationship between the infraorbital foramen, infraorbital nerve, and maxillary mechanoreception: implications for interpreting the paleoecology of fossil mammals based on infraorbital foramen size. Anatomical Record 291:1221-1226.

Robert Boessenecker said...

Re: the maxillary foramina - I hadn't realized they were homologous to the infraorbital foramen in other mammals, and I feel like an idiot for not! Totally makes sense, though. Pinnipeds all have an enlarged IOF, and it has been used as a synapomorphy of pinnipeds (or pinnipedimorphs). However, Kolponomos and lutrine mustelids also all have enlarged IOFs, as well as Puijila - which, who knows if it's a lutrine or a pinniped (yet).

Thanks for the extra info about Llanocetus - I'm looking forward for that description, as well as Mark Uhen's new chaeomysticete from the Alsea Fm. of Oregon, collected by the man himself, Doug Emlong...

Anyway, my next post will be about pinnipeds, or maybe I'll post about this balaenopterid skull I'm preparing.