Tetrapod Zoology

Given that we’re all enjoying gawping at Caperea so much, I may as well finish up and use the rest of the photos that Joy Reidenberg kindly provided. First off, here’s the skull from the side.

i-d9e9222a1fa05be43867bdc6756fc4be-Caperea-skull-right-lat-Oct-2010.jpg

It’s really weird: that lower jaw shape is utterly unlike anything else present in baleen whales, being comparatively deep and ‘heavy’ in appearance, with a downward flexure near the tip (is it just me, or does this look a bit like a flamingo mandible?). In comparison, the lower jaws of rorquals, grey whales are kin are laterally bowed, beam-like bones with cylindrical cross-sections. Those of right whales are much deeper, but they’re still not much like the Caperea lower jaw; they’re fairly straight and lack the down-curved anterior portion. As usual we have no idea why Caperea’s lower jaw is like this: remember that (to my knowledge) nobody knows what it eats [UPDATE: not entirely true - be sure to see the comments below], or how it eats (is it a skim-feeder, like right whales, or a lunge-feeder like most rorquals, or both, or neither?). Anyway, here’s the skull seen from below…

i-7fb27cb5be06eb90fd55354d00efab51-Caperea-skeleton-ventral-NZ-Reidenberg-skull-Oct-2010.jpg

Here’s the whole skeleton in right lateral view…

i-eb8e703ae3b43ab4f06385b2379af9be-Caperea-skeleton-right-lat-NZ-Reidenberg-Oct-2010.jpg

And here’s the ribcage, again showing nicely the peculiar, thick posterior ribs…

i-384fd2b9cea4190199d2d87931fd52ea-Caperea-skeleton-left-lat-ribs-NZ-Reidenberg-Oct-2010.jpg

Finally, here’s the distal end of the tail. Emily Buchholtz (2010) said that determining the precise number of vertebrae in Caperea is difficult because some of the distal-most caudals are virtually always missing. As everyone who knows whales knows… the caudal vertebrae that support the tail flukes have a distinctive cross-sectional shape, as does the vertebra positioned at the very start of the fluked part of the tail (it’s called the baseball vertebra). This is all great osteological correlate-type stuff, and has allowed people to show that fossils whales like Basilosaurus had tail flukes (Gingerich 1998). If we knew more about the shape and relative size of the fins and tail flukes of Caperea, we might be able to make some interesting inferences about its manoeuvrability, ability to accelerate, swimming speed and hence possible prey preference, since recent work (Woodward et al. 2006) has confirmed good correlation between these factors.

i-4fb6b15dcf5e25c3adba215963f529a0-Caperea-distal-tail-Reidenberg-NZ-Oct-2010.jpg

As usual, we long to know so much more. Many thanks to Joy for sharing the photos you see here (they were taken in New Zealand). For previous articles on Caperea and other baleen whales, see…

Refs – -

Buchholtz, E. 2010. Vertebral and rib anatomy in Caperea marginata: implications for evolutionary patterning of the mammalian vertebral column. Marine Mammal Science 10.1111/j.1748-7692.2010.00411.x

Gingerich, P. D. 1998. Paleobiological perspectives on Mesonychia, Archaeoceti, and the origin of whales. In Thewissen, J. G. M. (ed) The Emergence of Whales. Plenum Press (New York), pp. 423-449.

Woodward, B. L., Winn, J. P. & Fish, F. E. 2006. Morphological specializations of baleen whales associated with hydrodynamic performance and ecological niche. Journal of Morphology 267, 1284-1294.

Comments

#1 heteromeles
October 24, 2010

The geneticists are going to have a field day with this one. Some day. I’m willing to bet that there’s one mutant/hyper-expressed gene that drove all that bone enlargement. Maybe it’s homologous with the bone thickening gene in the hero shrew?

One serious question: I wonder how the jaw cross-section compares with that of other baleen whales? My hypothesis is any baleen feeder needs to have a certain minimum thickness of the jaw so that the lower jaw doesn’t break under the strain of feeding. The strain is unique among mammals, because the water is torquing the jaw as it moves through the baleen. Because Caperea is so small (body length shorter than a blue whale skull), this “jaw of minimum cross section” looks massively over-developed.

If there is a minimal cross section limit on the jaw, then Caperea may be so small because it has hypertrophied bone development, which allowed it to have a functional baleen jaw on a small body. The ribs and spine are then (perhaps) side effects of the jaw adaptation, rather than direct adaptations to some specialized lifestyle.

This is all hand-waving, of course, but I do want to know more about how that jaw works.
#2 Boesse
October 24, 2010

While it is safe to say that the jaw of Caperea is unique, It is important to note it has similarities with both Eschrichtius and balaenids, more or less to the exclusion of all other mysticetes, fossil or modern. Eschrichtius also has a laterally compressed dentary, albeit not as extreme as in Caperea, whereas balaenids (Morgan, correct me if I’m wrong) have a less laterally compressed dentary. Unlike balaenopterids and many cetotheres and cetotheriids sensu stricto, they are fairly straight and not twisted longitudinally, or arched in lateral view (and again, these are similarities with Eschrichtius and balaenids).

More importantly, Caperea has atrophied coronoid and angular processes; altogether, the lack of these features and the straight, unarched, untwisted horizontal rami all strongly argue against lunge feeding.

There’s no two ways about it – Caperea’s a real freak in terms of its head, jaws, and axial skeleton, and its phylogenetic position has been really difficult to nail down (resulting from its morpholohgical freakiness).
#3 LeeB
October 24, 2010

The online “Australian Government Biodiversity Species Profile And Threats Database” has a page on Caperea.
This references five papers which state that Caperea have stomach contents consisting of calanid copepods and small euphausiids.
It also mentions a Caperea seen inshore apparently feeding on copepods and larval euphausiids.
That reference might be worth tracking down as it may give insight into how it was feeding.

LeeB.
#4 David Marjanović
October 24, 2010

Here’s the whole skeleton in right lateral view…

Holy shit, this beast has no lumbar region!!! The ribs from one side and the haemal arches from the other side almost meet! And there are lots of ribs!

And the hands are tiny. Have they really got only four fingers?
#5 Anonymous
October 24, 2010

Is it just me, or does this look a bit like a flamingo mandible?.

I was about to say the same thing. Maybe a curved flamingo or Caperea-like lower mandible is the biomechanical compromise that happens when you try to shorten the snout of a filter feeder. The only benefit I can think of off the top of my head is that it allows you to keep the same length of jaw while still shortening the skull, though I can’t see how that would be useful.

You know Darren, there seem to be a lot of other animals out there with weird plates on their axial skeleton. In addition to Caperea, I can think of Sauropleura with weird shaped dorsal processes and hemal arches, Thrinaxodon, several bipedal Cretaceous ornithischians (Talenkauen for one), and Icthyostega (all three of the former have rib plates). Maybe you should do an article series on that.
#6 Allen Hazen
October 25, 2010

David (re #4)– the after ribs are swept back: I think there are 3 or 4 “lumbars” (= vertebrae with no ribs in front of the first that has a haemal arch). The ribs are hard to count in the photos, but I think got 15 or 16 pairs in my most careful counts (as opposed to 14 in the right whale skeleton shown for comparison in the previous blog-post).

I think there are other Cetaceans (Balaenopterids, maybe) with only four “fingers” per hand, though the right whale seems to be pentadactyl. (Right whales have proportionally broader flippers than Balaenopterids.)
#7 Dartian
October 25, 2010

David:

Have they really got only four fingers?

What Allen said; yes, they do, and they’re not the only extant tetradactylous mysticetes either. For further details, see

Cooper, L.N., Berta, A., Dawson, S.D. & Reidenberg, J.S. 2007. Evolution of hyperphalangy and digit reduction in the cetacean manus. The Anatomical Record 290, 654–672.
#8 John Scanlon, FCD
October 25, 2010

‘Caperea‘ is a nice example of J.E. Gray’s linguistic aesthetic, also responsible for a lot of euphonious (and mostly feminine) genera of reptiles and amphibians here in Australia.

Definitely flamingo-like jaws; I therefore predict that the whale feeds in an inverted position, dorsal surface of snout against the sea floor. (That would hardly be more bizarre than those side-feeding rorquals or a lot of other weird stuff out there. I mean, microhylids and spiders, fer chrissake…) The flamingo model would also predict rapid lateral oscillation (bringing food-laden water in the sides of the mouth rather than all at the front); this would be very expensive in a big whale, but maybe not out of the question for this guy. Anything unusual about neck vertebrae and occipital muscle scars?
#9 David Marjanović
October 25, 2010

I think there are 3 or 4 “lumbars” (= vertebrae with no ribs in front of the first that has a haemal arch).

A total number of 3 or 4 “lumbars” and “sacrals” is still very, very low. And that’s assuming that the first haemal arch is in front of the first caudal; I don’t know how widespread that is in mammals.
#10 Dartian
October 25, 2010

John:

‘Caperea‘ is a nice example of J.E. Gray’s linguistic aesthetic

I hope I won’t ruin that name for you by telling that whenever I see it, I can’t help thinking of a guinea pig… (Cavia aperea –> C. aperea –> Caperea – geddit?)
#11 C. M. Kosemen
October 25, 2010

Wow. Can you imagine how modern artists would reconstruct this beast if it was found only as a fossil? Most drawings would possibly look like a swimming armadillo!