Friday, December 12, 2008

What’s wrong with the hands of Steller's sea cow

When talking about species driven to extinction in historic times we automatically think of the Dodo, Carolina Parakeet, Tasmanian tiger, Caribbean monk seal among others. We might as well think of the Steller’s sea cow, Hydrodamalis gigas (picture below of one of the specimens at the NMNH).

H. gigas was a sirenian (sea cows: manatees & dugongs) that lived in the northern Pacific until about 240 years ago. This was one of the largest sea cows that have lived, only surpassed by Hydrodamalis cuestae from the Late Pliocene of California, which is estimated to have reaches up to 9.03 meters (~30 feet!) whereas one of the H. gigas measured by Steller (the first person to describe live specimens) was about 7.51 meters (~25 feet) (Domning, 1978).
The picture below is of a mounted skeleton of H. gigas at the Museum National d’Histoire Naturelle in Paris. It is most likely specimen A.14516, which is the only mounted skeleton at the MNHNP (Mattioli & Domning, 2006). It is a nice mount, but there is something wrong with it……

Look at the hand/flipper, its huge, and very dugong or manatee like (see the more detailed picture below). You see, G. W. Steller was one of the few persons to give an account of H. gigas from observing live (or recently killed) specimens (and hence the name Steller’s sea cow). His description of the hand is significant because the morphology is unlike that of any other known sirenian (Steller, 1899). According to Steller’s description, the forelimb of H. gigas had no fingers, in fact he describes the ends of the limb as having a posteriorly oriented hook-like structure made up of, most likely, stratified squamous keratinized epithelium (thickened hardened skin). The habitat of H. gigas were shallow waters where feeding would have exposed them to higher wave action, therefore the loss of fingers as well as having a hardened pad or surface would have provided more traction when using the forelimbs as propulsion or stabilization in these shallower waters. Domning (1978) concluded from the osteology and inferred myology that additional forelimb adaptations are also present in H. gigas. These adaptations such as reduction of some muscles and modifications to the elbow joint, made the limb better suited for movement in a more parasagittal direction (Domning, 1978).

To sum it all up, Hydrodamalis gigas had no fingers, it also had other forelimb adaptations that permitted it to “walk” in shallow marine substrates when feeding. The Paris mount is nice, but wrong, in that it has huge flippers instead of fingerless stumps.
You can see the second part of this saga here!!

Domning, D. P. 1978. Sirenian evolution in the North Pacific Ocean. University of California Publications in Geological Sciences 118:1-176.
Mattioli, S. & D. P. Domning. 2006. An annotated list of extant skeletal material of Steller’s sea cow Hydrodamalis gigas (Sirenia: Dugongidae) from the Commander Islands. Aquatic Mammals 32:273-288.
Steller, G. W. 1899. The beasts of the sea. (Translated by W. and J. E. Miller); pp. 179-218 in D. S. Jordan (ed.), The fur seals and fur-seal islands of the North Pacific Ocean. Part 3, Article 8. Government Printing Office, Washington, DC.

7 comments:

Meralis said...

Que pirata se ve la patita!!!
Hey and how do you know that the description of Steller was accurate? Did others saw the same "morphology" on the little "patitas" or in the fossil record? If so why a very reputable Museum such as the one in Paris would commit that mistake?

Brian Lee Beatty said...

Domning (1978) also had some very interesting finds to note about the loss of those digits in the fossil record of hydrodamalines. Isn't it fascinating that only hydrodamalines lose digits when other Sirenia keep them even though they rarely use them in swimming (or steering)? Manatees and dugonsg use them to "walk" on the bottom some, but don't seem to reduce them.
On the other hand, cetaceans use their flippers only for steering, but never lose digits (they multiply them, in fact). The functional differences among flippers are so diverse, it would be interesting to see someone do something with that, eh?

J. Velez-Juarbe said...

Meralis, Steller's description is the best known, but it is not the only one, there were other people with Steller who also describe H. gigas, this is mentioned in Domning (1978). Another, older species of Hydrodamalis, H. cuestae was also fingerless (op. cit.). Why is the exhibit at the Paris museum wrong, well... old & outdated!?!?
Brian, there are certainly some interesting finds reported in that work, specially the modifications to the scapula and humerus possibly related to the loss of fingers and "new" use of the forelimb! I remember recently watching a video of a manatee in shallow water and using the flippers for "walking", lovely!! But why hydrodamalines lost their fingers... could it have been related a preference to look for food in shallower waters, closer to the surf zone, hence the need for limbs modified for "walking" and/or getting a grip on the substrate?
Whales, compared to sirenians, seem to have proportionately shorter humerus & radius+ulna (as well as loss of mobility), so maybe multiplication of digits is their way of having a longer flipper, which they need for steering.

Nathan said...

The Harvard Museum of Natural History has a very nice mount, without fingers.

Anonymous said...

Steller's description is quite precise: "the ulna and radius
terminate bluntly with tarsus and metatarsus [carpal and metacarpal]. There are no
traces of fingers". No specimen of these bones exist,

So H. gigas had tarsus and metatarsus, which were however most probably much more regressed than suggested in the Paris mount. The fingers indeed are wrong.

Btw. the Helsinki museum initially made the same mistake and fitted their specimen with man-made hand and finger bones. These have in the meantime been removed. An old photo can be seen on http://www.sirenian.org/sirenianevolution.html.

The skeleton mounted in the Naturhistoriska Riksmuseet in Stockholm had a bone mounted in the position of a metacarpal, which Domning later identified as the transverse process of a vertebra.

Hans Rothauscher
http://www.hans-rothauscher.de/steller/seacow.htm

Alice Bluegown said...

That Paris specimen looks a bit hemmed in - you'd think they've give it a bit more prominence, although I do love those old-school, crowded museum galleries!

J. Velez-Juarbe said...

Hi Alice and welcome,

The specimen is indeed kind of hidden; It should be more prominent (from my biased pro-sirenian point of view) as it is one of the few complete mounts of this particular species.