by Greg Mayer
Having emphazised the cartilaginous inhabitants of the Okinawa Churaumi Aquarium yesterday, let’s go to a distant part of the phylogenetic tree today: manatees. There are three species of manatees (Trichechus), all in the tropical Atlantic or Atlantic drainages; this is either the West Indian (T. manatus) or West African (T. senegalensis) species.
The manatees were feeding on aquatic plants. Note that this one is using it’s right forelimb to manipulate the food.
Their skin texture was interesting; I’m not sure what the white structures all over the skin are (hair?).
And, in this very interesting view, we see a manatee supporting itself off the bottom with its right forelimb. We can clearly see its ‘fingernails’. (They are true nails– but it sort of doesn’t have fingers.)
In the picture above, we can also see the limb is flexed. The most distal curve, nearest the nails, is the joint between the phalanges and metacarpals; this is an extension. A bit above this, there is a slight flexion of the wrist joint. The elbow joint is considerably higher, near the body, with a slight flexion. This shows that, though paddle shaped, the limb is not stiff, but retains considerable mobility distal to the shoulder joint, allowing the manatee to use the limb in balancing and propulsion on the bottom, and, as seen three pictures above, as an aid in feeding. The diagram below shows the manatee’s limb skeleton, which shows the familiar “one bone, two bones, many bones” pattern of tetrapods and their immediate lobed fin ancestors.
I’ve been fascinated by manatees and their relatives (the mammalian order Sirenia) ever since reading years ago about Steller’s sea cow (Hydrodamalis gigas), a giant sirenian of the cold North Pacific, which was discovered by scientists in 1741 and extinct by 1768. (There have been some intriguing late sight records, but none have panned out). Then, in graduate school at the Museum of Comparative Zoology, I walked underneath the following Steller’s sea cow skeleton almost every day (it hung in a different hall back then; it’s now in the main mammal hall). Note that this specimen lacks the distal parts of its forelimbs.
The evolution of sirenians from terrestrial ancestors is fairly well documented in the fossil record, much of the work being done by Daryl Domning of Howard University. The story is not as widely known as that of whales, and I don’t know of any single sirenian evolution website, but you could start learning the story here and here.
