Whales in the desert - page 2
A few things about whales & evolution
If you didn’t already know, then try not to forget - whales are mammals! Their fishiness is linked to their habitat and shape, but aside from that they are nothing like fish at all (box 1). The main characteristics of mammals: whales use lungs to breathe, are viviparous (give birth to live young), and suckle their young.
We cannot say much about the ancestors of the cetaceans with certainty. However, what we do know is that whales are the closest relatives to the present-day Artiodactyla (animals with two or four hooves). Yes it is true, whales are more closely related to cows, pigs and hippopotamuses than – say – fish or seals.
There are two hypotheses about how this relationship actually comes about. Based on the characteristics of the skeleton, particularly the anklebones, palaeontologists place the ancestors of the whale in a group of primitive hoofed mammals. During the Eocene epoch some of this group very gradually adapted to life in the water. Thus the nostrils moved further towards the top of the skull, the front legs became paddle-shaped structures, the rear legs more or less disappeared, and the tail evolved into a stiff fin. The first descendants were the now extinct Archaeoceti, which began to evolve into more modern groups, including the whales we know today, during the Oligocene epoch.
Diacodexis, one of the oldest known Artiodactyla, is a close ancestor of the cetacean (From: C. De Muizon, 2001)
Ambulocetus is among the best known whales. The animal still had well developed rear legs, si walking was not a problem. Yet, it could already swim better than terrestrial mammals. Well on its way of conquering the world in the water... (From: Thewissen et al., 1996)
However, recent molecular research, which compares the genetic materials of various groups, offers another option. Molecular biologists suggest the hippopotamus group (Hippopotamidae) as the most closely related to the cetaceans... To be continued!
Fishy whales
Even though fish are not at all closely related to whales and dolphins, they look very similar. Both groups have pairs of fins, a tailfin and a streamlined form. Since these characteristics are extremely successful for animals living in the water, these groups evolved along the similar lines. This is known as convergent evolution - i.e. similar characteristics evolving as the result of living in similar environmental circumstances. Wings are another example of convergent evolution; they can be found in birds, bats and insects, and these animals are not at all closely related!
