Cetacean Evolution
  
By: Gwen Helsel
 
May 7, 1998

The question of how cetaceans came back to the water has long been disputed due to a lack of evidence, namely fossils. They may look like fish and swim like fish, but whales, dolphins, and porpoises are actually mammals. Unlike their cold-blooded neighbors, these marine animals are warm-blooded, breathe through lungs, and give birth to live offspring. All of these traits are characteristic of terrestrial mammals. So how did they come to be in the sea? In his first edition of Origin of Species, Darwin later became most embarrassed with the following passage:

"In North America the black bear was seen swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered by natural selection, more aquatic in their structure and habitats, with larger and larger mouths, till a creature was produced as monstrous as a whale." (Gould, 1994) This we now know is not the mechanism by which whales, dolphins, and porpoises came to be, however the idea of a land animal evolving into a whale is not so absurd.

What are cetaceans most closely related to? Within the past ten years fossils have been recovered which suggest that members of the order Cetacea are actually descendants of terrestrial animals related to even toed ungulates, including ruminants, hippos, camels, and pigs (Milinkovitch, 1997). Morphological and molecular data suggest that these terrestrial animals and the cetaceans form a monophyletic group. Dr. Shimamura, a professor of Bioscience and Technology at the Tokyo Institute of Technology, conducted research using short interspersed elements (SINEs). With the help of his colleges, Shimamura characterized nine retropositional events that might have occurred during the divergence of whales and even-toed ungulate in order to determine the phylogenetic relationship. The results of his research confirmed an alternative monophyl of cetaceans, ruminants and hippopotamuses, including camels and pigs only with an older common ancestor in a larger monophlogenitic group (Shimamura, 1997).

The common ancestor of cetaceans was actually a land animal that walked on four legs. It was a meat eater that resembled a short, legged wolf with hoof like claws (Heyning, 1995). This ancient ancestor is called mesonychid. Scientists believe that some mesonychids began hunting along the shore, probably to catch fish. They could have began to find more food in the deeper waters, or escaped from predators by swimming. If they were able to swim better, they could find more food and as a result survive long enough to mate and reproduce. Consequently, the genetic traits for living in water were selected for and some mesonychids evolved into the most primitive group of whales called the Archaeocetes, a name which means "ancient whales". All Archaeocetans were originally fresh water animals that had teeth, just as their terrestrial ancestors did. Pakicetus, a type of Archaeocetan, is the oldest known cetacean that was found in river deposits in the Keldana Formation in Pakistan (Thewissen, 1994). A newer species of cetacean, called Ambulocetus natans has been found about 120 meters higher in the same river deposits.

Odontoceles (toothed whales) and Mysticetes (toothless -whales) have fossil records that can be traced back to their common ancestor, the Archaeocetes which lived in about the late Eocene (about 38 - 50 million years ago). These animals (the Archaeocete) had short cervical vertebra, a reduced femur, and a flexible sacrum. The first of the fossils were found in sediments deposited near or in the Tethys Sea (Gingerich, 1994). Pakicetus is the earliest cetacean, and Ambulocetus arose shortly afterward.

Ambulocetus was approximately the size of a male sea lion. It had long femurs just as its terrestrial ancestor. Due to its fossil structure, Ambulocetus' long feet are suspected to have been used as propellers for swimming just as sea otters do. The preserved tail vertebrae are elongated suggesting that Ambulocetus maintained a long thin tail, like that of a mammalian ancestor. Although Ambulocetus had not yet evolved a fluke, its limbs are thought to have aided the organism in movement. It swam by moving it's spine up and down, but like seals, its main propulsive surface was provided by it's feet (Gingerich, 1994).

It is thought that Ambulocetus could have also been able to function on land just as sea lions do. Having long femurs, compared to the modem day whale, Ambulocetus could have been able to move about on land by flexing its spine in a sort of flopping motion. As Thewissen (1994) reported, "On land, the semipronated elbow left the hands sprawling when the shoulder was abducted and the wrist extended, resembling the posture of otariidpinnipeds". This type of land locomotion is also characteristic of the sister taxa which consists of hippopotamuses. Hippopotamuses round stumpy legs are just adequate to carry its vast round body, just as the shortened legs in Ambulocetus functioned. Unlike modem cetaceans, Ambulocetus had flexible wrists, elbows and digital joints which enabled them to maneuver on land.

Rodhocetus kasrani, a descendant of Ambulocetus, was the first early Archaeocete with a complete thoracic, lumbar, and sacral vertebral column. It retained primitive features seen in land mammals, but also exhibited derived characters that are found only in later cetaceans (Gingerich, 1994). Rodhocetus lived approximately 46.5 million years ago and differed from its ancestors in several ways. When compared to the fossils of its ancestors, the femurs were much shorter, indicating a lose of mobility on land as a direct result of a shorter limbs. The skull of the Rodhocetus is relatively large in size relative to the rest of its skeleton. Aside from the larger skull size, the nostril holes are shifted farther back on the head relative to ancestors such as Ambulocetus.

The changes in the ancestors of cetaceans are consistent with the morphology of modem day whales, dolphins, and porpoises. Modem cetaceans do not have common nostrils. The air hole has shifted over time through intermediates from the snout of the Mesonychid, to the blowhole of the common whale and dolphin. The position of the blowhole is thought to have come about due to the animals center of gravity. When the cetacean is slumbering, the snout is underwater and the back of the head/middle of the back lies just above the surface of the water. Thus, this area provides the most suitable position for the nasal passage duct. Natural selection acted to push the blowhole back because as the body size and shape changed through time, the buoyancy changes. If the nasal duct had remained at the front of the snout, a cetacean would have had to work harder in order to breathe.

Another necessary evolutionary change *that took place within cetaceans is the development of underwater hearing. The lower jaw of modem cetaceans has a greatly enlarged mandibularforamen, which contains fat pads that extend into the ear region (Hussain, 1993). As the size of the jaw bone increased in length, so did the size of the fat pads. The fat pads are often assumed to be involved in underwater hearing. Sound vibrations received by the lower jaw are transmitted to the fat pads to the middle ear. The external ear of Pakicetus resembles that of terrestrial mammals and shows a slight inflation in the region that is now the fat pads. The hearing mechanism in ancestral cetaceans is similar to that of modem true seals and walruses. According to Hussain (1993), this shows that the adaptations necessary for aquatic life were "acquired stepwise, with initial retention of terrestrial adaptations."

As remarkable as the above changes in ancient cetaceans are, there are also several changes which are not reflected in the gross morphology of the fossils. From living on land to living in the water, cetaceans had to develop the ability to survive without fresh water. Bajpai and colleagues researched the 8 0" levels in the tooth phosphate of cetacean fossils. They used the fact that tooth phosphate of mammals which ingest fresh water have lower 8 0" levels than do the teeth of mammals which ingest sea water have lower 8 0" levels than do the teeth of mammals which ingest sea water (Bajpai, 1996). Bajpai studied four species of cetaceans and found the oldest fossil, Pakicetus, to have the lowest delta oxygen 18 level and the newest fossil, Indocetus, having the highest delta oxygen 18 level. Ambulocetus had an intermediate delta oxygen 18 level, which helped to support Bajapi's hypothesis that Ambulocetus lived as adults in marine environments, but depended at least for part of their lives on a freshwater source. As a result of the adaptation to salt water, cetaceans were able to quickly disperse across the oceans and colonize the globe, which can be by modem day cetacean distribution.

Evidence of many of the transition states from the ancestors to modem day cetaceans can be seen in embryos. To some extent it can be said that ontology recapitulates phylogeny. In the embryos of cetaceans, one can find clear outlines of posterior fins however, these disappear as the embryo gets bigger. Posterior limbs can also be seen, but these too disappear except in very rare cases. An example of just how rare it is for vestigial organs to appear occurred in 1963. A herd of about 450 blue white dolphins were caught by a fisherman off the eastern coast of Japan. Of these 450 dolphins only one was observed to have rudimentary hind limbs which protruded on either side of the mammary slit (Conrad, 1983).

In comparison to the hind limbs, the anterior limbs are always full grown and develop into flat flippers of varying size. These flippers include the same bones as the forelimbs of their terrestrial ancestors and humans. The scapula remains approximately proportional to the cetaceans body size with exact shape varying between species, while the humerus, radius, and ulna are much shorter than these ancestral bones. The carpals and metacarpals vary in size for each cetacean, depending on the size of its flipper. Dr. Timothy D. Smith studied the relative shape of the scapula between river dolphins and marine dolphins, and showed that although the size was relatively the same, the shape differed between the too groups, most likely due to the different environmental pressures placed on them (Smith, 1994). For example, the flipper of river dolphin Inia geofftensis is used in an oar-like fashion to maneuver in shallow complex river environments, while the flipper in marine dolphins such as Delphinus delphis is an important steering or hydroplaning organ.

Today there are two common groups of cetaceans, the Odontocetes (echolocating toothed whales) and Mysliceles (filter--feeding toothless whales), which descended from mesonychids. Archaeocetus had the same arrangement of teeth that exists in most common mammals today, with sharp, cone-shaped front teeth and blade-like back teeth (Heyning, 1995). The original thought was that the toothless whales and the toothed whales were completely separate, however, a surprising finding by Axel Meyer, and Michael Minkovitch, was that one group of toothed whales, the sperm whales, are more closely related to baleen whales than to other Odontocetes. Their studies hinted that toothed whales might not be monophyletic (Meyer, 1993). According to Milinkovitch's study of mitochondrial DNA, since mt DNA is nonrecombining, different portions of the

mitochondrial genome should share the same phylogenetic history and as a result their analyses should converge to the same result (Milinkovitich, 1996). Through his research, the sister relationship between sperm whales and all baleen whales is found to be the most convincing of all possible outcomes.

Just as for primates, there is only one evolutionary history for cetaceans. Until recently, this history was read -from blurred, fragmented fossil records. With the aid of new molecular techniques such as the comparison of SINEs and mt DNA, a clearer and more accurate _phylogeny can be constructed. The general consensus of these studies have put Mesonychid as the most ancient ancestor of the cetaceans which evolved in to Archaeocete and eventually into the two common groups of cetaceans today, the Odontocetes (echolocating toothed whales) and Mysticetes (filter-feeding toothless whales). However, other experiments have shown that these groups are not mutually exclusive as shown by the new classification of sperm whales. Proof of ancestry has been shown in newly discovered fossils and through the comparison of developing embryos. In order to answer all of the mysteries of cetaceans origin and evolution, much more data and analysis is needed. As for the future of cetaceans, only time will tell.
 
 

References

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