|
COPYRIGHT 2001 American Institute of Biological Sciences
Whales indisputably are mammals, which is clear from their means of oxygen intake (they breathe with lungs), their care of newborns (mothers nurse their calves with milk), and a host of other features. This implies that whales evolved from other mammals and, because ancestral mammals were land animals, that whales had land ancestors. What happened in the transition to life in the ocean has been hard to imagine for scientists and laypeople alike. In the first edition of the Origin of Species (1859), Darwin suggested that a bearlike animal swimming with an open mouth might be a precursor of a filter-feeding baleen whale. This statement attracted much ridicule; in a letter, Darwin observed, "It is laughable how often I have been attacked and misrepresented about this bear" (Gould 1995). In later editions, Darwin deleted this reference to evolution entirely and merely noted that a bear sifting water for insects is "almost like a whale."
Nearly 150 years later, we can fill in much of the gap that embarrassed Darwin. The last two decades have witnessed an explosive growth in the number of fossils documenting the origins of Cetacea (whales, dolphins, and porpoises). An excellent morphological series of transitional cetaceans is now available to document the transition from land to sea, and many sophisticated analyses detail the biology of these archaic cetaceans. The origin of whales now offers a spectacular example of evolutionary change, allowing us to chart changes in anatomy and physiology as whales first moved into the water and then gradually explored the open seas.
Although Darwin didn't have the details right--bears did not evolve into whales--his basic point was correct: We can now show that whales are in fact hoofed mammals that took to sea. Yet in spite of the wealth of new evidence, certain segments of popular and creationist literature continue to use cetaceans as examples of animals that could not possibly have evolved through modified descent. Much of the blame for these misconceptions is the deliberate spread of misinformation by those who deny evolution, as well as simple ignorance on the part of those unaware of published research. However, the sheer volume and pace of recent research also cause problems. For those outside of the circle of specialists actively studying whale origins, it is hard to keep up with all the new discoveries.
In this article, we first introduce the families of archaic cetaceans that lived in the Eocene (approximately 55 million to 34 million years ago), the oldest period from which cetaceans are known. After that, we discuss the several organ systems that underwent dramatic changes. Then, we put the functional morphology and evolution of two organ systems, locomotion and osmoregulation, in a broader perspective. We show that the differences among these extinct animals make sense only in the context of evolving adaptations to an aquatic environment. We cannot provide a comprehensive review of early cetacean evolution, as this would take up many pages. The two chosen organ systems make compelling examples of macroevolutionary change, showing a stepped transition from land to water for archaic cetaceans. From these beginnings, the order Cetacea expanded into a wide variety of aquatic groups, including mostly large, filter-feeding baleen whales (suborder Mysticeti), and predatory toothed whales (suborder Odontoceti, w hich includes, among others, dolphins, porpoises, sperm whales, beaked whales, and killer whales).
What we do know of early cetaceans amounts to an overwhelming and dramatic record of evolving adaptations. The field of whale origins is progressing quickly, and new finds will add new insights, so that this article cannot be the final synthesis of whale origins. But even as specialists continue to debate important questions such as that of the sister group of cetaceans and the age of the oldest whale, it is clear that we already know more than enough to trace the general outline of whale evolution. Though important, the remaining questions are essentially details of a broader story of adaptation to a new environment, and their eventual resolution will amplify the story described here without changing its essence.
The cast of Eocene cetaceans
The diversity of Eocene cetaceans can be summarized into six families that together document the transition from land to water. Their phylogenetic relations are uncontroversial (Luo 1998, Thewissen 1998, O'Leary and Uhen 1999): Pakicetids form the base group and may include some, but not all, descendants (i.e., they may be paraphyletic), followed by ambulocetids and then remingtonocetids. The next cluster is paraphyletic and is classified as protocetids. The youngest and most derived Eocene cetaceans are basilosaurids and dorudontids, the latter of which are the sister group to the modern suborders, mysticetes and odontocetes.
Pakicetid cetaceans are the most primitive and oldest cetaceans. They are about 50 million years old and only found in Pakistan and India (Figure 1). Some features of the pakicetid skull (Figure 2a) suggest an amphibious lifestyle; the eyes, for instance, are on top of the skull. The teeth suggest that they ate hard food and were carnivores. Pakicetids were small, varying from fox to wolf size, but no skeleton is known for them.
Ambulocetid cetaceans are slightly younger and more derived than pakicetid cetaceans. They were also much larger, similar in size to large sea lions. A nearly complete skeleton for Ambulocetus (Figure 3) shows that the animal had a large head, long muscular body, and a long tail. Its limbs were short but the feet long. In overall body shape, Ambulocetus looked somewhat like a crocodile, although its hind limbs and feet were considerably longer. It may have lived as an ambush predator of fish in shallow water. Ambulocetids lived in coastal environments such as bays and estuaries approximately 49 million years ago (mya) and are known only from India and Pakistan.
Remingtonocetid cetaceans (Figure 4) are more derived than pakicetids and ambulocetids in the shape of the teeth and the reduction of the limbs. They are only found in near-shore marine deposits of South Asia. Partial skeletons for remingtonocetids indicate that they had long snouts and small eyes (Figure 2b). Their (middle) ear was large, suggesting that they used hearing to detect prey (as do modern odontocetes). Known remingtonocetids had large and powerful tails and vertebral columns and relatively short legs, which were weight bearing. In this sense, they looked like long-snouted crocodiles. Remingtonocetids probably lived between 49 mya and 43 mya. They vary greatly in size; the smallest (Kutchicetus) are similar in size to Pakicetus, whereas the largest may have been as large as Ambulocetus.
Protocetids (Figure 5) are known from near-shore marine deposits, and they are the oldest cetaceans to have spread across the world. Several partial skeletons are known (e.g., Rodhocetus and Georgiacetus); they indicate that the limbs were short and not weight bearing in several taxa, implying that land Locomotion was slow and cumbersome. These cetaceans may have lived like seals, spending most of their active time in the water but hauling ashore occasionally. Their eyes are large and oriented laterally, unlike remingtonocetids, but similar to dorudontids and basilosaurids. Most protocetids are relatively large, similar to small modern dolphins. The oldest protocetids are approximately 46 million years old; the youngest may be 39 million years old.
Basilosauridae and Dorudontidae reached their highest diversity in the late middle Eocene, around 35 mya. Skeletons of these animals (Figure 6) are unlike those of the other Eocene cetaceans in that they are immediately recognizable as cetaceans. As in modern cetaceans, basilosaurids and dorudontids have a streamlined form, short neck, forelimbs shaped like flippers, and strongly reduced hind limbs. Basiosaurids had long, snakelike bodies, around 20 m long, whereas dorudontids were dolphinlike in body shape and size. Both families are found in shallow marine environments.
Evolutionary change
In this section,...
Read the full article for free courtesy of your local library.
|
Bookmark this article
Print this article
Link to this article
Email this article
Digg It!
Add to del.icio.us
RSS
