Whales

Whales are Cetartiodactyls, which means they are related to cows, pigs, hippos, and deer. Whales evolved from mammals that hunted in lakes and streams and gradually became completely aquatic. They developed ears adapted to hearing underwater and gradually lost their legs. Their front legs became pectoral fins, and they kept only a small remnant of one bone from their rear legs .In one of the greatest fossil discoveries in history, the Thewissen Lab in Pakistan uncovered the story of their evolution from small squirrel-like animals to seagoing whales such as the basilosaurids in the Banner above. The following videos show the sequence of whale evolution.

Whales and other artiodactyls evolved from tiny squirrel-like animals. Sometimes, scientists classify artiodactyls as cetartiodactyls. The path to whale evolution began about 50 mllion years ago as animals began to spend a considerable amount of time in the water. This chapter shows the many transitional forms between land animals spending some time in the water to whales spending all of their time in the ocean.

The oldest known artiodactyl, Diacodexis pakistanensis, appeared 54 million years ago and was approximately the size of a squirrel. It was found in the North-West Frontier Province of Pakistan in an early Eocene formation (Figure 12‑22).

The closest living artiodactyl to cetaceans (Figure 12-23) is the hippopotamus. They have a similar lifestyle to the ancestors of whales since they spend most of their time in the water. Paleontologists group the Cetacea and Hippopotamidae within the Whippomorpha.

Instead of becoming larger and heavier like a hippopotamus, the ancestors of cetaceans began to develop large webbed feet for swimming. After Indohyus or possibly before, the next fossil in the line of the evolution of whales was Pakicetus (Figure 12-26 and 12-27). Notice the extremely long digits in the front and rear legs in the Pakicetus (56-41 Ma) skeleton for swimming.

The link between whales and artiodactyls is proven by a series of changes in the larger skeletal structure but also by the structure of the bones in the ear, which changed to hear underwater. One of these bones is the tympanic bone shown in Figure 12‑28. Another change in the ear was the involucrum, which is a thickened covering over the space of the inner ear. Pakicetus had an ear like a whale's ear rather than a normal land mammal.

After Pakicetus, the next step toward the evolution of whales in the fossil record was ambulocetus (Figure 12-30). An ambulocetid (12 ft long) skeleton (Figure 12‑29) was found in marshy sediments by the Thewissen lab in 1994. The name ambulocetid means walking and swimming whale. Ambulocetus (49-48 Ma) represents one more step in the transition from land to water. Scientists think that its long mouth and short legs indicate that it was an ambush hunter like a crocodile. The amazing thing is that so much evolution took place in a short time although the fossil record might have missed some early changes. With the following list of transitional forms, the question is how many transitional forms would be enough for people who are concerned about transitional forms in the fossil record? People might argue about the fine details of some of the fossil ages, but there are clearly transitions between the land animals and ocean going whales during the period 50 - 45 Ma. With such a rapid transition, and with the stochastic nature of fossil discovery and evolution in different bodies of water, there is bound to be some overlapping species and data in time. Try to focus on the big picture of the transitional forms.

Remingtonocetids were contemporaries of ambulocetids and probably fed on fish in shallow water (Figure 12‑31 and Figure 12-32). They had shorter legs than ambulocetus, which indicates that they spent more time swimming than walking.

Rodhocetus is the best known transitional animal between land mammals and whales (Figure 12-33). it propelled itself through the water with huge webbed feet.

The protocetids (Figure 12-34) were the first cetaceans to enter the oceans of the world. The Thewissen lab found protocetid fossils in Pakistan between 48 Ma - 35 Ma, and their fossils are also found in Africa and North America, whereas the earlier taxa are only found in Pakistan. The protocetids evolved large tail fins so the need for legs vanished completely. The rear legs shrank and became useless appendages. They would eventually vanish except for a small useless internal bone that is a vestige of the former legs on the animal. The front legs were on their way to becoming pectoral fins.

Basilosaurids (Figure 12-35, Figure 12-36, and Banner) were the next step in whale evolution. These fossils are also found around the world between 41 and 35 million years ago. They were up to 60 ft long and only had a remnant of hind legs (Figure 12‑32), which they did not need to swim through the ocean. They eventually lost their hind legs completely and only a vestige of the leg bones remained in whales (Figure 12‑35). A basilosaurid skeleton is also shown in the Banner.

The following video shows an animation of the transitional forms between land animals and whales .

Molecular clock analysis (Figure 12-37) shows the phylogenetic tree of cetaceans beginning with the divergence between the Mysticeli (filter feeders) and Odonticeli (carnivores).

Even though the giant Mysticeli do not look like their evolutionary ancestors on the outside, the skeleton inside clearly links them with their forebears (Figure 12-38). The vestige of the leg bone is also seen detached from the main skeleton.

Whales have brains that are approximately the size of human brains and have a high level of intelligence and social behavior. University of Arizona student Vy Vo wrote the following description of whale social behavior:

Besides the physical evolution to adapt to life underwater, whales are well-known for their highly developed social structure. To portray this complex and developed social life, Eric Wagner wrote The social lives of whales (2015) published on Science News for Students. According to Wagner, “There are two types of whales: those with teeth, and those that filter food from the water using plates in their mouths called baleen (bay-LEEN). (Baleen is made up of keratin, just like your fingernails.) Baleen whales largely keep to themselves. Toothed whales instead tend to travel in groups called pods”. These groups are believed to gather to find food, to protect insecure mates or guard to prevent predators. In particular, the social interactions among toothed whales are separated into two groups. The first type is fission-fusion societies; small whales usually fall into this group. The second one is matriarchal pod – “groups led by the mother or grandmother of many of its members.” (Wagner, 2015). Therefore, we can see a connection between whales’ sizes and their social interactions. More specifically about matriarchal pods, “Pod identities can be both strong and unique. Different groups of killer whales and sperm whales, for instance, have their own sets of clicks, whistles and squeaks that they use to communicate with one another. Different pods might also hunt for different prey, even when they roam the same waters.”. Whales are believed to be capable of teaching each other new skills. Interestingly, even the most solitary ones like baleen whales, teach each other new sets of skills. For example, Humpbacks, a type of baleen whales, catch their preys by a practice known as “bubble-netting.”. According to Wagner (2015), “In 1980, whale watchers saw a single humpback off of the East Coast of the United States do a modified version of this behavior. Before it blew bubbles, the animal slapped the water with its tail. That slapping behaviour is known as lobtailing. For the next eight years, observers watched as more and more humpbacks picked up the practice. By 1989, nearly half of the population lobtailed the water before starting to bubble-net a dinner.” This proved that these Humpbacks teach and learn a new skill in predating from each other. In conclusion, besides an adaptive physical evolution, whales also have highly developed social lives.

References

Eric Wagner. (2015, March 13). The social lives of whales. ScienceNewsforStudents. Retrieved from https://www.sciencenewsforstudents.org/article/social-lives-whales

Banner

Basilorsaurid fossil at Smithsonian Museum of Natural History. Photo Credit: Tim/FunkMonk. Used here per CC BY-SA 2.0