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The Cetacen Evolution and Functionality Due to similar characteristics, “scientists have long known that cetaceans [...] descended from four-footed land mammals” (9). “Until the early [...] 21st century, most paleontologists thought that cetaceans were most closely related to mesonychians, [...] an extinct [...] group of hoofed mammals from the northern hemisphere” (9). However, scientists who studied DNA thought the cetaceans were more closely related to hippopotamids, an even-toed group of mammals (9). In 2001, the Thewissen-lab discovered two pakicetid skeletons that have now changed the opinions on what is considered the cetaceans’ closest land relation (9). “It is only since the 1990's that relatively complete skeletons of the archaeocetes have been found [...] [and helped to explain] the transition from land-to-water [...]” (9). The evolutionary process of transforming from land to marine mammals “took less than 8 million years” (9). The cetaceans that lived during the Eocene period are collectively called the archaeocetes and include; pakicetidae, ambulocetidae, remingtonocetidae, protocetidae, dorudontidae, and basilosauridae (9). The Pakicetids

“Pakicetids were the first cetaceans, [and] their fossils are only found in Northern Pakistan and Western India” (6). “Pakicetids did not look like whales at all, [but] resembled land mammals. However, the skulls of pakicetids have an ear region that is highly unusual in shape, and only resembles those of modern and fossil whales. These features [...] are found in all cetaceans, and in no other animals” (6). “The rocks in which their fossils are reserved indicate that the bones were buried in a shallow stream, and that the climate was hot and dry. [...] Their bones are unusually thick, possibly an adaptation to make the animal heavier counteracting the buoyancy of the water” (6). The Ambulocetidae

“Ambulocetidae are large, powerful animals, with short limbs, [...] big feet, and a strong tail. They are only found in Northern Pakistan and Western India in rocks that indicate that the environment was nearshore marine and swampy” (1). The complete post-cranial skeleton of the species ambulocetus natans was discovered and found to be amphibious. The meaning of the name is “the walking and swimming whale” (1). The Remingtoncetid

Remingtoncetid fossils were found in Southern Asia and have been determined to have lived about 49-43 million years ago (8). “Their heads were very different from other whales [due to their small eyes, long snout, and long hind and forelimbs] and they had ears that transmitted sound similarly to those of modern whales” (8). Bone reconstruction has determined that this was a small animal that was no bigger than a river otter (8). The Protocetids

Protocetids were the first whales to become widely dispersed throughout the worlds oceans (7). The protocetids have been found with remingtoncetids and have been determined to have lived about 48-35 million years ago (7). This whale was the first to leave the Indian subcontinent and disperse to all shallow sub-tropical oceans. The fossils from this whale have also been found in Africa and on the Eastern Coast of North America (7). “Protocetids [...] had large feet which they used in swimming [...][and] it is possible that other protocetids had a tailfluke” (7). These whales also had large laterally placed eyes under the supraorbital plate, which is a large bony extension of the frontal bone (7). The basilosaurids and dorudontids

The whales that evolved from the protocetids branched into two different species, basilosaurids and dorudontids and lived about 35-41 millions years ago (2). They have mainly been found in the Eastern United States and Egypt. However, their distribution is thought to have been worldwide (2). “Basilosaurids were enormous, [...] had long snake-like bodies, [...][and] a tailfluke,[...] but they probably swam using wavy movements with their bodies” (2). Based on fossilized stomach contents they are known to have eaten fish and sharks (2). “Dorudontids are closely related to basilosaurids but were proportionally more like dolphins [and] they probably swam using the fluke” (2). “Both [of these whales] had complete hindlimbs that included a mobile knee and several toes” (2). However, due to the tiny size they were not used for aquatic propulsion (2). The mysticetes and odontocetes

Modern day whales fall into two suborders, mysticetes and odontocetes (4). Mysticetes do not have teeth, but have a hair-like substance that hangs from the upper jaw called baleen (4). These whales take in large amounts of water and then filter the food through the baleen (4). Odontoceti, however, have peg-like teeth which helps them to hunt for larger prey (5). “There are coastal and deep-sea species, some are tropical and others [are] polar [...] and some inhabit all climate zones” (5). Odontocetes navigate through their environment and find food through the use of echolocation (5). “[T]hey have a sophisticated sound emission system located in the forehead [...] consisting of a number of air-filled cavities that are connected to the nasal cavity” (5). The melon is a fatty lense-shaped organ that is used as an acoustic lense (5). “The ears of odontocetes are highly specialized for the reception of reflected echolocation signals” (5). Mysticetes and Odontocetes have been around for about 40 million years and are derived from the Eocene cetaceans (5 and 4).

Functionality “The overall size of a terrestrial animal is limited by the maximum size its limbs can attain and still remain functional” (3). “[...] As an animal grows, its mass increases more rapidly than its skeletal strength” (3). “It is for this reason that an ant is proportionately much stronger than an elephant” (3). However, the density of cetaceans is very close to the density of water (3). Therefore there is no limit to cetaceans size due to fundamental weightlessness, unlike land mammals who are limited by gravitational forces (3). The only probable limit on cetaceans size would be the prey it seeks. A smaller size cetaceans would be better adapted to chase faster-moving prey (3). Due to the viscosity of water, cetaceans have had to develop adaptation in order to move through an environment that is considered to be very resistance to movement (3).

The first of these adaptations is the forelimb. The shoulder joint is the only moveable joint in the forelimb, the wrist and elbows have become fused together (3). The hand bones of a cetacean are not connected, but are wrapped in a rigid fibrous tissue (3). The study of the cetacean forelimb structure indicates it is used for balance and steering, but not propulsion (3).

The next adaptation the cetaceans had to develop was the nostril position to the top of the head (3). This beneficial adaptation allows the animal to breathe without having to lift its heavy head out of the water (3). This adaptation has also allowed the snout of some to stretch and become narrower which allows for more efficient movement through the water (3).

The vertebra of the cetacean has also adapted to its life in the ocean (3). The neck vertebra has become shorter and is sometimes fused together in order to prevent head bobbing during movement through the water.(3). Conversely, the lumbar vertebra has grown longer and has developed outgrowths in which the large muscle mass that is responsible for propulsion is attached (3).

Two final adaptations are the body shape and surface. The cetacean body shape has become cylindrical and tapered at both ends (3). With this body shape the whale can move through the water quickly and efficiently in order to conserving energy (3). The cetacean has been able to rid its body of all protrusions except the forelimbs and has developed soft, smooth skin (3).

All of these adaptations have helped in the ability of the cetacean to effectively navigate though and survive in its environment (3).

Citations:

1. Ambulocetidae, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/Ambulocet.html 2. Basilosaurids and Dorudontids, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/BasilAndDor.html 3. Fontaine, Pierre-Henry. Whales And Seals Biology And Ecology. Schiffer: Atglen PA, 2007, Pages 13-30. 4. Mysticetes, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/Mysti.html 5. Odontocetes, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/Odont.html 6. Pakicetidae, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/Pakicetid.html 7. Protocetids, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/Protocet.html 8. Remingtonocetidae, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/Remi.html 9. Whale Ancestors, http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale_origins/w_ancest.html