Evolution of Whales and Dolphins Term Paper
- Length: 7 pages
- Sources: 10
- Subject: Animals
- Type: Term Paper
- Paper: #56054258
Excerpt from Term Paper :
The function point-of-view fails to account for the presence of a femur in addition to processes from the pelvic bone. Altogether, these hind legs in this whale present for contemplation a most interesting instance of those significant parts in an animal -- rudimentary structures." [Struthers, p. 142-143]
The dolphins breathe through their nose, but whales' nostrils drifted back farther and farther onto the top of the head in the fossils found. Today it is just a double or single blowhole, and the ears are closed and internal, detecting only vibrations from the lower jaw. They "speak" through a "melon organ" or pad of fat.
Milinkovitch used results from molecular studies to determine that sperm whales were more closely related to the baleen whales (mysticeti) than to toothed whales (Odontoceti) (Milinkovitch. 1997). But his conclusion was not well accepted, as sperm whales have substantial teeth and a single, rather than double nostrilled blowhole. More recently, scientists came back to the traditional arrangement, that the sperm whale is an odontocete. The evidence and recent molecular analyses strongly indicate the mystecetes and all other extant cetaceans is the oldest division in the ancestral tree.
The pygmy sperm whale (Kogia beviceps) and dwarf sperm whale (Kogia simus), are much smaller. The kogiids seem to have separated from the lineage that led to the sperm whale at least 9 millions years ago, making the sperm whale the myse phylogenetically distinct of all the seventy-five-odd species of living odontocetes. (Chadwick. 2001)
Hind limb bud images, dolphin embryo and fetus development show that dolphins and whales are descended from mammals, as they have the characteristics of hind legs, which are visible in the remaining atavistic bones, an embryo that has hind legs which disappear. There are photographs of hind limb buds on a five-week-old Pantropical Spotted Dolphin embryo, and 1.5 to 4-month fetus development taken by Professor J.G.M. Thewissen which are available on the Web (Thewissen. 2006).
Sirenians and elephants are evolved from a common ancestor. Like whales, sirenians returned to the water. Though hind limbs on whales may be hard to find, many sirenians (Manatees) still retain vestigial toenails like their elephant cousins, and share other traits in common with modern elephants.
There seems to be a controversy over the origins of modern whales which retain vestigial hind limb rudiments. Creationists call this vestige bone disease and say these remnants were designed for copulation only (which they are used for), but science says this is how evolution works --finding new purpose for old structures. In an out of date publication by Creationists they claim that there is no bone in the whale descended from the thigh leg bones, but new fossil finds of early whales contain complete leg bones from basilosaurus, and a reconstruction of the creature's likely appearance shows it has rudimentary legs.
On questions on the dating of varied early whales one might suggest further reading on whale evolution, the evolution of eco-location, the nursing of the young (suggesting common ancestry with land mammals), and the brain of humans vs. cetacea (Hasegawa. 1997).
The transitional inner ear of whales evolved from land to sea mammals. Professor Hans Thewissen explained the differences of earbone fossils, the incus and tympanic, in his web publication, The Thewissen Lab (Thewissen, 2001). But scientists have many e xamples of hind limb rudiments, such as one found in 1958, caught in the Bering Sea, where an x-ray revealing bone structures and a photograph taken of the protrusions on the whale's body, showed the whale had small legs. Even better, in 1919, a historical record and photos of a female humpback with protruded hind limbs including femur, tibia and tarsus was made. In 1914, there was a study of protruded limbs on whale and dolphin embryos.
A female sperm whale was caught in 1956. Upon examining the interior of the limb, partially cartilaginous bones were found, corresponding to the pelvis, femur, and possibly to the tibia. As there are numerous examples of this feature, a list of catches between 1956 and 1963 of whales caught and discovery of hind limb protrusions, including the approximate percentage of whales with this feature may be found on the web. (Ogawa. 1957)
The Oligocene epoch was crucially important in the evolution of dolphins and whales. The Oligocene epoch, part of the Tertiary Period in the Cenozoic Era, only lasted from about 33.7 to 23.8 million years ago, though many major changes occurred during this short time. These include the appearance of the first elephants with trunks, early horses, and the appearance of many grass plants that would eventually produce vast tracts of grasslands in Mongolia and North America for these animals to eat in the following epoch, the Miocene. Two modern lineages of cetaceans had evolved during this epoch from archaeocete ancestors. Interestingly, in the late Oligocene, the whale Aetiocetus, from Oregon, developed skull and jaw features typical of baleen whales, so it is considered to be the earliest mysticete. Yet it also had a full set of teeth.
By the Miocene period, whales of both lineages were relatively common and are found as fossils in many marine deposits. As a result of the cooling trend prevalent throughout the Oligocene period, the lives and habitats of many organisms were directly affected. In the oceans, marine biotic provinces became more fragmented as sea dwellers capable of withstanding cooler temperatures congregated to places further from the warmer equator, where other species could better survive. The cooling trend was also responsible for the reduced diversity in marine plankton, the foundation of the food chain.
On land, mammals such as horses, deer, camel, elephants, cats, dogs, and primates began to dominate, except in Australia. The continuation of land mammal faunal migration between Asia and North America was responsible for the dispersion of several lineages onto new continents. The "bulk feeding" in the open grasslands and savannas that occurred in this period resulted in the increase of general herbivores' size. As an example, ungulates continued to get larger throughout the Oligocene period. (Thewissen. 2006)
Families of both toothed and baleen whales are known to have evolved by the late Miocene and are still found today. These include the baleen whale families Balaenopteridae (including the blue whale) and Balaenidae (right whales), and the toothed whale families Delphinidae (dolphins and killer whales), Physeteridae (sperm whales), Monodontidae (belugas and narwhals), Phocaenidae (porpoises), and Ziphiidae (beaked whales).
During this last century, whales became very important evolutionary study specimens and whaling came under strict scrutiny and criticism, considering the discovery of great intelligence and sociability of the whale. The Minke whale, a rare whale, is known for its uncanny ability to recover from blows to the head and other means of anesthetizing it to prepare it for the kill. Recently, a Norwegian whaling boat in northern waters was reeling in a harpooned minke whale, when the animal revived. It rammed the ship, causing the mast to break and sent two crew members in the crow's nest toppling into the sea, breaking the ribs of one. Then the whale escaped. The news spread around the world with the headline: "Don't Get Mad, Get Even." (Chadwick. 2006)
Chadwick, Douglas, Whales, Dolphins and Porpoises, 2nd ed. Checkmark Books, 1999 National Geographic.
Chadwick, Douglas, "Evolution of Whales." National Geographic, November 1, 2001
Green, John. Whales and Dolphins Coloring Book. Courier Dover Publications, 1990
Hasegawa, M., Adachi, J. & M.C. Milinkovitch. "Novel Phylogeny of Whales Supported by Total Molecular Evidence." Journal of Molecular Evolution, 44 (Suppl 1): S117-S120 (1997).
Humphrey, Nicholas. www.humphrey.org.uk/papers/1976SocialFunction.pdf" "The Social Function of Intellect," in Bateson, P.P.G. And Hinde, R.A., Eds. Growing Points in Ethology, (1976) chapter 9, pages 303-317. Cambridge University Press.
Milinkovitch, M.C. & J.G.M. Thewissen., "Eventoed Fingerprints on Whale Ancestry." Nature, (1997) 388: 622-624.
Ogawa, R., and Kamiya, T.A. (1957) "Case of the Cachalot [Sperm Whale] With Protruded Rudimentary Hind Limbs." Scientific Reports of the Whales Research Insititute, No. 12, p. 197-208.
Ogawa, R., and Kamiya, T.A. "Case of the Cachalot [Sperm Whale] With Protruded Rudimentary Hind Limbs." Scientific Reports of the Whales Research Insititute, No. 12, p. 197-208. (1957).
Rendell, Luke and Whitehead, Hal.
Behavioral and Brain Sciences (2001), 24: 309-324 Cambridge University Press
Struthers, John, M.D., Professor of Anatomy in the University of Aberdeen. (1881) "On the Bones, Articulations, and Muscles of The Rudimentary Hind-Limb of the Greenland Right-Whale (Balaena mysticetus)." Journal of Anatomy and Physiology (London), Vol. 15, p. 141-321.
Thewissen, Hans. "Digital Library of Dolphin Development." The Thewissen Lab. November 4, 2006. < tp:/ / www.neoucom.edu/DEPTS/ANAT/Thewissen/
Green, John. Whales and Dolphins Coloring Book. Courier Dover Publications, 1990