Term papers writing service

A discussion of the differences between cows and whales

It is a trick question. You cannot do it. There is no convincing someone who has his mind made up already. But sometimes, it is even worse. Sometimes, when you point out a fossil that falls into the middle of a gap and is a superb morphological and chronological intermediate, you are met with the response: You are losing ground! Duane Gish of the Institute for Creation Research ICR regularly trots out the "bossie-to-blowhole" transition to ridicule the idea that whales could have evolved from terrestrial, hooved ancestors.

There simply are no transitional forms in the fossil record between the marine mammals and their supposed land mammal ancestors. Of course, for many years the fossil record for the whales was quite spotty, but now there are numerous transitional forms that illustrate the pathway of whale evolution.

Recent discoveries of fossil whales provide the evidence that will convince an honest skeptic. However, evolutionary biology predicts more than just the existence of fossil ancestors with certain characteristics - it also predicts that all other biological disciplines should also reveals patterns of similarity among whales, their ancestors, and other mammals correlated with evolutionary relatedness between groups. It should be no surprise that this is what we find, and since the findings in one biological discipline, say biochemistry, is derived without reference to the findings in another, say comparative anatomy, scientists consider these different fields to provide independent evidence of the evolution of whales.

As expected, these independent lines of evidence all confirm the pattern of whale evolution that we would anticipate in the fossil record. To illustrate this approach, I will present the evidence from multiple fields for the origin of the whales from terrestrial mammals. This paper will examine mutually reinforcing evidence from nine independent areas of research. Of course, as a starting point, we need to describe what makes a whale a whale.

What is a whale?

Science Blogs

A whale is first and foremost, a mammal - a warm-blooded vertebrate a discussion of the differences between cows and whales uses its high metabolism to generate heat and regulate its internal temperature. Female whales bear live young, which they nurse from mammary glands. Although adult whales have no covering of body hair, they acquire body hair temporarily as fetuses, and some adult whales have sensory bristles around their mouths.

These features are unequivocally mammalian. But a whale is a very specialized mammal with many unique characters that are not shared with other mammals - many of these are not even shared with other marine mammals such as sirenians manatees and dugongs and pinnipeds seals, sea lions, and walruses. For example, whales have streamlined bodies that are thick and rounded, unlike the generally slim, elongated bodies of fishes. A whale's tail has horizontal flukes, which are its sole means of propulsion through the water.

The dorsal fin is stiffened by connective tissue, but is fleshy and entirely without supporting bones. The neck vertebrae of the whale are shortened and at least partly fused into a single bony mass. The vertebrae behind the neck are numerous and very similar to one another; the bony processes that connect the vertebrae are greatly reduced, allowing the back to be very flexible and to produce powerful thrusts from the tail flukes.

The flippers that allow the whale to steer are composed of flattened and shortened arm bones, flat, disk-like wrist bones, and multiple elongated fingers.

The elbow joint is virtually immobile, making the flipper rigid. In the shoulder girdle, the shoulder blade is flattened, and there is no clavicle. A few species of whales still possess a vestigial pelvis, and some have greatly reduced and nonfunctional hindlimbs. The rib cage is very mobile - in some species, the ribs are entirely separated from the vertebral column - which allows the chest to expand greatly when the whale is breathing in and allows the thorax to compress at depth when the whale is diving deeply.

The skull also has a set of features unique among mammals. The jaws extend forward, giving whales their characteristically long head, and the two front-most bones of the upper jaw the maxillary and premaxillary are "telescoped" rearward, sometimes entirely covering the top of a discussion of the differences between cows and whales skull.

The rearward migration of these bones is the process by which the nasal openings have moved to the top of the skull, creating blowholes and shifting the brain and the auditory apparatus to the back of the skull. The odontocetes toothed whales have a single blowhole, while the mysticetes baleen whales have paired blowholes. In the odontocetes, there is a pronounced asymmetry in the telescoped bones and the blowhole that provides a natural means of classification. Although teeth often occur in fetal mysticetes, only odontocetes exhibit teeth as adults.

These teeth are always simple cones or pegs; they are not differentiated by region or function as teeth are in other mammals. Whales cannot chew their food; it is ground up instead in a forestomach, or muscular crop, containing stones.

Unlike the rest of the mammals, whales have no tear glands, no skin glands, and no olfactory sense. Their hearing is acute but the ear has no external opening. Hearing occurs via vibrations transmitted to a heavy, shell-like bone formed by fusion of skull bones the periotic and auditory bullae. These, then, are the major features of whales. Some clearly show the distinctive adaptations imposed on whales by their commitment to marine living; others clearly link the whales to their terrestrial ancestors.

Others show the traces of descent from a terrestrial ancestor in common with several ancient and modern species. From all these features together, we can reconstruct the pathway that whale evolution took from a terrestrial ancestor to a modern whale confined to deep oceans.

Thinking about the ancestry of the whale In 1693, John Ray recorded his realization that whales are mammals based on the similarity of whales to terrestrial mammals Barnes 1984. The pre-Darwinian scientific discussion revolved around whether whales were descended from or ancestral to terrestrial mammals. Darwin 1859 suggested that whales arose from bears, sketching a scenario in which selective pressures might cause bears to evolve into whales; embarrassed by criticism, he removed his hypothetical swimming bears from later editions of the Origin Gould 1995.

Later, Flower 1883 recognized that the whales have persistent rudimentary and vestigial features characteristic of terrestrial mammals, thus confirming that the direction of descent was from terrestrial to marine species. On the basis of morphology, Flower also linked whales with the ungulates; a discussion of the differences between cows and whales seems to have been the first person to do so.

Early in the 20th century, Eberhard Fraas and Charles Andrews suggested that creodonts primitive carnivores, now extinct were the ancestors of whales Barnes 1984. Later, WD Matthew of the American Museum of Natural History postulated that whales descended from insectivores, but his idea never gained much support Barnes 1984. Later still, Everhard Johannes Slijper tried to combine the two ideas, claiming that whales descended from what Barnes aptly called "creodonts-cum-insectivores".

However, no such animal has ever been found. More recently, Van Valen 1966 and Szalay 1969 associated early whales with mesonychid condylarths a now-extinct group of primitive carnivorous ungulates, none bigger than a wolf on the basis of dental characters.

More recent evidence confirms their assessment. Thus Flower was basically right. The evidence The evidence that whales descended from terrestrial mammals is here divided into nine independent parts: Although my summary of the evidence is not exhaustive, it shows that the current view of whale evolution is supported by scientific research in several distinct disciplines.

Paleontological evidence The paleontological evidence comes from studying the fossil sequence from terrestrial mammals through more and more whale-like forms until the appearance of modern whales.

Although the early whales Archaeocetes exhibit greater diversity than I have space to discuss here, the examples in this section represent the trends that we see in this taxon.

  • Dorudon lacked the elongated vertebrae of Basilosaurus and was much smaller about 4-5 meters in length;
  • The incus of Pakicetus, preserved in at least one specimen, is morphologically intermediate in all characters between the incus of modern whales and that of modern artiodactyls Thewissen and Hussain 1993;
  • Modern whales often retain rod-like vestiges of pelvic bones, femora, and tibiae, all embedded within the musculature of their body walls;
  • Interestingly, both deciduous and permanent teeth of the animal are found in these sediments with about the same frequency, supporting the idea that Pakicetus gave birth on the land.

Although there are two modern suborders of whales Odontocetes and Mysticetesthis discussion will focus on the origin of the whales as an order of mammals, and set aside the issues related to the diversification into suborders.

Sinonyx We start with Sinonyx, a wolf-sized mesonychid a primitive ungulate from the order Condylarthra, which gave rise to artiodactyls, perissodactyls, proboscideans, and so on from the late Paleocene, about 60 million years ago. The characters that link Sinonyx to the whales, thus indicating that they are relatives, include an elongated muzzle, an enlarged jugular foramen, a discussion of the differences between cows and whales a short basicranium Zhou and others 1995.

The tooth count was the primitive mammalian number 44 ; the teeth were differentiated as are the heterodont teeth of today's mammals. The molars were very narrow shearing teeth, especially in the lower jaw, but possessed multiple cusps.

The elongation of the muzzle is often associated with hunting fish - all fish-hunting whales, as well as dolphins, have elongated muzzles. These features were atypical of mesonychids, indicating that Sinonyx was already developing the adaptations that later became the basis of the whales' specialized way of life.

Pakicetus The next fossil in the sequence, Pakicetus, is the oldest cetacean, and the first known archaeocete. It is from the early Eocene of Pakistan, about 52 million years ago Gingerich and others 1983.

Although it is known only from fragmentary skull remains, those remains are very diagnostic, and they are definitely intermediate between Sinonyxand later whales. This is especially the case for the teeth. The upper and lower molars, which have multiple cusps, are still similar to those of Sinonyx, but the premolars have become simple triangular teeth composed of a single cusp serrated on its front and back edges.

The teeth of later whales show even more simplification into simple serrated triangles, like those of carnivorous sharks, indicating that Pakicetus's teeth were adapted to hunting fish.

  • Although my summary of the evidence is not exhaustive, it shows that the current view of whale evolution is supported by scientific research in several distinct disciplines;
  • Basilosaurus was a long, thin, serpentine animal that was originally thought to have been the remains of a sea serpent hence it is name, which actually means "king lizard";
  • Journal of Molecular Evolution 1991; 32;
  • Similarly, dolphins have been spotted with tiny pelvic fins, although they probably were not supported by limb bones as in those rare sperm whales;
  • Dorudon lacked the elongated vertebrae of Basilosaurus and was much smaller about 4-5 meters in length;
  • As the example of Basilosaurus shows, whales of intermediate age have intermediate-sized vestigial pelves and rear limb bones.

A well-preserved cranium shows that Pakicetus was definitely a cetacean with a narrow braincase, a high, narrow sagittal crest, and prominent lambdoidal crests. Gingerich and others 1983 reconstructed a composite skull that was about 35 centimeters long. Pakicetus did not hear well underwater. Its skull had neither dense tympanic bullae nor sinuses isolating the left auditory area from the right one - an adaptation of later whales that allows directional hearing under water and prevents transmission of sounds through the skull Gingerich and others 1983.

All living whales have foam-filled sinuses along with dense tympanic bullae that create an impedance contrast so they can separate sounds arriving from different directions. There is also no evidence in Pakicetus of vascularization of the middle ear, which is necessary to regulate the pressure within the middle ear during diving Gingerich and others 1983. Therefore, Pakicetus was probably incapable of achieving dives of any significant depth. This paleontological assessment of the ecological niche of Pakicetus is entirely consistent with the geochemical and paleoenvironmental evidence.

When it came to hearing, Pakicetus was more terrestrial than aquatic, but the shape of its skull was definitely cetacean, and its teeth were between the ancestral and modern states. Ambulocetus In the same area that Pakicetus was found, but in sediments about 120 meters higher, Thewissen and colleagues 1994 discovered Ambulocetus natans, "the walking whale that swims", in 1992. Dating from the early to middle Eocene, about 50 million years ago, Ambulocetus is a truly amazing fossil.

It was clearly a cetacean, but it also had functional legs and a skeleton that still allowed some degree of terrestrial walking. The conclusion that Ambulocetus could walk by using the hind limbs is supported by its having a large, stout femur.

However, because the femur did not have the requisite large attachment points for walking muscles, it could not have been a very efficient walker. Probably it could walk only in the way that modern sea lions can walk - by rotating the hind feet forward and waddling along the ground with the assistance of their forefeet and spinal flexion.

When walking, its huge front feet must have pointed laterally to a fair degree since, if they had pointed forward, they would have interfered with each other.

Search National Geographic

The forelimbs were also intermediate in both structure and function. The ulna and the radius were strong and capable of carrying the weight of the animal on land. The strong elbow was strong but it was inclined rearward, making possible rearward thrusts of the forearm for swimming. However, the wrists, unlike those of modern whales, were flexible. It is obvious from the anatomy of the spinal column that Ambulocetus must have swum with its spine swaying up and down, propelled by its back feet, oriented to the rear.

As with other aquatic mammals using this method of swimming, the back feet were quite large.

  • It is also likely that Rodhocetus had a tail fluke, although such a feature is not preserved in the known fossils;
  • The characters that link Sinonyx to the whales, thus indicating that they are relatives, include an elongated muzzle, an enlarged jugular foramen, and a short basicranium Zhou and others 1995;
  • Modern whales often retain rod-like vestiges of pelvic bones, femora, and tibiae, all embedded within the musculature of their body walls.

Unusually, the toes of the back feet terminated in hooves, thus advertising the ungulate ancestry of the animal.