Vertebrates Term Paper

Pages: 9 (2619 words)  ·  Bibliography Sources: 1+  ·  Level: College Senior  ·  Topic: Animals

SAMPLE EXCERPT:

[. . .] About 20 million years later, according to Patton, Paton, Smithson, Clack, rose another tetrapod called Casineria kiddi. The earliest tetrapods lacked hands that could flex, as humans curl their fingers and toes because they have a notch in the flexor surface on the phalanges. Due to this, walking on a rocky land, that necessitates the ability to curl the paws around various obstacles, would have been difficult. Acanthostega and Ichthyostega could only bend their hands very little amounts. Thus, while they did actually have hands, they were just slightly evolved hands. The evolution of Casineria kiddi, led to these notches on each phalange.

Casineria also had other adaptations to life on land. Its vertebrae, for instance, connected to one another to create quite a stiff backbone that probably served as a means to hold up the animal's body weight. The earlier Devonian tetrapods, however, had a much flimsier backbone that was similar to those of fish. It offered considerably less support. Further, Casineria's humerus or upper arm bone bore a shaft in the middle and flared out at each end. The stubby humerus of the Acanthostega did not narrow at the middle as bones commonly seen today. Casineria's proportionately lengthier and more thin and shapely form would have held the creature up better and aided it while walking.

Finally, the evolutionary advances travel to the very end of the limbs where Casineria had a set of five digits on each hand and foot. This established the fundamental model that runs through the rest of vertebrate years to the tips of human fingers. With finds like that of Casineria, paleontologists have begun to fill in some of the missing evolutionary blanks between the Devonian swamps and the amniotes of the Carboniferous period.

These tetaprods are important because their structure can still be seen today. However, they were in fact not the first animals to make the transition from water to land.

A fossil discovery a few years back found that 80 million years before the dinosaur, about 290 million years ago, there may have been a reptile running around on two legs. The 25-centimeter-long herbivore Eudibamus cursoris is the earliest known vertebrate able to scurry on its hind limbs. The find suggests that bipedalism may be more common than believed, but not necessarily a definite route to evolutionary success (Stokstad).

Like the raptors in Jurassic Park these animals ran fast on their toes. The vertebrate found in Germany had hind limbs that were 64% longer than its forelimbs and 34% longer than its trunk, proportions similar to those of modern two-legged lizards. Feet with long digits would have given the animal considerable stride ability. In addition, its tail helped it move quickly. Muscles attached to the tail could have made Eudibamus's hind limbs powerful enough for two-legged sprinting. It also helped to keep the animal's center of gravity close to its hip that is essential for balancing a two-legged gait. Further, the early vertebrate had evolved a new form of knee joint, which let it run with its feet directly beneath its body (Stokstad).

In other vertebrates of that time, the legs jutted outward from the body, since one of the paired shinbones or tibia connected with the underside of the mostly horizontal thigh bone or femur, while the other shinbone or fibula attached to the end of the femur. In Eudibamus, however, both shinbones fit onto the end of the femur and formed a hinge-like joint that put all of the leg in one plane as in dinosaurs and humans. It gave the animal an energy-efficient posture that allowed the bones, as well as the muscles, to help support the animal's weight. Even though it may not have been the most delicate bipedal animal, it had great strides over the four-legged competitors. Unfortunately, this did not help the animal in the long run (no pun intended). Like other animals that developed two legs on the evolutionary line before the dinosaurs, birds, and mammals, it did not stay around long (Stokstad) and was a dead end in the scheme of things.

Once these animals started coming up on dry land, how did they survive without water, especially amphibians that have thin skin and can easily dry out? According to zoologists as Lillywhite, scientists have found frogs throughout the world that use a variety of different means to keep themselves wet. For example, the South American painted belly monkey frogs, Phyllomedusa sauvagii, secrete waxes from specialized glands in their skin and then wipe themselves with the wax. They sort of "massage" the wax all over their body from the top of their heads to the tip of the limbs. Wax frogs in other arid parts of the world have somewhat similar behavior.

Evolution is an ongoing process, and even today there are groups of animals in the very process of moving from the sea up onto the land. One of these is the crabs, another group of arthropods that are distantly related to the insects. Crabs are basically marine animals, but like the insects have a high probability for eventually adapting to the land. In Japan, for example, crabs are presently making the transition from marine to freshwater life, which can be seen as a first stage on the road to the land. All along the coast there are various species of crabs that live in the intertidal zone. When the tide is out, they run freely across the mud and rocks. They can spend significant amounts of time on land by keeping a reserve supply of water inside their carapace. Meanwhile, like the insects, these crabs' hard exoskeleton provides some protection from the outside world.

References Cited

Clack, J.A. "An Early Tetrapod from Romer's Gap." Nature (2002) 418: 72-76. [electronic version]

Clack, J.A. "From Fins to Fingers." Science 304.5667 (2004): 57-59. [electronic version]

Coates, M.I, and J.A. Clack. "Polydactyly in the Earliest Known Tetrapod Limbs"

Nature. (1990) 347: 66-69. [electronic version]

Graham, Jeffrey B., and Heather J. Lee. "Breathing Air in Air: in what ways might extant amphibious fish biology relate to prevailing concepts about early tetrapods, the evolution of vertebrate air breathing, and the vertebrate land transition?." Physiological and Biochemical Zoology 77.5 (2004): 720-732. [Electronic Version]

Jarvik, E. "The Devonian tetrapod Ichthyostega." Fossils and Strata, (1996) 40: 1-213.

[electronic version]

Lillywhite, Harvey B. "To wipe and wax: in dry times, some frogs employ a curious method for saving water. Natural History 110.10 (2001): 58-64. [electronic version]

Paton, R.L., T.R. Smithson, and J.A. Clack. 1999. An Amniote-like Skeleton from the Early… [END OF PREVIEW]

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