Evolution of Sharks

Topics: Shark, Great white shark, Lamnidae Pages: 6 (2093 words) Published: December 24, 2010
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Date: 08/July/2010

Sharks have been in existence for more than 300 million years. Their evolution occurred over 150 million years earlier than that of the dinosaurs. Most of the existing fossil data of early sharks was collected from fossilized teeth together with several skin impressions. primitive sharks (Cladodonts) , had teeth which were double-pointed, up to 2metres in length, ate fish as well as crustaceans, and existed approximately 360 million years ago. The earliest shark-like beings shown in the fossil records seem to have existed towards the start of the Silurian age, close to 450 million years ago, however the earliest fossil teeth known of actual sharks was not discovered until the Devonian age, about 400,000,000 years ago. Their teeth were not more than 4 millimeters or 0.16 inches across. A candidate for close comparison is Antarctilamna, whose bones have been discovered in the Antarctic continent rocks. This fish was forty centimeters long, and conspicuously shark-like. Its spine was located at the front of a long dorsal fin, additionally its teeth that had 2 huge splayed cusps, and smaller cusps positioned between them. The era of fishes During the Devonian period the first key radiation of sharks began. This is referred to as the 'age of fishes'—during this period all fishes, including sharks, began to diversify greatly. Cladoselache was one of the most primitive true shark skeletons discovered intact. Fossils of ray-finned fish were found intact in some of their stomachs, their direction shows that they were captured tail-first, and therefore Cladoselache must have been skilled in swimming faster than its prey. Close to twenty five per cent of species so far discovered had been feeding on Convaricaris crustacean which obscured itself in sediments on the sea floor and fed on the lifeless flesh of squid and fish. Sharks and Cladoselache existing in the Devonian seas had to struggle with the dreadful placoderm fishes which were armor-plated. Sharks had gained a tactical lead by having body shapes that was both streamlined, hydro dynamically and efficient. With a flexible skeleton of cartilage that provided them with strength, they were better suited than their rivals and destined to go on to even superior things. The era of sharks When the Carboniferous period began, approximately 360 million years ago, sharks evolved and proliferated at a fast rate to the extent that scientists named it the 'golden age of sharks'. Furthermore, to the apparent shark-like species, some can only be termed as bizarre. Stethacanthus almost certainly grew up to 3.4 meters long and was different from any other shark, dead or living.

it had a helmet of miniature teeth on its head and a probing structure which looked like a square shaving brush clearly visible out of its back, approximately where its frontal dorsal fin ought to be. Both male and female sharks had dozens of teeth, larger teeth at the back and small ones at the front. Such a noticeable and awkward formation must have had a significant purpose. It might have had a function in courtship, or defense, or perhaps it enabled Stethacanthus to secure itself, as in case of a modern remora, to the base of a bigger fish.

Early sharks Sharks and chimaeras fit in to Class Chondrichthyes, since they lack true bones. in its place, their skeletons (counting their jaws and vertebrae) are composed of cartilage. The cartilage is regularly calcified, causing it to be hard, similar to bone, but not really ossified. Chondrichthyes only bony part are their teeth and scales also known as dermal denticles. The 1st shark fossils are composed of small dermal denticles. They are...

Cited: James Seth. Voracious Evolution. New York: Scribner 's, 2008: 62-87.
Ezekiel, G. Robinson. Silent Hunters of the Deep. NewYork: Burdett, & co, 2008: 35-47.
Steve and Jane Parker. The Encyclopedia Of Sharks. London: Elsevier, 2006 : 96-123 .
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