Shark evolution Essay Example for Free

Shark evolution Essay Sharks, commonly referred to as fish are in fact quite different from the regular bony fish. Sharks have a cartilaginous skeleton, which is does not fossilize well, but despite this sharks have left a considerable fossil record. Shark evolution is in fact one of the most successful cases of phylogenic development. The evidence of sharks dates back to over 400 million years ago, almost 200 million years before the first dinosaurs, and before the evolution of any land vertebrates, in the Ordovician period, at the start of the Palezoic era. The only fossils of the oldest sharks are the scales and teeth, the teeth being quite durable and easily mineralized. The teeth in particular reveal that the oldest sharks were quite different from their modern descendants. And even through vertebrates like dinosaurs were eliminated centuries ago, sharks continue to exist today, with almost 1100 species. Among the most primitive sharks was the Cretaceous, discovered in Texas. The Cretaceous is believed to be about 65 million years old. The earliest life on earth dates back to almost 4 billion years ago, to a group called the prokaryotes, which includes the first bacteria. The prokaryotes did not even have a defined nucleus. The oldest eukaryotes, which were more advanced, and had specialized organelles evolved about 2 billion years ago. These included the famous amoeba. But the first multicelluar life form dates back to about 600 million years ago, to the â€Å"Edicarian assemblage† in Australia. Jawless fishes are believed to be the first vertebrates to have evolved, and date back to about 500 million years ago. Sharks then appeared about 450 million years ago. But even though paleontologists have considerable evidence, shark evolution posed many problems because of the fossils. With time, most of the evidence was eliminated. The scales are mostly very small in size, and have many impressions of soft tissues. But despite these drawbacks, studies reveal a rich history. The oldest shark scales go back to about 455 million years ago. But since they are quite different from those in modern sharks, most paleontologists agree that they are not from sharks. The first acknowledged fossil dates back to 420 million years ago in Siberia. These have been classified under the genus of Elegestolepis. Others, discovered from Mongolia have been classified in the genera Mongolepis and Polymerolepis. But apart for these almost classifications, little is actually known about these primitive sharks. But with the beginning of the Devonian period, shark record becomes quite varied. The first teeth of about 400 million years ago were discovered from Europe. These teeth belong to the ancient Leonodus. Leonodus had characteristic double cusped teeth. The oldest shark cranium was discovered in Australia, and dates back to about 380 million years ago, and is believed to be from a shark specie xenacanth. Earlier in fact the Leonodus and xenacanth were thought to be related because of their similar crown shaped teeth. The first partially fossilized shark shark was discovered by a geologist Gavin Young in Antarctica. But this fossil displayed an odd mix of features. Possibility exists that this too may be a xenacanth, but Young named it Antarctilamna. The Antarctilamna was 16-inches long, and was a diplodont, resembling the xenacanth. Antarctilamna’s fin though was directed dorsally. The xenacanth in contrast was exclusively a freshwater shark, and had a long fin directed rearward, just behind the cranium. But with the passage of time, sharks have endured tremendous changes of environment, and yet, have managed to survive. But like all other creatures, the species had to either adapt to the change, or end. Each time a large scale shift occurred; a number of species found the new environment incompatible and became extinct. Sharks though displayed immense diversity, and helping then expand into new habitats. â€Å"Such explosive increases in variety are termed ‘adaptive radiations[1]. ’† Over such a huge span of geologic time, sharks have has two adaptive radiations, and have survived up to five mass extinctions. Many also agree that since most shark fossils have been found either from Australia, or Antarctica, sharks may have originated in the southern hemisphere. The first radiation was in the Carboniferous period, roughly 30 million years ago. This was characterized by the flourishing of the stethacanthids. The second major radiation occurred in the Jurassic Period, about 170 million years ago. During this time both land and air was dominated by reptiles. This was the time when the hybodonts developed. Many believe that the hybodonts were the ancestors of all modern sharks, but evidence is scanty. While the hybodonts shows many obvious similarities to the modern shark, it has characteristic features not found in sharks today. One of these includes the horns behind the eyes. But now, the earliest known modern shark is believed to be the Mcmurdodus from the mid-Devonian time. The basis for this is the Mcmurdodus tooth. The tooth shows is of the same type found in modern sharks, but not in the ancient ones. But â€Å"there is a 190-million-year gap in the fossil record between the last Mcmurdodus and the first unquestionable cowshark [1]. † Even though research continues to unveil the evolution of this epic animal, a complete evolutionary record is difficult to draw. The earliest known sharks in fact were quite different from the modern counterparts. But in some respects they were very close to the modern sharks of today, having the same cartilaginous skeletons, replaceable teeth and multiple gill slits. And with time sharks have endured great geologic change, adapting to different environments, evolving to change, and have thus survived as one the most durable animals on the planet. References 1. Martin, R. Aidan. ReefQuest Centre for Shark Research. Biology of Sharks and Rays. Retrieved from (http://www. elasmo-research. org/education/evolution/evol_s_predator. htm)