Sunday, 03 July 2016 07:24

Timeline of Sharks

If you could go back in time and look at the first, unremarkable prehistoric sharks of the Devonian period (about 400 million years ago) you would never guess that their descendants would become such dominant creatures, holding their own against vicious aquatic reptiles like pliosaurs and mosasaurs and going on to become the "apex predators" of the world's oceans.

Over 400 million years ago, innumerable species of bony fish and marine organisms inhabited the Earth’s seas. The time period, known in history as the Devonian Era, gave birth to a family of fish unlike any other of the time, known as Chondrichthyes (Cartilaginous fishes). The Chondrichthyans evolved a truly unique skeleton composed almost completely of cartilage. Divergent evolution began to further separate the Chondrichthyans from the primitive fish, creating an array of beings that would come to be known as the ancestors of all shark species.

Chondrichthyans are considered to have been highly advanced for their time, possessing a great deal of the same characteristics seen in modern cartilaginous species. Now nearly 400,000 millennia on and evolution has generated over 1,200 highly advance species of sharks and rays.

Cartilaginous fish are so named because their internal framework is composed of almost entirely of cartilage rather than bone. Cartilage, unlike bone, does not fossilize well when an animal dies. The exposed collagen fibers in the cartilage degrade and break apart, fragmenting the skeleton into thousands of pieces, and leaving only teeth, scales and spines behind, making it extremely difficult to determine when these organisms first appeared on earth.

The Jurassic Explosion, beginning approximately 100 million years ago, marked the emergence of thousands of shark that slowly evolved into the species that we see today including the great white (Carcharodon Carcharias).

Around 65 to 60 million years ago, the white shark, Carcharodon Carcharias arrived. The white sharks were born into existence so highly evolved for their niche on the Earth that they have hardly been altered by nature. The great whites seen today are virtually a mirror image of the white shark of the late-Jurassic/ early Tertiary period.

Also around this time (60- 55 million years ago) the greatest of all sharks inhabited the Earth, Carcharodon Megalodon. Megalodon was the biggest predator known to have existed in the oceans of the planet and this immense beast could reach 65 feet in length and could weigh 30 tons, comparable to an average-sized sperm whale. The teeth of C. Megalodon closely resembled that of its smaller cousin the white shark, but three times the size. Due to similarities in serrated edges and size of the teeth, a popular theory emerged that the white shark and the Megalodon were very closely related, if not actually representative of the same species. The theory suggested that due to a reduction in prey size, Megalodon became smaller too and eventually evolving into the great white shark.

Currently a new popular theory maintains that the two enormous predators diverged from a common line and one containing the Megalodon became extinct and the other has survived to this day. Scientists have also found partially fossilized giant teeth of the Megalodon at great depths of the ocean causing some to believe that the Megalodon was still alive 2 million years ago.

Not long ago it was commonly accepted that sharks were simply primitive, mindless predators. We now know that this couldn’t be further from the truth. Humans have recently come to appreciate sharks for their long history and deep ancestry. Sharks have evolved from modest means hundreds of millions of years ago, to the present day apex predators of the sea. The fact that many modern shark species evolved several million years ago and have remained unchanged in all that time goes to show how efficient and well adapted these creatures are to their environment.





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