Evolution

Evolution: Origin of sharks



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The shark, notorious ruler of the sea, began its reign nearly 400 million years ago when its evolutionary lineage separated from that of its ancestor, the prehistoric bony fish. Today, more than 300 species of sharks still inhabit the oceans, many of which have existed in their present form for nearly 150 million years.

Tracking the evolutionary history of the shark, or Carcharodon, has been no easy task. Unlike their ancestors, sharks have a skeleton made of cartilage, rather than bone, which does not preserve well. Consequently, they have left behind a limited fossil record consisting mainly of the remains of discarded teeth.

During the first 200 to 300 million years of their unique evolution, some scientists believe that as many as 2,000 different species of shark may have existed. Many were strange creatures with odd characteristics quite unlike the common sharks of today. Most had rounded snouts, small brains, smooth teeth and less flexible fins. Early shark species like the Horn Shark and Cow Shark moved about the water very slowly. Competition for food forced adaptations that allowed the shark to evolve into the quick and agile predator it is today.

Weaker species eventually died out, while stronger species continued to thrive and adapt to an ever-changing ocean environment. As the Cretaceous Period drew to a close around 60 million years ago, catastrophic changes brought about the extinction of the dinosaur, as well as many species of sea life. The remaining shark species emerged as the most feared of all ocean predators.

Soon after, a shark of unmatched size and power evolved in Earth's waters. The Carcharodon Megalodon, likely the closest ancestor to the present-day Great White (Carcharodon Carcharias), reached an average length between 50 and 100 feet. The fossilized teeth of this giant predator often measure over six inches from base to point. Although scientists are not certain exactly how long the Megalodon survived, its extinction (possibly as recent as five million years ago) was most likely due to a limited food supply and changing climate.

Today, a wide variety of shark species still inhabits the oceans. The Whale Shark, measuring up to 49 feet long, is the largest known species in existence. The Cookie-Cutter Shark is the smallest, measuring only 19 inches. Although shark species can differ greatly in body size, appearance, habitat and diet, certain characteristics are common in all sharks:

•All sharks have cartilage skeletons
•All sharks shed their teeth at regular intervals, often producing more than 1,000 teeth each year
•All sharks have tooth-like scales called dermal denticles
•All sharks have at least five pairs of vertical gill slits
•Unlike fish, sharks do not require swim bladders to maintain buoyancy

Few sharks today truly live up to the notorious reputation earned by their cousin the Great White. Yet these mysterious creatures continue to be a symbol of power and a force to be reckoned with.

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