Evolution

The Evolution of Sharks



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In our short human life spans, it seems easy to look at the lifeforms around us - especially ones as strange as sharks - and think they couldn't have changed much over the millions of years they've existed. Yet sharks have been, and still are, incredibly diverse - over 2,000 species of fossilized sharks alone. Such diversity reflects evolution and adaptation of the species to its particular ecological surroundings.

Most knowledge about shark evolution comes from the fossilized teeth of sharks. Because sharks have a cartilaginous (composed of cartilage) skeleton, ancient sharks leave few reminders, and the most commonly used is their teeth. It is rare to discover a full set of shark teeth, and many shark teeth are eroded over time; therefore, scientists create reconstructed sets of teeth to study the creatures. This can be a tricky process, as it requires an expert to correct identify and sort shark teeth so that they are not combining species by mistake. In addition to shark teeth, shark scales, fin spines, and rarely, vertebra or craniums, can be used to determine more about the shark's evolution.

Ancient sharks share a few similarities with our modern sharks: they have similar scales, gills, fins, skeletons, and in some cases, teeth. However, they also possess many differences which show how the modern shark has evolved. Ancient sharks had shorter, rounded snouts with a longer jaw that attached to their skull, whereas modern sharks have a long, pointed snout with a short jaw that does not attach directly to the skull, meaning that ancient sharks had a smaller, weaker bite. Early sharks also had rigid fins (as opposed to the modern flexible fin), and spines that supported a weaker muscular system.

Sharks are an ancient creature, having appeared around 450-420 million years ago - almost 200 million years before dinosaurs on the geological scale. The human species, in comparison, dates back only about 60,000 years. The earliest known shark scale fossils date back to about 450 million to 420 million years ago. The earliest known fossil shark teeth are dated back about 400 million years, found in Europe, and belong to a strange shark known as Leonodus; these teeth were double-cusped, indicating the shark may have been freshwater. Early freshwater sharks, known as xenacanths, had bodies similar to eels, with tapering tails, a fin spine behind the skull, and two-pronged teeth. Cladoselache had a short, rounded snout, a mouth located at the front of the head, long jaws attached to the cranium under the snout, and a spine in front of each dorsal fin, along with strong keels and a curved tail fin, but was also scaleless, unlike most of its ancient and modern counterparts.

Around 360 to 280 million years ago, sharks experienced their first major evolutionary change, due to a mass extinction of many of the fish groups that existed during that period. During this time, shark species experimented with strange head and body structures, such as the Stethacantus, which developed a bristle-topped dorsal fin, or the Falcatus, which developed a long fin spine that projected over its head like a lance. At the end of this period, however, only 1% of marine species survived, leaving only a few shark species to survive into the next age. The next major evolutionary change occurred during the famous Jurassic Period, around 200 to 140 million years ago. Sharks such as Hybodus, which resembles today's bullhead sharks, came to rule in the seas, bringing a rise to the modern shark. During this time, shorter jaws, longer snouts, constricting spines, and dense pointed teeth became the choice of the shark's evolutionary path. It was from these structures that modern sharks evolved about 100 million years ago.

Although some may believe that sharks are stationary in the evolutionary scale, paleontology has shown this to be far from the case. Sharks - like many other modern-day species - are highly refined predators of the seas. They have survived several mass-extinctions, and continuously responded to environmental changes to remain active in our water's ecosystems. Indeed, they are a predator to be respected.

Credit for research due to the ReefQuest Centre for Shark Research (http://www.elasmo-research.org)

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