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The Theory of the Evolution of Sharks

Evolutionarily speaking, sharks are one of the oldest living organisms on the planet. It has been extremely difficult to identify the exact evolutionary path of the shark, because shark bones are made of cartilage and do not fossilize as easily as harder, calcium-based bones. This means sharks are hard to find in an already sparse fossil pool, but scientists have discovered many fascinating things.

Pre-Shark Forms
- Sharks began in the same lineage as fish and other marine life, with invertebrate, gummy-like creatures that most resemble jellyfish and worms. Sea animals with shells came next, and it's believed that these creatures are the most likely ancestors for boned fishes, the earliest of which are known as Agnathans. Agnathans appear in the fossil records 500 to 450 million years ago, and the first cartilage-boned fish came about 100 million years later. Scientists believe these Chondrichthyes are the earliest shark ancestors.
First Sharks
- The oldest shark fossil is that of Cladoselache, a shark that lived in Lake Erie about 350 million years ago. While it's very possible that there were other sharks prior to Cladoselache, this fossil offers definitive proof of the next stage of shark evolution. While the Cladoselache's mouth was still out front, rather than the common underside mouth of the modern shark, the eye position, fin and body structure are very similar to that of a modern shark.
Species Differentiation
- Cladoselache marks a period of rapid diversification that continued for about 50 million years. During this period, shark species developed a number of bizarre adaptations, including brush-like protrusions, oddly shaped fins and strange dental structures. Changes in structure began to decrease about 300 million years ago, and most species of shark were wiped out in a mass extinction 250 million years ago. Those that remained were of two types: freshwater and marine, the latter of which were the direct ancestors of modern sharks.
Modern Sharks
- Most of the species of shark that are alive today have their origins in the Jurassic period. These sharks are fast and lean -- and they had to be, since they shared the sea with dinosaurs. Most notable of the early modern sharks was the species identified as Carcharodon Megalodon, an enormous creature whose teeth measured at least six inches in length, and whose body may have been up to 100 feet long. While it is long gone, it's very likely that the modern Great White shark evolved from this same genetic line. Sharks have experienced little evolution in the last 50 million years.

Animals & Science