Sharks have been present on Earth since the dawn of the dinosaurs and possibly even earlier than expected. This unique species has been branched off from most species for hundreds of millions of years. Over hundreds and millions of years, sharks were forced to adjust due to mercurial and an obscurely changing environment. The first major shark radiation occurred during the Carboniferous Period approximately 286-360 million years ago. The second largest blast of sharks occurred during the Jurassic Period, approximately 200-144 million years ago. Sharks branched off earlier than most modern fish today, which is evident in their non-homologous structure. Sharks have a skeleton made from cartilage, a study medium between bones and skin in a way. Most modern fish have bones of calcium, which indicates that they branched off later on the evolutionary cladogram.
According to Elasmo Research, nobody is really sure about which groups of ancient sharks their modern descendants evolved from, but most assume that they evolved from Hybodonts. The Hybodos, had the habitat of shallow seas about 180 million years ago and is the most famous out of this particular group of candidates. Some research indicates that these were the direct answer of modern bullhead sharks due to the homologous structure of their brow ridges. Another possible candidate may be the Mcmurdodus, which was first located in Devonian deposits aged 390 million years old in what is now western Queensland, Australia. Although early to be related to sharks, this possibility is based off of its structure tooth enameloid, which appears to be of a multi-layered type which is found in all living sharks but not in most ancient sharks. But what throws some possible obstacles in this theory can be found in the fossils. Mcmurdodus is known from its fossilized teeth, which in overall form resemble the saw-like lower teeth of cowsharks. But this could be due to convergence, rather than evolutionary relatedness. Many issues are concerned with the large gap between modern sharks and makes it difficult to obtain evidence of direct ancestry. Modern sharks however can find ancestral footing when the neoselachians underwent massive bursts of adaptive radiation throughout the Jurassic and Cretaceous periods, which produced a massive amount of diversity of new body forms and lifestyle behaviors. Sharks have survived all five mass extinctions on Earth and have continued to evolve going extinct.
The most modern growth of shark diversity, which produced the Golden Age of sharks during the Carboniferous, came about 100 million years earlier. But majority of modern lineages is a result of the “Jurassic explosion” where sharks have continued to evolve on separate paths for nearly 200 million years. One of the most modern sharks to appear in the fossil record is the hammerhead, which date to be only 35-50 million years old, which is still older than the ancestors of modern humans. Megalodons were even more modern, with their presence in the fossil record dating back to 16 million years ago and then stopped at 1.6 million years ago. Today, there are approximately 440 types of existing shark species, but many are in danger due to environmental concern.
- “450 Million Years of Sharks.” Shark Savers ::. N.p., n.d. Web. 26 Feb. 2016