Why it pays to be a biologist

I have nothing against theoretical physicists or mathematicians, but it's a lot safer being a biologist. Here's why

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Deep Sea Killers-C. megalodon

Many consider the Great White Shark (Carcharodon carcharias) to be among the most incredible creatures to roam the oceans today. Growing up to a length of 6m (20ft), it is the infamous creature made famous by the success of the movie Jaws.

The Great White Shark as a species can be traced back to the early Pliocene (5 million years ago). It's fossilized teeth (Fig.1) can be found almost anywhere, worldwide, in marine sediments of the correct age. Teeth found in the Miocene (15 to 13.5 million years ago) are slightly rolled and have an eroded edge. Common synonyms for these teeth are Carcharodon rondeletti and Carcharodon sulcidens, but the teeth are identical to those of the living species not regarding intraspecific variation.

Fig. 1. Fossilized tooth of Carcharodon carcharias. Height: 5 cm. Early Pliocene, Sacaco/Peru © L. Andres

About 16 million years ago (during the Miocene), a distinct species appeared in the world's oceans. Carcharodon megalodon (or C. megalodon) was possibly the biggest shark species to have inhabited the oceans. It may have attained an astonishing maximum length of 15 m (50ft) and weighed as much as 50 tonnes (Fig.2)

Fig. 2. Comparing C. megalodon (13 m) and the Great White Shark (6.5 m) © L. Andres

Such estimates are obtained from teeth and certain skeletal components (as sharks have a skeleton made out of cartilage that does not fossilize easily; the teeth however are very durable). Traditional research holds C. megalodon as an ancestor of the white shark. Recent research suggests that it may have been a close relative. It's triangular teeth may have reached a maximum height of 17 cm (Fig.3). It may have hunted in the same stealthy manner as white sharks do, stalking beneath it's prey and rising upwards at great speeds to deliver a forceful, and often fatal, first bite.  It's prey probably included primitive whales and other large marine mammals.

Fig. 3. Fossilized tooth of Carcharodon megalodon. Height: 13 cm. Middle to Late  Miocene, Florida/USA © L. Andres

Around 1.5 million years ago (towards the end of the Pliocene), C. megalodon became extinct. The development of megatooth sharks can be traced back until the Cretaceous period. It is directly linked with the development of other animals. In the Cretaceous not only sharks, but also marine reptiles ruled the waters. This condition changed, however, after the extinction of the dinosaurs (65 million years ago) in favour of the sharks. Sharks now occupied the ecological niches for predators. Additionally, the basis for a more energy-rich nutrition was created by the rise of marine mammals (such as the cetoheriids; ancestors of baleen whales) in the Eocene (55 to 33 million years ago). During the Late Oligocene (30-25 mya) the climatic conditions were much more favourable. Temperatures were significantly higher than they are today, tropical and subtropical waters reached much further into the higher latitudes of the polar regions. During the following epoch, the Miocene (25-5 million years ago), modern baleen whales (Mysticeti) developed and spread more and more. There was an increase in size in whales and simultaneously in megatooth sharks. During the spreading out of the whales they presumably reached cooler polar waters that provided them with a richer food supply to which the whales adapted themselves. The whales were migrating between cool water feeding grounds and warm water breeding grounds. The climate became colder at the end of the Miocene and the beginning of the Pliocene (about 5 million years ago). The ice cover of the Antarctic polar region grew bigger and the mean sea level dropped. Living conditions and habitat for C. megalodon, who probably loved warm waters, obviously were restricted to such a degree that the species became extinct during the Pliocene. 

Reference:

C. megalodon-Megatooth Shark by Lutz Andres

Roesch, Ben S. 1998. A Critical Evaluation of the Supposed Contemporary Existence 
of Carcharodon megalodon. The Cryptozoology Review 3 (2): 14-24.