Key Takeaways

• Sharks emerged over 400 million years ago, which is about 200 million years before dinosaurs.
• The earliest chondrichthyans scales date back 440 million years, the earliest shark scales date about 425 million years, and the earliest shark teeth about 410 million years.
• After the mass extinction, sharks began to grow (in terms of size) again. Around 45 million years ago, the Great White shark evolved "from broad-toothed mako sharks."

Introduction

The Devonian Period

• Life on earth began evolving from single-celled organisms around 545 million years. Around 400 million years ago, bony fish and marine organisms began to flourish in marine environments.
• Chondrichthyes is a family of fish with cartilaginous skeletons that evolved from bony fish. It is recognized as the progenitor of all shark species.
• The chondrichthyans scales date back 440 million years, the earliest shark scales date about 425 million years, and the earliest shark teeth about 410 million years. Sharks emerged about 200 million years before dinosaurs.

The Carboniferous and Permian Periods

• About 320 million years ago, cartilaginous fish were in their prime. Cartilaginous fish were found in the shallow and warm seas across Europe and North America. Sharks thrived in this climate.
• Sharks were found in many distinctive niches and adaptations that are not found in present-day sharks. Adaptations of sharks of this age included specialized secondary sexual features and rapid replacement of teeth.

• The entire Carboniferous period is known as "the golden age of sharks." After the mass extinction event that concluded the Devonian Period, 75% of marine species perished, leaving sharks to dominate.
• The next mass extinction, 259 million years ago, killed 96% of marine species. Again, selected species of sharks survived.

Neoselachian Radiation

• During the late Triassic (237-201 million years ago) and earliest Jurrasic period (201-174 million years ago), Neoselachian radiation occurred that led to the diversification of modern sharks. Shark species emerged in response to the abundance of food sources in the marine environment.
• A conducive marine environment after the carboniferous era led to an explosion of fish in the seas, which meant more food for predators. They adapted to have speed, flexible jaws, and maneuverability to catch the bony fish prey in marine waters.
• According to Detlev Thies and Wolf-Ernst Reif, sharks began to diversify from preying on fish in near-shore hunting. Rather, they began evolving to be able to compete for fish prey with larger marine reptiles.

Jurrasic Explosion

• About 100 million years ago, thousands of shark species emerged that are closer to today's species.
• From that date, all shark families known today started emerging. It is also when scales and rays evolved.
• Squalicorax is one example. It grew to 16 feet and preyed on large marine reptiles, though it also ate dead mosasaurs.

The End of the Cretaceous Period

• The Cretaceous started 145 million years ago and concluded with an asteroid strike, the fifth mass extinction event, around 66 million years ago.
• While, unlike non-avian dinosaurs and most other creatures, sharks prevailed, they were still deeply affected. All the largest species disappeared. Only small deep-water species that mostly ate fish persisted.
• To give an idea about how different the sizes of large marine predators were before the mass extinction, the image below compares a megalodon's tooth to a tooth of a modern-day Great White shark.

 

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• Overall, diversity is likely the key factor that allowed sharks to survive five mass extinctions. They are able to function in different parts of the water column (including the deep sea) and eat a variety of foods.

65 Million Years Ago - Present

• After the mass extinction, sharks began to grow (in terms of size) again. Around 45 million years ago, the Great White shark evolved "from broad-toothed mako sharks."
• Today's sharks remain relatively unchanged from their earlier species millions of years ago. It is indicative of their efficiency and adaptation as predators in marine environments. The Hammerhead shark is an exception and is one of the youngest shark species, having evolved around 23 million years ago.
• However, even in the case of the Hammerhead shark, its earliest ancestors date back 45 million years ago.

 

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• Some scientists predict that in line with their past abilities to adapt, some shark species will survive changes in the ocean environment. 

Research Strategy

Key Takeaways

• According to the report published by Britannica, "environmental changes, in conjunction with rising walls of land, almost certainly disrupted marine food chains, including those that supported baleen whales (which feed on krill and zooplankton), which scientists believe were the main prey of megalodons."
• Scientists have hypothesized that the emergence of great white sharks significantly impacted the extinction of megalodons (Carcharocles megalodon), which occurred between 4 and 3.2 million years ago.
• Studies conducted on the teeth of both species (living sharks and 13 fossil megalodon teeth) suggested that both predators once occupied the same position in the food chain/web and thus, may have preyed and competed for the same food, which included whales, dolphins, and porpoises.

Introduction

Factors that Possibly Contributed to the Extinction of Megalodons

Environmental Factors

•  Environmental factors were the basis of some of the past theories and hypotheses on the extinction of megalodons.
• Fossil evidence suggests that the megalodons went into extinction before about 2.6 million years, which coincides with a period of cooling and drying across the world. "These alternating changes may have led to the closing of the seaways separating North America from South America and Eurasia from Africa, such as the emergence of the Isthmus of Panama."
• This phenomenon is said to have deflected ocean currents from their usual paths and apparently split the population of predators and prey.
• Furthermore, according to the report published by Britannica, "these environmental changes, in conjunction with rising walls of land, almost certainly disrupted marine food chains, including those that supported baleen whales (which feed on krill and zooplankton), which scientists believe were the main prey of megalodons."
• According to Paleobiology Ph.D. Student Jack Cooper, of Swansea University, U.K., "During the Pliocene, between 5.3 and 2.6 million years ago, sea levels were changing constantly and quite dramatically. This will have naturally reduced coastal habitats, which we know megalodons lived in to find food and use nursery areas for their young. With these habitats reduced, there was less area for megalodon to live in and less food available. Given the megalodons' enormous size, and its main prey likely being whales, less food would have meant megalodon likely couldn't have met its energetic demands."
• In addition, with increasing competition from white sharks and killer whales, which scientists revealed to have taken place around the same time or period, megalodons found fewer prey; therefore, their population crashed further.
•  Climate change and other environmental pressures may have also contributed to the extinction of megalodons according to scientists.

The Emergence of Great White Sharks

• Scientists have hypothesized that the emergence of great white sharks significantly caused the extinction of megalodons (Carcharocles megalodon), which occurred between 4 and 3.2 million years ago.

 

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• According to Robert Boessenecker, of the College of Charleston's Geology and Environmental Geosciences Department, "We think that C. megalodon was out-competed by the modern great white shark (Carcharodon carcharias), which evolved serrated teeth around 5 to 6 million years ago, and became a worldwide-distributed species around 4 million years ago."

 

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• Studies conducted on the teeth of both species (living sharks and 13 fossil megalodon teeth) suggested that both predators once occupied the same position in the food chain/web and thus, may have preyed and competed for the same food, which included whales, dolphins, and porpoises.

 

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• Given that great whites were three times smaller than megalodons, it's likely that they were nimbler, lighter, and quicker than the megalodons when hunting for food.
• In summary, scientists believe this to be the primary reason behind the extinction of megalodons, as the rise of competitors (great white sharks and killer whales) upset and shifted the food-chain dynamics, as the availability of its primary food source decreased significantly.

Additional Findings

• For additional context, scientists appear to agree that the extinction of megalodons may provide insights into the future of sharks and in consequence, the marine ecosystem. The case of the megalodon shows that large species of sharks are likely to be impacted by rising sea levels and fluctuations in food availability.
• At the same time, like the disappearance of megalodons, the extinction of large sharks would destabilize the food web due to the sudden abundance of species that tend to be their prey.
• Boessenecker also believes that if the hypothesis that great white sharks contributed to megalodons' fate is accurate, the ecology and survival of different predatory sharks likely depend on the great white, as the apex predator (at least among sharks).

Research Strategy