Sharks, the iconic ocean predators, exhibit a staggering diversity, with over 500 living species that vary greatly in shape and size. From the massive 20-meter-long whale sharks to the tiny bioluminescent lanternsharks, these underwater beings exhibit an incredible variety of shapes and colors. Other familiar ichthyic faces include the flattened angel sharks, hammerheads, sawsharks, goblin sharks and wobbegongs. Despite their prominence in marine ecosystems, scientists have long grappled with understanding how, when, and why such a vast diversity of shark body types evolved.
During much of their evolutionary history, sharks were restricted to environments close to the benthos. For about 200 million years, their lives were limited to these extreme depths. Large, pelagic sharks like today’s top predators first shot to prominence in the Jurassic. This was a major extinction event that took place about 145 to 201 million years ago. The development of all these various body forms among sharks didn’t occur in a vacuum, or overnight. Rather, it happened like a theater performance played in slow motion over millions of years.
Understanding Shark Diversity
Sharks have adapted to develop incredible diversity in shape and size, showing their ability to adapt and thrive within a variety of environments. The vast range includes the massive whale shark, which can reach lengths of up to 20 meters, showcasing the upper limits of size in this group. On the other end of the size spectrum, the pygmy lanternshark, which can be as small as a smartphone, illustrates the small-scale end of things. This huge range exemplifies the ecological niches that other shark species fill.
Various other species add to this diversity. Hammerhead sharks are one of the most easily recognizable animals in the ocean, thanks to their distinctive head shape, which helps them hunt. In much the same way, the sawshark has an unusual long snout with saw-like teeth on both sides that it uses to hunt its prey. Goblin sharks are famous for their stretching jaws and nightmarish look. They reveal the incredible diversity of adaptations that evolution has produced in these amazing animals.
Even given this incredible diversity, sharks’ incredible evolutionary journey through time has mostly gone unexplored. Shark scientists have gathered data on body shape from over 400 extant (living) species from historic scientific illustrations. This priceless research opens a window to better understand the ecological adaptations these fishes acquired, how they evolved, and what they evolved to do.
The Evolutionary Path of Sharks
Sharks can be traced back to a time before the first dinosaurs ever arose on earth, endowing us with a deep historical perspective into their evolution. Once into the water column, sharks began to explore new body shapes and sizes. This geological constraint remained in place for dozens of millions of years, until soon after the onset of the Cambrian revolution.
The change from bottom-dwelling species to open-ocean predators was a major turning point in shark evolution. As sharks started to frequent more of the open ocean realm, the researchers observed a shift in body shape. To do that, they evolved deeper bodies and more symmetrical tails. These unique adaptations made them extremely efficient swimmers in open water.
For that, scientists have turned to a statistical method known as ancestral state reconstruction. Using this technique, they’re able to interpolate the body forms of extinct sharks. This approach has let researchers make educated guesses about the appearance and behavior of extinct species just by analyzing their modern relatives. Using this method, they have traced back the timing and cause of the striking changes in shark shape.
Three Key Shifts in Body Shape
The adaptation of sharks to become open-ocean dwellers took place through three major changes in body shape. Each transition was accompanied by morphological modifications that allowed sharks to be successful linearly in open ocean habitats. Holding such a complicated organism together These changes weren’t capricious or sudden—they were the product of slow evolutionary forces that played out over millions of years.
The second shift constituted intentional alterations that perfected the practice of swimming. As sharks were forced into open waters, the pressure of natural selection favored those with relatively deeper bodies and more torpedo-like shapes. Such adaptations helped them travel faster through the water and become highly successful predators in wide open ocean environments.
The second shift focused on tail symmetry. Smaller sharks started to have increasingly symmetrical tails, allowing them to push themselves more forcefully through the water. This evolutionary improvement significantly increased their capacity to efficiently pursue prey. This flexibility would become essential as their unique development opened them up to novel pressures in a fast changing world full of competitors and predators.
Lastly, the third shift pointed out that diversification between species occurred as they fit into different ecological roles in a changing ocean. As they adapted to diverse hunting techniques and prey, sharks developed specialized features. It was this adaptation that caused the wonderful diversification we experience today.