Mass Extinctions and Adaptive Radiations

Throughout history there is evidence of catastrophic events that dramatically changed the environment and resulted in the mass extinction of a significant percentage of species. In the worse such event, 251 million years ago, as many as 95% of species were killed, perhaps due to either a comet impact or volcanic activity. Five major mass extinction events have occurred during the past 439 million years.

For surviving species, these mass extinctions can provide an opportunity to flourish if previous predators disappeared or new, inviting habitats opened up. These, in turn, have sometimes given rise to ‘adaptive radiations’, or a dramatic increase in new diverse species. Sharks have survived all five of these mass extinctions, a testament to their versatile design. They have also benefited from a few adaptive radiations, giving rise to many interesting sharks. The first major shark radiation occurred 360 to 286 million years ago.

A few Prehistoric sharks

Cladoselache. Illustration by R. Aidan Martin

Even as long as 370 million years ago, sharks would have been recognizable to us as sharks, although many, such as the Cladoselache, had characteristics that would seem unusual to us, now. A four foot shark that was prevalent at that time, Cladoselache still had a mouth in the front of its head, like other ancient sharks, whereas most modern sharks have mouths that are located at the bottom of their head. Unlike most ancient and all modern sharks, Cladoselache lacked tooth-like scales that provide protection and allow muscles to attaché to the skin more strongly. Cladoselache also lacked the claspers that male sharks use to mate with females.

Ctenacanths are sharks that developed around the same time as Cladoselarche and continued on for over a million years. Ctenacanths are best known by their fin spines, which were long and cylindrical, with unique comb-like ridges that gave rise to its name.

After a  mass extinction left the oceans with far fewer fish and many ecological niches left open, one of the more interesting sharks evolved. Stethacanthus was a two-foot long shark that lived in warm, shallow seas. Male Stethacanthus had a flat-topped dorsal fin with enlarged scales that resembled a brush, and similar enlarged scales on top of the head. These strange features must have held some purpose, and suggestions have ranged from being courtship related to offering some sort of protection.

Stethacanthus. Illustration by R. Aidan Martin