If you went back in time and looked at the first, unremarkable prehistoric sharks of the Devonian period (about 400 million years ago) you would never guess that their descendants would become such dominant creatures, holding their own against vicious aquatic reptiles like pliosaurs and mosasaurs and going on to become the "apex predators" of the world's oceans.
Time Line of Shark Evolution
The First Sharks (The Devonian Era - 400 MYA)
Over 400 million years ago, innumerable species of bony fish and marine organisms inhabited the Earths seas. The time period, known in history as the Devonian Era, gave birth to a family of fish unlike any other of the time, known as Chondrichthyes (Cartilaginous fishes) (Huxley, 1880; Long, 1995). The Chondrichthyes evolved a truly unique skeleton composed almost completely of cartilage. Divergent evolution began to further separate the Chondrichthyans from the primitive fish, creating an array of beings that would come to be known as the progenitors of all shark species (Bonaparte, 1838). Chondrichthyans are considered to have been highly advanced for their time, possessing a great deal of the same characteristics seen in modern cartilaginous species. Now nearly 400,000 millennia on and evolution has generated over 1,200 highly advance species of sharks and rays, encompassed within the Elasmobranchii (sharks, rays and skates) and Holocephali (chimaeras, also known as ghost sharks), which have all diverged from the Chondrichthyans (FAO species catalogue, 2002).
Cartilaginous fish are so named because their internal framework is composed of almost entirely of cartilage rather than bone. Cartilage, unlike bone, does not fossilize well when an animal dies. The exposed collagen fibres in the cartilage degrade and break apart, fragmenting the skeleton into thousands of pieces, and leaving only teeth, scales and spines behind, making it extremely difficult to determine when these organisms first appeared on earth.
In a historic scientific find last century,
scientists discovered several entire shark skeleton imprints dating
back to the mid-Devonian Era, persevered in the Cleveland Shales in
The Carboniferous Era (320 MYA)
During the Carboniferous era, 320 million years
ago, most of present day Europe and
The Hybodontidae came into existence in the Carboniferous era and may be considered a more immediate ancestor to modern sharks. They possessed many key similarities, including copulatory claspers, which the more primitive Cladoselache did not (Romer and Williams, 1976). The Hybodonts became the dominant group during the Mesozoic Era spanning the late Permian to the Jurassic period (220 to 65 million years ago). The Hybodus was a common genus of the Hybodont group and is the oldest known representative of the order Selachii (Romer and Williams, 1976). Its fin orientation and body structure may show links between the more primitive Cladoselachii order and Neoselachii or modern sharks such as the bullhead (Heterodontus). The Hybodontidae are believed to have become extinct about the same time as the dinosaurs, around 65 million years ago (Romer and Williams, 1976). Scientists have also found fossils of mako and mackerel sharks dating back to the late Jurassic period, putting the arrival of most modern sharks somewhere in the lower Cretaceous period (130- 60 million years ago) (Long, 1995).
Two palaeontologists, Detlev Thies and Wolf-Ernst Reif, proposed the theory of Neoselachian radiation, which stated that the development of the modern shark or Neoselachian was the result of an opportunistic response to an abundant new food source (Thies and Reif, 1985). Populations of these new bony fish exploded in the seas providing plenty of food to predators that possessed the speed, flexible jaws and manoeuvrability necessary to catch them (Thies and Reif, 1985). Thies and Reif (1985) went on to suggest that these sharks began to branch off from near-shore hunting and began competing with the large marine reptiles of the time in pelagic predation.
Jurassic Explosion (100 MYA)
The Jurassic Explosion, beginning approximately 100 million years ago, marked the emergence of thousands of shark that slowly evolved into the species that we see today including the great white (Carcharodon carcharias). Around 65 to 60 million years ago, the white shark, Carcharodon carcharias arrived (Long, 1995). The white sharks were born into existence so highly evolved for their niche on the Earth that they have hardly been altered by nature. The great whites seen today are virtually a mirror image of the white shark of the late-Jurassic/ early Tertiary period.
Megalodon (60 MYA)
Also around this time (60- 55 million years ago) the greatest of all sharks inhabited the Earth, Carcharodon megalodon. C. megalodon was the biggest predator known to have existed in the oceans of the planet (Applegate and Espinosa-Arrubarrena, 1996). The immense beast could reach 65 feet in length and could weigh 30 tons, comparable to an average-sized sperm whale. The teeth of C. megalodon closely resembled that of its smaller cousin the white shark, but three times the size, over 6 inches long. Due to similarities in serrated edges and size of the teeth, a popular theory emerged that the white shark and C. megalodon were very closely related, if not actually representative of the same species (Ferrari and Ferrari, 2002). The theory suggested that due to a reduction in prey size, C. megalodon became smaller eventually evolving into the great white shark (Ferrari and Ferrari, 2002). Currently a new popular theory maintains that the two enormous predators diverged from a common line and one containing C. megalodon became extinct and the other has survived to this day (Ferrari and Ferrari, 2002). Others still think that it is possible that C. megalodon may still inhabit the most remote parts of the oceans. Scientists have also found partially fossilized giant teeth of C. megalodon at great depths of the ocean causing some to believe that C. megalodon was still alive 2 million years ago (Applegate and Espinosa-Arrubarrena, 1996).
Not long ago it was commonly accepted that sharks were simply primitive, mindless predators. We now know that this couldn’t be further from the truth. Humans have recently come to appreciate sharks for their long history and deep ancestry. Sharks have evolved from modest means hundreds of millions of years ago, to the present day apex predators of the sea. The fact that many modern shark species evolved several million years ago and have remained unchanged in all that time goes to show how efficient and well adapted these creatures are to their environment. During the millions of years of evolution, the present day Chondrichthyes have developed some of the most sophisticated hunting systems ever known (Bright, 1999). The success of the Chondrichthyes as predators is largely due to their highly developed sensory systems (Bozzano and Colin, 2000).