Shark Evolution

 

Misconceptions and Interpretations

There are many misconceptions regarding the evolution of sharks and rays. Contrary to popular belief they have not remained unchanged for 300 million years. However, many of the families we have today have been in existence for perhaps the last 150 million years. Compared to our own paltry 3.5 million years this makes the elasmobranch lineage very ancient indeed. The fossil record is sketchy at best when it comes to sharks. Cartilage is preserved very poorly so the body structures of many early sharks are purely speculative. Fortunately for the palaeontologists, sharks discard their teeth on a regular basis and these teeth which fossilize well are often enough to allow accurate identification of individual groups and help place them correctly in the evolutionary time line.
One of the pitfalls of using fossilized teeth for identification is that the teeth of sharks sometimes vary significantly depending on which area of the jaw they have come from. In the past this has led to palaeontologists inventing multiple species of extinct sharks that were actually the same animal. During a shark’s lifetime its dentition also may change which only adds to the confusion. In recent years there has been a revision of extinct species that paid more attention to the hetero-dental nature of modern species, and consequently the list of extinct species has been paired down.

A World Before Fish

How life first came about on earth is a burning question that many scientists and theologists have dedicated their entire careers to solving. Be it by chance or divine intervention, it is likely that super heated compounds developed a complexity at which point they were able to grow by absorbing the chemicals around them. Complex compounds are a far cry from even the most basic fishes but at some point (around 3.5 billion years ago) simple organisms started evolving. These most ancient forms of life on earth were called stromatolites and they may still be seen today. Stromatolites are simple organisms that cluster together and form mounds not unlike coral reefs but with a far less complicated structure.

1.4 billion years later the first cells with a nucleus were ooching around looking for a way to take advantage of both the chemicals in their environment and each other. As is the nature of evolution, a radiation took place after this point that enabled these basic creatures to fill every niche that was available to them. By about 600 million years ago, multi-cellular organisms such as jellyfish, worms, sea-pens, and more obscure soft bodied invertebrates had taken over the ocean deserts.

540 million years ago near the beginning of the Phanerozoic Eon (which lasts to this day and is split into the Cambrian, Palaeozoic, Mesozoic, and Cenozoic Periods) Most of the modern groups of animals were developing. This era heralded the development of shelled creatures which have been preserved very well in the fossil record.

There is some conjecture regarding how primitive fishes managed to develop from these invertebrates. Most likely, fishes evolved from the larval forms of their predecessors. Most invertebrates release eggs and sperm which after combining, develop into free swimming, fish-like larvae. These larvae drift within the planktonic soup until they reach a point of metamorphosis. In many groups such as corals and tunicates this metamorphosis is highly dramatic, involving a complete restructuring of the body to adapt to a sessile lifestyle. However, if the planktonic environment were rich enough it would be prudent for some larvae to remain as free swimming organisms permanently. Which group of invertebrates are responsible for spawning the first fishes is not known but it is possible that it may have been the echinoderms (seastars and sea cucumbers) as they share an early cell developmental process with vertebrates. Some scientists believe that sea squirts are responsible. Recent fossil evidence also points to a now extinct animal called a conodont that was characteristically fish-like and may have been the true predecessor.

The Agnatha

Around 500 to 450 million years ago the first primitive fishes appeared in the fossil record. They were the Agnathans and they were characterized by two (dorsal and ventral) bony shields on the head with many trunk scales tapering towards a primitive caudal fin in which the notochord turned upwards rather like the sharks of today. One notable difference was that the first agnathans lacked paired fins (pectoral and pelvic fins).

Agnathans were very successful and diversified to dominate every niche available until well into the Devonian period towards the late Palaeozoic. Eventually, some groups developed many of the characteristics associated with vertebrates today including paired limbs, cellular bone, a complex sensory line system, dentine, and complex eyes and inner ears. Sadly these pioneers of all that represents our ancestry, mostly vanished during the Devonian extinctions. Today the two remaining groups of agnatha are represented by the Hagfishes and Lampreys. They are jawless, limbless, and have rudimentary, cartilaginous back bones.

The Rise of Jawed Fishes

At some point during the Ordovician Period also known as the early Palaeozoic, the major groups separated from each other. Between about 400 and 350 million years ago the seas were beginning to fill. The major groups were:

  • Osteichthyes – the bony fishes which eventually outnumbered all other vertebrates by species.
  • Crossopterygii – which split to form the Porolepiformes which evolved into salamanders and the Osteolepiformes which eventually evolved into frogs and toads, reptiles, birds, and mammals.
  • Dipnoans – the lungfishes
  • Acanthodians – also called the spiny sharks – Superficially shark like with strong broad spines strengthening all of their fins except the caudal fin which was upturned (heterocercal) as in the true sharks. Their bodies were covered with small flat bony scales. The Acanthodians survived into the early Permian about 200 million years ago.
  • Placoderms – a large group of fishes covered with big bony plates and blade like jaws that proliferated during the Devonian and then died out at the beginning of the carboniferous period.
  • Chondrichthyes – the cartilaginous fishes that evolved into today’s sharks, skates, rays, and chimaeras.

Most of these evolutionary experiments were probably adaptations to the demands of life in an ever more competitive environment. During the Carboniferous Period, the ranks of the sharks swelled to their greatest diversity ever but towards the beginning of the Permian Period many ancient forms became extinct along with the majority of the more experimental forms.

Enter the Neoselachians

As the Permian Period was drawing to a close the seas were filling with Actinopterygians – the ray finned fishes. This was a food source that could not be ignored by the ocean’s predators. In response, the elasmobranchs began to radiate again and during the early Triassic a shark appeared in the fossil record that was similar enough in appearance to modern day sharks to be considered one of the first of the “modern sharks”. The name of this shark was Palaeospinax.

Palaeospinax was morphologically similar to the dogfish of the family Squalidae. It had a calcified sectioned vertebral column instead of a continuous notochord, its two dorsal fins had supportive leading edge spines, and most notably it had the under slung mouth of a modern shark.

Neoselachian Radiation

Amongst the first of the presently extant sharks to swim in the seas were the slow swimming Horn sharks and the Cow sharks but towards the mid cretaceous the food supply in the mid oceans was enough to push the development of fast moving predators that could pick off large, schooling, off shore fishes. At the time the seas were ruled by enormous ichthyosaurs and plesiosaurs so this new food source did not come without risk to the sharks.

During the Cretaceous, most of the present genera were firmly established and then around 60 million years ago at the end of the Cretaceous a catastrophe occurred which wiped out the dinosaurs and many other species, leaving the remaining sharks as the supreme rulers of the oceans.

Otodus Megalodon

About 50 million years ago, a super-predator evolved, the size of which the world had not previously seen. Megalodon was similar in shape and dentition to the white shark (Carcharodon carcharias) the most notorious shark of today’s temperate oceans. Its size however was spectacular. The largest of its fossilized teeth that have been found to date have measured over six inches long from point to base. Extrapolating this information and using the relative proportions of Carcharodon carcharias as a blue print, it would not be unreasonable to assume that megalodon reached somewhere between 15-30m in length.

Sadly, but perhaps just as well for us, no megalodon have been seen for some time. Estimates on the time of its extinction vary widely. Some popular fiction writers would like us to believe that megalodon is still down there somewhere lurking in the shadows. More likely, megalodon faded away some time within the last 30 million years due to a combination of a waning food supply and a changing climate.

Ray Radiation

Meanwhile, back at the end of the Triassic, at about the same time as Palaeospinax was swimming around the coastline of the super continent Pangaea, another group of sharks were adapting well to the bottom terrain of the shallow slopes. By the upper Jurassic Period the first guitarfishes were grubbing around for food and blending into the bottom sediments. These rays were a little more primitive than those of today. The main differences being a more shark like skeletal structure and the presence of fin spines. It has been suggested that all modern rays were derived from primitive guitarfishes but it is unclear exactly where the families are linked.

The most recent addition to the batoid tree are the stingrays which showed up a mere 60 million years ago. So far so good; stingrays have adapted extremely well to the changing state of our modern oceans. They fill the shallows of most tropical and temperate continental waters.

References

The Rise of Fishes – John Long.

Phyletic Relationships of Living Sharks and Rays – Leonard J.V. Compagno. Amer. Zool. 17:303-322 (1977)

Ichthyology – Karl F. Lagner, John E. Bardach, Robert R. Miller, Published by John Wiley and Sons 1962

The Book of Sharks – Richard Ellis. P20-23, 84-88. Knopf 1989

The Encyclopedia of Sharks – Steve and Jane Parker. P16-19. Firefly 2002