SHARK EVOLUTION

MISCONCEPTIONS AND INTERPRETATIONS
Source: http://elasmodiver.com/elasmobranch_evolution.htm

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 paleontologists, 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 paleontologists inventing multiple species of extinct sharks that were actually the same animal. During a shark's lifetime it's 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 herero-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 complex 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 last 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 notocord turned upwards rather like the sharks of today. Agnathans were very successful and diversified to dominate every niche available until well into the Devonian period towards the late Paleozoic. The various 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 ancenstry, 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 cartilaginous skeletons.

THE RISE OF JAWED FISHES
At some point during the Ordovician Period also known as the early Paleozoic, the major groups separated themselves from eachother.

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.

THE FIRST SHARK - Cladoselache
Although Chondrichthyes are rooted in the Ordovician period the first, well preserved early shark fossil to be discovered was Cladoselache dating from approximately 350 million years ago. The fossil of this shark was found miraculously intact in the Cleveland Shale of Lake Erie. It was so well preserved that its muscle fibers were visible as were its kidneys. Cladoselache Had two low dorsal fins both with prominent spines, broad based pectoral fins and eyes set far forward on the head. The mouth was at the front of the head as opposed to the under slung mouths of modern sharks, and the teeth had a large central pointed cusp with a smaller point on each side. Although cladoselache was almost certainly not the first ever true elasmobranch, armed with Cladoselache, Paleontologists were able to categorically state that elasmobranchs had arrived.

TWO VISIBLE SUBCLASSES
At the end of the Devonian another group of cartilaginous fish became evident. These fish known as Chimaeras or Ghost sharks had distinct mobility differences in their skeletal structure. The upper jaw was fused to the skull, the pectoral fins were large and able to flap unlike the rigid fins of sharks and their bodies tapered to a thin whip like tail. Due to the great distance between the chimaeras and the modern elasmobranchs, extant chimaeras are considered in the separate subclass Holocephali and presently consist of three families.

AN EXPLOSION OF FORMS
During the evolution of chrondrichthyes there have been many groups with bizarre appearances. Sometimes these families are collectively referred to as "paraselachians" . Many fossil skeletons contain unusual appendages. Most of which have as yet not been conclusively explained.
Some examples of these paraselachians include:

Stethacanthus - a Cladodont which lived through the Silurian Period between 380 and 300 million years ago. It had a modified first dorsal fin that terminated in a spine covered pad reminiscent of an inverted scrubbing brush. Its forehead also had a similar surface. These surfaces may have been used for pinning prey or for mating.

Helicoprion - from the Permian Period, had a conveyor belt of teeth that spiraled out of its lower jaw and a thin corresponding line of sharp teeth in the upper jaw. The lower whorl of teeth rotated out of the jaw as the shark grew. Unlike most sharks it retained the smaller previous teeth which rotated back into the jaw forming a spiral or whorl not unlike the growth pattern of a shell. The two dermal surfaces sliced against each other giving it a formidable shearing weapon.

Falcatus - from the carboniferous period had a curving, forward facing appendage in place of its first dorsal fin. It has been suggested that only the male may have had this sword like structure.

Xenacanthus - a member of the pleurocanthids. It had a long backward facing spike extending from the back of its skull and an eel like or ribbon like fin running down the length of its back.

Iniopteryx - Iniopterygians lived from the Devonian into the Carboniferous period. More closely related to modern day chimaeras, they had flexible pectoral fins which were disproportionately long and rayed for strength. It is unclear whether these "wings" were used to glide above the water or to paddle under it. The leading edge of the wings were covered with sharp toothy denticles.

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 oceans 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 fair to be had 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 icthyosaurs 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.

CARCHARODON 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 Carcharodon carcharias as a guide, it would not be unreasonable to assume that Megalodon reached somewhere between 50 to 100 feet 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 literature 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 yet fill the shallows of most tropical and temperate continental waters.