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Shark Evolution

400 million years ago began the evolution of what would become the most efficient undersea predator

 

 

 

 

Inhabitants Of Ancient Seas. There were sharks in the oceans of earth long before the first animals had begun to colonise the land surface. Their history stretches back for at least 400 million years, a vast period of time which makes the 2-million-year-old history of humans ( genus Homo) seem insignificant by comparison

 

Earth is at least 4500 million years old. The earliest signs of life date back 3000 million years, and by 600 million years ago quite complex animals and plants existed. 

Ancient Marine Vertibrates

The story of the backboned (vertebrate) animals, the fishes and other forms that eventually led to humans - began about 600 to 500 million years ago. The first definite fossil of a creature with a stiffening notochord along its back was a Cambrian animal called Pikaea, which was discovered in 560 million year old rocks in Canada. The oldest complete fish fossil discovered to date was found in central Australia, and was a jawless, armoured fish called Arandaspis, which dates from the early Ordovician period. This small filter-feeding or mud- grubbing fish lived in shallow coastal waters over 480 million years ago.

In the next 100 million years all the major fish groups that are found on earth today evolved, although their origins and relationships to one another are still uncertain. In the Silurian Period some 400 million years ago, the jawless fishes were the most numerous, although primitive bony and cartilaginous jawed fishes had begun to appear at about that time.  Fossils of the so-called 'spiny sharks, which had large fin spines, and one of the earliest true sharks have also been found in Silurian rocks. By the Devonian period, about 30 million years later, fishes had diversified and spread into all parts of the world.  By the end of the Devonian, some 25 million years further on, certain fishes had moved onto land. From these first amphibians evolved the four-legged animals, including reptiles such as dinosaurs, and eventually mammals. 

LEFT:    Two of the earliest backboned animals on earth. These ancient fishes, which lived in seas that covered the earth between 350 and 440 million years ago, had primitive, sucking mouths. 

Shark Evolution

The early evolutionary history of sharks and shark-like fishes is still poorly understood. Until recently scientists thought that there were no shark fossils older than those from the Middle Devonian. Now, however, it is certain that sharks did not evolve at that time; researchers were looking for the wrong kind of evidence. Microscopic examination of ancient sediments has revealed fossilised remains of sharks which may push their origins back at least 50 million years earlier.

Traditionally, sharks have been regarded as primitive vertebrates, so-called 'living fossils', but recent work suggests that they are highly specialised. Their highly evolved and complex biology ranks them with birds and mammals.  Even Devonian sharks, which were once considered to be the most primitive, are now thought to have been relatively specialised with a longer evolutionary past than was once supposed. 

The evolution of sharks remains unclear partly because it has been difficult to analyse particular characteristics, such as the shape of brain-cases or fin structures of modern sharks, which are an extremely diverse group, adapted to many habits and habitats. Any analysis of fossil sharks has had to rely almost solely on hard parts, such as teeth. Other features such as fins are only rarely preserved. Much more needs to be known about ancient sharks before any assessment can be made as to which characteristics were primitive and which were advanced.

Shark Fossil Record

All that is known about ancient sharks and their evolution has been gleaned from the fossil record.  Fossil remains of sharks have been known for many centuries, although their true nature was not always recognised. Until the 17th century many scholars regarded such fossils as sports of nature, and thought that fossilised shark's teeth were bird's or snake's tongues. Some large fossil teeth, called glossopteris (literally tongue-stones), were used as amulets to ward off evil and to protect against poisoning. It was not until about 150 years ago that the study of fossils, palaeontology; became a science, and the ancient remains of plants and animals were systematically classified. 

Sharks are rarely found as complete fossils because their skeletons are made of cartilage. Normally only the hard parts, such as teeth, scales and fin spines, are found. However, under certain special conditions, complete fossil sharks are preserved, and these provide scientists with vital information. One such deposit, found last century in Upper Devonian Cleveland shales from the USA, yielded entire shark carcasses which had been preserved in a bacteria-free environment so that even muscle and kidney tissue could be examined.

Minute Teeth and Scales - Earliest Evidence of Ancient Sharks

The very earliest signs of sharks are minute fossil scales and teeth which are found in rocks from the Late Silurian to Early Devonian period (around 400 million years ago). It becomes more and more difficult, however, to identify shark scales in older rocks because they closely resemble those from jawless fishes called the lodonts, which lived at the same time. Only microscopic differences separate shark and the lodont scales, and the two kinds seem to become more and more alike the further one goes back.

A similar problem exists with ancient shark teeth, which did not seem to be present in rocks older than those from the Middle Devonian. It now seems that the reason for this was that scientists were not looking in the right places, and that early shark teeth were often very small. In 1986 teeth were found in Lower Devonian rocks from Spain which belonged to a group of sharks called Xenacanthids.

What, then, were the origins of the shark-like fishes? One possibility is that different cartilaginous fishes (the group which includes sharks, skates, rays and chimaeras) evolved from placoderms (now extinct, bony- plated jawed fishes). Alternatively the placoderms and the cartilaginous fishes might all have shared a common ancestor at some time in the early Silurian period, some 430 million years ago. A third possibility is that the cartilaginous fishes and a group of primitive jawless fishes, such as the lodonts, both had a common ancestor. The lodonts (now extinct) had a skeleton made of cartilage and were covered with scales, which may have lined their mouths as well. They also had paired fins and eight pairs of branchial structures for supporting their gills. One region where the lodonts evolved rapidly was around Tuva, in Siberia, which is also where the oldest known shark scales have been found.

Common Ancestor 

The common ancestor of sharks and the lodonts (a fossil of which is yet to be found) would probably have been a small fish with a long slender body, one dorsal fin, no fin spines, paired pectoral fins and at least seven pairs of gill supports. Its mouth would have been either at the front of its body, or slightly beneath it, and the creature would have been covered with small scales which varied in size and shape according to their position.  

LEFT:  The earliest complete fossil sharks so far found belong to this genus They have very distinctive three-pointed teeth which first appear in rocks from the mid-Devonian period, and gradually disappear by the Permian period.  Specimens range from 0.5 to 2 m (15 to 6 ft) in length.

 

 

 

LEFT: The first hybodont sharks appeared during the Carboniferous period and members of this group only died out about 65 million years ago. at the end of the Cretaceous period. Specimens of Hybodus grew to around 25 m (75 ft) in length.

 

 

 

Ancestoral and Modern Forms

The history of how sharks developed from these ancestral forms is complex and confused. Scientists do not have a set of fossil remains which show a smooth transition from species to species, connecting ancient forms to their modern descendants. Instead they have some isolated pieces from an immensely complex jigsaw. Some fossils are of evolutionary experiments which led nowhere and eventually became extinct. Others may have features which seem to explain gaps in our knowledge, but also have other features which raise more questions. Some new discoveries provide little information at present, but may become vital when other pieces of the jigsaw are in place. Many theories exist to explain the evidence that is available, and these are constantly being modified as new information comes to light.

Although there is evidence of earlier sharks, the first complete fossils of shark-like fishes have been discovered in mid-Devonian rocks. Most frequently found are members of the genus Cladoselache, streamlined fish that grew to a length of about two meters (6 ft). Complete specimens of Cladoselache have been preserved in the remarkable Cleveland shales, so quite a lot is known about them. 

Cladoselache had five pairs of gill slits, a fin spine and all the same fins as modern sharks, except for an anal fin. These fin spines, which become more common and elaborate in later sharks, and which still persist in some species today, were positioned in front of the dorsal fins and acted as cutwaters to the first dorsel. Cladoselache had distinctive teeth with a large central cusp flanked by several smaller points, and apparently they lived on small fish - the remains of which have been found in the stomachs of fossilised specimens. These sharks are not now considered to be the main line leading to the modern species.

LEFT:   Perhaps the most curious of all ancient generated at the front of the creature's jaw teeth are those from the shark-Like creature Helicoprion, which first appeared during the Carboniferous period, some 345 million years ago. Fossil teeth in the form of spirals or whorls are found in rocks worldwide and gave rise to many fantastic reconstructions animal's life. Almost nothing is known about before partially complete fossil specimens how these strange creatures lived, and were found which enabled researchers to see how the teeth might have fitted inside the fish's jaws. 

Dominant Shark Groups

In the Devonian period a new group of sharks became common, the hybodonts. These became the dominant group during the Mesozoic era (from about 220 to 65 million years ago), when nearly all other shark groups died out. Although they were not as streamlined as modern oceanic sharks, and were probably not as accomplished as swimmers, they were nevertheless an advance on Cladoselache.

RIGHT:  The curious teeth of a frilled shark Chlamydosalachus anguineus -one of the so-called 'primitive' sharks, which closely resembles some extinct species. This shark has some 300, three-pointed teeth, arranged in 27 rows, to provide it with about 1000 hooks with which to grasp the fish it eats.

A typical genus of this group is Hybodus which was found all around the world from the Triassic period to the mid- Cretaceous. This shark grew to at least 2.5 m (7.5 ft) in length, and had a blunt snout with an elongated body The arrangement of its teeth, with sharp pointed ones at the front and blunt ones at the back, seems to suggest that it caught swimming prey as well as eating shellfish, such as molluscs.

Although sharks and shark-like fishes have a long history, the modern sharks (Cneoselachians) did not rise to dominance until after the Jurassic period, when, for some reason that is not yet clear, many of the more ancient forms had become extinct. Some Jurassic sharks are closely related to modern sharks, and this gives many present-day shark families histories which stretch back for 135 million years or more. The skates and rays, another group of cartilaginous fishes, also appeared in the mid-Jurassic, but they did not really come into their own until the Tertiary period, between 65 and 2 million years ago, when they were able to exploit a dramatic rise in the numbers of bivalve shellfish in the oceans.

Sharks Today

The fossil record of modern sharks is fairly good, but again it normally consists only of hard parts such as teeth and scales. The appearance and relationships of present-day groups are well understood compared with the situation in the Paleozoic and Mesozoic eras.  Oceans in the Cretaceous period (140 to 65 million years ago) were dominated by goblin, sand, probable, nurse, cow and angel sharks. Early sawfishes appeared and later evolved into the modern saw sharks. At the beginning of the Tertiary period (about 65 million years ago) the grey nurse sharks (Odontaspididae) were found in large numbers. All modern forms of sharks were present in seas of the Miocene epoch (25 to 5 million years ago), including the giant Carcharodon megalodon, perhaps the most awesome of all sharks, now extinct.

Many researchers are working on the problems presented by ancient sharks, and recent discoveries have included some of the oldest shark fossils found.

Among international research has been work in Germany which has concentrated on examining the fine structure of shark teeth and scales, and has revealed that the outer enamel on many teeth has three layers -a feature which can be used to identify certain groups of sharks. Other work, in the USA, has been delving into the nature of the supposedly primitive modern species; the frilled shark Chlamydoselachus anguineus, the Port Jackson shark Heterogonous portusjackson, and the six-gill and seven-gill sharks (family Hexanchidae), in an attempt to understand their relationships to other modern sharks, and to uncover any links with earlier fossil forms.

Missing Link

One particularly important recent discovery has been the recognition of a new class of fossil sharks called Iniopterygians.  Remains of this strange group have been found in Upper Carboniferous rocks from North America which are 300 million years old.

Examination has shown that these creatures are a missing link, and that they combined certain characteristics of both elasmobranches (sharks, skates and rays) and Holocephalians (chimaeras or rat tails). The life styles of these fishes remain uncertain, although their complex tooth plates seem to suggest that they lived on shellfish.

There has been an upsurge of interest in fossil sharks in recent years as more information has become available from new fossil discoveries, especially in the southern continents, and from a microscopic examination of existing fossil remains. In many cases these discoveries have helped scientists to understand some of the finds made last century.  There are still few definite answers about the origins and relationships of all the known shark fossils, however, progress is being made.

Iinterested Further? 

Paste this link into your browser to see what is probably the best educational site on sharks on the Internet.    http://www.elasmo-research.org/education/evolution/evol_s_predator.htm

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