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  Seawater’s saltiness is a result of rainfall weathering exposed rocks and mountains, and washing their life-giving minerals into the rivers which feed the oceans. The continual evaporation of pure water from the surface of the oceans ensures that their salinity level remains more or less constant. On average, one kilogram of seawater contains 34.7 grams of salt (about one heaped tablespoon), half of which is chlorine, and a third sodium. Four other elements found in seawater are sulphur, magnesium, calcium and potassium. Seawater also contains dissolved sediments from atmospheric dust, marine biological processes and seafloor volcanic activity.

  Salinity and temperature are critical for the establishment of life forms, because they affect the rate at which chemical reactions take place. Furthermore, many of the ocean’s salts, especially calcium salts, are used to build skeletons, and seawater’s nitrates and phosphates are extracted by plants, the first link in the oceanic food chain. Photosynthesis enables plants to grow at the depth to which sunlight penetrates. Phytoplankton—single-celled drifting algae, mainly dinoflagellates and diatoms—feed on dissolved organic and chemical detritus. They in turn are consumed by the filter-feeding zooplankton—the copepods (tiny, shrimp-like crustaceans) and euphausiids (krill), collectively known as the ‘insects of the sea’ because of their abundance and their importance as a food source. A primary feature of zooplankton is their vertical migration, as they rise to the ocean’s upper layers by night to feed, and descend into safer, dark water by day.

  There are also large, predatory plankton such as the gelatinous jellyfish. Beyond plankton, the oceanic food web becomes bewilderingly diverse: worms, crustaceans (including thermal vent spider crabs), bivalves (such as mussels), echinoderms (sea cucumbers and starfish), nudibranchs, cnidarians (corals, anenomes), cephalopods (octopus, cuttlefish, squid), reptiles (turtles, snakes, the saltwater crocodile), teleosts, elasmobranchs, pinnipeds, cetaceans (such as whales), mammals and seabirds.

  Where do the elasmobranchs fit in? Almost everywhere, through their evolved ability to feed off most other oceanic life forms. Evolutionary processes ensured the early development of biologically complex and sophisticated predators and scavengers, preventing the oceans from becoming overpopulated and unhealthy. The description that follows of the evolution of the elasmobranchs requires some reference to the Geologic Timeline, which is structured primarily according to fossil records and measured in millions of years (mya).

  Precambrian Time

  4500–540 mya

  • Hadean Eon (4500–3800 mya). Earth formed from colliding planetoids. Unstable crust, very little free oxygen in the atmosphere, constant cosmic bombardments.

  • Archaean Eon (3800–2500 mya). Stabilising biosphere and oceans, single-celled stromatolites become the earliest life forms.

  • Proterozoic Eon (2500–540 mya). Oceans shaped by proto-continents. A single supercontinent, Rodinia, breaks up about 600 mya. Emergence of complex multicellular organisms.

  Lower Paleozoic Era

  540–407 mya

  • Cambrian Period (540–505 mya). Explosion of life forms: earliest aquatic plants, earliest aquatic animals such as corals and molluscs.

  • Ordovician Period (505–440 mya). The Cambrian–Ordovician transition is marked by the first mass extinction event.

  • Silurian Period (440–407 mya). The second mass extinction event, the Ordovician–Silurian transition, gives rise to the evolution of jawless fishes and the first land-based plants. The first jawed fishes date from the end of the Silurian and also the first sharklike dermal denticle fossils.

  Upper Paleozoic Era

  407–245 mya

  • Devonian Period (407–360 mya). Amphibians, insects, proto-forests. First shark teeth fossils in the Lower Devonian Period. Emergence of freshwater and marine predators including cladoselache, the best-known Paleozoic shark. This period ends in the third mass extinction event, the Devonian–Carboniferous transition.

  • Carboniferous Period (360–286 mya). Seed-bearing plants, ferns, earliest land reptiles. Major shark radiation, including emergence of the eugeneodont and hybodont sharks and the holocephali (the extant chimaeras).

  • Permian Period (286–245 mya). The end of this period is marked by the fourth mass extinction event, the Permian–Triassic Transition.

  Mesozoic Era

  245–65 mya

  • Triassic Period (245–210 mya). Reptiles diversify, first dinosaurs, earliest mammals. Emergence of the neoselachians, the modern sharks. The fifth mass extinction event, the Triassic–Jurassic transition.

  • Jurassic Period (210–144 mya). Dinosaurs dominant, first birds.

  • Cretaceous Period (144–65 mya). Earliest modern mammals, flowering plants, reptile domination. Continuation of the hybodonts. The sixth mass extinction event 65 mya terminates many land and aquatic life forms.

  Cenozoic Era

  65 mya–present

  • Tertiary Period (65–1.8 mya). Mammals dominant, birds diversify, first apes, first upright hominids.

  • Quaternary Period (1.8 mya–present). Humans become dominant. The seventh mass extinction event, the Holocene Extinction, largely human-induced, is generally considered by the international scientific community to be well advanced.

  The planet’s earliest life forms included the stromatolites—rocklike structures built up of microorganisms, especially blue-green algae—which can be seen in the shallow saline waters of Western Australia. By the end of the Precambrian, Earth’s oceans hosted primitive marine invertebrates such as those found in South Australia’s Flinders Ranges, ‘the first appearance of large, architecturally complex organisms in Earth history’.11

  The fact that life originated in water threw up its own set of complications. Compared to movement through air, swimming is difficult:

  When a body moves through a fluid a hydrodynamic force acts on it. A component of the force acts backwards along the direction of motion; it resists the progress of the body and is known as the drag . . . the force also has a component at right angles to the direction of motion. This is known as the lift . . . When a fish swims drag acts on its body and must be overcome by a forward propulsive force . . . When the body bends not only does the tail move to the side but the head moves a little to the side as well; the tail wags the fish.12

  The Cambrian Period gave rise to the evolutionary development that would allow active rather than passive aquatic movement: a flexible internal vertebral column to support an animal’s shape and to enable sinuous forward movement—swimming. The evolution of a skeletal structure meant that the future choice of internal building materials—bone, cartilage, enamel, dentine—would be influenced by an organism’s surrounding environment.

  The first swimming animals to emerge during the Cambrian Period included:

  • conodonts, eel-like creatures with a stiffening notochord, that is, a rod providing backbone firmness, the muscles around which would have moved the notochord when they contracted: the origin of swimming. Their teeth structure, and terminal eyes, suggest that they may have been predators;

  • the arandaspis, the oldest known vertebrate. Its fossil was first discovered in central Australia in 1959 and named after the area’s Aranda Aboriginal tribe. Finless and armoured, it probably foraged on the seabed, hoovering in food and using its tail for locomotion;

  • agnathas, the first true vertebrates, which appeared towards the end of the Cambrian Period. The name means ‘no jaws’. They generally had stout bodies, paired fins and external protective bony shields around their heads and gills. Unlike the extant cartilaginous agnathas—the hagfishes and lampreys—most prehistoric agnathas had bony skeletons; and

  • ostracoderms. Typical agnathas, many species were about 30 centimetres long although there are some fossil specimens exceeding a metre in length. The jawless mouth meant that the ostracoderm either hoovered in food or was a filter feeder—again, unlike the hagfishes and lampreys, which are parasitic suction feeders. Behind the bony head plates
the body was covered with dermal scales.

  The known shark fossil record begins in the form of scales and teeth. To have teeth, an animal must have jaws. Jaws evolved from the front pairs of gill arches over about 50 million years, and the first jawed fishes date from the Silurian Period. It is thought that the front gills initially became modified to move the jawless mouth up and down to pump water more effectively over the gills. Teeth evolved over the same period from skin scales—dermal denticles—migrating permanently into the ‘new’ mouth.

  The Devonian Period, which followed the emergence of the first jawed species, has been called the ‘age of the fishes’. Some of the first to emerge were:

  • xenacanths. Freshwater predator–scavengers, with an upper jaw attached at front and back to the skull and therefore fairly inflexible;

  • ctenacanths. More evolved predators with firmer keels and pectoral fins made flexible by cartilaginous supports, both of which improved pursuit swimming. Like modern sharks, males had claspers;

  • placoderms. With heavy external bony plating over the head and thorax, and toothlike crushing structures in their simple jaws, the placoderms were the main predators of the Devonian Period, which has led some researchers to claim them as the planet’s proto-sharks, although their skeleton was bony, not cartilaginous;

  • acanthids. With two spined dorsal fins, they are often called ‘spiny sharks’, though as their skeletons have bony elements, it is more likely that they are the true ancestors of the teleosts rather than the sharks; and

  • cladoselaches. So-named because of their multi-structured teeth, the cladoselaches first appeared about 370 mya. They had cartilaginous skeletons and are possibly the true ancestors of sharks. They grew to about two metres, had two dorsal fins with short, strong spines, large pectoral fins, a strong caudal fin and sharp teeth for grasping prey—all the ingredients of a fast-moving predator. Cladoselache fossils are rare, but a number of almost perfectly preserved specimens were found on the southern shore of Lake Erie in the United States.

  If the Devonian Period was the ‘age of fishes’, the Carboniferous Period has been called the ‘Golden Age’ of sharks, as they radiated into a variety of experimental forms. This may be because fossil evidence indicates that during much of this period the seas were relatively ‘empty’, following the extinction event that ended the Devonian Period, the cause of which remains unknown. The demise of the once-dominant placoderms made available niches for new forms of predator, the eugeneodontida.

  Among the evolutionary experiments of these early elasmobranchs were appendages whose use has not yet been explained and probably never will be. Ichthyologists struggle to describe their forms and explain their functions: one species had an ‘enormous, flat-topped dorsal fin bristling with enlarged scales. Basically, it looked like a fish with a brush sticking out of its back . . .’13 Another had an ‘outrageous dorsal fin—the shape of an ironing board—that it seems was part of courtship display as it is found in the males only. The top of this fin was covered in rough, tooth-shaped scales . . . Was this supposed to mimic a huge mouth and make the creature appear more frightening?’14 The teeth of some of these proto-sharks were even more baffling:

  They formed a whorl erupting from the back of a semicircular ‘conveyor belt arrangement’, but the teeth did not fall away at the front as in modern sharks. Instead, they were rotated under the apex of the lower jaw, and then back up into a cavity under the jaw where they were stored in a tight spiral.15

  The teeth . . . were arranged in a single row, the bases of each tooth being greatly enlarged and joined together in a curved symphysical whorl . . . the entire whorl grew outwards as each new tooth was formed. This gave the shark a pair of saw-edged shears that projected from the mouth, each composed of a series of teeth and tooth-bases with the youngest teeth at the base and the oldest at the tips.16

  As evolutionary experiments they represent the birth of elasmobranch diversity:

  Falcatus and Damocles are two separate genera of sharks, each with long spines protruding from the head that were directed forward. One [fossil] limestone slab of Falcatus actually has two sharks, one on top of the other. The shark on top of the slab, which is devoid of any spine, is actually biting the spine of the shark beneath it. This may be an indication of courtship behavior.17

  And we have to guess at some other anatomical oddities:

  Squatinactis seems fairly convincing as a torpedo-style hunter . . . the torpediniforms are largely ambush predators who lurk on or in bottom sediments. When they encounter suitable prey, they ‘jump’ on their expanded dorsal fins, sucking the victim in under the mantle, which is then folded over the prospective lunch. Unlike modern rays, squatinactids presumably lacked electric cooking, which may well have dictated a somewhat different design. This hunting strategy makes sense out of what might otherwise seem a bizarre architecture. The anteriorly directed dorsals make little hydrodynamic sense for swimming. However, if the objective is to surge upward and forward over a slow-moving Paleozoic fish, in a single movement, Squatinactis is very well designed.18

  The fourth mass extinction event ending the Permian Period wiped out 90 per cent of the planet’s life forms. It may have been caused by a meteorite hit, or extensive volcanism, the latter possibly causing the oceans’ oxygen levels to fall dramatically. Some marine species did survive, one group in particular becoming dominant in both saltwater and freshwater systems. These were the hybodonts, which were probably better able than placoderms to prey upon the rapidly evolving teleosts, although the fossil record is not certain. Nor is there certainty over their eventual demise:

  Among the vast array of modern type sharks that occur at the end of the Cretaceous, Hybodus was a carry over from the older Carboniferous. The hybodont sharks were one of the first modern types of sharks to appear in the fossil record. Some paleontologists believe that during the Jurassic, the hybodont line diverged into today’s modern shark lineage. The hybodonts though displayed some primitive and different characteristics than the main body of sharks of that time. Hybodont sharks . . . went into a worldwide decline during the Paleocene [early Tertiary].19

  One way through the uncertainty of the elasmobranch lineage is to consider not so much from which ancestors modern sharks evolved, but how form evolved. Thus cladoselaches had:

  • a long anterior mouth extending from the front of the snout to the gill slits;

  • a simple and relatively weak jaw joint; and

  • an undeveloped vertebral column.

  The later hybodonts had:

  • an underslung mouth;

  • a protrusible jaw;

  • strengthened jaw joints; and

  • pectoral and pelvic fin cartilage structures increasing their flexibility.

  Hybodonts lacked:

  • solid non-compressible vertebrae.

  The earliest freshwater fossils of teleosts date back to just under 400 million years ago, whereas the marine teleost record is considerably younger, dating to the mid-Triassic Period some 230 million years ago. Although the original ancestors of teleosts may have been the acanthids, they also bear an uncertain relationship to the placoderms, held by some to be the sharks’ ancestors!

  Proof of the success of the evolving elasmobranch form—cartilage, variegated teeth in increasingly flexible jaws, powerful paired fins, claspers for internal fertilisation, a heterocercal tail fin—came with its ability to also radically diversify by flattening and adapting to a benthic lifestyle. Again, this was to take advantage of new prey, in this case on or near the oceans’ floors, especially bivalves. The first batoid fossils date back about 200 million years (following the fifth mass extinction event, which eliminated about 20 per cent of marine families), and they continued to evolve into the Tertiary Period, when a few species of stingrays began colonising freshwater systems about 50 million years ago. And at about this time the largest sharks of all, the filter feeders, also began to evolve, to take advantage of an inexhaustible food so
urce, plankton.

  Earth’s sixth major extinction event, 65 million years ago, famously brought an end to the planet’s domination by land dinosaurs and sea reptiles. With the disappearance of these giants, mammals began to increase in size, diversify in form and spread rapidly. Some returned to the seas, those mammals becoming yet another shark food source. An outsized shark, Carcharodon megalodon, which can be roughly translated as ‘megatooth’, lived between 20 and one million years ago (or less) as a specialist whale predator. Megalodon’s record is limited to rare fossilised vertebrae, and more plentiful teeth, measuring as much as 18 centimetres in length. Despite the scarcity of the fossil evidence, palaeontologists estimate that megalodon grew up to 15 metres long and weighed as much as 50 tonnes. (The largest great white sharks are about 6 metres long and weigh up to 2000 kilograms.)

  The fossil record suggests that by the beginning of the Miocene Epoch, about 23 million years ago, megalodon had become dominant as a super-predator, but it died out during the Pliocene–Pleistocene extinction event 1.6 million years ago (when the first modern humans appeared). Some people, however, believe that ‘megatooth’ may not be extinct, while shark scientists continue to debate its lineage and genus, including its relationship to the extant great white shark, Carcharodon carcharias, which first appeared about 11 million years ago and evolved into a specialist predator upon sea mammals, mainly pinnipeds. (The Carcharodon lineage can be traced back to about 65 mya.)

  A tooth of Carcharodon megalodon, the giant shark that grew to about 15 metres. This tooth is 12.5 centimetres long and ten centimetres at its widest. (From The Elasmobranch Fishes, J. Frank Daniel, University of California Press, 1928)

  Evidence for the continued existence of megalodon is slim. The well-known Australian ichthyologist David Stead wrote a lengthy account in his 1963 book, Sharks and Rays of Australian Seas, of a 1918 encounter between New South Wales fishermen and a gigantic shark. Stead linked the eyewitness accounts of these ‘prosaic and rather stolid men, not given to “fish stories”’, with his study of seemingly non-fossilised giant teeth dredged up from the Pacific Ocean and concluded that sharks of ‘80 to 90 feet long’ might still inhabit the depths.20 While there is still a degree of uncertainty as to what lives in those depths—as evidenced by the relatively recent discoveries of the coelacanth, the megamouth, and the colossal squid—it is possible that once megalodon’s main prey, whales, evolved to inhabit cold as well as warm waters, this huge temperate-water shark died out from natural causes.