THE OCEANS

By far the greatest area of the planet Earth – 362 million square kilometres (140,000,000 sq m) or over 70 per cent – is covered with ocean water. Compared with the land areas, however, the ocean is quite barren. It is largely a cold and dark place, with prolific life appearing only in the uppermost few hundred metres, where sunlight can penetrate and support a flora of drifting algae. With an average ocean depth of 3.8 kilometres (2 1/4 m), this topmost fertile layer does not represent a very large proportion of the whole. Animals also live in the chill dark ocean depths, feeding on organic debris raining down from the productive layers, but they are not numerous.
When the dinosaurs first evolved in late Triassic times all the landmasses were fused into one supercontinent called Pangaea. The rest of the planet’s area was covered by one continuous ocean called Panthalassa. The division of Pangaea and the separation of all the continents has meant that Panthalassa no longer exists and the great basin that once occupied it has been shrinking ever since. Even so, its remnant – the ocean basin that stretches between the eastern end of the Palaearctic, the Oriental and the Australasian realms and the western flanks of the Nearctic and Neotropical realms - still covers almost half the globe. The other ocean areas are new, having grown from rift valleys that ripped the supercontinent apart. The Tethys has gone now – swallowed up by the movements and collisions of continents along the southern edge of the Palaearctic realm.
The continents are awash at the edges. The continental edges are never at the shorelines of the landmasses – they usually lie far out at sea. The resulting shallow shelf of land is called the continental shelf and may be as deep as 100 metres (333 ft). Thereafter the bottom topography drops off quickly and the sea bed slopes down to the abyssal plain, the surface of the oceanic plates. Deeper portions still are found in the oceanic trenches, elongated indentations in the ocean floor where one plate is being swallowed up beneath its neighbour. Most plant and animal life is found on the continental shelves, as the sea bed here is within the zone illuminated by the sun. The shelf is usually narrow along continental edges where an oceanic plate is being swallowed up, and broad where the continents are moving apart. In the late Cretaceous period, as the last parts of Pangaea broke up, the shelves were much broader than now.
Unlike the continents, the oceans have no well-defined zoogeographic realms. The great water areas are continuous with one another and have few barriers to migration.

Shorelines

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SOAR

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Cicollum angustalum

The waters of the world’s oceans are in constant circulation, driven by the force of the winds across the surface. At certain places, particularly where the prevailing wind is blowing off the land or parallel to the coasts, the strong swirling currents bring cold nutrient-rich water up from the deep ocean floor. These areas of upwelling are found off the west coasts of the Nearctic, Neotropical and Ethiopian continents. The nutrient-rich water combined with the warm sun gives rise to blooms of floating plants in the surface waters, bringing in great shoals of feeding fish. Huge flocks of birds and pterosaurs also congregate in these areas and they hunt the fish.
One of many types of pterosaur that have evolved a fishing way of life is the soar. With a wingspan of over 4 metres (13 ft), it can hover for long periods over likely ocean areas, looking for signs of fish shoals near the surface. When a shoal is sighted, whole flocks of soars settle on the surface, and start to fish by clipping their long heads and necks quickly under the water. The animal itself is never fully submerged. After gathering a full crop it takes off again, rising clumsily into the wind from the wave tops and flying unerringly back to its nesting island. While it is hunting in flocks in the open ocean, the soar occasionally falls prey to the bird-eating plesiosaurs (page 105).

The soar nests in flocks, forming rookeries on rocky islands. Hunting trips may take several days, after which the adult brings a crop full of fish back to feed the young.

The soar is graceful both in the sky and on the water. While swimming, it holds its wings above its back, never wetting them. Only the head and the long neck are plunged underwater.

For all its aquatic adaptations, the plunger still needs to come ashore to breed. It nests in vast rookeries on isolated rocks, but only in areas where the nearby ocean currents ensure a constant supply of fish.

Shorelines

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PLUNGER

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Pinala fusiforme

A windswept chain of rocky islands rises above the green and white swell of the southern ocean. On the rock pinnacles, above the height at which waves surge and break, lie a number of glossy shapes, basking in the distant watery sun. These black and white creatures flop around clumsily, seeming to wobble on their bellies, pushed along by their stubby limbs, apparently quite incapable of any fast or graceful movement. Yet, when they reach a cliff edge, they rise to their hind legs and plunge head first into the foam. There they are transformed into elegant streamlined creatures, turning and darting, chasing after the shoals of fish on which they feed. In many areas the seas are so rich in fish that the fishing animals do not need to travel long distances to find them.
The plunger is a fishing pterosaur that has lost its powers of flight. Its wings are still present but modified into hydrodynamic organs that allow it a mobility in the water that its ancestors had in the air. Layers of fat have built up under the skin, and these not only insulate the animal from the chill waters but also give it the streamlined shape that allows it to swim easily. The plunger’s lungs have special adaptations to withstand the great pressures found at the depths at which they hunt their food.

A development of the wing membrane between the hind legs and tail has produced the plunger’s powerful swimming paddle. The swimming motion is augmented and steered by the wings, which are strong flaps of gristle. Fatty insulation is reinforced by the sleek pelt of fine hair, with its striking black and white pattern.

The huge pliosaurs developed specializations that enabled different species to exploit different foods. The whulk consumes the ocean’s plankton. Its teeth have become thin, fine and tightly packed, forming a sieve structure along the jaws. A voluminous pouch has developed beneath the lower jaw. The pouch is filled as the whulk opens its vast mouth (a). With the mouth closed the pouch collapses, forcing the water out between the teeth and straining the plankton from it (b).

Temperate sea

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WHULK

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Insulasaurus oceanus

In the Mesozoic period the reptiles established an early mastery of the sea. Among them were the ichthyosaurs, which were the most well-adapted sea reptiles of all, with their fish-shaped bodies and tails. These no longer exist, having become extinct and been replaced by the lizard-like mosasaurs during the Cretaceous period. The turtles, slow-moving shelled herbivores, were also successful adopting a marine existence. The other main line of marine reptiles was the plesiosaurs. These quickly evolved into two main lines: the short-necked types, or pliosaurs; and the long-necked types, or elasmosaurs. Both types still exist in the modern oceans. The largest marine reptile alive today is the whulk, a pliosaur. At 20 metres (67 ft) long, the whulk cruises the oceans of the world, but where its ancestors fed on ammonites and squid and other cephalopods, this pliosaur consumes much smaller creatures. During the Cretaceous period the shallow seas over the continental shelves produced vast volumes of plankton, tiny animals and plants that drifted in the warm nutritious waters. The shallow seas are not so extensive nowadays, but the plankton is still there. The whulk feeds on it by swallowing great volumes of water and straining out the plankton through its thousands of tiny teeth.

Temperate sea

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BIRDSNATCHER

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Raperasaurus velocipinnus

Sea birds wheel over the green ocean, concentrating their attention on a shoal of fish feeding in the surface waters. One by one the birds dive, splashing into the depths and surfacing again with their catches. Suddenly a great turmoil churns up the water, and in a cloud of spray, pointed heads on long thin necks burst from the surface and shoot skywards, snapping and snatching at the wheeling flock. When they subside once more into the sea the flock of birds has scattered in panic, and many of their number have disappeared.
The necks belong to birdsnatchers, the specialized, bird-eating elasmosaurs. In appearance they are very similar to conventional elasmosaurs that have existed for the past 100 million years. The seas have remained relatively unchanged, as has the fish population, and so the elasmosaur shape has proved a successful and long-lasting one. The main adaptation evolved by the birdsnatcher is its ability to catch sea birds. It lives chiefly on fish, but now and again a school of birdsnatchers will work together to seize a flock of birds from the sky. With necks held back they approach an area where birds are fishing. Then they all break the surface at the same time and thrust their necks quickly upwards, each one grabbing a bird from the wheeling flock.

The conventional elasmosaur shape is evident in the outline of the birdsnatcher. There is a bulbous, streamlined body with paddles for limbs, and a shortish tail with a vertical diamond-shaped fin. The neck is extremely long and flexible and consists of more than 70 vertebrae.

The birdsnatcher’s jaws are long and narrow, armed with pointed teeth that are angled outwards. This has been the shape of the elasmosaur head and jaws since Cretaceous times and, like the ancient elasmosaurs, the birdsnatcher lives mainly on fish. Its jaws and teeth, however, enable it to also catch birds right.

Open sea

PELORUS

Piscisaurus sicamalus

The pelorus is a small pliosaur and, like its relatives, it feeds on ammonites. This is the only type, however, that has the ability to attack and kill the largest ammonite of the oceans. It hunts in the warm placid waters of the doldrums, where its prey is most abundant.
There is an oily swell, and dotted here and there are the floating bulbous shapes of kraken shells, their long tentacles streaming out just below the surface, passively entrapping any small creature that happens into their entangling fibres. A splash! And a spray of water! The tranquil scene is shattered by a frenetic burst of activity around one of the bobbing shells. A pelorus has broken the surface and is wrestling with a kraken, struggling to reach its vulnerable parts. The weak tentacles loop and coil out of the water, trying to entangle the attacker, but they have little real strength. The pelorus clambers over them, wriggles through their coils, and finally plunges its sharp jaws down into the fleshy head. Again and again it stabs, until at last the tiny brain is destroyed. The writhing tentacles gradually become still, and the pelorus begins to feast, tearing off chunks of flesh as quickly as it can before the air seeps from the shell's chambers and the massive corpse sinks to the depths.

The internal structure of the kraken is a ‘scaled-up’ version of the internal structure of any shelled cephalopod. The shell has a number of chambers, and the animal occupies the last one. As the kraken grows it produces more shell, moves forward and lays down a wall to create a new chamber behind. A blood vessel connecting all the chambers controls the air pressure and hence buoyancy.

The twelve tentacles with their trailing curtain of stings and hooks, radiate from the mouth at the shells entrance.

The pelorus is immune to the stings of the kraken, but it could easily become entangled in the tentacles and drown. Its method of attack is to swim swiftly up through the curtain of fibres and clamber along the upper surface of the arms until it reaches the head. There it can stab at the soft body found in the shell chambers until the ammonite is dead.

Open sea

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KRAKEN

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Giganticeras fluitarus

The ammonites of the Mesozoic seas were mostly swimming animals that moved freely about in the ocean waters chasing small swimming creatures that they caught with their tentacles. In Cretaceous times they developed into a number of different forms. There were those with heavy shells produced in irregular coils, that spent their time crawling along the sea bed. Others were freely drifting animals, filtering microscopic food particles from the water using very fine tentacles. This second trend has reached a pinnacle with the modern kraken.
The shell of the kraken is truly enormous, some specimens reaching 4 metres (13 ft) in diameter. The shell acts as a protective armour, as well as a float to keep the animal in the fertile surface waters. It has twelve tentacles that it spreads out around itself, and each of these has thousands of trailing fibres that are armed with stings and hooks. The whole arrangement forms a deadly net that covers an area of about 20 metres (67 ft) in diameter. The kraken will eat almost anything that becomes entangled in its traps, from microscopic floating plants, to fairly large fish. Many krakens often drift in the same area of productive waters, their floating shells acting as perches for migrant birds and pterosaurs.

The air-filled chambers of the kraken’s shell mean that it floats with the living animal and the tentacles just submerged (a). It moves by expelling waste water through its syphon, propelling the shell backwards and allowing the tentacles to trail behind (b).

CONCLUSION

The world is a complete ecosystem. The grassy plains are grazed by rajaphants and sprintosaurs. The fruits and berries in the trees of the tropical forests are eaten by crackbeaks. The twigs of the temperate and coniferous woodlands are browsed by brickets and coneaters. Mountain plants are scraped up and eaten by balaclavs and hanuhans. These plant-eating animals are, in turn, preyed upon and killed by fierce hunters like northclaws, cutlasstooths and arbrosaurs, and their dead remains are devoured by scavengers such as gourmands, and many of the pterosaurs.
Yet the numerous animals described in these pages can only provide a selective and superficial account of life on the planet. In particular, the animals are nearly all vertebrates. Many millions of other species exist, especially among the invertebrates that have been only briefly mentioned here. The grasslands, forests, deserts and mountains contain a unique fauna of tiny creatures that are just as essential to the completeness of the ecosystem as are the big spectacular beasts.
What would happen if the large reptiles were to become extinct suddenly, as has happened to the dominant forms of life many times in the past? What would develop to take the place of the large land-living vertebrates? Would it be the tiny mammals that scamper around the dinosaurs’ feet, or would it perhaps be the versatile and adaptable birds, or even some novel development of the reptile stock that we just cannot imagine? As the continents are now split apart and widely separated, it is possible that the evolution would be different in each of the zoogeographic realms, thus developing a world fauna that would be more diverse than at any time in the geological past.
Whatever happens, life will survive and progress. For as long as our planet can support life, life will develop and adjust to the changing conditions.