Swimming

Plesiosaurs

The locomotion of the plesiosaurs has formed the subject of at least three different hypotheses, outlined by Taylor (1986), Halstead (1989), McGowan (1991), Ellis (2003) and Benton (2004). Because their tails were relatively small, their paddles large and powerful, it can reasonably be assumed that the latter were used for creating thrust. At first, it was thought that plesiosaurs swam by beating their limbs forward and backward as though they were oars. The paddles would, of course, have had to be 'feathered' or rotated to a horizontal position like the feet of a swimming duck during the backstrokes, since they could not be lifted above the surface of the water like the oars of a rowing boat or skiff. However, the fossils do not indicate that such rotation actually took place.

A solution to the problem was proposed by Robinson (1975) who suggested that the plesiosaurs 'flew' through the water - as do turtles and penguins today. In their case, the paddle is flat, having an aerofoil cross section, like that of a bird's wing, with a rounded leading edge and a tapering rear end. It is tipped at an angle to the horizontal so that, in the downstroke, it generates lift and forward thrust. In the upstroke, the angle to the horizon is tipped in the reverse direction so that a smaller forward thrust and lift force are generated. The tip of the paddle describes a figure-of-eight pattern, and each stage of the cycle produces lift and forward movement (Bauplan III: Fig. 17). More recently, Godfrey (1984) has refined this concept by suggesting that the tip of the plesiosaur paddle may have described a crescent-shaped path (Fig. 34) as do the flippers of sea lions. He claimed that the plesiosaurs would not have been able to have moved their paddles up and down in a figure-of-eight. This was because the pectoral and pelvic girdles were heavy, flattened units of bone connected by a dense series of gastralia or belly ribs to form massive, immovable ventral plates. To these were attached the powerful muscles that operated the limb. At the same time, the limb girdles were too weak for strong vertical movements of the flippers to have been made.

Beverly Halstead (1989) suggested that plesiosaurs could have used all four fins in propulsion because their trajectory through the water was undulating. Consequently. the motion of the rear flippers would not have been affected by the turbulence of the vortices created by the power strokes of the front flippers.

The long necks of many plesiosaurs would have made it difficult for them to swim under water and maintain a straight course, because if they veered slightly to one side the water, in striking their necks obliquely, would have caused them to veer even more. For this reason, they are usually depicted as swimming on the surface. This involves the use of more energy than does swimming under water, but the difference would not have been so great when the plesiosaurs swam slowly. If they fed on worms or clams, using their long

■ Fig. 34a-c. Plesiosaur locomotion, three hypotheses: a rowing under water like a duck; b flying under water like a penguin; c intermediate style swimming like a sea lion. (Redrawn from Benton 2004 after Taylor 1986). Further explanation in the text

necks to reach down to the bottom, this would have solved the problem. Their piscivorous teeth, however, would have been more suitable for catching fishes and squid-like molluscs. However, these animals were too speedy to have been caught easily - unless the plesiosaurs darted out their long necks to snatch such animals that swam past them (Alexander 1989).

From the fossil records it can be seen that the plesiosaurs, like crocodiles (Sect. 4.6.3), swallowed stomach stones. No doubt these served to counteract the natural buoyancy of the animals. As they were inshore reptiles, finding suitable stones would have caused no problems. Furthermore, the gastroliths may have served to grind up shellfish and similar food items, just as does the grit in the gizzards of grain-eating birds today (Taylor 1992; Ellis 2003). In contrast, ichthyosaurs, which tended to inhabit deeper water far from the shore, did not make use of gastroliths. Not only would it have been more difficult for them to find suitable stones to swallow, but a diet composed largely of squids and fishes would have caused them no digestive difficulties. As we shall see later, many herbivorous dinosaurs did use stomach stones to digest their food (Sect. 10.2).

The discovery of numerous baby and juvenile fossil plesiosaurs in South Australia suggests that this was perhaps a breeding area to which the adults migrated seasonally, like the migration of whales up and down the coast today (Ben Kear in Ellis 2003).

Ichthyosaurs

There can be little doubt that the ichthyosaurs would have been very efficient swimmers (Massare 1988). Their adaptations to marine life were even more extreme than those of the plesiosaurs and their appearance closely resembled that of fishes (Bauplan 1; Fig. 17). They swam as fishes do (Sect. 4.2), their bodies and tails were specialised for propulsion, their limbs were flattened into paddles for balancing, while a fleshy dorsal fin kept their bodies from rolling. Although ecological analogues of porpoises and dolphins, they moved their tails from side to side, not up and down like the flukes of a cetacean.

Christopher McGowan (1991) discussed the mechanics of swimming with special reference to ichthyosaurs. He pointed out that he had initially taken a traditional view of the reversed heterocercal tail as a device for generating a downthrust to overcome the natural buoyancy of an air-breathing vertebrate. (A heterocercal tail is one in which the two lobes are not alike.) When the tip of the vertebral column extends into the dorsal lobe, as in sharks, this is usually larger than the ventral lobe. However, because the lower lobe is deflected more than the rigid upper lobe, its vertical force is greater and the tail generates a net upward thrust. Elasmobranch fishes do not have swim bladders and are therefore heavier than seawater. When not actually moving, they rest on the bottom but, when swimming, the comparatively rigid pectoral fins - whose cross section resembles that of an aeroplane wing - gives lift to the forepart of the body while the heterocercal caudal fin lifts the tail. In contrast, a reversed heterocercal tail would drive the rear part of the body down into the water (Fig. 35).

McGowan (1991), however, wrote that he had changed his mind when he read a paper by Taylor (1987a) who, like Wall (1983), argued that ichthyosaurs may not necessarily have been much lighter than seawater. Many modern aquatic animals, such as otters, hippos and some seals, have densities as great or even greater than that of water. Their weight is incurred by possessing very dense bones. The advantage of being heavy is that these animals do not need to expend energy in order to remain submerged, nor do they need to swallow gastroliths. Although some ichthyosaurs may have had dense bones, the Lower Jurassic forms examined by McGowan did not. In this, they were similar to whales and other cetaceans that must exhale before diving. Taylor (1987a) stressed that the increasing water pressure as an ichthyosaur dived would soon have neutralised any buoyancy. A tail that generated a downwards thrust would thus be disadvantageous. An ichthyosaur swimming at the surface may have initiated a dive by a downward flexure of the tail. This would have produced a downward movement of the head, driving the animal deep into the wa-

Shark Swim Mechanics

■ Fig. 35a-c. Ichthyosaur locomotion showing comparison with that of a shark. a Shark with excess weight counteracted by caudal and pectoral fins; b ichthyosaur swimming horizontally with natural buoyancy counteracted by downward thrust from caudal and pectoral fins; c ichthyosaur diving. Arrows indicate vertical thrust. Further explanation in the text ter where changes in the horizontal swimming level would have been effected by altering the inclination of the pectoral fins. The pelvic fins would probably have acted as stabilisers, maintaining the body on a straight course, correcting both up and down as well as side-to-side movements.

The effectiveness of an inclined plane depends on the ratio of lift to the drag force. Both the lift force and the drag increase with increased area, so the lift-to-drag ratio cannot be enhanced merely by altering the size of the plane. A long narrow plane, like the wing of an aeroplane, has a much higher lift-to-drag ratio than does a square plane. The relative narrowness of an inclined plane or wing is expressed by the 'aspect ratio' - the ratio of length to width. For example, if a plane were 10 units long and 10 wide, it would have an aspect ratio of 1. On the other hand, a plane 30 units long and 5 units wide would have an aspect ratio of 6. Planes with high aspect ratios generate less turbulence at their tips than do planes with low. Furthermore, the lift-to-drag ratio is increased by having a streamlined profile (McGowan 1991).

The movements of animals both in water and in air are strongly influenced by the density of the medium in which they are travelling. Although water is over 800 times denser than air, swimming is, somewhat surprisingly, relatively inexpensive in energy. This is partly because water provides more buoyancy than air does. Swimming animals cannot influence the density of the water that surrounds them, but flying animals can do so by selecting the altitude at which they are cruising. (That is why migrating birds usually fly at relatively high altitudes where the density of the atmosphere is lower.)

Since drag increases with the cross-sectional area of the body as well as with the number and size of external projections such as fins, speedy aquatic animals tend to be elongated and to be able to retract the fins or flatten them against their bodies when swimming fast.

■ Fig. 36. a Stenopterygius megacephalus. b S. hauffianus (Ichthyosauria; Lower Jurassic and Middle Jurassic; length ca. 3 m). The aspect ratio of length to width was moderately high in S. megacephalus, but even higher in S. huffianus. Further explanation in text b b

The pectoral and pelvic fins of the ichthyosaurs were tilted obliquely downwards, like the flight feathers of a dart. They would have acted as inclined planes whenever the body veered from its intended direction, and generated correcting forces to bring it back onto a straight course. Since their orientation was oblique, they would have corrected both vertical and lateral movements (McGowan 1991). A streamlined body with a high aspect ratio was not characteristic of all ichthyosaurs. Some of the early forms, such as Stenopterygius megacephalus (Fig. 36a) from the Lower and Middle Jurassic of Europe, had large tails with only moderate aspect ratios. They were clearly not adapted for cruising as were most of the later ichthyosaurs. The congeneric S. hauffianus (Fig. 36b), in contrast, was a fast endurance swimmer. Its tail was slender and had an aspect ratio as high as that of a modern swordfish.

Dolphins and porpoises are well known to leap repeatedly out of the water as they swim. This is called 'porpoising' and is necessary for breathing when travelling at high speed. If only enough of the body were exposed to bring the blow hole above the surface of the water, as happens at lower speeds, the turbulence created would be so great that the resulting drag would slow the animals below the critical level for efficient swimming. Moreover, porpoising may actually save energy by reducing drag which is very much greater in water than in air, even at high speed. Although it costs energy to leap, the faster the animal is swimming, the longer the leap and the greater the amount of drag avoided. The calculations necessary to assess the critical speed, above which leaping saves energy, are extremely complicated and the factors involved are not readily estimated. Nevertheless, Alexander (1989) believed that ichthyosaurs porpoised like dolphins. They may also have dived quite deeply to find their prey. Today, the tetrapod that dives most deeply is the sperm whale (Physeter macro-cephalus), whose deepest recorded dive is 2.25 km (1.4 miles). The stomach contents of another individual included a species of dogfish found only on the seafloor, suggesting that a depth of over 3 km has been reached. The various adaptations for deep diving are both physiological and mechanical; but which, if any of them, evolved among the plesiosaurs and ichthyosaurs is quite unknown.

Plesiosaurs

The 19th century palaeontologist, W.D. Conybeare, who became Dean of Llan-daff, was one of the first to study the plesiosaurs in detail and gave them their name. (He also gave its name to the first of the mosasaurs because it was found near the River Meuse.) Conybeare described plesiosaurs as 'snakes threaded through the bodies of turtles'. This maybe an apt portrayal of their general appearance, but it does not help to explain either their relationships or their modes of life. Brown (1981) reviewed their phylogeny and classification.

Plesiosaur Diversification

The first true plesiosaurs of the Upper Triassic period were larger than the nothosaurs to which they were so closely related, if they were not actually descended from them (Sect. 5.1). Their body lengths ranged about 2-15 m. The order Plesiosauria contained four main families: Plesiosauridae, Crypto-clididiae, Elasmosauridae and Pliosauridae (Brown 1981; Ellis 2003; Benton 2004). Some had long necks, others short. The plesiosaurids, known mainly from the Lower Jurassic of Europe, had small skulls and fairly long necks. Ple-siosaurus (Fig. 37a) is a typical example. The cryptoclidids, such as Crypto-

Cryptoclidus

■ Fig. 37. a Plesiosaurus (Plesiosauridae; Lower Jurassic; length ca. 2.3 m), b Cryptoclidus (Crypto-clididae; Upper Jurassic; length ca. 4 m), c Muraenosaurus (Elasmosauridae; Upper Jurassic; length ca. 6 m), d Macroplata (Pliosauridae; Lower Jurassic; length ca. 4.5 m) e Peloneustes (Pliosauridae; Upper Jurassic; length ca. 3 m). (After Palmer 1999)

■ Fig. 37. a Plesiosaurus (Plesiosauridae; Lower Jurassic; length ca. 2.3 m), b Cryptoclidus (Crypto-clididae; Upper Jurassic; length ca. 4 m), c Muraenosaurus (Elasmosauridae; Upper Jurassic; length ca. 6 m), d Macroplata (Pliosauridae; Lower Jurassic; length ca. 4.5 m) e Peloneustes (Pliosauridae; Upper Jurassic; length ca. 3 m). (After Palmer 1999)

clidus (Fig. 37b) from the Upper Jurassic of Europe, had even longer necks in proportion to body. There were 30 cervical vertebrae and the skulls had pointed snouts with nostrils set back from their tips. The elasmosaurids from the Upper Jurassic of Europe had longer necks still. For example, in Muraeno-saurus (Fig. 37c), the neck was as long as the body and tail combined, and contained 44 vertebrae; and Elasmosaurus (Fig. 38) from the Upper Cretaceous of Asia and North America had an even longer neck that consisted of 71 vertebrae. These were undoubtedly ambush, or 'sit-and-wait' predators.

In contrast, the pliosaurs had necks that became progressively shorter while the paddles grew larger and more powerful during the course of evolution. The neck of the Lower Jurassic Macroplata (Fig. 37d) from Europe contained 29 slightly shortened vertebrae. The slender, crocodile-like skull was only slightly larger in proportion to those of the plesiosaurids. Although smaller than Macroplata, the pliosaur Peloneustes (Fig. 37e) from the Upper Jurassic of Europe and America, had an even shorter neck with only about 20 vertebrae, and a larger head. It was more streamlined than Macroplata, which must have fed

■ Fig. 38. Elasmosaurus (Elasmosauridae; Upper Cretaceous; length ca. 14 m). (Cloudsley-Thompson 1994).
■ Fig. 39. Kronosaurus (Pliosaudridae; Lower Cretaceous; length ca. 5 m). (Cloudsley-Thomp-son 1999)

mainly on fishes, and its teeth were fewer and blunter. They were better adapted to catching soft-bodied squids and for crushing hard-shelled ammonites. Kronosaurus (Fig. 39) from the Lower Cretaceous of Australia is the largest known pliosaur. (The name comes from the mythological Kronos who devoured his children.) Its skull measured about a quarter of its total body length and was larger and even more powerful than that of Tyrannosaurus rex!

The plesiosaurids, cryptoclidids, and elasmosaurids fed mainly on molluscs and fishes. As we have seen (Sect. 5.2.1), they would have used their long necks to shoot out their heads after fast-moving prey that swam near them unaware of imminent ambush. Their long, pointed conical teeth interlocked when the jaws were shut - an adaptation for retaining slippery fishes and cephalopods in their mouths when the jaws closed (Benton 2004). In contrast, the pliosaurs were adapted for long-distance cruising and fast swimming. Kronosaurus (Fig. 39) and Liopleurodon spp. (Fig. 40) from the Upper Jurassic of Europe, with heavy heads, short necks, and streamlined bodies, were highly manoeuverable and certainly not sit-and-wait predators like the earlier pliosaurids and all other plesiosaurs. They probably fed on cephalopods, sharks and other fishes as well as on ichthyosaurs and smaller plesiosaurs. With their powerful jaws and sharp, broad-based teeth, the Pliosauridae had skulls designed like a lattice reinforced with girdles that resisted the great bending moments generated during biting (Taylor 1992). The mandibles were also like box girders and for the same reason, but retained a streamlined shape.

Plesiosaurs almost certainly came ashore in large numbers to lay their eggs in nests excavated in the sand - as do marine turtles today. As already mentioned, the discovery of numerous baby and juvenile fossil plesiosaurs in South Australia suggests that this was perhaps a breeding area to which the adults migrated seasonally (Sect. 5.2.1). When on land, they would have been much more vulnerable to predation from other Mesozoic reptiles than when they were swimming in the ocean; while the newly-hatched young would have faced a perilous journey down the beach to the sea. No doubt safety depended on

■ Fig. 40. Liopleurodon (Pliosauridae; Upper Jurassic; length ca. 12 m; after Palmer 1999) with ichthyosaurs

numbers, so hatching must have been synchronised. In addition to terrestrial reptiles, the enemies of baby plesiosaurs probably included crocodilians and pterosaurs. Adults in the water must have faced mosasaurs (Sect. 4.5), crocodilians, larger plesiosaurs and ichthyosaurs.

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