The infinite variety: the beginning of life
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- LORDS OF THE AIR
- EGGS, POUCHES AND PLACENTAS
Assignments IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS Discuss the origins, morphology and lifestyle of animals belonging to the suborder Serpentes. Discuss the adaptive radiation of reptiles living in the Mesozoic period. Review the evidence we have that dinosaurs were warm-blooded animals. Describe the adaptions that pterosaurs required in order to fly. In what ways is the pterosaur wing similar and different to the wings of birds and bats. What adaptations allowed reptiles to better colonize the terrestrial environments than their amphibian counterparts? LORDS OF THE AIR Many characteristics of birds show close resemblance to those of reptiles and in particular the early bipedal reptiles before they evolved into the great dinosaurs. In the early Triassic (225-200 million years ago) small pseudosuchians such as Saltoposuchus showed the essential characteristics of birds including bipedalism. There are no fossils detailing the change from the ectothermic bipedal reptiles into endothermic flying birds except for five fossil specimens of the upper Jurassic (about 150 million years ago) found in the lithographic slates of Solnhofen, Bavaria. These Archaeopteryx lithographica probably achieved some degree of gliding, and are certainly the earliest known animal to possess feathers. Anatomically these animals are much less specialized than the modern birds but does represent the earliest animal classified as a member of Aves and is in its own subclass Archaeornithes. All other birds were extinct or living belong to the subclass Neornithes. In 1860 the first fossilized feather was found, and a year later the first Archaeopteryx was found. The whole body axis was elongated, the dorsal vertebrae were not fixed and only five were fused to form the sacrum. There was a long tail, with feathers arranged in parallel rows along its sides. The fore-limbs ended in three clawed digits, with separate metacarpals and carpals. This limb was used as a wing since feathers were attached to the ulna and hand, but the wing was small and the shape rounded. The pelvic girdle and hind-limb resembled that of the archosaurs. In the skull there were sharp teeth in both jaws, and the eyes and brain were considerably smaller than modern birds. The bones were not hollowed and since the sternum bone (keel) was not well developed, it could not have had muscles that could achieved flapping flight. It has been suggested that it used its feathers which probably originally evolved as some form of insulation, as a kind of net to trap insects while running fast across land. Alternatively it was suggested that it was arboreal and the feathers which were originally derived from reptilian scales, enabled Archaeopteryx to glide short distances much as gliding lizards do today (e.g. Draco volans). Thus the two theories that flight evolved 'from ground up' and 'from trees down' have been proposed. The descendants of Archaeopteryx and other ancient birds underwent a dramatic adaptive radiation during the Cretaceous period when both aquatic and terrestrial habitats were invaded. Hesperornis was a loon-like diver that possessed teeth, and had already lost its power of flight since the wings had become functionless and is the only other bird species known to have teeth. That Archaeopteryx almost definitely used its claws on the front wings to climb is clearly paralleled by the Hoatzin (Opisthocomus) a heavily built bird occurring in South America and belonging to the cuckoo family. Its young possess conspicuous claws on the digits of the wings with which it is able to climb away from possible predators. These claws are usually considered to be a secondary development, however, their resemblance to the claws of the Archaeopteryx is remarkable. When the Hoatzin chicks grow up they lose these claws, but the adult birds are nevertheless poor fliers. The debate as to whether Archaeopteryx could or could not fly still continues. It has been argued that Archaeopteryx was too heavy and that its muscles were to light to power it, and that they used their feathers for gliding or cooling themselves. Some researchers have argued that Archaeopteryx had the muscles of a cold-blooded reptile. These are twice as powerful per unit weight as those of warm-blooded animals, and may have allowed Archaeopteryx to fly short distances which makes more ecological sense than a warm-blooded Archaeopteryx possessing wings and feathers but not the ability to fly. Now with the discovery of a fossil bird in northeastern China which has provided the first evidence that fairly modern tree-perching birds had evolved by 135 million years ago, only 15 million years after Archaeopteryx. This sparrow-sized bird, which is as yet unnamed, has an opposable first digit and slender claws on its legs. This would have allowed it to firmly grasp a tree banch and to "perch" (the flat forward pointing claws of Archaeopteryx mark it as a ground-dwelling animal). This small bird had a well developed keel on its sternum which was the anchor site for strong flight muscles and also possessed a pygostyle (fused cluster of tail vertebrae to which long tail feathers are attached). This gave the bird a centre of gravity in the centre of the wings, whereas the long-feathered tail of Archaeopteryx puts the centre gravity well back of the wing and just above its feet which is a better position for an animal that runs. This Chinese bird did, however, retain some primitive traits. These include small remnants of claws and fingers, stomach ribs and the bird may have had teeth. All of these are present in Archaeopteryx and carnivorous dinosaurs but not in modern birds. This Chineese fossil does present problems such as how an animal like Archaeopteryx could have evolved into this bird like animal within 10 - 15 million years? It is now almost certain that Archaeopteryx was not a direct ancestor to the modern birds, but would have been an offshoot. The fossil of a 4 metre long coelurosaur called Deinonychus showed an anatomy almost identical to Archaeopteryx except that it lacked wings and feathers and was around 50 million years older than Archaeopteryx. Other bird-like dinosaurs include Avimimus, a 1,5 metre bipedal fossil found in Mongolia. This animal had a short deep head, toothless beak, long neck and tail and possibly feathers, which would make it the most ancient of feathered animals. Yet another fossil discovered in North America called Protoavis, may have been a bird or a dinosaur, but certainly pre-dates Archaeopteryx. However, the fossil that has attracted the most amount of interest in relation to the links between birds and dinosaurs is Mononychus, a turkey-sized predator equiped with sharp teeth and a long tail and looked very similar to other theropods. Mononychus does share some anatomical features with birds that are not found in any of the other bird-like dinosaur fossil including Archaeopteryx. For example Archaeopteryx has a fibula (the thin bone in the lower hind limb) that touches the ankle, in birds and Mononychus this does not happen. All birds have a keeled sternum for attachment of wing muscles. Mononychus also has a keeled sternum and some of its wristbones are fused together which is also an adaptation for flight. This evidence suggest that Mononychus evolved from a flying animal, just as ostriches are descended from flying birds. If this is the case Mononychus probably had feathers and the real ancestor of birds goes back still further in the fossil record. Although we have a poor fossil record describing the evolution of birds there is little doubt that they evolved directly from a small coelurosaurian dinosaur. However, the conquest of the air by birds was not only achieved with the adaptation of the feathers and powerful wing muscles, but also necessitated considerable weight reductions. The bones of birds are extremely thin and hollow inside, with structural strength being created by cross struts. The heavy extension of the spine that supported Archaeopteryx's tail has been replaced with stout quilled feathers. The heavy jaw with teeth has been replaced with a beak composed of lightweight protein called keratin. The basic bird plan of structure originating in the Jurassic has been modified to produce over 8600 living species. The factors that promoted such species radiation are unclear since there is a poverty of fossil records and it is not possible to trace individual lines, which you can do for other vertebrate groups. It is clear that the process of change has been radical and accomplished in an extremely short evolutionary period. In particular the bill structure appears to be easily and quickly moulded by evolutionary processes. From an ancestral finch-like bird with a short straight beak, the Hawaiian Honey-creepers (Family Drepanididae) have evolved bill structures that are adapted to feeding on insects, nectar, fruit and seeds in a period of a few thousand years. Darwin noted similar variation in the bills of the finches of the Galapagos islands. Elsewhere in the bird world the evolution of bill structure has occurred for a much longer time and we therefore see bills adapted to seed-eating (sparrows; Ploceidae), fruit-eating (hornbills and toucans; Bucerotidae and Ramphastidae respectively) insect-eating (nightjars; Caprimulgidae), tearing (eagles and hawks; Accipitridae), probing (stilts; Recurvirostridae), filtering (flamingoes; Phoenicopteridae) and capturing of fish (cormorants; Phalacrocoracidae). The feet of birds also show adaptations to scratching for food (pheasants; Phasianidae), wading (heron; Ardeidae), grasping (eagles), perching (warblers; Muscicapidae) and swimming (ducks; Anatidae). Feathers are also highly evolved in the differentiation of different feathers (primary and secondary wing, tail, inner and outer contour feathers, down and filoplume) as well as adaptations to meet different habitats due to the unequalled insulation properties of feathers which permit the Emperor Penguin (Aptenodytes forsteri) to be the only animal that can endure winter on the Antarctic ice cap. Most birds have an oil gland near the base of the tail. The bird takes this oil with its beak and coats individual feathers to waterproof them and maintain their insulation. Other birds, including herons, parrots (Psittacidae) and toucans lack this gland and condition feathers with a fine talc like dust, powder-down, that is produced by the continuous fraying of the tips of special feathers. Cormorants and darters, spend a great deal of their time diving in water, their feathers are not waterproofed, permitting them to get completely wet. This is of advantage since it reduces buoyancy and they can dive deeper and more easily in pursuit of their fish prey. After foraging they stretch their wings to dry. Feathers are unique to birds, but were derived from scales and arise to form papillae. A papilla consists of a projection of vascularized dermal tissue that grows out of an epidermal pit, called the feather follicle. A typical feather consists of a stiff axial rod, or shaft. The proximal portion of the shaft, the quill is hollow whereas the distal end is solid. The shaft bears two rows of branches, or barbs, which in turn support two rows of smaller, numerous barbules. The feathery vane is composed of a double series of barbs and barbules. The barbules on the side of the barb towards the tip of the feather bear hooklets or barbicels, that form bridges with ridges on the adjacent proximal barbules. The vane is thus lightweight and pliable, but also extremely strong and resilient. At least once a year each feather is shed and a new feather develops from the same papilla. Birds usually shed, or moult, their old feathers during late summer. There may be partial or complete moult in spring when the bird assumes a more colourful breeding plumage. The acquisition of breeding plumage may also result from wear or the breaking-off of feather tips, thus exposing different colours beneath. Feather coloration is due to two basic pigments known as melanins which are pigment granules of brown, black or yellow and the carotenoids which are either red or yellow. Green, blue and iridescent markings on sunbirds and other species are due to the peculiar surface and (Nectarinidae) internal structure of their feathers. Absence of pigments result in partial or complete albinism. The first juvenile plumage of birds is usually replaced before the first winter. This winter plumage usually resembles that of an adult female irrespective of whether the juvenile is male or female. Only in the second year does differentiation of plumage between males and females occur. Mature male and female plumages frequently differ in colour (sexual chromatic dimorphism), especially during the breeding season, when the male may be particularly brightly coloured (eg Red Bishop birds Euplectes orix). Such colour changes are used during courtship with male birds advertising themselves. Breeding plumage may facilitate mate recognization within a species, and is particularly important when many related species coexist in the same area. In particular striking combinations of colour are used in finches (Fringillidae) and parakeets/parrots. Worldwide ducks assemble in multispecies flocks, but during breeding each drake (male) species will acquire a unique colour and pattern combination particularly in the head regions which will distinguish that species from other duck species in his quest to find a mate. Colour may also be used to effectively camouflage birds. The most striking being the ptarmigan (Lagopus mutus; Tetraonidae), this grouse is white during winter when snow is on the ground, but mottled brown during the rest of the year. Feathers have become enlarged and specialized and are used with or without changes of plumage colour to attract mates. The Pennant-wing Nightjar (Macrodipteryx vexillaria) acquires long pennants from the primary feathers. In the Crested Grebe (Podiceps cristatus; Podicipedidae) both sexes develop elongated chestnut-brown feathers on their cheeks, a deep brown ruff beneath the beak and a pair of horn-like tufts of glossy black feathers on the head. Sexual difference has been taken to the most extreme for any animal with the male pheasants, peacocks, grouse, manakins, and birds of paradise all of which grow feathers to a great size. The Great Argus pheasant (Argusianus argus) has wing feathers that are over a metre long and are lined with huge eye spots. The Peacock (Pavo cristatus), which is basically a pheasant, has tail feathers up to 1.8 m long, with a conspicuous pattern that resembles large eyespots. The most spectacular bird plumages occur in the Birds of Paradise (Paradisaeidae) from the island of New Guinea. The King of Saxony (Pteridophora alberti)has two long quills from his forehead each bearing a line of enamelled blue pennants; the Superb Bird (Lophorina superba) has an immense emerald shield which it can expand until it is as broad as the bird is tall; the Twelve-wired Bird of Paradise (Seleucidis melanoleuca) has a shimmering green bib and a huge inflatable yellow waistcoat with bare quills, the wires of its name, curling down behind it. The most celebrated birds of paradise are those possessing plumes arising from beneath their wing coverts. There are several species, each with a plume of a different colour (yellow, red or white). These birds display communally, with dance displays being held in a prominent position on a branch which has had twigs and leaves stripped off it. In this way a dull coloured female is attract and she flits across to the branch where one of the male birds jumps aggressively onto her back. Copulation is quick, and the female returns to the nest that she has already prepared for her now fertilized eggs. The male birds which had been burdened with the plumes for several months now losses them. Although bright colours are important for courtship in some birds. other birds have used behavioural patterns to attract their mates. The Satin Bower Bird (Ptilonorhynchus violaceus; Ptilonorhynchidae) bird Australia constructs an avenue of twigs on which he attaches a variety of objects which are either yellow-green, or preferably a shade of blue that closely matches his plumage colour. The nature of the objects collected is unimportant and may include berries, feather from other birds and even pieces of plastic. These birds are even known to steal desirable objects from a neighbouring nest and certainly mash blue berries with his beak and uses the blue-purple pulp to paint the walls of his bower. With this bower he tries to lure the female bower bird for courtship and copulation. Copulation in birds is generally clumsy, since the male birds with few exceptions have no penis. The mating birds cling and may twist about until the two vents are brought together and sperm is transferred to the females. Unlike other tetrapods birds only lay eggs, a characteristic inherited from the archosaurian ancestors. It is possible that vivipary would have been too great a load for a female to carry in flight throughout the weeks necessary for their development and therefore the eggs within the females are laid soon after fertilization. Birds now have to pay the penalty for being endothermic, for reptiles can bury their eggs and abandon them. Bird's eggs like the adults themselves, need to be kept at a constant temperature which is usually several degrees above ambient temperatures. Birds therefore incubate their eggs. Some birds just before egg-laying moult a group of feathers on their undersides and expose a bare patch of skin which becomes distended with minute blood vessels. The eggs are kept against this patch and kept at the same temperature as the parent bird. But not all birds produce this patch by moulting. Ducks and Geese mechanically pluck out their own feathers. The blue-footed Booby (Sula nebouxii; Sulidae), not only uses its feet for display but also uses them as insulators. The other disadvantage of egg laying is the need to build a nest, or in some way to safe guard the eggs. This puts both eggs and parents at risk. Vertical cliffs being almost inaccessible represent one safe site, providing the eggs do not roll off. This is minimized by producing eggs that are pointed at the one end which permits them to roll in a circular direction. Other birds, particularly those belong to the plover group (Order Charadriiformes) lay their eggs on open fields and gravel plains, but are usually highly cryptic and not easily found. More commonly birds construct nests to provide some form of protection. Woodpeckers (Picidae) excavate or enlarge holes in trees, kingfishers (Alcedinidae) use holes in river banks. The Tailor bird of India, (Orthotomus sutorius), a warbler, sews together the growing leaves of a tree by piercing holes in their margin and tying them together with strands of plant fibre. The weaver bird weaves plant material together to form an almost basket-like structure which is attached to a thin twig and hangs upside down. Other species of weaver birds collaborate and build elaborate community nests. The oven bird of Argentina (Furnarius rufus; Furnariidae) builds its nest out of mud and against fence posts and bare branches. Hornbills, also nests in holes in trees and incaserates using mud the incubating female and feeds both the female and the young hatchlings through a small hole in the mud wall. Cave swiftlets (Collocalia inexpectata; Apodidae) in southeast Asia construct artificial nests from glutinous spittle which is attached to the walls of the cave. Several bird species, including the famous cuckoo (e.g. Cuculus carnosus; Cuculidae) have escaped the labour of incubation and chick rearing by depositing their eggs in the nest of other birds and allowing foster parents to rear its young. Adaptations for such parasitism include close mimicry of eggs between the cuckoo and its host, and the more rapid development of the cuckoo chicks so that they hatch first and can dispose the legitimate eggs of its foster parents. However, all hatchling bird species do have a small egg-tooth at the tip of their beak which they use to break the egg. The egg has provided a small air sac at the end of the egg to provide the first air for the chick. Hatchlings can be divided into two categories. Chicks that can run away almost immediately from the nest and are fully covered with down feathers and can feed on their own but still have parental supervision are said to be precocial. This type of hatchling is most common to birds that do not build nests, but lay their eggs in the open such as the plover group. Chicks that at birth are naked and helpless and need to be fed by the parents are said to be altricial and restricted to bird species that construct nests. The anatomy of birds is intimately connected to their ability to fly and this is apparent in the bird shape which offers minimum resistance to the air. Several adaptations result in a low centre of gravity, which tends to prevent the body from turning over during flight. The wings are attached high up on the trunk, as are the light organs such as lungs, whereas heavy flight muscles and muscular digestive organs are positioned ventrally. The pattern, speed and endurance of flight are reflected in the shape of wings. Highly aerial birds; which includes swifts (Apodidae), swallows (Hemiprocnidae), terns (Laridae) and albatrosses (Diomedeidae); have long pointed wings which enable them to soar in the air for long periods using the minimum amount of energy. Other bird species have short rounded wings that enable them to take off quickly and fly rapidly for short distances (eg sparrows). Vultures (Accipitridae) which fly in circles at low speeds using thermal air currents have broad rectangular wings that permit slow flight. Humming birds (Trochilidae) are even able to achieve hovering flight, by tilting their bodies so that they are almost upright and they can beat their wings as fast as 80 times per second. Flight has, however, permitted birds to be both the fastest moving animals and the animals that travel the most distance. The Carrier Pigeon (Columba livia; Columbidae) attains a maximum racing speed of 96 km/h, ducks can reach 145 km/h and the swift (Apus apus) 170 km/h in level flight. The swift may travel up to 900 km each day to collect aerial insects which is its only source of food, and this species even copulates in flight. The Peregrine Falcon (Falco peregrinus) during a dive can achieve speeds of 267-290 km/h, and has swept its wings back to reduce drag even further. No other creatures can fly as fast or as far as birds. Many species of bird make long journeys. The White Stork (Ciconia ciconia; Ciconiidae) travels every autumn down to Africa and returns to Europe in the spring navigating with such accuracy that the same pair, year after year will occupy the same nest on the same roof top. However, the Arctic Tern (Sterna paradisea), holds the record for long-distance migration. The extremes of its Arctic nesting and Antarctic wintering ranges are 16 700 km apart. Since the routes taken are circuitous, these birds may fly 40 300 km each year. During the autumn, many birds gather in flocks and fly southward, returning the following spring. A lesser and opposite movement occurs in the Southern Hemisphere, where the seasons are reversed. Some other birds perform altitudinal migrations into mountainous regions for the summer and return to the lowlands to winter. In Africa young Starred Robins (Pogonocichla stellata; Turdidae) moves from the high interior forests to the warmer river valleys with the onset of autumn and winter. Most species used established routes for migration and travel more or less on schedule, arriving and leaving regularly. Migration, breeding, and moult are phases in the annual cycle of birds that are regulated by the endocrine system. Migration is a semiannual event, dependent especially on the pituitary gland in the brain. Usually prior to migration fat reserves, not present at other times, are accumulated rapidly for extra fuel during the long flights. Also, many strictly diurnal birds become nocturnal during migration. Seasonal differences (photoperiodism) influence migratory behaviour of some northern species. Generally birds migrate close to the earth's surface, although some bird species may migrate at more than 1 km altitude. Most birds migrate at between 50 and 80 km/h and stop and feed as they proceed along the migration front. Although some birds use obvious landmarks such as coasts, rivers and mountain ranges other birds will migrate without the aid of directional features. Evidence suggest that migration in daytime is guided by the position of the sun and at night by the patterns of stars. This would necessitate that migrations need to be done on clear nights. On cloudy nights birds tend to get lost and if they are released in a planetarium where the constellations have been rotated so that they no longer match the position of the stars in the heavens, the birds will orientate with the visible, artificial constellations. Still other bird species appear to be able to use the earth's magnetic field as a guide. Despite the large amount of adaptation required for flight, there are nevertheless a large number of birds that have abandoned flight. The older bird fossils dating some thirty million years after Archaeopteryx including gull-like forms (Ichthyornis) which were skilled flyers with a keeled chest bone and no bony tail. In essence they were modern birds. At the same time, however, lived huge swimming birds Hesperornis, which were nearly as big as a man and had already ceased to be able to fly. Fossils of those other non-flying birds, the penguins, also appeared around this time. Fossils of another large flightless bird Diatryma stalked the plains of Wyoming, while a similar bird Phororhacos. This bird was about 2m tall, carnivorous, and equipped with a huge bill. It is possible that this group was successful in the absence of other large carnivores representing either reptile or mammal classes. Large carnivores in the former class were already extinct in the former class and were yet to evolve in the latter class. Diatryma may have been the early ancestor to Gruiformes group of birds (Rails and Cranes) which even today have representatives (eg flightless rails of Gough Island) that showed a marked tenancy to lose flight when they colonize islands that have few or predators. The cormorants of the Galapagos Islands have such small wings that they cannot fly any longer. On the Madagscarene Islands, the dodo (Raphus cucullatus), was a very large pigeon that adopted a terrestrial habit and was exterminated by the human introduction of dogs to the island in the seventeenth century. The Elephant bird Aepyornis was about 3m tall and possessed the largest known eggs for any bird species (148 times the size of a hens egg by volume). Moas (Diornis) were another giant flightless bird over 3m tall and occurred on New Zealand. Currently four orders of birds species fall into the general category of wingless and flightless terrestrial birds. These include ostriches (Struthioniformes), rheas (Reiformes), cassowaries and emus (Casuariiformes) and kiwis (Dinornithiformes) Assignments IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS Discuss the general adaptations birds have evolved for flight. Your answer should include sections on anatomical modification, physilogical adaptations, feathers and wings. Describe the modifications that have occured in the beak and feet of birds. Discuss how such a diversity of forms evolved, relating these forms to environmental factors and or food items that they forage. Birds have been described as the living relatives of dinosaurs, briefly discuss the validity of such a statement. EGGS, POUCHES AND PLACENTAS The duck-billed platypus Ornithorhynchus anatinus (bird-billed) from Australia is animal belonging to the most primitive order of mammals (Monotremata). This animal is the size of a rabbit, possesses thick fur, webbed and clawed feet, a cloaca combining both excretory and reproductive functions and a large pliable flat beak like a duck's. It lives in the rivers of eastern Australia, swimming using its webbed flat feet and steering with its hind-limbs. When it dives, it closes its ears and tiny eyes with little muscular flaps of skin and hunts for aquatic invertebrates using its bill, which is rich in nerve endings and very sensitive. It is also a powerful burrower, excavating tunnels up to 18 metres in length through the river banks. These animals roll back the webbing of their fore feet into their palms and this frees the claws for burrowing. Within these tunnels the female constructs an underground nest of grass and reeds and lays two eggs that are nearly spherical, the size of marbles, and soft shelled and therefore similar to a reptile's egg. Since platypuses have fur, they are warm blooded and possess rudimentary mammary glands; they definitely belong to the class Mammalia and is one of only two primitive living mammal families which lays eggs. The female platypus develops on her belly special glands, that are similar in structure to sweat glands but are enlarged and produce a thick rich milk which oozes into the fur. The young platypus suck the fur. This is the beginning of the true mammary gland found in all higher mammals. The other important mammalian feature of endothermy is also incompletely developed and the platypus allows its body temperatures to fluctuate more greatly than other mammals (viz can drop to 300C). The only other animal that can parallel this mixture of primitive features is the spiny ant-eater called Echidna which taxonomists renamed Tachyglossus (swift-tongued). These animals are spiny with a long tube-like snout that has no teeth but does possess a long tongue which flicks out to catch insects. Its front legs are equipped with long digging claws. At the beginning of each mating season the female develops a small pouch into which she later transfers her single egg. The mammary glands discharge directly into the pouch and the milk is sucked of the hairs. The Echidna and Platypus are of great antiquity, but we have no hard evidence to indicate which fossil reptiles were their ancestors. Our knowledge of many of the candidates is based to a considerable degree on its teeth, one of the most durable parts of any animal's anatomy. Fossilized teeth provide information about an animal's diet and habits. They are also highly characteristic of a species and similarities between teeth are strong evidence of genealogical relationships. Both Platypus and Echidna became highly specialised for underwater foraging and ant eating respectively and consequently lost their teeth (although young Platypuses still produce three tiny ones soon after birth which are lost in a very short time). We therefore have virtually nothing to help us link these creatures to any group of fossil reptiles. This is further complicated since the features that characterize mammals are hair, warm bloodedness and milk producing glands which cannot be easily deduced from fossils. We know that dinosaurs, such as Stegosaurus undoubtedly developed very effective methods of absorbing heat quickly from the sun and thereby maintained higher than ambient body temperatures. Mammals, however, evolved from an earlier group of reptiles (the Synapsids, often referred to as mammal-like reptiles). One of the earliest group of synapsids, the pelycosaurs, also had similar adaptations to the Stegosaurus dinosaurs. Dimetrodon, grew long spines from its backbone which supported a sail of skin which must have served as a solar panel in a similar way to the Stegosaur's plates. Although the pelycosaurs persisted for a considerable time their sail like crests disappeared in later forms. In seems extremely unlikely that even if there was a warming of the climate, the forces of evolution would allow an animal to lose such a valuable method of heat control unless it was able to replace it with an adaptation that is more efficient. It has been hypothesized that the pelycosaurs and their successors, the therapsids, were to some degree endothermic. One of the therapsid lines were the theriodonts which were small carnivorous animals (less than 1m) and were almost certainly the evolutionary line that lead to the mammals. There is some doubt as to whether theriodonts should be classified as reptiles or as very primitive mammals. An example of which is Cynognathus, an animal approximately a metre long, possessing a large dog-like skull with highly specialized and differentiated teeth (remember reptiles are generally characterized by simple, undifferentiated peg-like teeth). These teeth suggested that Cynognathus teeth were for chewing and cutting food rather than swallowing it whole. There is also a well developed secondary palate, which separates the nasal passage from the mouth, which permits continued eating while the mouth is filled with food. All these features suggest that the animal was very active and probably requiring an endothermic metabolism. To maintain such a metabolism would require some form of body insulation, possibly even fur. These fossils indicate that some theriodonts were far advanced towards the mammals in certain characters, but still remained comparatively primitive in other respects. The mixture of conservative and advanced features makes it difficult to identify the final line that evolved towards the mammals. The environmental conditions that stimulated such changes may have produced similar adaptations in more than one group of animals. It is likely that mammalian traits were acquired by several separate reptilian groups. It was originally hypothesized that the line of reptiles from which the platypus and echidna stemmed was not necessarily the same as that which was to give rise to other mammals. In other words mammals had a polyphyletic origin (derived from more than one ancestor) rather than a monophyletic origin (derived from a single ancestor). Recent evidence based on the skull morphology of Probainognathus is argued for monophyletic origin for the mammals. Much of this debate depends on whether the advanced theriodonts were reptiles or represented the first mammals. What is certain that monotremes diverged from the main mammalian line during the Triassic, whereas the other major division in the mammals, namely differential of placental and marsupial forms only occurred during the late Cretaceous period. Whatever the exact shape of the genealogical tree, at least one group of the reptiles completed the transition to a mammalian status some 200 million years ago. A fossil from the upper Triassic of a small animal (Megazostrodon) discovered in 1966 in southern Africa, is possibly the earliest true mammal. This creature was only about 100 mm long and resembled in body form a modern day shrew. Details of its jaw and skull link it firmly with true mammals and its teeth were specialised for eating insects. There is little doubt that it must have been both warm-blooded and fury. What we cannot determine is whether it laid eggs like a platypus or gave birth to live young and suckled them by means of a breast. Even with the advantage of warm bloodedness the first small mammals were quite over shadowed in both numbers and size by the dinosaurs until 65 million years ago. Equipped with warm bloodedness, mammal were able to be active at night when the great reptiles became torpid and therefore survived in the shadow of the dinosaurs. The earliest mammals were probably like the opossums that today live in the Americas particularly those belonging to the genus Didelphis. The Virginia opossum Didelphis marsupialis of North America is a large rat shaped creature, with small eyes and a long naked tail which it can wrap round a branch with sufficient strength to support its own weight. It has a large mouth that opens wide and is equipped with a great number of small sharp teeth. It is a tough adaptable creature that has spread through the Americas, from Argentina in the south to Canada in the north. One of the most extraordinary aspects of this animal is its manner of reproduction. The female has a capacious pouch on her underside in which she rears her young. The young are extremely small and without fur and have attach themselves to the mother's teats. The method by which they get there is one of the most fascinating. The opossums copulate and fertilization of the female's eggs occurs internally. The young embryos, however, have only enough yolk to maintain themselves for the first few days of their life. At twelve days and eighteen hours the animals are expelled into the outside world. This represents the shortest gestation period known for any mammal species. These young are born so premature that they are no larger than bees, and so unformed that they are not called infants but are rather referred to as neonates. As the neonates emerge from their mothers cloaca, they haul themselves through the fur of her belly to the opening the pouch, a distance of some 80 mm. Only about half of the neonates reach the pouch and each animals attaches itself to one of thirteen nipples and starts to take milk. If more than thirteen complete the journey, only those that attach themselves to a teat will survive. Nine or ten weeks later, the young clamber out of the pouch. They are now fully formed, the size of mice, and cling to their mother's fur. In about three months they leave their mother for an independent life of their own. Mammals that bread in this way (by means of a pouch) are all placed in the order Marsupialia. There are seventy six species of opossum (Family Didelphidae) in America, with the smallest (Marmosa murina) being mouse sized and not possessing a pouch (the young simply cling to the teats between their mother's hind legs. The largest is the water opossum or Yapok (Chironectes minimus) and is almost the size of a small otter, and possesses webbed feet for swimming. Its young are saved from drowning in the pouch when their mother goes, a sphincter (ring-shaped muscle) which closes the entrance the entrance of the pouch. The young inside are able to endure several minutes of submergence and breathe air within the pouch that has a higher concentration of carbon dioxide than most mammals could survive. The earliest mammalian fossils that have been positively identified as being marsupial were found in the Americas and this may be where the group originated, however, the greatest assemblage of marsupials occuring today is in Australia. The earliest marsupials (Alphodon and Eodelphis from Cretaceous North America) closely resemble the living Didelphis opossums that occur in the Americas. From didelphid ancestors certain South American marsupials specialized into aggressive carnivores during Tertiary times. These were the borhyaenids, of which the Miocene genus Borhyaena was typical and resembled a large wolf. The skull was very dog-like, with the canines enlarged as piercing and stabbing teeth, and some of the molars modified into shearing blades. The body was long, limbs exceptionally strong and the feet were equipped with exceptionally sharp claws. Others such as Thylacosmilus was as large as a tiger; possessed a short skull and tremendously elongated bladelike upper canine tooth, whereas in the lower jaw there was a deep flange of bone to protect this tooth when the mouth was closed. These carnivorous marsupials became extinct with the influx of placental carnivores from North America. In order to explain how the marsupials got from South America where they originally radiated to Australia we have to return to the period when the dinosaurs were still at the height of their dominance. At that time, the continents of the world were grouped together in a single large land mass. Consequently fossils of closely related dinosaurs have been found in all of today's continents. The early mammal like reptiles would have similarly widespread distributions. About 135 million years ago the large single land mass (Pangaea) split into two a northern supercontinent called Laurasia comprising today's Europe, Asia and North America; and in the south, another super-continent called Gondwana made up of South America, Africa, Antarctica and Australia. The primary evidence for this grouping and the subsequent splitting and drifting is geological. It comes from studies of the way in which today's continents fit together, the continuities of the rocks between their opposite edges, the orientation of magnetic crystals in rocks which shows the position that they held when they were first formed, the dating of the mid ocean ridges and their islands and drillings taken from the ocean floors. The distribution of many animals and plants adds corroborative evidence. Giant flightless birds provide a particularly clear case since they appeared very early in the history of the birds. One group which included the ferocious Diatryma, evolved in the northern super-continent are all extinct. The other group called ratites evolved in the southern supercontinent, and are represented by the Rhea (Rhea americana) in South America, the Ostrich (Struthio camelus) in Africa, the Emu (Dromaius novaehollandiae) and Cassowary (Casuarius spp.) in Australia and the Kiwi (Apteryx) in New Zealand. These birds are so similar that it seems very probable that they are descended from a single flightless ancestor which had distribution right across the Gondwana supercontinent. When the land masses separated the different groups of flightless birds continued to evolve independently of each other into their present-day forms. Other evidence for the splitting up of the super-continents comes from fleas, which are parasitic and travel with the animals they live on but readily develop into new species and move on to new hosts. Some families of highly characteristic fleas are found only in Australia and South America, with the most probable explanation being that they originated on group of animals that had a wide distribution across Gondwana. Botanical evidence is found with the southern beech, a forest forming tree that flourishes only in the temperate lands of the southern hemisphere. This distribution can also be explained by the break-up of Gondwana. During this break-up Africa separated and drifted northwards and Australia and Antarctica remained joined to one another and were linked either by way of a land bridge or a chain of islands, to the southern tip of South America. At this point, it seems, the pouched animals (marsupials) were developing from the early an mammal stock. If these developments took place in South America, as some evidence suggests, then the early marsupials could have spread across into the Australian Antarctic block by way of these land-bridges or by island hopping. Fossil evidence supporting this theory comes from two very closely related marsupial animals; Polydolops and Antarctodolops being found in South America and Antarctica respectively. Meanwhile, primitive mammals were also evolving in the northern super-continent. They were to develop a different way of nourishing their young. Instead of transferring them at a very early stage into an external pouch, they retained them within the body of the female and supported them by means of a device called the placenta. We will examine this technique of reproduction later. The South American marsupials flourished greatly while they had the continent to themselves since the southern supercontinent was fragmenting and drifting apart and South America was moving slowly northwards. In due course, it connected with North America by way of a land bridge in the neighbourhood of Panama. Down this corridor came the placental mammals to dispute the possession of South America with the marsupial residents. In the course of this rivalry, most species of marsupials disappeared, leaving only the tough, opportunistic opossums. One of these has even invaded North America, the land from where the placental invaders had come from. That marsupial invader is the Virginia opossum. None of the marsupials that lived in the central part of the southern super-continent which became Antarctica survived. By that time Antarctica had drifted over the South Pole where it was so cold that it developed an immense ice cap and life on the land became insupportable. The eastern section of the super-continent, which became Australia had drifted in a north- east direction into the emptiness of the Pacific basin and has since remained totally separate from any other continent. The marsupials that occurred on this section of the super-continent have continued to evolve without any invasion from placental animals until man introduced them. During this time, the marsupials radiated into a great number of different forms in order to take advantage of the wide range of environments and opportunities available to them. Fossil remains of some spectacular species that once existed have been discovered in the limestone caves of Naracoorte, 250 kms south of Adelaide. Among them are the remains of a huge marsupial the size of a cow, with a head like a small giraffe that browsed on the branches of trees. One specimen Thylacoleo was originally thought to be a carnivore due to the back teeth that were elongated into formidable shearing blades, and called a marsupial lion. More recently the front legs have shown that this animal was well suited for a tree climbing existence and used its elongated back teeth to cut down hard fruits. Australian marsupials still survive within a dozen main families and are represented by nearly two hundred species. Many of these creatures parallel the placental forms that evolved in the northern hemisphere. For example there are carnivorous marsupials that will tackle reptiles and nestling birds and are called marsupial cats (Dasyurus) and until very recently there was also a marsupial wolf called a Thylacine. Since this animal took to preying on newly introduced sheep it was hunted and eventually exterminated by local farmers. Sometimes the resemblance between placental and marsupial forms is so close that you need to examine the animals closely in order to distinguish them. The sugar gliders Petaurus spp. are small leaf and blossom eating marsupials that live in eucalyptus trees. They have a parachute of skin connecting its fore and hind legs which enable them to glide from branch to branch and resemble almost exactly the North American flying squirrel (Petaurista alborufus). The similarity is based on similar lifestyles requiring similar forms. For example in order to have lifestyle that relies on gliding you will need to have structures that function as parachutes. A burrowing lifestyle also demands particular structures that are similar for marsupial and placental animals alike. Placental moles (e.g. Cape Golden Mole Chrysochloris asiatica) and marsupial moles (Notoryctes) both have short silky fur, reduced eyes, powerful digging forelegs and a stumpy tail. The distinguishing feature is that the female marsupial mole possesses a pouch, which unlike other marsupials opens from the rear and therefore does not fill with earth when she burrows. Not all marsupials have such close placental equivalents. The koala (Phascolarctos cinereus) is a medium sized tree-living creature that feeds on leaves and is comparatively slow moving. Its ecological equivalent are monkeys which are far more athletic, active and intelligent. The numbat (Myrmecobius fasciatus) is an ant eating marsupial possessing a long sticky tongue used to collect its food items; a feature common to all ant eaters. Further adaptations for ant-eating are not nearly so extreme for the numbat as those of other ant-eaters, e.g. the giant ant eater (Myrmecophaga tridactyla) of South America which has evolved a long curving tube-like snout and lost all its teeth. The numbat jaw is are not nearly so elongated and it still possesses all its teeth. Other marsupial forms are more unique in their adaptations for example the boodie (Bettongia lesueur) a shy, strictly nocturnal rat kangaroo, possessing small pointed canine teeth to help fed on other small animals. It makes its nest in a burrow, industriously collecting material for it in a most ingenious way. It picks up a few straws in its mouth, stacks them in a bundle on the ground and then pushes them back over its long tail with its hind legs. The tail then curls up tightly so that the straw is effectively baled and the boodie move away by hopping. Boodies locomote using only their back legs which have very long feet. An animal like the boodie may have been the ancestor to the spectacular radiation of bipedalism that resulted in the kangaroos and wallabies The development of the kangaroos may be related to Australia's continuing drift northwards and the consequent drying and warming of its climate. This would have caused a reduction in forest cover and replacement by grasslands. Living in an open grassland would require that the herbivores feeding on the grass an ability to escape predators. In kangaroos the hind legs have become enormously powerful and the long muscular tail is held out stiffly behind to acts as a counterbalance which gives the animals a potential to reach speeds of 60 kph and to clear fences nearly 3 metres high. The second difficulty that grass eaters must overcome is the wear and tear on their teeth. Grass is tough, due to the silicates that occur in them, and breaking it down into a pulp in the mouth is very abrasive on the teeth. Grazers elsewhere have molars with open roots so that wear can be compensated by continuous growth throughout the animal's life. In kangaroos the roots of the teeth are closed, and they have evolved a different system of tooth replacement. There are four pairs of cheek teeth on either side of the jaws. Only the front ones engage. As they are worn down to the roots, they fall out and those from the rear migrate forward to take their place. By the time the animal is fifteen or twenty years old, its last molars are in use. There are some forty different species in the kangaroo family. The smaller ones are usually called wallabies. The largest is the red kangaroo Macropus rufus which is as tall as a man and the largest living marsupial. Kangaroos reproduce in much the same way as the opossums. The egg which is still enclosed in the vestiges of a shell a few microns thick and has only a small quantity of yolk within it, and descends from the ovary into the uterus. There, lying free, it is fertilised and begins its development. If this is the first time that the female has mated, the fertilized egg does not stay there long. In the case of the red kangaroo it is only thirty three days before the neonate emerges. Usually only one is born at a time. It is a blind, hairless an only a few centimetres long; its hind legs are mere buds, but its forelegs are better developed and with these it hauls its way through the thick fur on its mother's abdomen. The neonate's journey to the pouch takes about three minutes. Once there, it fastens on to one of four teats and starts to feed. Almost immediately, the mother's sexual cycle starts again. Another egg descends into the uterus and she becomes sexually receptive and she mates and the egg is fertilised. But then an extraordinary thing happens, the egg's development is temporarily halted. Meanwhile, the neonate in the pouch is growing prodigiously. After 190 days, the baby is sufficiently large and independent to make its first foray out of the pouch. From then on it spends increasing time in the outside world and eventually, after 235 days, it leaves the pouch for the last time. If there is a drought at this time, as happens often in central Australia the fertilised egg in the uterus still remains dormant. But if there has been rain and there is good pasture, then the egg resumes its development. Thirty three days later, another bean sized neonate will emerge from the mother's cloaca. The female will then immediately mate again. But the first-born does not give up its milk supply so easily. It returns regularly to feed from its own teat. The female kangaroo in effect has three young dependents on her, each at a different stage of development. One active young at foot which grazes but comes back to suckle, a second, the tiny neonate, sucking at her teat in the pouch; and a third the fertilised egg waiting further development. It is a commonly held notion that the marsupials are backward creatures, scarcely much of an improvement on those primitive egg layers, the platypus and echidna. That is a long way from the truth. The marsupial method of reproduction must certainly have appeared very early in mammal history, but the kangaroos have refined it marvellously. No other creature anywhere can compare with the female kangaroo who, for much of her adult life, supports a family of three in varying stages of development. The mammalian body is a very complicated machine that takes a long time to develop. Even as an embryo it is warm blooded and burns up fuel very quickly. Both these characters demand that the developing young should be supplied with considerable quantities of food. All mammals have found methods of providing far more than could ever be packed within the confines of a shelled egg. We do not know whether the early mammals in the northern supercontinent ever passed through a marsupial stage before developing the placenta. It could be that they sprang from a branch of the mammal like reptiles that never acquired pouches. The placental and marsupial forms probably arose independantly from a common ancestor, and they evolved side by side. Certainly the fossil record of the placentals is as ancient as that of the marsupials, and they arose sometime during the Cretaceous period. During the early stages of their evolutionary histories they were probably well matched, so that marsupial adaptations were about as efficient in evolutionary terms as placental adaptations. However, during the Cenozoic, the placental animals came to dominate in all areas of the world except the large island of Australia, which until the advent of many had never witnessed placental mammals. In Australia the marsupial animals achieved the sophisticated levels of efficiency occurring in the Red Kangaroo. In the northern continents the placental method of mammalian reproduction evolved with many ensuing benefits. The placenta allows the young to remain within the uterus for a very long time. It is a flat disc that becomes attached to the wall of the uterus and is connected by the umbilical cord to the foetus. The junction with the uterine wall is highly convoluted so that the surface area between the placenta and the maternal tissues is very great. It is here than that the interchange between the mother and foetus takes place. Blood itself does not pass from mother to young, but oxygen from her lungs and nutrients derived from her food both dissolved in her blood, diffuse across the junction and so enters the blood of the foetus. There is also traffic in the other direction. The waste products produced by the foetus are absorbed by the mother's blood and then excreted through her kidneys. All of this makes for great biochemical complications. But there are further ones. The mammalian sexual cycle involves the regular production of a new egg. This causes no problem to the marsupial, for in every species, the neonate emerges before the next egg is due to be produced. In the placental animal the foetus, however, stays in the uterus for a very much longer period. So the placental foetus secretes a hormone which suspends the mother's sexual cycle for as long as the placenta is in place so that no more eggs are produced to compete with the foetus in the uterus. There is also another problem. The foetus' tissues are not the same genetically, as the mother's. They contain genetic material from the father. So when it becomes connected to the mother's body, it risks immunological rejection in the same way as a transplant does. Just how the placenta prevents rejection is not completely understood. So by these means, the babies of placental mammals can remain in the uterus until, if necessary, they are so well developed that they can be fully mobile as soon as they are born. The placental breeding technique spares the young the hazardous journey outside their mother's body at a very early stage that a marsupial neonate has to undertake, and allows their mother to supply their every want during the long period they remain within her. So whales and seals can carry their unborn young even as they swim for months through freezing seas. No marsupial with air breathing neonates in a pouch could ever succeed in doing such a thing. It is possible that the placental technique of reproduction was to prove one of the crucial factors in the mammals' ultimate success in colonising the whole of the earth. Download 2.01 Mb. Share with your friends:
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