The infinite variety: the beginning of life
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- THE INVASION OF THE LAND
- A WATER-TIGHT SKIN AND THE SHELLED EGG
Assignments IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS Discuss the development and modifications of the eye that have occured in the fish. Also discuss why sharks and skates are generally dull-coloured, whereas many bony fish have bright colours. Describe the evolutionary transitions from the earliest chordate (e.g. amphioxus) to the most advanced bony fish. Describe the morphological differences that exist between the cartilaginous and bony fish. THE INVASION OF THE LAND The first fish may have crawled onto land during Devonian times (350 million years ago) and probably did so, in response to drying swamps. This required that two problems must be overcome, how to move without the support of water and how to obtain oxygen from air rather than water. The mud skipper (eg Periophthalmus sobrinus which occurs in our mangrove swamps) suggest adaptations that ancestral fish may have developed in their quest to colonize land. Each pair of front flippers has a fleshy base supported internally by bones and is able to be used to lever the animal forward. Another animal which showed the beginnings of limbs is the coelacanth Latimeria chalumnae (order Crossopterygii), the living fossil that was thought to have been extinct for 70 million years. In order to breathe in air the mudskipper retains water in its mouth which swills over the limning of it. The African Lungfish Protopterus (Order Dipnoi) can burrow into mud, curls itself into a ball and secretes mucous which creates a parchment-like case around the hole it has encased itself in and avoid desiccation during the dry season. The lungfish has a pouch opening from the gut (similar to the primitive bichir fish) which functions as a lung and extracts air from the tube it created when burrowing through the mud. By flexing its throat muscles the fish draws air into its pouch which is supplied by numerous blood vessels which absorb gaseous oxygen. With the termination of the dry season the fish returns to an aquatic existence and breathes with its gills, but like the Bichir may take gulps of air if a lack of oxygen develops. Although all of these animals have been regarded as possible ancestors of the first tetrapods which colonized the land, their skull morphology is unlike the first fossil tetrapods which were amphibians. Neither Coelacanth nor Lungfish have a passage linking the nostrils with the roof of the mouth, a characteristic feature of all land vertebrates. However, another lobe-fin fish called Eusthenopteron, which only exists today as fossils, possessed such a passage and well developed lobes. Careful examination of the fins of these fossils revealed that the base of the lobe was supported by one stout bone close to the body, two bones joined to it and at the terminal end a group of small bones; an arrangement found in the limbs of land vertebrates. A link between lobe-fin fish and amphibians has been found in the fossilized Ichthyostega found in Greenland in 1938. The swamps through which such animals waded was thick with horsetails and club moss trees which became fossilized as coal and also contained the first fossils of the terrestrial vertebrate (tetrapods) which belong to the class Amphibia. These animals had evolved only 50 million years after the first bony fish and reached greatest expansion some 100 million years later in the Upper Carboniferous period. Some of these early forms grew to four metres in size and possessed jaws spiked with cone-like teeth. Today relatively few amphibians have survived, but they are nevertheless distributed in tropical and temperate areas of the world and in a variety of habitats. The modern amphibians differ considerably from their large ancestors. The living amphibian that most resemble early forms are the salamanders and the newts which collectively are called Caudata ("tailed ones"). The largest member of this group comes from Japan and has a body length of 1,7 m (Megalobatrachus). In general amphibians are only partly successful in their colonization of land, since their limbs are short and they need to flex their body laterally in order to take reasonable strides. Amphibian skin is permeable and in a dry atmosphere would quickly dehydrate, they even do not have the mechanisms to drink water. A moist skin is also required to supplement respiration, since the lungs are comparatively simple and not totally adequate for its needs. These limitations restrict amphibians to moist environments. For reproduction amphibians are also almost entirely dependent on water since, like fish, their eggs have no water-proof covering and their larvae (tadpoles) are quite fish-like. These larvae initially have no legs swim using a long tail and breathes using external feathery gills. The two life phases of some species of Caudata (entirely aquatic and semi-terrestrial) have been used to exploit a greater variety of habitats. A Mexican salamander (Ambystoma mexicanum) regularly changes from an aquatic form to a land form. If there is a particularly wet season and/or the lake does not shrink greatly the larval stages are maintained and the larvae may become as big, or bigger than the land-living forms. A lack of iodine in the water may have prevented metamorphosis. Another species of Caudata has reverted permanently to an aquatic life. It always breeds in a larval condition and its external gills develop into branching bushes on either side of the neck. Using a thyroid extract it can be induced to lose its external gills, develop lungs, and turn into an animal that resembles a burrowing salamander that lives in Florida. However, another species called the mud-puppy Necturus maculosus has reverted irrevocably to water-living and has external feathery gills and very reduced lungs. The Greater Siren (Siren lacertina) is more elongated, has lost its back legs altogether and also breathes using gills. The Three-toed Amphiuma (Amphiuma means tridactylum), from southern USA is extremely elongated with tiny legs that have no function and is known locally as a Congo eel. This tendency to retain larvae characteristics in adult forms is called paedomorphosis. The abandonment of lungs and limbs, the cornerstone adaptations that permitted colonization of the land is not entirely restricted to aquatic amphibians, but even occurs in animals that live almost entirely on land. Such animals breathe through their skin and the moist membranes lining their mouths. The elongated body forms permit maximum surface area, but they nevertheless remain only a few centimetres in size and are restricted to very moist environments. Another group of Amphibians called the caecilians are also limbless, but are adapted to a burrowing existence and almost resemble earthworms. Their anatomy is so different from the salamanders that they are classified in the order Gymnophiona. They have several primitive features such as the retention of small scales in the skin and a very short tail. The solidly build skull, the elongated body comprising as many as 270 vertebrae, no internal girdles for supporting limbs and blindness (compensated by having extendable, sensory tentacles) are all adaptations to a burrowing existence. However, they are carnivorous and have mouths with a large gape. There are about 300 species in the order Caudata and 160 in Gymnophiona, but the most numerous group of amphibians belongs to the order Anura (tail-less ones) with about 2600 species. The Anurans include frogs which are generally characterized by smooth, moist skins and inhabit moister environments, and toads which have a drier, warty skin and often occur in drier environments. Unlike the members of Gymnophiona, this group has shortened the body and have even fused vertebrae together and have developed their hind legs enormously to become accomplished jumpers. The Goliath Frog (Gigantorana) can achieve 3m and the tree-living frog Rhacophorus reinwardti can achieve fifteen metres by gliding. To do this they increased the size of toes and with it the web of skin that unites them to form a parachute on each leg. Jumping represents a major way of escaping predators. Since amphibians are generally soft bodied, they are sought after as food items by larger predators, however, many rely on having a cryptic coloration of green camouflaged with blotches of brown and grey. The common European Toad (Bufo bufo) inflates its body and stands on its toes to appear as large as possible and thereby discourage any potential predator. More active defence occurs in the fire-bellied toad whose mucous which keeps the skin moist is also extremely bitter tasting. The poison arrow frogs (Family Dendrobatidae) include some species whose mucous is lethal to mammals and local people used its poison to tip their arrows. Such defences are of little value if their attacker dies after they themselves have been eaten, and therefore are often accompanied by bright warning colours and patterns (aposematic coloration). Amphibians are all carnivorous, and indeed some are quite formidable such as the horned toads (Family Leptodactylidae) which can easily prey on a nestling bird or a small vertebrate such as a mouse. Although these frogs have a large mouth serviced with sharp teeth, their purpose is for defence and to grip the prey item, but does not help with breaking the prey item into smaller parts for digestion. Most amphibians are smaller and restrict their prey items to invertebrates. For this purpose an extendible tongue attached to the front of the mouth has evolved. The tongue is sticky at its tip and when it is flipped out it adheres to the small prey item which is bodily brought back into the mouth. The tongue helps with swallowing since it produces mucous which lubricates the food before passing it into the gut. Amphibian eyes are fundamentally similar to their fish ancestors, however, they do require a membrane that can be drawn across the eyeballs to keep the surface clean. However, the mechanisms that fish use to perceive sound using resonances generated in their swimbladder will not work in air, and consequently ear drums have evolved to detect sound waves. With their increasing ability to detect sound waves frogs have also developed the ability to produce sound using the huge swelling of their throats to amplify the sound produced by air blown from their lungs over simple vocal cords. Such sounds are unique to individual frog species and are used during courtship (a prelude to mating) and to recognize frogs of the same species. Mating for most amphibians still takes place in water, with the males grasping the females and fertilization taking place externally, with the sperm cells swimming to the egg cells. Large numbers of eggs are produced to offset high mortalities of eggs and tadpoles. Other frog species have a different strategy whereby comparatively few eggs are laid, but considerable parental investment protects them from predators. Some tropical pond-dwelling frogs (e.g. Dendrobates pumilio and Osteopilus brunneus) find safety for their tadpoles by depositing eggs in centres of plants such as bromeliads which create small reservoirs of water in forests where the rainfall is high. These sites are safe from aquatic predators. In Dendrobates pumilio males and females divide parenting duties; males guard the eggs until they hatch (10 to 12 days) thereafter the females assumes care for the young. The females begin by transporting each newborn tadpole to a bromeliad, at the base of which a small pool of water has collected. Although protected from desiccation and predation, the tadpole has no food supply and is entirely dependant on its mother for nutrition until it metamorphoses into a froglet which takes six to eight weeks. In Brazil, another small frog builds its own ponds at the margins of forest pools, constructing a crater ringed with low mud (100 mm in height). The eggs are laid and the tadpoles stay in their exclusive water residence until the rain raises the level of the main pool and floods the crater created by the parent frogs. When the first amphibians appeared, the terrestrial environment would have been a much safer site for the development of their offspring than an aquatic environment which has many predators (especially fish). As a consequence anurans evolved mechanisms to exploit the terrestrial environment for the breeding of its young. The midwife toad Alytes obstetricans (Family Discoglossidae) of Europe lives in holes close to water and mates on land. After fertilization the long strands of eggs are twisted around the hind leg of the male toad. The male carries them around until the tadpoles are ready to hatch and then takes them to water. The South American Centrolene frogs defend calling sites which are leaves overhanging streams. Such sites are where the eggs are laid, and parent frogs attend the eggs until they hatch and the tadpoles fall into the water below. In Africa some species of frog (e.g. Chiromantis) breed on branches of trees above ponds. The female excretes a liquid which is beaten into a ball of froth by the male frog. The eggs are then laid and the outside surfaces of the froth harden into a crust which retains moisture. The female frogs may bring up additional moisture to the nest. The eggs hatch and tadpoles develop within the hardened froth. The tadpoles are released when the lower part of the froth ball liquifies and they fall into the water below. Frogs producing foam nests occurs in five anuran families (Rhacophoridae, Hyperoliidae, Myobatrachidae, Hylidae and Leptodactylidae). In some tropical American frog species (Eleutherodactylus) a considerable yolk is provided in each egg which makes it possible for all stages of larval development to take place within the eggs and fully developed froglets emerge directly from them, this is term direct development and occurs in nine families of frogs. Many frogs invest considerable parental effort. The toad Pipa carvoelhi have a normal anuran copulation in water, however, only a few eggs are fertilized and the male frog using his webbed hind feet gathers the eggs and spreads them onto the female's back. This process is repeated until about a hundred eggs are gathered. The skin below begins to swell and embed the eggs and a membrane develops over the top of the eggs and covers them. After 14 days the female's back is rippling with the movements of hatched tadpoles. After 24 days the young break holes and are released from their mother. The frog species Gastrotheca have brood patched on their backs where fertilized eggs develop into tadpoles. When the tadpoles are ready to be released the female finds a shallow pond to sit in and deposit her young. In Gastrotheca ovifera more yolk is provided with the eggs, and the tadpoles remain until they are froglets before leaving the pouch. Egg brooding is usually done by the female frogs, although in the Australian hip-pocket frog Assa darlingtoni it is the male who broods them. The stage at hatching for egg-brooding frogs, also called marsupial frogs, is determined by the amount of yolk in each egg, which in turn reflects the number of eggs produced per clutch. Species in which the eggs hatch as small tadpoles produce 100 or more ova, each ca. 2 mm in diameter. Where froglets emerge directly, only about six ova are produced, each ca. 10 mm in diameter. A West African frog Nectophrynoides has taken parental effort even further. These frogs have internal fertilization with the fertilized eggs retained in the oviduct. Tadpoles develop, complete with mouths and external gills and they feed within the oviduct on tiny white flakes excreted from its walls. After nine months development which is co-ordinated with the arrival of the first rains, the female gives birth, by bracing her body against the ground with her forelegs and then inflating her lungs to full capacity which in turn swells the abdomen and squeezes the young out by pneumatic pressure. The tiny frog Rhinoderma found in southern Chile deposits her eggs on moist ground, the males sit in groups around the eggs and guards them. When developing eggs move within the gelatinous coats, the males take the eggs into their large vocal sacs where they continue developing until they are fully-formed froglets. Phyllobates subpunctatus, one of the South American poison dart frogs (Family Dendrobatidae), also lays the eggs on moist ground in close proximity to a guarding male frog. When the tadpoles hatch they wriggle themselves onto the male's back, where his copious quantities of mucous keeps them attached and prevents them from drying out. These tadpoles have no gills, but obtain oxygen by absorbing it through the skin of their body and from the surface areas of their greatly enlarged tails. However, the most bizarre form of parental care is the Australian frogs Rheobatrachus silus and Rheobatrachus vitellinus. In these species the female frogs swallows the eggs after fertilization and broods them in her stomach for six weeks. Such a breeding system presents interesting problems, such as how the eggs and hatched tadpoles escape being digested within the mother's stomach? The nurturing females appear to cease feeding during the breeding period. The production of hydrochloric acid and pepsin are halted in the stomach by a hormone-like substance prostaglandin E2 which is secreted by the egg capsules and then by the tadpoles. With this shutting down of normal stomach activities, the stomach's digestive functions are transformed into that of a protective gestational sac. The eggs, which range from 21 to 26, are relatively large, ca. 5 mm and rich in yolk. Consequently, the tadpoles do not need an external source of nutrition but feed exclusively on yolk throughout their six-week development period. During birth the female's oesophagus dilates in a manner analogous to the vaginal canal of mammals, and the young froglets are propelled from her mouth. Within a few days after expulsion of the young, the stomach begins to function again as a digestive organ, and the frog resumes feeding. Unfortunately neither of these two species has been found recently, and it is sadly concluded that these interesting frogs are now extinct. From these patterns it is clear that size and complexity of parental investment reflects clutch size. Another trend is that development time appears to reflect climatic conditions. Frogs within tropical regions develop relatively rapidly, sometimes spending only two or three weeks in the tadpole stage, whereas those living in cool temperate climates develop much more slowly. One such species is the spotted frog, Rana pretiosa, which lives in the cold streams that cascade down the Rocky Mountains. Because the cold water in which the frogs live slows their metabolism, more than one year is needed to produce fully yolked eggs, and the females lay eggs only once every two or three years. Tadpoles also metamorphose more slowly in cooler areas. For example bull frogs in northern USA (Rana catesbeiana) typically spend two years in the tadpole stage and another species Ascaphus truei needs three years to reach adulthood. In arid habitats, development is limited not by temperature but by moisture. One example are the rainfrogs Breviceps (Family Microhylidae) which lives in arid regions of Africa. These animals only emerge above ground during heavy downpours. Although much of the biology of this elusive group of frogs is unknown, it appears that they form pairs during the breeding season. Adults emerge from their underground burrows and absorb rainwater through their skins, thus replenishing their body fluids. In particular the bladder is filled with water. The male is far smaller than the female and is unable to clasp the female in order to copulate with her. Instead the male glues himself to the female's back. With the male riding on her back, the female burrows into the ground and proceeds to lay eggs that are then fertilized by the attached male partner. Periodically the female wets the eggs from her extended bladder, keeping them moist until the froglets hatch. This breeding process takes place on only one or two nights per year, when there is a sufficiently heavy downpour. Once fertilization occurs, growth proceeds rapidly. The Spadefoot toads (Scaphiopus) in southwestern deserts of the USA, have tadpoles that develop into frogs in less than two weeks. Such rapid development is necessary in a habitat where the water will only last for a few weeks. The zenith of amphibian's adaptations to minimize their dependence on water under arid conditions is the water-holding frog, Cyclorana which inhabits the central desert regions of Australia. During the brief and infrequent periods of rain these frogs feed on the flush of insects, they mate and lay their eggs in tepid shallow pools of water, the eggs hatch and tadpoles rapidly develop into froglets. As the rain soaks away the frogs and froglets absorb as much water as possible and bury themselves deep into the sand where they secrete a membrane around themselves to prevent moisture loss. They remain in this condition until the first significant rains, which could be in several years time. Assignments IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS Discuss the adaptations required to make the transition from an aquatic to a terrestrial life using the amphibian group as an example. What limitations to a terrestrial life do amphibians exhibit? Why have some amphibians after evolving limbs then lose them to become limbless? Support your answer with both terrestrial and aquatic examples. The anurans have evolved a variety of reproductive strategies to reduce predation of eggs and tadpoles and to exploit arid regions. Discuss such adaptions with special reference to the amount of parental investments existing between different anuran species. A WATER-TIGHT SKIN AND THE SHELLED EGG The reptiles evolved from an early ancestral group of amphibians (Subclass Labyrinthodontia) which have been extinct for 175 million years. Terrestrial Labyrinthodontia had strong limbs, robust bodies. The first animal with a dry skin was probably Seymouria which lived in the Permian (230 million years ago) is thought to be the link between the amphibians and the reptiles and was probably the first animal to have a hard-shelled egg that entirely freed its reproduction from water or extremely moist habitats. All amphibians require a moist environment to survive and reproduce, but the reptiles can occupy a dry environment due to their water-tight skin and the shelled eggs. A modern example of how reptiles manage to survive in hot dry conditions can be found in the marine iguanas (Amblyrhyncus cristatus) which are able to survive on barren larva fields on the tropical arid islands of the Galapagos Islands. These animals bask in the sun which helps raise their body temperatures without the risk of desiccation. Physiological processes of animal's body, like other chemical reactions, are affected by temperature. Up to about 40oC the higher the temperature the quicker the physiological processes and the more energy they produce and the more active the animal can be. Neither reptiles nor amphibians generate their heat internally like we do, but they draw it directly from the environment usually in the form of solar radiation. The daily activity cycle of these marine iguanas maintains the body at the most efficient temperatures. At dawn, when ambient temperatures are lowest, they climb to ridges and expose themselves broadside to the rising sun. As temperatures rise, the risk of overheating increases, the iguanas respond by lifting their bodies off the ground and positioning themselves so that air currents can pass below them. They can also pack themselves into the few shady places that exist (such as rock crevices). The sea surrounding the islands is influenced by the cold Humboldt Current, and is only entered to feed on green alga at the hottest time of day (noon). During foraging their bodies would cool-off rapidly and they will need to conserve as much heat as possible. These animals therefore constrict the arteries near the surface of their bodies so that blood circulates only in the centre of the body. Nevertheless body temperatures will drop up to 10oC before they have to return to land. On land they stretch-out their bodies and absorb warmth from the black larva surfaces. With the setting sun they again congregate on the ridges with their bodies broadside to the last of the solar radiation for the day. These behavioral sequences maintain the body temperatures close to 37oC, although it varies considerable more than in endothermic animals (eg our bodies). Animals like iguanas are ectothermic since their body temperatures tend to fluctuate. Endothermy has advantages since it permits greater independence of the prevalent temperatures (eg can maintain activities at night and in cold regions), but is energetically expensive. About 80% of daily calories is invested in maintaining body temperatures constant in endothermic animals. In contrast an ectothermic animal uses only 10% of the energy that an equivalent endothermic animal would use. As a consequence they survive in desert conditions were endothermic animals would have more difficulty surviving. The ability to breed under dry conditions is achieved by a gland located in the lower part of the oviduct and secretes a parchment-like shell which prevents desiccation of the shell. However, the shell still needs to be supplied with sufficient yolk to support the development of the embryo and the shell needs to be porous to enable oxygen to diffuse through. Clearly fertilization of eggs needs to be internal (male reptiles therefore evolved a penis) and to be completed before the shell is deposited. The Tuatara Sphenodon punctatus (Order Rhynchocephalia) an ancient lizard that occurs on New Zealand has no penis and males and females press their genital openings close together in order to achieve internal fertilization in a way similar to amphibians. These lizards have another amphibian feature that is an ability to be active down to 7oC, a much lower temperature than for any other reptile. Fossilized bones of these creature have be dated to 200 million years ago and may represent one of the most basic four-legged (tetrapod), tough skinned, egg-laying ectotherm that was a predecessor to the great dinosaurs that conquered all parts of the earth (except the polar region). The diversity of dinosaurs also included forms that returned to the sea (ichthyosaurs and plesiosaurs). The amphibians and earliest reptiles that evolved from them are often referred to as cotylosaurs, and the stem reptiles themselves are called captorhinomorphs. Less than 100 million years after their first appearance, the captorhinomorphs had already divided into three major divisions (Subclasses) based on the skull structure. One lineage referred to as the Anapsids has turtles and tortoises (Order Chelonia) as living representatives. The anapsids are characterized by a solid skull roof with no temporal openings in the skull (viz. area behind the orbits of the eyes). A second lineage referred to as the Diapsids produced the most diverse and spectacular radiation of animals. Diapsids skulls primitively possess upper and lower temporal openings behind the orbit of each eye. Living representatives of this group include snakes and lizards (Order Squamata) and the Tuatara (Order Spheodontida). Extinct forms within this group included the marine reptiles (ichyosaurs and plesiosaurs) which are sometimes referred to as Euryapsids. However, the largest group within this lineage are the Archosaurs (ruling reptiles) most of which are now extinct except crocodiles and alligators (Order Crocodylia). Extinct Archosaurs included the famous dinosaurs represented by two orders; Saurischia (lizard-hipped dinosaurs with a triradiate pelvis) and Ornithischia (bird-hipped dinosaurs with a tetraradiate pelvis), the flying pterosaurs (Order Pterosauria) and thecodonts the ancestral stock o f all archosaurs and birds. Thecodonts were relatively small and often bipedal reptiles that had a resemblance to the first crocodiles. The third lineage refers to the Synapsids, which possess skulls with a single (lower) temporal opening behind the orbit of each eye. These were the first group of reptiles to colonize land during the Permian period and are referred to as the mammal-like reptiles. Within the synapsids two orders have been identified. The primitive Order Pelycosauria was characterized by animals which developed elongated spines from the vertebrae and are commonly referred to as sailbacks. The most spectacular example was Dimetrodon with vertebrae projecting more than a metre above the back at their highest point. These vertebral spines supported a web of skin and probably served as a temperature-regulating device that added a great area of skin surface for warming up and cooling off. The other group of synapsids are classified in the Order Therapsida. The therapsids developed into animals that resembled dog-faced tanks, for their limbs extended beneath their bodies, rather than to the sides, they may have had fur, and exhibited specializations of bone and teeth structure. These mammal-like reptiles suffered at least six distinct mass extinctions during the last eight million years of the Permian. The survivors of each extinction appeared to be more warm-blooded, to have more specialized jaws and teeth and to possess a more efficient respiratory system. Although this line ultimately lead to the evolution of the mammals, they came to dominate only fairly recently during the Tertiary period (starting some 65 million years ago). The Triassic period produced new forms of reptiles (archeosaurs), the ichthyosaurs, crocodiles and the flying pterosaurs and the first of the dinosaur line, which were small active animals about the size of a pheasant, many of which were bipedal and had probably evolved high metabolic rates. Some may even have been covered with down and later feathers, an evolutionary line that ultimately evolved into birds. These dinosaurs remained in the shadow of the dominant group which were the mammal-like reptiles. Towards the end of the Triassic at about 220 million years ago, a mass extinction of the mammal-like reptiles may have facilitated the radiation of the other reptile group (archeosaurs) during the next million years (Jurassic). The oldest true dinosaur Eoraptor has been dated at 230 million years ago. This animal was a primitive, small (ca. 1 metre), carnivorous dinosaur. Like the more recent and better known dinosaur Tyrannosaurus rex, Eoraptor belonged to the saurischian group of dinosaurs (lizard-hipped). Eoraptor is considered primitive because it has an exceptionally simple jaw, and probably evolved shortly after saurischians and ornithischians diverged. Only 10 million years after Eoraptor the entire dinosuar group had already diverged, whereas the other reptile groups, such as the crocodiles and mammal like reptiles were declining rapidly. The richest deposits of dinosaur fossils have been found in the midwestern states of North America. Although recent excavations to Mongolia suggest that this region will provide the greatest number of new fossil dinosaur species. Some of these dinosaurs were no bigger than a chicken called Compsognathus, whereas others represent the largest land animals that have existed on the earth such as Apatosaurus which measured 25 m long and weighed at least 30 tonnes. A fossil dinosaur, Seismosaurus, unearthed in 1986 appeared to have been 43 metres long and weighed about 100 tonnes. Another dinosaur called Ultrasaurus, may have been heavier still with an estimated mass of 150 tonnes. The simple peg-like teeth of these animals meant that food, particularly plant material such as the tough leaves of the cycads that existed at that time, had to be broken down in the stomach. Mammals have specialized teeth that break-down and grind food to a considerable extent before entering the stomach for further processing. Consequently herbivorous dinosaurs probably needed large guts and even used stones (gastroliths) to process their food and this may have been the reason for them becoming so large. Carnivorous dinosaurs, like Tyrannosaurus, would also need to be large to prey on these mega-herbivores. Many Apatosaurus fossil bones have teeth marks which correspond to the fit of carnivorous dinosaur's jaws such as Allosaurus. Some scientists have re-interpreted such findings and have suggested that these apparently carnivores were more likely to have fed on the large carcasses of the mega-herbivores. The large size may also reflect temperature control. The bigger the body the more heat it retains and the more constant the temperature will remain for the animal. Evidence for warm-bloodedness is that the chest cavities are large enough to hold huge hearts, like birds do today. The dinosaurs were known to migrate, and both their northerly and southerly limits to these migration routes would have not been possible for a cold blooded animal. The bone histology of dinosaurs (particularly the more advanced thecodonts) suggest that they may have regulated their temperatures the way birds and mammals do today. Specialized structures such as the parallel rows of plates on Stegosaurus have been interpreted as additional temperature-control mechanisms. These plates, although made of bone, are spongy and probably carried many blood vessels which could either dissipate excess heat or absorb heat from the environment. Anatomical analyses of many dinosaurs suggested that they were active, fast-moving animals, and therefore probably possessing endothermic metabolisms. Finally the ratio of predator-prey ratios of fossilized dinosaurs do not correspond to the expected ratios assuming them to be ectothermic but does more closely resemble those of endothermic mammals. It is recognized that endothermy may take several forms and that some dinosaurs may have fell short of fully fledged endothermy. It has even been speculated that Tyrannosaurus rex underwent three vastly different growth stages and may have been equipped with a variable metabolism. A 2 metre juvenile would have been very active, capable of scampering around like some groundbirds do today. By contrast, mid-sized individuals, averaging 3.5 to 4.5 metres were probably less agile, and may have traveled in packs. A fully grown 12 metre adult weighing 8 tons would not have been agile, and may have reverted to a solitary life-style scavenging on carcasses. Further, all, but a few highly specialized endotherms have some kind of heat insulation in the form of hair, fat or feathers. Without it, the demands on energy are so extravagant, that it is difficult for such an animal to survive. However, the only fossil impressions of a dinosaur skin discovered suggests that their hides were not furry or leathery, but scaly and covered with bony bumps. It has even been suggested that the large herbivorous dinosaurs (sauropods) would have required hundreds of kilos of vegetation a day to sustain their enormous bulk and that they had a unique endothermic metabolism fueled by the heat given off by non-stop digestion. The dinosaurs had several extinction phases, with the gigantic dinosaurs, being replaced by smaller, low browsing, beaked dinosaurs at the end of the Jurassic and early Cretaceous. Again, another extinction occurred and marked the late Cretaceous period. These dinosaur extinctions may have been related to the radiation of angiosperm plants (viz plants possessing flowers) which attracted animals to disseminate their pollen and seeds. A new generation of low browsing dinosaurs may have promoted the spread of these plants. Overgrazing by dinosaurs may have threatened many low-growing plants with extinction, except for the angiosperms which possessed reproductive superiority. The late Cretaceous period witnessed the Hadrosaurs or duckbill dinosaurs (Anatosaurus, Lambeosaurus, Corythosaurus and Parasaurolophus) occupying swamps and forests and large herds of Triceratops and their relatives on the grass plains together with Tyrannosaurus rex. The discovery of fossilized egg-filled dinosaur nests belonging to the Hadrosaur Maiasaura gives new light on the life-styles of dinosaurs. Grouped nests were found in a single layer of sediment, implying that they were all built in the same year. These nests were spaced at an average of 7 metres apart:- about the size of an adult Maiasaura. Some bird species lay their eggs close enough together for maximum mutual protection, yet far enough apart so that they can move easily past their neighbours. Tiny eggshell fragments within the nests suggested that baby dinosuars remained in the nests to be cared for and fed by their mother. Had the Maiasaur simply hatched and wandered off to fend for themselves, the shells would be broken in a few large pieces rather than smashed into fragments. It is now accepted that these hadrosaurs nurtured and protected their young, probably feeding them by mouth like young birds until they were strong enough to leave the nests. The amazing aspect of these mesozoic reptiles were their exploitation of not only the terestrial surface but their conquest of the air by pterosaurs and their recolonization of the aquatic environment by Ichthyosaurus and Plesiosaurus. The ichthyosaurs were completely adapted to a marine life, like mammal such as dolphins are today. Fossil evidence suggested that egg-laying on land had been abandoned, and that the young were born alive and at sea. The body sahpe was completely reconverted to that of a fish; the neck telescoped to give a fusiform body shape, the limbs shortened into small steering devices. LOcomotion was performed, fish-like, by undulations of the trunk and tail; a fishlike fin was developed on the back (but like that of dolphins, it lacked the skeletal support found in dorsal fins of fishes), but the tail became a powerful swimming organ, in appearance like that of a shark. In this last regard, however, there is a notable structural difference; for whereas in a shark the end of the backbone tilts into the upper lobe of the tail fin, that of the ichthyosuar turns sharply down at the back, with the fin expanding above it. Most ichthyosaurs were presumably fish-eaters, but some feed on ammonites. The plesiosaurs were less extreme in their adaptations and probably were able to wadddle up on to a beach for egg-laying rather like marine turtles do today. They possessed a long neck or long snout or both; the body was short, broad, and relatively flat. Reversion to a truely fishlike means of locomotion was impossible, for the trunk was inflexible and the tail short; instead the limbs were developed into powerful oarlike structures, with which the creatures "rowed" its way through the sea. The pterosaurs were probably the first flying vertebrates, and evolved from an early line of thecodonts. Although pterosaurs were not ancestral to birds they did share some traits that indicate similarities in anatomy and physiology such as hollow bones. In addition, both bird and pterosaur skulls have relatively larger cerebellar and optic lobe capacities than the skulls of modern reptiles. Many of the earlier pterosaurs were small animals not even as large as crows. However, pterosaurs of the late Jurassic and Cretaceous periods grew to be the largest ever flying animals. Pteranodon had a 7 metre wingspan and a weight of ca. 17 kg, and the discovery of fossilized fish within their fossilized ribs, indicated that they must have flown great distances over water. What is difficult to explain is how they kept from crumpling their wings if they splashed into the water after prey, and even more difficult to understand how such large animals regained altitude. One suggestion is that they scooped up fish pelican-fashion and soared on ocean breezes. Even so, the lack of a stabilizing tail and the position of the wings behind the centre of gravity made them aerodynamically unstable. The rudderlike head may have provided some lateral stability, but other pterosaurs such as the largest Quetzalcoatlus (which had a wingspan of 16 metres and a weight of 65 kg) were even more unbalanced and lacked such stabilizing devices. Flight in Quetzalcoatlus has been compared to shooting an arrow backwards, even so this large beast must have had some means of contolling its flight since it evidently feed on the carcasses of other dinosaurs. The pterosaur wing was supported from an enormously extended fourth digit (finger) on the front limb. From this the wing was extended, in somewhat batlike fashion, a great wing membrane. Manipulation of a wing of this sort would appear to have been an awkward matter, and flight was originally considered to be mostly achieved by soaring rather than flapping. Further since there are no intermediate fingers extending into the wing memebrane, it was originally thought to have been very fragile. The hind legs of pterosaurs, in stark contrast to most birds, were feeble structures, and it is difficult to see how these creatures could have stood up, let alone get a running take-off as birds do today. Some recent findings have required some radical changes to our thinking on the pterosaurs. Some Soviet scientists have reported that one of the smaller pterosaurs (Sordes pilosus) had fur like mammals; implying that they were endothermic. Recent analysis of pterosaur Sandactylus (5 metre wingspan) the skin of the pterosaur wing was quite thick, with epidermal, dermal and muscle fibre layers, and therefore not just a membrane. Within the upper dermal layer were blood vessels. This antomy and arrangement of blood vessels is similar to that of a bats wing which uses its blood vessels to cool itself while flying. If the pterosaur needed to cool down, the flying must have involved energy expenditure, and therefore be active (flapping) rather than gliding flight. The lack of stiffeness in the pterosaur wing is difficult to interpret if they flapped their wings. It is, however, hypothesized that Sandactylus kept its wings at a constant tension by moving its hind legs, which were also attached to the wing. The implications of these findings is that pterosaurs had more control over their flight than scientists had previously thought, and that their flight was not limited to passive gliding. These pterosaurs were obviously fascinating animals which dominated the skies for 100 million years, unfortunately they left no descendants for us to study. Although the fossils of dinosaurs during the entire mesozoic era suggest a high diversity of organisms adapted to a variety of habitats, the reason for their final wholesale extinction some 65 million years ago is not completely resolved. However, this extinction does correlate with a thin band of iridium-enriched clay that marks the boundary between Cretaceous and Tertiary periods (nicknamed the K-T boundary). Because iridium is rare on earth, but common in meteorites, it was proposed that the earth was hit by an asteroid 10 km in size. More recently proof of such a meteorite has been found in the Gulf of Mexico (off the continental shelf of the Yucatan Peninsula). This impact site has formed the Chicxulub crater. To have formed this crater the meteorite would have needed to be at least 10 km in diameter. The impact of such a meteorite would have caused massive impact earthquakes, perhaps hundreds of times greater than the largest measured earthquake. Massive tsunami waves (tidal waves) would have radiated out. When such a meteorite struck the earth, dust would have blanketed the globe, darkness would have occurred for one to three months and land temperatures would have plummeted. Since the meteorite very likely hit the sea, the water vapour could have created a greenhouse effect, making the short-term climate exceptionally hot, although in the long-term the temperature declined. Hot nitric acid would have rained out of the atmosphere and threatened many organisms with death, particularly those possessing shells. Recent evidence of large amounts of soot in the K-T sediments suggest that large-scale fires accompanied such a catastrophe (as much as 90% of the world's forests may have burned). Such events would have had a profound effect on the ecosystems of the world. One theory suggests that mammals, which were on the brink of a great radiation during the Cenozoic, may have been predators of dinosaur eggs, or in some other way outcompeted the dinosaurs for resources. At this time mammals were only represented by shrew-like creatures, a few centimetres in size. Numerous, but tiny cone-shaped teeth from these mammals were found together with the gigantic fossilized bones of the great dinosaurs. In the fossil records of the Montana Badlands there is a black marker of coal and some excellently preserved fossilized tree stumps. Below this marker was the last of the cycad and tree fern forest, but the tree stumps represent the coniferous redwoods (Sequoia). These later plants prefer a much cooler climate than the cycads and tree ferns. Although a large body can retain heat more efficiently, if it becomes cooled, it becomes increasingly more difficult to gain heat. In contrast very small animals can find micro-habitats that reduce exposure to unfavourable conditions and can more quickly warm their bodies up during favourable conditions. Aquatic animals also have a greater buffer against temperature since water maintains heat more efficiently than land. Consequently the three main types of reptiles that endured the late Cretaceous extinction were lizards, tortoises and turtles and crocodiles, all either small-sized or aquatic animals. Crocodiles (Order Crocodilia) are the largest living reptiles and possibly the most advanced, having a nearly complete four-chambered heart. The nostrils are at the end of the snout and the eyes protrude from the head so that these animals can float near the surface of water with only these parts exposed above the water. It was possibly these features that allowed them to survive the sudden global cooling that almost definitely occurred at the end of the Cretaceous period. Under hot conditions crocodiles open their mouths and air passes over the soft skin on the inside of the mouth and cools the animal down. The crocodile eyes are unusual in that the photo- pigments receptive to light are different in the upper and lower hemispheres of each retina. The upper retinal hemisphere which looks down into the water has a photopigment similar to that of freshwater animals (porphyropsin), whereas the ventral retinal hemisphere has the pigment of terrestrial animals (rhodopsin). The skin is thick and covered with horny epidermal scales and dorsal bony plates (osteoderms) which may extend to the ventral surface and are like those in turtle shells. The social lives of crocodiles is complex. Male Nile crocodiles establish and defend breeding territories adjacent to the water and courtship occurs in the water. As the females approach; the males roar with such intensity that their flanks vibrate throwing up clouds of spray from the water, and their jaws clap furiously. Mating lasts for a few minutes with the male clasping the female. Their jaws and tails become intertwined during copulation. The female excavates a hole in the bank close to the waterline and lays about forty eggs in several batches. She ensures that the eggs are buried so that temperature remains relatively constant to within 3oC. Saltwater crocodiles build mounds of vegetation as a nest and sprays urine to cool it if it becomes to hot. The alligators occurring in the New World piles up rotting vegetation into a nest which is regularly turned over in order to provide the eggs with appropriate temperature and moisture conditions. Just before hatching the female Nile Crocodile waits and when she hears the pipping calls of the hatching babies she will scrap the earth away and will pick her young up and put them into a pouch at the bottom of her mouth and will transfer them to the water. The male will escort these baby crocodiles to a nursery area where they will remain for the next few months with the parents closely guarding them. The Crocodiles and its allies invest considerable parental care in the rearing of its young after their hatching. Many dinosaurs were also thought to invest in considerable parental care, since they built fairly elaborate nests out of mud which would have retained the young dinosaurs until they were large enough to climb over the perimeter of the nest edge. Tortoises, terrapins and turtles belong to the Order Chelonia and have an ancestry that is even older than the crocodiles. The strengthened bony plates occurring in crocodiles (ossicles) have in tortoises become modified to form a continuous dorsal carapace and a ventral plastron. This represents the most effective armour developed by any vertebrate group, and this pattern has changed very little since it first evolved. The turtles reverted to an aquatic life style where the heavy shell was less of an impediment to locomotion. However, the shelled egg, an essential adaptation to terrestrial life, did become an impediment since the membrane beneath the shell by which the embryo breathes through the shell pores functions by gaseous exchange and cannot work in water. Consequently turtles come on to beaches to lay their eggs in a terrestrial environment. However, when the young turtles hatch they have a perilous journey from where they hatched (above spring high tide) to the sea, and many succumb to predation. The third group of reptilian survivors are the lizards (Order Squamata) and are very much more numerous (3000 species) and have many more modifications arising from their ancestral stock than either of the other surviving reptile groups. Snakes are essentially highly specialized lizards that have elongated bodies through increasing the number of vertebrae and have lost their limbs and even have a reduction of the left lung. Lizards belong to the suborder Sauria and includes geckos (Family Gekkonidae), iguanas (Iguanidae), chameleons (Chamaeleonidae), skinks (Scincidae), worm lizards (Amphisbaeridae) and monitor lizards (Varanidae). They have all enhanced their water-tight integument with the development of scales, which have become highly modified. The Australian shingleback skink (Trachysaurus) has stout polished scales, the Gila monster (Neloderma) has round pink and black ones (and has additional protection by being venomous) and the horned lizards occurring in arid areas have enlarged them into spiny appendages which are scored with fine grooves which allow dew to condense on them and be collected in the mouth. Spines in the chameleons have also become horned with one to four occurring in the head region. The scales on the underside of the toes of geckoes have become highly modified with numerous microscopic hairs (lamellae) which enable them to climb smooth surfaces (including glass) with relative ease by each hair engaging on the smallest irregularity of the surface. Many lizard families have members with reduced limbs that may even be lost altogether and parallels the amphibian groups Gymnophiona and Caudata. Skinks show a progression of limb reduction. The snake lizards of South Africa (Family Pygopodidae), even within their single genus, have some species with a complete complement of functional legs each with five toes; another species possesses very small limbs, with only two fully developed toes on each foot and a third species has hind legs with a single toe and no externally visible front limbs. A hundred million years ago limb reduction occurred among ancient lizards and resulted in the evolution of snakes (Suborder Serpentes) of which about 2300 species live today. They differ from lizards in the following respects: (1) the right and left halves of the lower jaw are not firmly united, instead they are connected by an elastic ligament; (2) there is no pectoral girdle; (3) a urinary bladder is absent; (4) the braincase is closed anteriorly; (5) the eyelids are fused over the eyes but a transparent window exists which allows the snake to see; and (6) no external ear openings exist. These adaptations and loss of structures suggest that the snake's ancestors had previously adopted a burrowing existence, and their surface dwelling is secondary. The loss of legs for locomotion on the surface has been overcome with the development of flank muscles that flex in alternate bands so that their body is drawn up in a series of S-shaped curves. As the contractions travel in waves down the body the flanks are pressed against obstacles on the ground such as stones and the snake is able to push itself forward. When snakes hunt they are able to creep up on their prey without oscillating its body. The scales on the underside are shaped like narrow rectangles running across the width of the body and overlapping one another with their free edges to the rear. The snake is able to hitch these scales up and forward in groups by contracting its belly muscles. The back edges catch the ground and as the contractions pass downwards in waves, the snake advances smoothly and silently with no lateral movement. Snakes are predators with prey being seized with their mouths. In boas and pythons they swiftly coil themselves around the body of the prey and suffocate it. With the backward pointing teeth the snake engages onto the prey and the snake draws it into the mouth by using the loosely connected lower jaw. Other snakes deliver venom via specially modified teeth to kill the prey before ingesting it. In back-fanged snakes a poison gland lies above the teeth and the venom trickles down a groove in the tooth. The snake therefore has to drive its fangs deep into the prey before it is able to deliver its venom. Other snakes have their fangs placed in the front of the upper jaw and have an enclosed canal through which the venom is delivered. Cobras (Naja) and mambas (Dendroaspis) have short immobile fangs which inject the venom, whereas vipers have long fangs which are kept hinged back and are rotated forward when it attacks it prey. Still other snakes spit poison into the eyes of it prey. Possibly the most advanced snakes are the pit vipers (Family Cotalidae) which include the rattle snakes (Crotalus) of the southwestern regions of the United States. These animal invest heavily in parental care and like some amphibians retain their eggs inside their body. The shell is reduced to a thin membrane so that the embryos, as they lie inside the oviduct, not only feed on their yolk but draw sustenance from their mother's blood diffusing from the walls of the oviduct pressed against them. Such a system for nourishing of its young is functionally analogous to the placenta used by mammals. The mother snake will also safeguard her young after they have hatched, warning intruders with sound of the vibrating rattle at the end of the tail. Each time a rattle snake sheds its skin a special, hollow scale remains and accumulates at the end of their tail. Up to twenty scales may accumulate. Rattle snakes are nocturnal hunters and use a pit located between the nostril and the eye to detect infra-red radiation. The detection of heat given by a small mammal is also directional, and therefore it is able to attack its prey even in pitch darkness. Being ectothermic, food requirements are, however, small and therefore less time is spent foraging than the equivalent sized endothermic mammal. This ensures their success even in the most inhospitably dry regions of the world. Download 2.01 Mb. Share with your friends:
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