Reptiles - Order Squamata PDF

Order Squamata These are diapsid reptiles (together with crocodilians and tuataras) and have two pairs of temporal skull openings. This group (containing snakes, lizards and worm lizards) is the most recently evolved and diverse reptiles, representing approx 95% of modern reptiles. Lizards appeared...

Order Squamata These are diapsid reptiles (together with crocodilians and tuataras) and have two pairs of temporal skull openings. This group (containing snakes, lizards and worm lizards) is the most recently evolved and diverse reptiles, representing approx 95% of modern reptiles. Lizards appeared as early as the Jurassic, but they remained a small insignificant group until the cretaceous period of the Mesozoic (when the dinosaurs were at the height of their dominance) before they started to radiate. Snakes appeared during the late Jurassic, probably from a group of lizards whose modern descendents included the monitor lizards and Gila monsters. Two particular specialisations characterise snakes. Firstly, there is the extreme body elongation and the accompanying rearrangement of internal organs, and secondly, they have specialisations for consuming large prey. Amphisbaenians (worm lizards), first appeared in the early Cenozoic, and have structural specialisms associated with burrowing. Live births (viviparity) in living reptiles is limited to the summates, and is thought to have evolved at least 100 different times. Evolution of viviparity is usually associated with cold climates, occurring by increasing the length of time eggs are kept within the oviduct. Developing young respire through extra embryonic membranes and obtain nutrition through the yolk sack (lecithotrophy) or via the mother (placentotrophy) of a combination of each. Most lizards and snakes have skulls modified from the ancestral diapsid form. This involves having a jointed skull, which together with jaw modifications allows the snout to be tilted upwards, allowing the animals to seize and manipulate prey, and increases the effective closing force. Suborder Sauria: lizards This is a very diverse group, having terrestrial, burrowing, aquatic, arboreal and aerial species. Included in this group, are Geckos (small, agile, mostly nocturnal forms with adhesive toe pads helpful in their arboreal habits), iguanids (most familiar new world lizards, usually brightly coloured with ornamental frills and crests, includes the Galapagos island marine iguana), skinks (elongated bodies and reduced limbs), and chameleons (arboreal lizards, which catch insects with a long sticky tongue). Most lizards have moveable eyelids (snakes eyes are covered with a permanent transparent cap), keen daylight vision, and an external ear, which snakes lack. The inner lizard ear is variable in structure, but hearing does not play a major part in the animal's senses. Many lizards live in arid, hot regions and water loss is reduced by their lack of skin glands and so they produce semi solid crystalline urine. Gila monsters and their relatives, beaded lizards, are the only lizards to have a venomous bite. Lizards, like most reptiles, are ectotherms, adjusting their body temperature by moving amongst different microclimates. As ectotherms require considerably less energy than endotherms, reptiles are successful in ecosystems with low productivity and warm climates such as tropical desserts and grasslands. Thus, ectothermy is not an 'inferior' characteristic of reptiles, rather a successful strategy for coping with specific environmental challenges. Suborder Amphisbaenia: worm lizards A highly specialised and poorly understood group due to their burrowing lifestyle and general rarity. Only one species exists in the United States, with most of them prevailing in Africa and South America. Little is known of them outside of their anatomy, and even that is difficult to study due to the mechanics of dissecting something so small. Most species are less than six inches long. The head is stout, not set off from the neck, and either rounded, sloped, or sloped with a ridge down the middle. Most of the skull is solid bone, having a distinctive single median tooth in the upper jaw. Eyes are deeply recessed and covered with skin and scales. The body is elongated, and the tail truncates in a manner that vaguely resembles the head. Their name is derived from Amphisbaena, a mythical serpent with a head at each end. The Mexican mole lizard is unusual in having a pair of forelimbs, but all limbless species have some remnants of the pelvic and pectoral girdles embedded within the body musculature. The skin of amphisbaenians is only loosely attached to the body, and they move using an accordion-like motion, in which the skin moves and the body seemingly just drags along behind it. Uniquely, they are also able to perform this motion in reverse, just as effectively. Suborder Serpentes: snakes Snakes are limbless and usually lack both pectoral land pelvic girdles. The numerous vertebrae of snakes (shorter and wider than those of tetrapods) allow quick lateral undulations over rough terrain. Ribs increase the rigidity of the vertebral column, providing more resistance to lateral stresses, and elevation of the neural spine gives the numerous muscles more leverage. Phylogeny of snakes is poorly known as snake skeletons are typically small and fragile, making fossilisation uncommon. However, 150 million year old specimens readily definable as snakes with lizard-like skeletal structures are known. Evidence suggests that snakes may have evolved from burrowing lizards during the Cretaceous Period. An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum. A modern analogy of these ancestors is the Monitor Lizard Lanthanotus of Borneo. As these ancestors became subterranean, they lost their limbs and their bodies streamlined for burrowing. According to this hypothesis, features such as the transparent fused eyelids and loss of external ears evolved to combat subterranean conditions such as scratched corneas and dirt in ears. An alternative hypothesis, based on morphology, suggests that the ancestors of snakes were related to mosasaurs (extinct aquatic reptiles from the Cretaceous), which in turn are thought to have derived from lizards. Under this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), while external ears were lost through disuse in an aquatic environment. Fossil snake remains are known from early late Cretaceous marine sediments, which is consistent with this hypothesis, particularly as they are older than the terrestrial examples. The great diversity of modern snakes appeared in the Palaeocene, correlating with the adaptive radiation of mammals following the extinction of the dinosaurs. There are more than 2900 known species of snakes ranging as far northward as the Arctic Circle in Scandinavia and southward through Australia and Tasmania. Snakes can be found on every continent with the exception of Antarctica, dwelling in the sea and as high as 16,000ft (4900m) in the Himalayan Mountains of Asia. There are numerous islands from which snakes are conspicuously absent such as Ireland, Iceland, and New Zealand. Many lineages of lizards and amphisbaenians exhibit reduction and loss of limbs, but none have radiated to the extent of snakes. The increased mobility of the skull to allow ingestion of large prey may be the reason for their success. Unlike lizard jaws, the two halves of the lower jaw (mandible) are joined only by muscles and skin, allowing them to spread wide apart. As the animal must be able to continue to breathe, during the sometimes lengthy process of swallowing, its tracheal opening (glottis) is thrust forward between the two mandibles. For most snakes, chemical reception is the primary sense (they have reduced hearing and poor vision). In addition to the usual olfactory areas of the nose, which are not well developed, there are vomeronasal organs (Jacobson organs), a pair of pit-like organs in the roof of the mouth. The forked tongue picks up scent molecules from the air and conveys them to the mouth, drawing the tongue over the richly innervated chemosensory epithelium in the pits. Snakes usually move by lateral undulation, with movement following an 'S-shaped' path, the snake propels itself by exerting lateral force against surfaces. Occasionally, concertina movement enables a snake to move in a narrow passage, as when climbing a tree. To advance in a straight line, many heavy bodied snakes use rectilinear movement whilst two or three sections of the body rest on the ground to support the snake's weight, intervening sections lift free from the ground and are pulled forward by muscles. Side winding is a fourth movement method enabling dessert vipers to move across loose sandy surfaces with minimum contact with the hot substratum. The snake\'s three-chambered heart is encased in a sac, called the pericardium, located at the bifurcation of the bronchi. The heart is able to move around, due to the lack of a diaphragm protecting it from potential damage when large ingested prey is passed through the oesophagus. The cardiovascular system of snakes is also unique due to the presence of a renal portal system in which the blood from the snake\'s tail passes through the kidneys before returning to the heart. The vestigial left lung is often small or even absent, as snakes\' tubular bodies require all of their organs to be long and thin. In the majority of species, only one lung is functional. This lung contains a vascularised anterior portion and a posterior portion, which does not function in gas exchange. This \'saccular lung\' is used for hydrostatic purposes to adjust buoyancy in some aquatic snakes, but its function remains unknown in terrestrial species. Many organs that are paired such as kidneys or reproductive organs are staggered within the body with one located ahead of the other. Order Sphenodonta: tuataras The tuatara is considered the most unspecialised living amniote; the brain and mode of locomotion resemble that of amphibians and the heart is more primitive than other reptiles. The two modern species of Tuatara are greenish brown, up to 80cm in length with a spiny crest along the back, more pronounced in males. Their unique dentition consists of two rows of teeth in the upper jaw overlapping one row on the lower jaw. They also have a pronounced parietal eye, dubbed the 'third eye', its function is uncertain. They are able to hear although no external ear is present, and they have a number of unique features in their skeleton, retained from fish. Tuataras, like many of New Zealand's native animals, are threatened by habitat loss and introduced non-native species such as the rat. Modern Tuatara has been referred to as living fossils, remaining mostly unchanged throughout their entire history (approx 220 million years ago). Order crocodilia Modern crocodilians are the only living-surviving reptiles of the archosaurian lineages that gave rise to the dinosaurs and birds. Modern crocodilians represent a lineage that began its radiation in the late Cretaceous, differing little from primitive crocodilians of the early Mesozoic. All species have an elongated, robust and reinforced skull and massive jaw musculature arranged to provide a wide gape and rapid powerful closure. Teeth are set in sockets and are similar to all archosaurs and the earliest birds. They have the avian and mammalian trait of a four-chambered heart. Crocodiles and alligators are distinguished on the basis of head morphology, with crocodiles having a narrower snout and a visibly lower tooth when the mouth is closed. Temperature influences the sex of unborn hatchlings, with low temperatures producing females (opposite to turtles). Crocodilians have the most complex social behaviour of any modern reptile, exhibiting significant parental care. Introduction to class: aves With over 9000 species, birds outnumber all vertebrate groups apart from fishes. Birds are found all over the earth from poles to tropics, forests to deserts, mountains to prairies and on all oceans. Some species even live in total darkness in caves, navigating by echo location. Others dive up to 45m to prey on aquatic life. In terms of size, they range from the tiny 1.8g bee hummingbird to the Ostrich weighing up to 135kg. Their single unique feature is their flight feathers. Some flightless theropod dinosaurs possessed feathers, although none of them ever supported flight. Birds show a remarkably uniform morphology. All birds have modified forelimbs (wings), all have hind limbs adapted for walking/perching/swimming, all have keratinised beaks, a high metabolic rate, a four-chambered heart, and a lightweight, but strong skeleton and they lay shelled eggs. A bird's entire anatomy is evolved for flight; so much of this section will deal with this aspect, rather than details of individual bird groups. Origin of birds There is substantial evidence to suggest that birds are theropod dinosaurs. The oldest known bird, the Late Jurassic Archaeopteryx, is one of the first transitional fossils to be found in support of evolution in the late 19th century, although it is not now considered a direct ancestor of modern birds. Although ornithischian dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard hipped) dinosaurs, and evolved their hip structure independently. Birds diversified into a wide variety of forms during the Cretaceous Period. The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or 'opposite birds', so- named because the construction of their shoulder bones was in reverse to that of modern birds. They occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed eaters. More advanced lineages also specialised in eating fish, like the superficially gull-like subclass of Ichthyornithes ('fish birds'). One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. Despite their extreme specialisations, the Hesperornithiformes represent some of the closest relatives of modern birds. Containing all modern birds, the subclass Neornithes is now known to have evolved into some basic lineages by the end of the Cretaceous and is split into two super orders, the Paleognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites (large flightless birds). The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans) and the Galliformes (the pheasants, grouse, and their allies). It is agreed that the Neornithes evolved in the Cretaceous, and that the split between the Galloanseri from other Neognathes occurred before the tertiary extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs. The classification of birds is a contentious issue, scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders. Characteristics of birds Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight. Feathers are a feature unique to birds, facilitating flight, providing insulation, and are used in display, camouflage, and signalling. There are several types of feathers, each serving its own purpose. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae. The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex. Parts of a feather: 1. Vane 2. Rachis 3. Barb 4. Afterfeather 5. Hollow shaft, calamus There are two basic types of feather: vaned feathers, which cover the exterior of the body, and down feathers, which are underneath the vaned feathers. The pennaceous feathers are vaned feathers. Also called contour feathers, pennaceous feathers are distributed over the whole body. Some of them are modified into remiges, the flight feathers of the wing, and rectrices, the flight feathers of the tail. A typical vaned feather features a main shaft, called the rachis. Fused to the rachis are a series of branches, or barbs; the barbs themselves are also branched and form the barbules. These barbules have minute hooks called barbicels for cross-attachment. Down feathers are fluffy, because they lack barbicels, so the barbules float free of each other, allowing the down to trap much air and provide excellent thermal insulation. At the base of the feather, the rachis expands to form the hollow tubular calamus, or quill, which inserts into a follicle in the skin. Moulting is annual in most species, although some may have two moults a year. Large birds of prey may moult only once every few years. Moulting patterns vary across species. Some drop and re-grow wing flight feathers, starting sequentially from the outermost feathers and progressing inwards (centripetal, pheasants), whilst others replace feathers starting from the innermost ones (centrifugal, woodpeckers). A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless. Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. The skin there is well supplied with blood vessels and this helps with incubation. Feathers require maintenance and birds preen daily, taking an average of 9% of their waking time. The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland at the base of the tail, which protect the feathers\' flexibility and act as an antimicrobial agent. The skeleton consists of very lightweight bones, which have large air-filled cavities (called pneumatic cavities) often connected to the respiratory system. The ribs are flattened and the sternum is keeled for the attachment of the pectoralis flight muscles (except flightless birds), which account for 15% of the total mass of the bird. The nervous system is large, relative to the bird\'s size. The most developed part of the brain is the one that controls the flight-related functions, and the cerebellum coordinates movement. The cerebrum controls behaviour patterns, navigation, mating, and nest building. Most birds have a poor sense of smell with the exception of kiwis and vultures. The avian visual system is usually highly developed. They are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones. This allows for the perception of ultraviolet light, which is involved in courtship, sexual and species recognition, but is not visible to the human eye. Ultraviolet vision is also used for foraging (kestrels detect UV reflective urine trails left by rodents). The avian ear lacks external pinnae, but is covered by feathers, and can be highly developed. Birds have one of the most complex respiratory and circulatory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac, which extends from the lungs, and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When exhaling, the used air flows out of the lungs and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, the lungs receive constant supplies of fresh air during inhalation and exhalation. The heart has four chambers, the right aortic arch gives rise to systemic circulation (in mammals the left arch is involved). Unlike in mammals, avian red blood cells have nuclei. Flight, migration and navigation Flight is the primary means of locomotion for most bird species and is used for breeding, feeding, and predator avoidance and escape. Wing shape and size generally determine a bird species\' type of flight; many birds combining powered, flapping flight with less energy-intensive soaring flight. About 60 modern bird species are flightless, as were many extinct birds. Flightlessness often arises in birds on isolated islands, probably due to limited resources and absence of land predators. Many bird species migrate, taking advantage of global differences in seasonal temperatures, therefore optimising availability of food and breeding habitat. Many species undertake annual long distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the temperate or Arctic/Antarctic regions and a non-breeding season in the tropical regions or opposite hemisphere. Before migration, birds substantially increase body fat and reserves, and reduce the size of some of their organs. Landbirds have a flight range of around 2500km and shorebirds can fly up to 4000km, Seabirds also undertake long annual migrations, some seabirds disperse after breeding, travelling widely but having no set migration route. The Bar tailed Godwit has the longest known non-stop migration of any species, up to 10,200km. Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, involving only a fraction of the population, usually females and subdominant males. Altitudinal migration occurs when birds spend the breeding season at higher altitude elevations, moving to lower ones during suboptimal conditions. Some species are neither territorial nor migratory, but are nomadic, moving according to weather and food availability. Birds navigate during migration using a variety of methods. Diurnal migrants use the sun day and a stellar compass at night. Birds using the sun compensate for its changing position during the day by the use of an internal clock. Orientation with the stellar compass depends on the position of the constellations surrounding Polaris. Some species, additionally, have the ability to sense the Earth\'s geomagnetism through specialised photoreceptors. Food, feeding and digestion Birds\' diets can include; nectar, fruit, plants, seeds, carrion, and various small animals, including other birds. As birds have no teeth, their digestive system is adapted to processing unmasticated food items, swallowed whole. Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists; others that concentrate on specific food items or have a single strategy to obtain food are considered specialists. Nectar feeders have specially adapted brushy tongues and, in many cases, bills designed to fit co-adapted flowers. Kiwis and shorebirds with long bills probe for invertebrates, with individual species bill lengths and feeding methods resulting in the separation of ecological niches. Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as solids, kingfishers and terns plunge dive after their prey. Flamingos, three species of prion, and some ducks are filter feeders. Geese and dabbling ducks are primarily grazers. Some species, including gulls, and skuas, engage in kleptoparasitism, stealing food items from other birds, although this method is thought to supplement direct hunting. Other birds are scavengers; some, like vultures, are specialised carrion eaters, whilst others, like gulls and some birds of prey, are opportunists. The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food. Most birds are highly adapted for rapid digestion to aid with flight. Some migratory birds have the additional ability to reduce parts of the intestines prior to migration. Like reptiles, birds are primarily uricotelic (kidneys extract nitrogenous wastes from the bloodstream, excreting semi solid uric acid). This material, as well as the output of the intestines, emerges from the bird\'s cloaca. The cloaca is a multi-purpose opening, combining waste and reproductive functions. Social behaviour and reproduction Birds have two sexes: male and female. A bird's sex is determined by Z and W sex chromosomes, rather than the X and Y-chromosomes seen in mammals. Males carry two Z chromosomes (ZZ), and females carry a W chromosome and a Z chromosome (WZ). In nearly all species, sex is determined at fertilisation. However, temperature-dependent sex determination occurs in Australian brush-turkeys, with higher temperatures during incubation resulting in a higher female-to-male sex ratio. At hatching, chicks range in development from helpless, small, blind, immobile and naked, (termed altricial), to independent, mobile and feathered, (termed precocial) depending on species. Altricial chicks need help thermoregulating and must be brooded for longer than precocial chicks. The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in megapodes ends at hatching; the newly hatched chick digs itself out of the nest mound without parental assistance, fending for itself immediately. At the other extreme, many seabirds have extended periods of parental care, the longest being that of the great frigate bird, whose chicks take up to six months to fledge and are fed by the parents for up to an additional 14 months. In some species, both parents care for nestlings and fledglings; in others, such care is the responsibility of one sex. In some species, other members of the same species (usually close relatives, e.g. previous offspring) will help with the raising of the young (termed alloparenting). In birds, male parental care is more common than in any other vertebrate class. In most species, chicks leave the nest just before, or soon after, they are able to fly. The amount of parental care after fledging varies; albatross chicks leave the nest on their own and receive no further help, whilst other species continue some supplementary feeding after fledging. Chicks may also follow their parents during their first migration. Modern bird groups Subclass Neornithes Paleognathae: Struthioniformes: (ostriches), Rheiformes (rheas), Apterygiformes (Kiwis) Tinamiformes---(tinamous) Neognathae: Anseriformes (waterfowl), Galliformes(fowl), Gaviiformes (loons), Podicipediformes (grebes), Procellariiformes (albatrosses, petrels, and allies) Sphenisciformes (penguins) Pelecaniformes (pelicans and allies), Ciconiiformes(storks and allies), Phoenicopteriformes (flamingos) Falconiformes(falcons, eagles, hawks and allies), Gruiformes(cranes and allies) Charadriiformes (gulls, button-quails, plovers and allies) Pteroclidiformes (sand grouse) Columbiformes (doves and pigeons) Psittaciformes (parrots and allies), Cuculiformes (cuckoos, roadrunners), Strigiformes (owls), Caprimulgiformes (nightjars and allies) Apodiformes (swifts and hummingbirds), Coraciiformes (kingfishers and allies) Piciformes (woodpeckers and allies), Trogoniformes (trogons) Coliiformes (mousebirds) Passeriformes (passerines)