Animal Diversity: Chordata

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The complex system of tetrapod limb muscles is arranged in two series that are derived from the simpler musculature of the upper and lower aspects of fins. A comparison of the bony and skeletal structures of crossopterygians and early amphibians shows that their limbs are very closely allied. Ancient tetrapods, the Labyrinthodonts retained bony scales in the abdominal region. Grooves in the skull of some juveniles carried the lateral line system, which however was absent in the adults of the same species. Thus, many ancient tetrapods, like modern amphibians, were probably aquatic as juveniles and terrestrial as adults. These labyrinthodonts / stegocephalia originated no later than the Upper Devonian. Their important features include: 1) the loss of bones rigidly linking the skull to the shoulder girdle. This was accompanied by the appearance of a mobile neck allowing the head to be moved relative to the trunk. 2) The operculum was lost, as it was no longer needed in these early choanates since they had lost the internal gills of their early ancestors. 3) A reduction of the notochord and a rigid spine. The thick centra constricted the notochord. Special articulatory surfaces called as zygapophyses linked the neural arches to each other. Also the notochord was shorter and did not extend into the braincase. A sacral rib connected the axial skeleton to the pelvic girdle, allowing the weight of the tetrapod body to be transmitted to the hind limb. The dermal fin rays that were no longer needed on land were lost. 4) There was the development of four muscular limbs with discreet digits. The conquest of land also depended on some means of aerial respiration. The primitive lung is a characteristic of ancient fishes; it came first and from it evolved the swim bladder, an organ of specialized hydrostatic and other functions found only in bony fishes. In the Devonian, the development of a respiratory sac, capable of absorbing atmospheric oxygen, would be of immense help to early fishes that were compelled to live in water that became periodically low on oxygen level and clogged with rotting vegetation. The amphibians suffered a loss of true biting teeth. They were forced by virtue of their imperfect adaptation, to confine their feeding to slow moving prey and later to insects that could be reached with a sudden flick of the muscular, sticky and protrusible tongue that they later developed. The lateral line system, although retained in the larval forms, was soon lost in the land-living adults. Terrestrial life depended not only on the development of efficient lungs and walking legs, but every part of the animal body was involved. Not only did animals require to breathe atmospheric air and to walk; they had to withstand desiccation, rid themselves of the lateral line system, detect air-borne substances of a much greater dilution, see and hear predators and prey at much greater distances. However, no new organs were formed. The Devonian was an age of great climatic instability. The fresh water streams and lagoons of that time were alternately filled and dried out. Such conditions of periodic desiccation would have resulted in the extinction of numerous species, and resulted in the survival of those possessing the physiological and structural adaptations suitable to function in the new conditions. Ancient bony freshwater fishes had developed a bony supporting skeleton, osmotic homeostasis, internal nares, and lungs. In the crossopterygians, the appearance of two pairs of highly mobile, muscular, lobe-like lateral fins supported by bones, gave an indication of later development of walking legs of the modern tetrapods. This is a classic example of Pre-Adaptation: the possession by an organism of characters that are conducive to its survival under altered conditions. Thus those types that had locomotory, respiratory, integumentary, excretory and sensory specializations related to drought survival, would prosper and reproduce. Those whose adaptations were directed towards purely aquatic efficiency would fail. A comparison of the earliest Amphibia with Palaeozoic fishes shows many similarities between the embolomerous labyrinthodonts and the osteolepids of the 20
Devonian. This is particularly marked in the general structure of the skulls, in the similar labyrinthodont pattern of the teeth, in the possession of the large palatal tusks, and in the structures of the girdles. The amphibian skin became heavily keratinized. This assisted water retention, but at the same time allowed cutaneous respiration. Yet, amphibians in general cannot live away from moist situations. The permeability of the amphibian skin has thus imposed serious limitations on the choice of habitat as well as upon geographical distribution. Secondly, the inevitable failure of the amphibia to develop temperature control further limits their chances of land colonization. From the time of their emergence they have remained imperfectly adapted to terrestrial life: most land-going species remained dependent upon fresh water for reproduction. Majority of amphibians require a damp environment in which to breed because their eggs and embryos must extract oxygen and food from the surrounding water and at the same time excrete waste material directly into it. They have developed no protective shell, as have reptiles, birds and primitive mammals. They also lay down little yolk for the nourishment of the growing young. The earliest known anuran ancestor, Protobatrachus, did not appear until the early Mesozoic. Although it had a tail in the adult, but the skull was similar to those of modern frogs. The ribs were short, the presacral vertebrae were reduced in number and there were free caudal vertebrae. There was an elongate ilium and a fairly long femur. However, the radius and ulna, and the tibia and fibula were separate. The earliest true frog remains occur in the Upper Jurassic. The earliest known salientia is Triadobatrachus massinoti, from early Triassic. This “proto-frog” is around 250 million years old and does not have the combination of characters normally associated with frogs. The earliest true frog is Vieraella herbsti from early Jurassic, while the fossil from middle Jurassic is Notobatrachus degiustoi, which is around 155-170 million years old. Another well-preserved Jurassic fossil known as Eodiscoglossus santonje, has 8 pre-sacral vertebrae and thus is clearly within Anura. The Urodeles, too, have not been found before the Jurassic period. No fossil Apoda has been discovered. Today, the most terrestrial amphibia are the Anura. The Urodela have retreated once more to the water and the degenerate Apoda into moist holes on the ground. Although amphibians were the dominant land fauna in the Carboniferous, little more than 2000 species live today, although some remain plentiful in appropriate areas. Of the three surviving amphibian groups, the Anura are abundant in all the greater zoogeographical regions but are absent from most oceanic islands. The Urodela are almost exclusively Palaearctic and Nearctic forms, occurring in North America, Europe, Asia, and North Africa. A few species extend southwards into the Neotropical and Oriental regions. The Apoda, on the other hand, are mainly tropical, occurring in the Neotropical, Ethiopian and Oriental regions. They are absent from Madagascar, Australasia, and the Pacific Islands. IV. AMPHIBIA: GENERAL ORGANIZATION a. External Appearance- In the Anura, the head is large and depressed with a wide mouth and large tympanic membranes in most genera. The eyes are large and prominent, provided with an upper eyelid and a nictitating membrane. The trunk is short, the tail absent and the cloacal aperture is terminal. The hind limbs are much longer than the forelimbs (Figure 3a). The forelimb consists of an upper arm or brachium, a forearm or antebrachium, and a hand or manus, ending in four tapering digits. The hind limb consists of the thigh or femur, the shank or crus, and the foot, which consists of a tarsal region and five slender digits. The arboreal forms have plate-like adhesive discs at the termination of the digits of all four legs. The discs hold on to the substratum with the help of capillary action and adhesion by means of 21
Page 1 and 2: List Of Contents I. Introduction An
Page 3 and 4: Tetrapoda The class Amphibia is com
Page 5 and 6: II. CLASSIFICATION Basal tetrapods
Page 7 and 8: Microsaurs represent a diverse grou
Page 9 and 10: espiration. Amphiuma is an elongate
Page 11 and 12: latter living on land have a warty
Page 13 and 14: Family Character BRACHYCEPHALIDAE V
Page 15 and 16: Family Character Rhacophoridae Very
Page 17 and 18: mineralized tissue in their mesoder
Page 19: available led to an extensive remod
Page 23 and 24: The median fin is completely lost a
Page 25 and 26: special development for water absor
Page 28 and 29: Respiratory movements involve the a
Page 30 and 31: heart they are separated by a peric
Page 32 and 33: capillaries. The ventral branch cal
Page 34 and 35: Skull of frog (Figure 12) 1. Skull
Page 36 and 37: Typical second vertebra 1. In frog
Page 38 and 39: 10. The sternum lies in the mid-ven
Page 40 and 41: Hind-limbs bones of frog (Figure 17
Page 42 and 43: diencephalon with the medulla oblon
Page 44 and 45: tympanic membrane or tympanum. The
Page 46 and 47: kidney with peritoneal connections
Page 48 and 49: desert-dwelling frogs like Cycloran
Page 50 and 51: the ova as they pass down the ovidu
Page 52 and 53: C. Eggs carried by the parents- I.
Page 54: 4. Walter, H.E. and Sayles, L.P. Bi

Animal Diversity: Chordata

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