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ORIGIN OF TETRAPODA, , , , , , The tetrapods (Gr., tetrapods, “four-footed”), refers to four(eres. The most important achievement by the vertebrate, invasion of land, )The change from a body plan for breathing and, ‘ating it water to a body plan enabling the animal to move on land, navigi of the most profound evolutionary changes known. The first, , e of : :, srapod t© conquer the land are seen in the sediments of Scotland and, tel, , Greenland as fossils. |, Tetrapods evolved from a group of animals known as the Tetrapodomorpha, who evolved, ‘ent Sarcopterygii lobe-finned fishes around 390 million years ago in the middle Devonian, , from anc’ Seat :, res) is also becoming increasingly ch fms as a result of more transitional fossil, , jibe, js the, , , , findivigs and improved phylogenetic analysis, Their forms were transitional between lobe finned, , fishes and the four lim! he first tetrapod are the amphibians and that amphibi, , aquatic in their larval stages adapted to water life due to the presence of gills) The gills of the, hen analyzed showed similarities with fishes. Under these Circumstances it was, , amphibian larva w!, believed that the aquatic adaptation was directly inherited from the piscine ancestors{ Therefore, , the statement “amphibians were derived from fishes’ gained ground for belief., , vonian, 367.5 million years ago; the, by which they colonized the earth’s, f active research and debate, , Modern tetrapod groups appeared by the late De, specific aquatic ancestors of the tetrapods, and the process, land after emerging from the waters, remain unclear, and are areas 0, , among palaeontologists at present., , Tetrapods can be defined as the nearest common ancestor of all living amphibians (the, , lissamphibians) and all living amniotes (reptiles, birds, and mammals), along with all of the, , descendants of that ancestor., , , , Fons Itis universally accepted that the first tetrapod were the ancestors of modern amphibians., ossil evidence suggested that the earliest tetrapods must have originated in Devonian period. It, jated with fresh water deposits., , ae been found that earliest remains of tetrapods are associ, that - early tetrapods originated from ancestors living in fresh water. Anatomists also opined, nave me necessities for land life had developed in the bony fishes of the Devonian period that, , Tise to the modern amphibians/In the Devonian period of palaeozoic era they were originated, , from, fishes, hence that period is called ‘Age of amphibians’ >

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_— Blology, , ( Modem amphibians generally remain semiaquatic, with the first Stage, like aqvati e later stages of life as partly terrestrial and pa:, most tetrapodes in the present day are amniotes, being mainly terrestrial tetrapods. The sng, are more advantageous over amphibians, by the fact that they can lay their eggs on land, op "ot, , fertilized eggs within the mother. Crepronalanaice a into the reptiles (which ng 2, lizards, dinosaurs, crocodilians, turtles), birds and extinct relatives; while another Neg, , L. jspil ‘Oup of, diverged into the mammals and their extinct relatives, The amnioies further evolved fey, , ee r fli, such as birds from among the dinosaurs, and bats from among ‘the mammals, Bh., , a, , ° Of their Live, tly aquatic. ie S, , Several groups of tetrapods, such as the caecilians, snakes, cetaceans, sirenians, and, have lost some or all of their limbs through further evolution; some have only concealed Vest, bones as a remnant. Many tetrapods have returned to partially aquatic or fully aquatic lives,, tetrapods that evolved back to aquatic life include mammalian species such as cet, , ree : e ACean, (like whales and dolphins), sirenians (like the sea cow), pinnipeds (including seals); and Aquat, birds like penguins., , = coed, , am earliest amphibians developed in the Devonian period, around 370 million, from sai, , pterygian fish with lungs and bony limbed fins, features that were helpful i, , to dry land. These lobe finned fish were similar to the modern coelacanth and lungfish. These, ancient lo i joi, , ev leg like fins with digits abled them to, crawl along the s, , crawl along the sea bottom. E i into limbs and they wou, ee a including modern amphibians, reptiles, birds, an, , I > . mammals), espite being able to crawl on land, these prehistoric tetrapodomorph fish still spent most of th, , cir, time in the water. They developed lungs, but still breathed with gills., , YEATS ago,, in adapt, , , , , , , They diversified and became dominant during the Carboniferous and Permian periods,, but were later removed by reptiles and other vertebrates. Over time, amphibians shrank in size and, decreased in diversity, leaving only the modem subclass Lissamphibia., , Fegan —<—<——, , , , There is no doubt that amphibians have originated from fish like ancestors. This has been, , accepted universally. This is due to certain features that have been shared by both fish and amphibians., They are:, , Both are cold blooded., , Both respire by gills (early life in amphibians)., Both have air bladders that serve as lungs., Both lay eggs in water to prevent desiccation., Initial developments take place in water., , An affinity between the amphibians and the teleost fish is the multifolded structure of the teeth, , and the paired supra occipital bones at the back of the head, neither of these features being, found elsewhere in the animal kingdom., , FISHES, , , , During Devonian period there were 3 existing forms of bony fishes which could have, been the ancestors. They are :- )Actinopterygians ii) Dipnoans _ iii) Crossopterygians., , >

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i. and jimb might have developed. Thus b, Hal and this group is out of consideration, Tate could not have originated from such, , Dipnoans :- According to Romer Dipnoj, wman“Dipnoi and amphibiaT Wan Tshes are the “uncles of Amphibians”, , an, , 4 According, ” The dipnoans were considered, ‘Osest to amphibians on several, , iito Ne ‘ame grand father.’, , probable ancestors because the living lung fishes are cl, , aspects like :similarity in brain — Cerebral hemisphere &, : Ability to breathe air by lungs ee, a similarity in pectoral girdle, tructure & development of eggs a: i ‘, d) : : @gs are alike and the larvae posses internal gills like tadpoles of, Inspite of similarity there are certain characters, , allow them to be the ancestors. These are: Posseated by the Dipnoi that does not, , a) Skull is cartilaginous, , b) Dentition different, , ¢) Vertebral column has no vertebrae (Notochord), , d) The skeleton of their fins is not reminiscent that of an ancient tetrapod., , Their similarity may be due to convergent evolution., , iii. Crossopterygians: - Crossopterygians hold a very important position for origin of tetrapod., However crossopterygians evolved into 2 sub-orders :- Coelacanth & Rhipidistia, , Origin from Coelacanth, , Inspite of many similarities like skull made of bony plates, brain situated far back in the, cranium and nucleated RBC their lies differences in the limbs. The fins of these fishes are much, smaller than the legs of the amphibians. Hence they are discarded., , Origin from Rhipidistians, , The Rhipidistians were the fresh water fishes. They resembled amphibians in the skull,, Palate, and teeth pattern')The Rhipidistians underwent dichotomous evolution into Rhizodontidae, & Holoptychidae. . :, , Holoptychidae were living in Devonian but did not survive in the end, hence they were, tuled out. However the Rhizodonts represented by Eusthenopteron exhibited many morphological, features common with early amphibians, which are evidence of their close phylogenetic relationship., i., ii,, iii,, , Skull pattern similar. Jaws are similar, Vertebrae alike and advanced, Notochord consist of series of rings,, homologous with neural spine of amphibian. ;, Paired fins of Eusthenopteron were homologous to the tetrapod limb., Both thipidistians and primitive tetrapods were predators, armed with sharp, strong teeth,, With a peculiar labyrinthine infolding of the enamel., , sition of external and internal nares is similar., , with spine being projected from each ring, which is, , iy,, v., , Vi.

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oo hows a trend towards ih, , idistians represented by Eustheaopteron st amphip, ( src eaies was ty Sanity evidence of a transitional stage between them, ung, : very of late Devonian deposits of East Greenland, such as. Icthyostega..., isc ‘, , Unti (:, aracteristics of Ichthyostegids, SL Ichthyostega was one of the first primitive amphibians, with nostrils, , and more efficient Tun, It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe, finned fish, Eusthenopteron., , Skull is solid about 6" in length. Roofing pattern resembled crossopterygian fishes,, erculum was lost. ., , ; a in front of the eyes had become elongated similar to that of amphibian skull., , The vertebrae are greatly developed whereas in caudal region fin rays of tail are retained,, , The pelvic & pectoral girdle with which the limbs articulate are cai, , pable of carrying the animal, around on the ground. ., ¢ Thus Ichthyostega presented a strange mixture of fish and amphibian characters, and therefore, it is an ideal ancestor of tetrapod., e, , They had a semirigid ribcage of overlapping ribs, which may have substituted for a rigid, spine. |, In conjunction with robust forelimbs and shoulder girdle, Ichthyostega may have had the, ability to locomote on land in the manner of a seal, with the forward portion of the tors |, elevated, the hind part dragging behind. ), , Now, their has been s;, , |, peculations that the Crossopterygians invaded the land through the, transitional Ichthyostegids. But why did they develop, , forms capable of emerging on to the land) |, It is certainly not to breathe air for that could be done by coming to the surface of the pool. |, Not because, in search of food, as there was at first little on land for the carnivores., Not to escape enemies, as there fish ancestors were among the most powerful vertebrates, found in fresh water forms. ', , ( One theory suggested that Overpopulati, indepe:, , ay have caused them to evalve some, ee 'trom the aquatic habitat and to explore terrestrial food Source for part of their, lifetyete. — =e, ——————, , According to Romer (1958) dessication of ponds durin Devoni © ancestral |, ae to migrate over land to larger pobIsandUMOS squire gradually tenestial eae, including, imbs., , phibians evolved adaptations that, periods. Their |, , ; c allowed them to Stay out of the water for longer |, ‘ungs improved and their skeletons became stro, bodies on land. They developed “hands” and, , etons bec nger to support the weight of their, T : feet” with five or more digits; the skin became more, capable of retaining body fluids and resisting desiccation,, , , , , , t, , Sea Sts, , SO

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ion - In order to get atmos, , of Pheric O,, they d i, fish inhabiting anoxic waters = Y developed lungs. In middle and late, , (very low i, tionary pressure to develop their air- Font oxygen) would have been under, , /pessication - They developed thick skin layer and certain glands in order to check evaporation., , « feision i 2 For a good vision on land the early tetrapods developed eyelids & nictitating, membranes for its protection) Changes in the eye came about because the behavior of light at, , the surface of the eye differs between air and water environment due to the difference, , in refractive index. The eye was now exposed to a telatively dry environment, so eyelids, developed and tear ducts evolved to produce a liquid to moisten the eyeball. Early tetrapods, inherited a set of five rod and cone opsins known as the vertebrate opsins., , earing- The ear for equilibrium in fishes has been modified for hearing in tetrapods, in, form of tympanumSThe spiracle retained as the otic notch, eventually closed by the tympanum,, a thin, tight membrane. The hyomandibula of fish was reduced in size to form the stapes., Situated between the tympanum and braincase in an air filled cavity, the stapes was now, capable of transmitting vibrations from the exterior of the head to the interior. Thus the stapes, , became an important element, coupling airborne sound waves to the receptor system of the, inner ear., , ip Senses: Lateral line system - Fish have a lateral line system that detects pressure fluctuations, in the water. Such pressure is non-detectable in air, but grooves for the lateral line sense, , organs were found on the skull of early tetrapods) suggesting an aquatic habitat. Modern, amphibians, which are semi-aquatic, exhibit this feature., , Aaa skeleton - With the move from water to land, the spine had to resist the bending conaed, y body weight and had to provide mobility where meting RierkiaN it could bend along its, entire length. Strong interlocking vertebral column develOped to face the force of gravity., , , , | Locomotion - In order to satisfy the locomotory need on land the early tetrapods developed, ‘U’ shaped pectoral &*V’ shaped pelvic girdle, with strong broad limb bones having hands &, feet bearing 5 toes., a) Girdles: The shoulder girdle was disconnected from the skull, resulting in improved, terrestrial locomotion. The upper portion of the girdle had a flat, scapular blade, with the glenoid, Cavity serving as the articulation surface for the humerus. The pelvic girdle also was much, larger than that found in fishes, accommodating more muscles. The hind legs were somewhat, Specialized in that they not only supported weight, but also provided propulsion. The three, bones ilium, ischium and pubis meet at a single point in the center of the pelvic triangle called, , the acetabulum, providing a surface of articulation for the femur., , >) Limbs: Fleshy lobe-fins supported on bones seem to have been developed. The paired, had bones distinctly homologous to the humerus, ulna, and radius in the fore-fins and to, , the femur, tibia, and fibula in the pelvic fins. Most of the animal’s strength was used to just, , Pl