1. B.SC(H) ZOOLOGY PART II Origin & Evolution of Land Ectotherms

2. Amphibians They have a 2-phase life history. There is usually a free-living aquatic developmental stage. There is usually a terrestrial juvenile/adult stage. NOTE: this represents a transition from water to land, just as in the evolution of the group.

3. Introduction……………….. The group Tetrapoda is divided into four classes made up of amphibians, reptiles, birds and mammals. The living representatives of the class Amphibia include salamanders, newts, frogs, toads, and the caecilians. The amphibians lead a double life, that is, first in the water, and then on the land. The result of this ambitious attempt is that they present a medley of makeshift adaptations, which leave them still a long way from vertebrate perfection.

4. Introduction……………….. Among the dual adjustments that they make, are those associated with locomotion and protection against desiccation. In water, an elongated fishlike body, propelled by a muscular tail, has proved to be the most efficient mechanism for locomotion. However on land, the weight of the body is no longer supported by the surrounding aqueous medium, so that the two pairs of appendages become modified into legs, which act as levers to lift the body away from the ground. Such levers are equipped with adequate muscles without adding excessively to the body weight. However the amphibians are not particularly successful at locomotion on land. Even in frogs and toads, where amphibian legs reach their highest development, such locomotor appendages are so inefficiently anchored to a single vertebra of the supporting backbone that these animals cannot bear their weight upon them in the sustained manner necessary for standing or walking, and can progress only by the momentary exertion of hopping or jumping.

5. Introduction……………….. The problem of dessication arises from the fact that the surrounding air, takes up moisture rapidly from any moist surface. Amphibians not only utilize gills and primitive lungs in respiration, but also exchange gases to a very large extent directly through the skin. Consequently, these animals can live only in moist places. In comparison, the higher land animals, in which an efficient pulmonary system is formed, are not restricted because they develop a thick, relatively dry integument, which is resistant to dessication. Thus, relatively inefficient respiratory organs, together with other anatomical handicaps prevent amphibians from maintaining a body temperature independent of that of the surroundings

6. Introduction……………….. The difficulty of avoiding dessication is also involved in the breeding habits of amphibians because they have not made the changes required of true land vertebrates. No amnion (liquid-filled sac) is produced by the embryos of lower vertebrates including the Amphibia. The latter must therefore go back to the water to breed in most cases. Furthermore, the metamorphosis of such an amphibian as a frog or a toad, necessitated by its emergence from water to land, works profound changes both in its structure and in its feeding habits. For instance, during its lifetime a toad changes its diet six times. While in the egg it absorbs the yolk; upon hatching it develops a temporary mouth and eats the jelly of the egg envelopes; next it becomes the free swimming tadpole feeding mainly upon the aquatic vegetation; the juvenile stage has fat bodies provided to meet the intervening demands of hibernation; with the warmth of spring the young toad catches slugs and insects for a living.

7. Characteristics of Amphibians…..

8. 1. Amphibians are ectothermal vertebrates. 2. They have varied body forms – ranging from elongated forms, with a distinct head, trunk and tail; to a compact, depressed body with a fused head and trunk and no intervening neck. 3. Limbs are usually four in number, although some forms are limbless. 4. Skin is smooth and moist with many glands including pigment cells. Poison glands are sometimes present but scales are mostly absent. 5. Mouth is usually large, with small teeth in either upper or both jaws. Teeth are bicuspid and pedicellate. In some forms, teeth are completely absent. The nostrils open into the anterior part of the mouth cavity. Characteristic of Amphibians

9. Characteristic…………. 6. Skeleton is mostly bony, with varying number of vertebrae; ribs are present in some forms but absent in others. Ribs if present do not encircle the body. Centra of vertebrae are cylindrical. Similar type of vertebra is also found among several groups of early tetrapods. There is the presence of double or paired occipital condyle. The posterior skull bones have been lost. Small, widely separated pterygoids are found. A small bone in the skull called operculum is present and is fused to the ear bones in most anurans; it is perhaps involved in hearing and balancing. 7. Ability to elevate the eye with specially developed levitator bulbi muscle. There is also the presence of a special type of visual cell in the retina known as the green rod. (This however is absent in Apoda). 8. Respiration occurs by lungs, skin and gills, either separately or in combination. A forced pump respiratory mechanism exists. The larval forms have the external gills that may persist throughout life in some forms.

10. Characteristic 9. Presence of a three-chambered heart having two atria and one ventricle. A double circulation takes place through the heart. 10. The excretory system consists of paired mesonephric kidneys and urea is the main nitrogenous waste. 11. Sexes are separate; fertilization is mostly internal in salamanders and caecilians but generally external in frogs and toads. Amphibians are predominantly oviparous, rarely ovoviviparous. Eggs are moderately yolky with jelly-like membrane coverings. Metamorphosis is usually present. Fat bodies are associated with gonads.

11. Cause of shift…….. The development of vertebrates which lived on land, started about 350 million years ago in the Devonian period. At this time, some fish began to crawl out of the water and started walking on land and breathing air. The climate of the world at the end of Devonian became hot and arid. This would have caused the water in shallow pools and lakes to become warmer, and many small water bodies may have evaporated during the seasonal droughts. The fish ancestors of the first land vertebrates must have had two important features i.e. firstly: the presence of lungs as simple pouches leading from the throat, which developed a rich supply of blood vessels. And secondly: the development of limbs from the bony supports of the fins.

12. Search for Ancestery……………….. The first tetrapods evolved from rhipidistian crossopterygian fishes. The fossil remains of primitive tetrapods have been found in the eastern parts of Greenland in Devonian deposits about 350 million years ago. These specimens have features intermediate between late crossopterygians and early amphibians.

14. Search for Ancestery……………….. The most likely ancestors of the amphibians were the rhipidistians that were common in the Permian. Unfortunately, the fossil record of the origin of amphibians is very poor. Rock deposits from the middle Devonian period contain typical rhipidistian fish; while early amphibian ancestors appear in the late Devonian. However no fossil species, which directly link the two groups, have been found during the intervening period of about 30 million years. Until more fossil species are found, which show the transitional forms between fishes and amphibians, this important period of vertebrate evolution will remain uncertain. The earliest fossil amphibians that have been found had already solved the problems of living on land. They were the Labyrinthodontia, and Ichthyostega is a typical example.

15. Early Amphibians The earliest of amphibians in the fossil record called Icthyostegids show relationship to rhipidistians in a number of features: Arrangement of Dermal Bones in the skull Fins and their supporting girdles Tooth Structure- complex folding of enamel layer of tooth - labrynthodont Extension of lateral line system into the skull Caudal tail Structure of Vertebrae

21. Early Amphibians

22. Ichthyostega Hindlimb

23. Ichthyostega Skull

24. The anatomical innovations of Icthyostegids indicates that and their contemporaries had evolved successful solutions to at least some mechanical problems faced by land dwelling vertebrates. One important terrestrial problem for these early tetrapods was the need to prevent compression of internal organs because of pressure transmitted by the limbs and limb girdles. A second problem pertaining specifically to forelimbs was that of preventing the impact of terrestrial locomotion from being transmitted to the brain case since rhipidistian pectoral fins are connected to the skull. These problems were solved by attaching first the pelvic fin and then the pectoral fin to the main axis. The spine thus became a suspension bridge that absorbed the impact of terrestrial locomotion, freed internal organs from pressure and enabled the head to turn independently of the body.

25. Search…………….. On the basis of the morphology of their vertebrae, paleontologists have been of the opinion that fossil amphibians with stereospondylous and embolomerous vertebrae were not in the amniote line. Labyrinthodonts had many features seldom seen in modern amphibians. These included minute bony scales in the skin dermis; a fishlike tail supported by dermal fin rays; and skull similar to those of rhipidistian fishes. Labyrinthodonts, like their aquatic ancestors, had a sensory canal system of neuromast organs. One or another of the labyrinthodonts was ancestral to the first amniote

27. Evolution of Amphibia Modern amphibians are specialized animals, which do not resemble primitive amphibians very closely. They are so specialized that it is not clear when they separated from the primitive amphibia, from which group they derived, or how closely related the modern forms are to each other. Three distinct groups of modern amphibia remain, which as adults feed on insects or other small invertebrates. These groups clubbed together as Lissamphibia include: Urodela (newts and salamanders); Anura (frogs and toads); and Gymnophiona (caecilians).

28. Evolution of Lissamphibia………….. By the Carboniferous, many groups are recognized including Temnospondyli, Anthracosauria, and Microsauria. It is not clear as to what is the relationship of living amphibians known as Lissamphibia to these groups. One hypothesis suggests that lissamphibians are the sister group to Temnospondyli. Alternatively, lissamphibians may have evolved from a temnospondyl ancestor. It is hypothesized that lissamphibians are either monophyletic (a common temnospondyl ancestor) or diphyletic (apodans descended from a microsaurian ancestor). What we can say from the knowledge of amphibian relationships is that the class Amphibia, as traditionally defined, is a paraphyletic group that omits its amniote descendants. Successive mutations and natural selection increasingly adapted basal amphibian descendants for terrestrial life culminating with the origin of the amniotes.

29. Origin of Lissamphibia………….. Temnospondyls was a group that was common in the Permian with its fossil record extending back to the Mississippian. Members of the temnospondyls have achieved skeletal similarities to modern frogs and salamanders, suggestive of their close relationship. A number of lissamphibian skeletal features and their relatively smaller size can be explained as the retention of juvenile ancestral temnospondyl features. The condition in caecilians does not fit easily into this scenario, possibly suggesting an independent origin from microsaurs.

30. Origin of Lissamphibia………….. Microsaurs represent a diverse group of fossil forms known from the Pennsylvanian to the lower Permian. They share a number of skeletal features with caecilians, which may suggest either a close relationship or convergence on an elongate body form specialized for burrowing. Anthracosaurs Anthracosauria is a small Paleozoic group, thought to be in direct line to the amniotes. Their fossil record extends from the Mississippian to the Triassic. Lissamphibians Living amphibians of approximately 2000 species may be grouped in three orders: Apoda, Urodela and Anura.

31. ORDER 1. APODA (GYMNOPHIONA / CAECILIA). Members of the order are pantropical in distribution. The caecilians are burrowing forms, with worm like bodies, lacking limbs. The tail is very short suited to their mostly terrestrial habits and the anus is almost terminal. The skull is solid and bony, again suited for a burrowing lifestyle. The animals are blind, but carry special sensory tentacles. Unlike other amphibians, some caecilians have dermal scales. Adults lack gills and gill slits. The very small eyes are buried beneath the skin or under the skull bones. Because of the presence of an intromittent organ in males, internal fertilization is assumed. In some caecilians, eggs are laid, which hatch into free-living larvae. The eggs are large, yolky and cleavage is meroblastic; they are laid on land in Ichthyophis, and the embryos develop around the yolk sac, but often have long, plumed gills. The female guards the eggs until the larvae hatch and move to the aquatic habitat. Other genera skip over the aquatic larval stage and a few have specialized external gills. In still other genera, the eggs are retained within the female, metamorphosis occurring before birth. Viviparity is common in the aquatic form.

32. ORDER 2: Urodela (Caudata) These include the salamanders and newts, the latter being small, semi-aquatic forms. Urodeles are found in temperate and subtropical climates in the Northern Hemisphere but do not reach the tropics in the New World. The elongated body consists of head, trunk, and a well developed tail, the latter being retained throughout life. Two pairs of limbs occur in most species. Larvae resemble adults except for the presence of gills, and like adults, have teeth in both the upper and lower jaws. The urodeles have a greater tendency to show generalized characters of the class amphibia, in comparison to the much more specialized Anura. The group shows different types of forms, varying from the terrestrial salamanders, such as Salamandra maculosa, which is viviparous, to the fully aquatic forms, such as Necturus. Furthermore, there is a tendency to retain larval characters in the adults of certain aquatic forms, the process known as paedomorphosis / neoteny. Examples include Megalobatrachus, which has no eyelids but loses its gills in the adult.

33. ORDER 2: Urodela (Caudata) In Cryptobranchus, the spiracle remains open being used for expulsion of water during respiration. Amphiuma is an elongated form with very small legs, no eyelids and four branchial arches. An extreme example of neotenous forms is Necturus, which has external gills but has such a reduced lung that the animal can live as a permanently aquatic form. Similarly, Siren shows all larval characters and has no hind limbs. The terrestrial newts are of different types: some are definitely terrestrial like Triturus vulgaris, although it is not able to live in very dry habitats. The limbs support the body weight, their soles being applied to the ground and turned forwards. The tail shows reduction to form a rod-like organ but when the animal returns to the water for breeding purposes, the tail develops a large fin. On the other hand, is the genus Ambystoma, which has eleven species, in which some races become mature without metamorphosis, because of lack of iodine in water, whereas others are genetically neotenous.

34. ORDER 3. ANURA Frogs and toads, which come under this category, show fused head and trunk, and no neck region. Two pairs of well-developed limbs occur, the hind pair being particularly adapted for leaping. The feet may be webbed and adapted for swimming or resemble long fingers with suction pads for climbing. Frogs and toads are the first vertebrates to have vocal cords for sound production. The anuran larva or the tadpole has head and body fused into a single, egg-shaped mass and a long tail with a median fin. Metamorphosis is clearly defined, involving the loss of gills as the lungs develop; resorption of tail and the appearance of legs

35. ORDER 3. ANURA There are many distinctive features of living frogs. Frogs have not more than nine vertebrae in front of the sacrum, and the 3-4 vertebrae behind the sacrum are fused into a rod-like structure called the urostyle. In comparison are the urodeles and the apoda that have many more vertebrae and lack the urostyle. Furthermore, anurans lack a tail in the adult stage, unlike the other two groups. Frogs also have a radio-ulna, which represents a fused radius and ulna (bones of the forearm), and a tibio-fibula, the fused tibia and fibula (bones of the shank). The ankle bones of frogs: the tibiale and fibulare, also known as the astragalus and calcaneum respectively, are greatly elongate. Thus there is an additional lever system that frogs can utilize in jumping. Frogs also have a distinct life phase called as the tadpole: a highly specialized feeding form. Although urodeles and the apoda do have a larval stage in their life cycle, but these larval forms do not have the diverse specializations that the frog tadpole has. Even the most primitive frogs have the beginnings of a unique method of tongue projection, associated with extreme modification of the gill arches into a fused hyobranchial plate.

36. ORDER 3. ANURA Although there is no scientific difference between frogs and toads, the former live in water, are mostly smooth-skinned and possess long hind limbs for leaping; while the latter living on land have a warty dry skin, and shorter hind limbs for hopping. Anurans inhabit a wide variety of habitats, ranging from arid deserts to mountainous regions to swampy areas to tropical rain forests. Temperature and water regulation are critical to amphibians generally, and the anurans particularly. Being ectothermal, frogs and toads depend on the ambient temperature for body temperature regulation. In winters, frogs in temperate zones hibernate or enter into a state of extremely reduced activity. On the other hand, they avoid the extreme heat of summer months in the tropics, by remaining underground during daytime and being active at night. Anurans are also susceptible to the loss of body moisture due to extremely hot or dry conditions. Those in temperate climates maintain moist skin to assist in evaporative cooling. In addition, their permeable skin, gives the frog an ability to absorb water simply by jumping into water. In contrast are the frogs in arid regions, which have the skin impermeable to water so as to prevent rapid evaporation and dehydration. Instead, they cover their body with a mucus film, or burrow to avoid the heat altogether.