UNIT 3 REPTILES AND BIRDS Structure Introduction Objectives Anarnniotes and Amniotes Reptiles Origin and Adaptive Radiation of Reptiles Changes in Traditional Classification of Reptiles Classification of Extant Reptiles Main Characteristics of Reptiles Distinguishing Features of Reptiles as Compared to Amphibians Characteristics of Extant Reptilian Orders Bird- Aves Ancestry and Evolution of Birds Classification of Birds Characteristics of Birds Form and Function of Birds Social Behaviour Summary Terminal Questions Answers 3.1 INTRODUCTION You have already read in the previous unit (Unit 2, Block 1 of the LSE-10 course) about the class Pisces, members of which are completely dependent on water, throughout their life. You have also studied the amphibians, some of which may be able to live on land but have to go back to water to breed, indicating that they are not totally adapted to a terrestrial mode of life. In this unit and the next unit you will learn about those vertebrates like reptiles, birds and mammals which are totally land animals, and do not need to go back to water to lay eggs or to produce young ones. In the present unit, you will study reptiles and birds. In reptiles which belong to the class Reptilia you will learn about the characteristics, classification, evolution, and the salient features of the four extant orders: Rhynchocephalia Chelonia, Crozodilia and Squarnata and their adaptation to terrestrial life. In birds which belong to the class Aves, will study in brief their characteristics, classification, evolution and adaptations to flight . Se Objectives I After studying this unit you should be able to: describe briefly the evolution and affmities of reptiles (Class Reptilia) and birds (Class Aves), t . vt 1, describe the adaptations of reptiles to terrestrial life, I I 7 + $it list and describe the characteristics of reptiles, > outline the classification of the four extant orders of reptiles, describe the salient features of the four extant orders: Rhynchocephalia, Chelonia, Crocodilia and Squsmata, distinguish between poisonous and non-poisonous snakes, list and describe the characteristics of birds, outline the classification of extant birds upto the level of order, explain and describe the adaptations of birds particularly in relations to flight, * describe the social behaviour of birds. Diversity in Chordates 3.2 ANAMNIOTES AND AMNIOTES The adaptations of reptiles, birds and mammals to life on land were made possible by evolving a reproductive pattern that freed them completely from-dependence on external water sources. This was achieved by adopting essentially a mechanism, of internal fertilization (fertilisation of the egg within the body of the mother) and most importantly by developing a harder egg that could be laid on land. Such ay egg that is surrounded by an air rather than an aquatic medium evolved the following features (Fig. 3.1) : a tough protective yet permeable horny or occasionally calcareous outer shell and a series of embryonic membranes that form a complete set of life support system. Among the membranes, the yolk sac encloses enough yolk to provide nourishment to the embryo. 'The chorion membrane sheathes externally both the yolk and the embryo. The allantois allows for gas exchange and stores excreted metabolites in the allantoic cavity. 171e amnion encloses the developing embryo in a liquid filled-space - a replica of the ancestral pond. As a result of the evolution of such an egg, aJso called cleidoic egg, the vulnerable aquatic larval stage, so characteristic of fishes and amphibians, was suppressed and the embryo developed within the safety of the hard egg which was laid outside the body of the mother, or within the mother's body. These three animal groups, the reptiles, birds and mammals are called amniotes (Amniota) because they all share the presence of the amnion. Fishes and amphibians lack such extra embryonic membranes including the amnion and so are called Anamniotes (Anamniota). Amniotic cavity Amnion Allantoic blood vessel Yolk sac blood v Fig.3.1 : Cross section of a bird embryo within the shelled egg, showing the four membranes of the amniote egg which surroun'd and protect the embryo and its food supply. The shell permits gases to pass between the egg and the outside environment. 'The study of reptile? is called herpetology (Gr-herpeton: REPTILES reptiles) Reptiles belong to the class Reptilia (rep-til'ea-L. repto, to creep) and were the first truly terrestrial vertebrates. They continued the colonization of land begun by the crossopterygian fishes and amphibians and became well adapted for existence on land. 3.3.1 Origin and Adaptive Radiation of Reptiles Evolutionary biologists, in general, agree that a group of amphibian like tertrapods the labyrinthodonts, which evolved a reproduction strategy that freed them completely from dependence in external water sources gave rise to a monophyletic group of-terrestrial amniote sometimes during the early Carboniferous period of the Paleozoic er? (Fig. 3.2). This group separated into three lineages by the late Carboniferous that gave rise to vertebrate$ - the reptiles, birds and mammals. This as you have read in section 3.2 was made possible by the evolution of the cleidoic egg. Fig. 3.3 shows the evolution of the amniotes. Reptiles and Birds The labyrinthodont (Diplovertebron) shown here may have been the primitive amphibi-an from which both the reptiles and amphibians arose, although the lineage remains obscure. J Ine phylogeny, geological distribur~onand evolution of the amniotic reptiles. The remains af the early stem The transition from certain labyrinthodont amahibian ancestor to earliest reptiles have been found in amnitoes was made due to the development of'an amniotic (cleidoic) egg, which the fossilized stumps of primitive trees, dating back to made land existence possible although this egg may well have developed before the middle Carbonifems about the earliest amniotes had attempted to move far on land. The amniotic 32n million years ago. The assemblage, which includes the reptiles, birds and mammals, evolved from a best known of the cotylosaurs lineage of Small, lizard-like forms known as captorhinids, which retained the is the Seymouria which so skull pattern of the amphibians. This was the anapsid lineage that led to the closely resembled the laby- turtles. The first to diverge from the primitive stock were the mammal-$ke rinth~don: amphibian that it reptiles, characterized by a skull pattern termed the synapsid condition. All has not been easy for other amniotes including bird and all living reptiles except turtles, have a skull paleontologists to set the two pattern called diapsid. The great Mesozoic radiation of reptiles may have been groups apart. partially due to the increased variety of ec~logicalhabitats into which they could move. Note that reptiles are not a monophyletic group, as they encompass parts of three distinct evolutionary lines. Groups commonly assigned t6 the class Reptilia are those amniotes that are not bird6 or mammal; 1)ivrrsity in Chordates Reptiles diverged phylogenetically from the labyri~ithodontamphibians during the late Paleozoic era, some 300 mill~onyears ago. he oldest stem reptiles from which the reptiles evolved are believed to be the captorhinomorphs or captorhinids (Fig. 3.4) which are placed in the Order Cotylosauria . These creatures resembled lizards superificially. Their skeleton was more thoroughly ossified than that of labyrinthodonts living at that time, and their limbs suggest that they were more agile. They probably fed upon small terrestrial arthropods that were becoming abundant at about the same time and were undergoing adaptive radiation of their own. Reptiles being the first vertebrates that could penetrate terrestrial environment far beyond the shores of bodies of fresh water, encountered few competitors upon land. They multiplied rapidly, spread into terrestrial niches available to them and specialized accordingly. Much of their adaptive radiation involved different methods of locomotion ' and feeding. Varied feeding patterns involved, among other things, modification of jaw muscles which in turn affected the structure of the temporal region of the skull from which the muscles originate. Skull morphology, therefore provides a convenient way to sort out the various lines of reptile evolution (Fig. 3.5). The skull of the captorhinids had a solid roof of dermal bone covering the jaw muscles dorsally and laterally (Fig. 3.5 a). This type of skull is called anapsid condition, for there are no temporal openings behind the orbits in the temporal roof bounded by bony arches. In other words the skull behind . the orbits is completely roofed over with dermal bones. Captorhinids and their close relatives are placed in the subclass Anapsida. This group is represented today only by the turtles. Their morphology is an odd niix of primitive and specialized characters that has scarcely changed at all since the turtles first appeared in the fossil record in Triassic. As feeding patterns and jaw muscles underwent transformation, various types of fenestrae or openings evolved in the temporal roof overlying the jaw muscles. Some parts of the dermal roof were apparently not stressed and bone did not develop in such areas. Muscles can attach more firmly to the edges of openings and to bony ridges than they can to flat surfaces and they can bulge through the openings when they contract. U lm -Fig.3.4: Captorhinid skeleton (fossil) from the Permian, about 285 million years ago. This specimen was about 25cm long. The amniotes that evolved from the Captorhinids thus diverged by the late Carboniferous into three well defined line of evolution (Fig. 3.3) as given below : I. Anapsida: as you know are represented today by turtles, in which the ancestral anapsid skull is retained. 2. Synapsida: include the mammals-ljke reptiles. Synapsid skull has a single pair of laterally placed temporal openings which originally lay ventral to the postorbital and , Some diapsids as they evolved squamosal bones.