(a) Give an account of the origin as well as adaptive radiation of reptiles. (20 marks) (b) Describe general structure of a…

Origin and radiation of reptiles. The reptilian stock is traced to early amniotes such as captorhinomorphs and cotylosaurs. The decisive split was between synapsids, with one temporal fenestra, which led to mammals, and diapsids, with two temporal fenestrae, which gave rise to most reptiles. Reptiles became truly terrestrial because of the amniotic egg with extra-embryonic membranes, a water-resistant keratinized skin, and improved jaw articulation with stronger tooth implantation, allowing efficient feeding without cranial kinesis. The diapsid skull, with enlarged temporal openings, lightened the skull and gave space for jaw muscles, facilitating ecological diversification. Early reptiles had a rigid, akinetic skull; cranial kinesis is a later derived feature in some squamates. Their Mesozoic radiation was extensive. Testudines evolved a protective shell and occupied slow-moving aquatic and terrestrial niches, as in the Indian star tortoise and softshell turtles. Squamata radiated into lizards and snakes, exploiting terrestrial, arboreal, fossorial and aquatic habitats; Indian cobra and monitor lizards illustrate venomous and non-venomous adaptations. Crocodilia became semiaquatic ambush predators, represented by the Indian crocodile and gharial. Rhynchocephalia, represented by tuatara, is a relictual, slow-changing lineage. Reptiles dominated Mesozoic ecosystems, but many remained ectothermic; endothermy evolved mainly in the therapsid-mammal transition, giving mammals greater activity and parental care. Mass extinctions, especially the Permian and Cretaceous events, removed many amniote groups, while survivors persisted through aquatic life, small size, burrowing and ecological flexibility. This combination of amniotic reproduction, desiccation resistance and efficient feeding underlies reptile success.

Mammalian tooth structure and classification. A mammalian tooth is a thecodont, heterodont organ. The crown is covered by enamel, the hardest tissue, formed by ameloblasts; beneath it is dentine, formed by odontoblasts of neural-crest ectomesenchymal origin; the central pulp cavity contains nerves, blood vessels and connective tissue. The neck marks the junction of crown and root, while the root is embedded in the jaw and covered by cementum, which anchors the periodontal ligament. Structurally, teeth are divided into crown, neck and root. By shape, mammalian teeth are incisors for cutting, canines for tearing, premolars for grinding and shearing, and molars for crushing; some mammals have reduced or homodont dentitions. By attachment, mammalian teeth are normally thecodont, set in sockets; pleurodont and acrodont attachment are comparative conditions in which teeth are fixed to the inner jaw ridge or fused to the ridge, respectively. By succession, most mammals are diphyodont, having a deciduous and a permanent set, as in humans, cats and dogs. Polyphyodont succession, with continuous replacement, occurs in manatees and some other sirenians. Monophyodonty is rare in mammals; adult edentulism in platypus is not a replacement pattern.

Herdmania: habitat, features and development. Herdmania is a marine tunicate living in subtidal benthic habitats, attached to rocks, mud or seaweed, where it filter-feeds on plankton and organic particles. Its habit is sessile and filter-feeding; tunicates are often colonial, and Herdmania is an attached ascidian with a protective tunic of tunicin and collagen. The body has two siphons: an oral siphon for water entry and an atrial siphon for exit. Water passes through the branchial basket into the branchial chamber, where food is trapped; the gut lies mainly in the branchial chamber, while the heart and gonads lie in the atrial chamber. Herdmania is hermaphroditic, with male and female gonads in one individual. Development is ovoviviparous: fertilized eggs develop inside the female and release a free-swimming tadpole larva. The larva has a notochord, dorsal nerve cord, tail and sensory organs, showing chordate characters. On settlement, it attaches by its oral end, the tail is absorbed, and the notochord and nerve cord regress; this retrogressive metamorphosis produces the sessile adult. Thus Herdmania illustrates a free-living chordate larva and a simplified adult body plan.