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

(a) Origin and adaptive radiation of reptiles

Origin. Reptiles arose in the Late Carboniferous, about 320–310 million years ago, from advanced reptiliomorph (labyrinthodont-like) amphibians. Hylonomus (about 318 million years ago, Nova Scotia) is among the earliest known amniotes. The first reptiles were the stem group Cotylosauria, with a solid (anapsid) skull. Success on land rested on:

• the amniotic egg with a shell and extra-embryonic membranes (amnion, chorion, allantois, yolk sac), which freed breeding from water;
• dry, keratinised, scaly skin that prevents water loss;
• internal fertilisation, stronger lungs, and limbs and girdles that support the body.

Adaptive radiation. The lineages that diverged are grouped by the temporal openings of the skull:

• Anapsida (no opening): cotylosaurs and Testudines; molecular evidence now places turtles within the diapsids.
• Synapsida (one lower opening): pelycosaurs, then therapsids, which led to mammals.
• Diapsida (two openings): Lepidosauria (Rhynchocephalia; Squamata, the lizards and snakes) and Archosauria (crocodiles, dinosaurs, pterosaurs and birds).
• Euryapsid-type marine reptiles (one upper opening): ichthyosaurs and plesiosaurs.

Lepidosaurs and archosaurs diverged in the Permian. After the Permian–Triassic extinction (about 252 million years ago) archosaurs became dominant in the Triassic, and dinosaurs flourished through the Jurassic and Cretaceous. In the Mesozoic, the age of reptiles, they occupied every habitat:

• on land, dinosaurs, from small bipedal theropods to huge herbivorous sauropods and armoured ornithischians;
• in the air, the pterosaurs;
• in the sea, fish-like ichthyosaurs, plesiosaurs and mosasaurs, with paddle-like limbs;
• lizards, snakes and crocodiles that burrow, climb or swim.

The end-Cretaceous extinction, 66 million years ago, removed pterosaurs, plesiosaurs and non-avian dinosaurs. Four orders survive: Testudines, Crocodilia, Squamata and Rhynchocephalia. Birds, descended from theropod dinosaurs, are the other surviving archosaurs.

(b) Mammalian tooth: structure and classification

A mammalian tooth has a crown above the gum, a neck at the gum line and one or more roots set in a socket (alveolus) of the jaw bone and held by the periodontal ligament.

• Enamel covers the crown. It is the hardest tissue of the body, mostly hydroxyapatite secreted by ameloblasts, and is not replaced.
• Dentine forms the bulk of the tooth. It is secreted by odontoblasts and is softer than enamel.
• The pulp cavity at the centre holds connective tissue, blood vessels and nerves, which enter through the root canal.
• Cementum, a bone-like layer, covers the root and anchors the ligament fibres.

Classification:

• By shape: mammals are heterodont, with incisors for cutting, canines for piercing and tearing, and premolars and molars for crushing and grinding. Toothed whales are homodont. Crown patterns include bunodont (rounded cusps: pig, human), lophodont (transverse ridges: elephant, rhinoceros), selenodont (crescent ridges: cattle, deer) and secodont or carnassial (shearing: carnivores).
• By attachment: mammalian teeth are thecodont, set in sockets. In other vertebrates they may be pleurodont, fused to the inner side of the jaw (many lizards), or acrodont, fused to the top of the jaw (Sphenodon, agamid lizards).
• By succession: monophyodont, one set only (toothed whales); diphyodont, a deciduous (milk) set replaced by a permanent set (most mammals; humans have 20 milk and 32 permanent teeth); polyphyodont, continuous replacement (manatees).

(c) Herdmania: habitat, habits, features and development

Herdmania (H. pallida) is a solitary sea squirt (Urochordata, Ascidiacea).

Habitat and habits. It is exclusively marine and lives in shallow coastal water, attached to rocks, shells and other hard surfaces. The adult is sedentary and a filter feeder; only the larva swims.

General features.

• The sac-like body is covered by a thick, leathery tunic (test) of tunicin, a cellulose-like substance. It has two siphons: an oral (incurrent) siphon and an atrial (excurrent) siphon.
• Beneath the muscular mantle is a large pharynx (branchial sac) perforated by ciliated stigmata. The endostyle secretes mucus that traps food. Water passes through the stigmata into the atrium and leaves by the atrial siphon, carrying wastes and gametes.
• A cerebral ganglion lies between the siphons.
• It is hermaphrodite, with testis and ovary in the same animal, and cross-fertilisation is external, in sea water.

Development. Cleavage is holoblastic and leads through the blastula and gastrula to a free-swimming tadpole larva. It has a trunk and a tail with a notochord, a hollow dorsal nerve cord with a brain vesicle bearing an ocellus and otolith, a pharynx with gill slits, and adhesive papillae at the front. The larva attaches by these papillae and undergoes retrogressive metamorphosis: the tail and notochord are resorbed, the nerve cord and sense organs degenerate to the ganglion, the pharynx enlarges and the siphons form. The notochord, nerve cord and gill slits of the larva show the chordate affinity of tunicates.