Class Reptilia Lecture Notes
Introduction to Class Reptilia
Taxonomy and Species Count: * Class: Reptilia. * Status: Recognized as true terrestrial vertebrates. * Species Diversity: Approximately documented species.
Evolutionary History and Phylogeny
Geologic Timeline of Amniotes: * The evolutionary history of reptiles and their descendants spans from the Carboniferous period ( million years ago) through the Permian, Triassic, Jurassic, Cretaceous, and into the Cenozoic (Tertiary to present).
Early Lineages (Paleozoic): * Stem amniotes: The common ancestors of all amniotes appearing in the Carboniferous. * Synapsids: Lineage leading to Pelycosaurs, Therapsids, and eventually modern Mammals. * Anapsids: A lineage including Mesosaurs and Captorhinids; modern Turtles were historically linked here, though their placement is debated. * Diapsids: The vast lineage originating from stem diapsids that split into two major groups: Lepidosaurs and Archosaurians.
Lepidosaur Lineage: * Includes Plesiosaurs, Ichthyosaurs, and the modern Lepidosaurs which consist of Tuataras, Lizards, Amphisbaenians, and Snakes.
Archosaurian Lineage: * Includes Thecodonts, Pterosaurs, Saurichians, Ornithischians, Crocodilians, and modern Birds.
Form and Function: Integument and Egg Structure
Integumentary System: * General Characteristics: The skin is tough, dry, and highly keratinized to prevent desiccation. * Epidermal Scales: Derived from the epidermis. Key components include: * Scales: The visible keratinous outer plates. * Osteoderm: Bony plates located within the dermis that provide additional protection. * Melanophores: Pigment-containing cells located in the dermis responsible for coloration. * Epidermis and Dermis: Distinct skin layers separated by a flexible hinge that allows for movement.
The Amniotic Egg (Fig 26.3): * Shell Variations: * Hard-shelled: Calcareous in nature. * Soft-shelled: Leathery texture. * Membrane Structures: * Amnion: A fluid-filled sac that encloses and protects the embryo. * Allantois: Functions in respiration and the storage of nitrogenous waste. * Chorion: The outermost membrane involved in gas exchange. * Yolk sac: Provides nutrients to the developing embryo. * Embryo: The developing organism itself.
Physiological and Anatomical Generalizations
Skeletal and Morphological Traits: * Possess toes on each limb. * Occipital Condyle: A single bony protrusion at the base of the skull that articulates with the first vertebra.
Circulatory and Respiratory Systems: * Heart: Most reptiles possess a -chambered heart; however, Crocodilians possess a specialized -chambered heart. * Lungs: Respiratory system functions by pulling air into the lungs through thoracic expansion.
Metabolism and Waste Management: * Poikilothermic: Internal temperature fluctuates according to the environment. * Excretion: Nitrogenous waste is excreted primarily as uric acid to conserve water. * Salt Glands: Located near the nose or eyes to assist in salt excretion and water conservation.
Nervous and Sensory Systems: * Possess pairs of cranial nerves. * Jacobson’s Organ: Also known as the vomeronasal organ or Ruysch’s organ. This is an olfactory chamber located in the roof of the mouth where the tongue delivers chemical particles for scent detection.
Internal Anatomy (General Reptilian Model): * Brain segments: Cerebrum, Cerebellum, Midbrain, Medulla oblongata, Olfactory bulb, and Pituitary gland. * Major vessels: Carotid artery, Jugular vein, Aortic arch, Aorta, Left ventricle, Right ventricle, Posterior vena cava, and Hepatic portal vein. * Digestive/Excretory track: Esophagus, Trachea, Stomach, Liver, Spleen, Small intestine, Colon, and Cloaca. * Urogenital: Testis and Metanephric kidney.
Order Testudines: Turtles
Shell Anatomy: * Carapace: The dorsal (top) part of the shell. * Plastron: The ventral (bottom) part of the shell. * Integration: The vertebrae and ribs are fused to the carapace.
Cognition and Senses: * Possess a well-developed cerebrum; studies show they are capable of navigating mazes. * Senses of smell and vision are good, and they possess the ability for color perception.
Ecological and Reproductive Traits: * Reproduction: All are oviparous (lay eggs). * Marine Turtles: Known for long-distance migrations between feeding and nesting grounds. * Predation Example: Snapping turtles utilize a worm-like tongue to lure prey.
Order Squamata: Lizards and Snakes
Overview: Represents approximately of all living reptile species.
The Kinetic Skull: Features a movable, jointed skull involving the quadrate bone. This flexibility allows for an increased gape and more efficient prey handling. * Structure includes the Pterygoid, Upper jaw, and Lower jaw.
Sub-order: Sauria (Lizards): * Characterized by extreme diversity in form and niche.
Sub-order: Serpentes (Snakes): * Body Plan: Limbless with a highly flexible skeleton. * Hinged Jaw: Lateral halves of the lower jaw are joined by flexible tissues, enabling the consumption of prey much larger than the snake’s head. * Ocular Adaptation: The eye is covered by a spectacle, a transparent membrane; snakes lack eyelids and do not blink.
Locomotion in Serpentes
Lateral Undulation: The standard S-shaped movement where the snake exerts side-to-side force against surface irregularities.
Concertina Movement: Used in narrow channels or climbing; parts of the body bunch up and anchor while other parts stretch forward.
Rectilinear Movement: A slow, straight-line motion where the skin and muscle move in a wave-like pattern (Move → Contract → Fix → Stretch → Move).
Side-winding: A specialized motion for move across loose sand where the snake throws its body forward in loops.
Snake Predation and Sensory Organs
Feeding Strategies: * Most snakes utilize ambush tactics and swallow prey whole. * Constriction: Wrapping the body around prey to suffocate it.
Pit Vipers: * Nocturnal predators equipped with pit organs, which are specialized heat sensors. * Pit Structure: Consists of an outer chamber and an inner chamber separated by a membrane containing receptor nerve endings packed with mitochondria.
Venom Apparatus: * Fangs: Specialized teeth used to inject venom; often released after the strike. * Mechanism: Venom gland → Venom duct → Discharge orifice in the fang. * Venom Types: Can be neurotoxic (affects nervous system), hemotoxic (affects blood/tissue), or a combination of both.
Order Crocodilia and Specialized Study
Physical Traits: * Thecodont Dentition: Teeth are set into bony sockets (thecodont), a trait shared with early archosaurians.
Life History and Reproduction: * Nests: Juveniles and nests are highly vulnerable to predation. * Temperature-dependent Sex Determination (TSD): The incubation temperature of the eggs determines the sex of the offspring. * American Alligator specific pattern: Known as F-M-F; females are produced at lower and higher temperatures, while males are produced at intermediate temperatures during the critical period ( to weeks into incubation).
Academic Field: * Herpetology: The formal scientific study of reptiles and amphibians.