The Systems of Reptiles

Taxonomic Classification and Evolutionary Lineages of Reptiles

• Veterinary science identifies four primary groups of reptiles: Crocodiles, Snakes, Lizards, and Turtles.

• These groups originated from two distinct primitive lineages:

  • Lineage One: Encompasses turtles, terrapins, and tortoises.

  • Lineage Two: Encompasses crocodilians, lizards, and snakes.

• Snakes are classified as highly evolved lizards.

• While distinct, these groups share many physiological similarities across their body systems.

The Reptilian Integumentary System

• Scales and Keratin:

  • Scales are overlapping folds of skin primarily composed of keratin, which is also the chief constituent of hair, nails, and horny tissues.

  • Beta keratin: A stiff material providing durability and resistance to water loss.

  • Alpha keratin: A more pliable material that facilitates movement between scales.

• Scale Variation:

  • Snakes: Scales are often small and smooth to aid in movement.

  • Turtles: Scales are thick and hard for defense and durability.

  • Burrowing species: Often possess slippery, smooth scales.

  • Desert reptiles: May have spiny scales on the head and tail for extra defense.

• Osteoderms: Body plates located in the skin beneath the scales found in crocodilians and some lizards. These can fuse with the skull to form a rigid skull cap.

• Growth and Ecdysis:

  • Turtles and crocodilians experience continuous skin growth (unless hibernating).

  • Lizards and snakes experience periodic growth, followed by periods where growth stops, necessitating ecdysis (the process of sloughing or shedding skin).

• Coloration and Scent:

  • Chromatophores: Specialized pigment-containing cells controlled by hormones, nerves, or environmental factors (light and temperature).

  • Pheromones: Substances secreted to elicit specific behaviors in members of the same species.

  • Glandular Structures:

    • Turtles/Tortoises have musk glands in the throat, groin, and armpits.

    • Male lizards have prefemoral pores (in front of the femur) for pheromone secretion.

    • Snakes have scent glands at the base of their tails.

The Reptilian Skeletal System

• General Anatomy: Includes a bony skull, vertebral spinal column, ribs, and (except for snakes) limbs. Boas possess vestigial hindlimbs.

• Limb Bones:

  • Humerus and femur are typically smooth, long, and cylindrical.

  • In the front limb, the ulna is larger than the radius and bears most weight.

  • In the hindlimb, the tibia is larger than the fibula and bears most weight.

• Skull Characteristics:

  • Most reptiles (excluding turtles) have an opening in each temple for flexibility and possess long, conical, identical teeth.

  • Snakes: Have flexible skulls with mandibles that can disconnect (dislocate) into two halves to swallow large prey.

  • Crocodilians: Skulls consist of $30$ fused bones; teeth are socketed in the bone.

  • Lizards: Teeth attach to the edge or surface of the jaw.

  • Turtles: Possess inflexible skulls and lack teeth; they use keratinous jaws to cut and crush food.

• Turtle Shell Anatomy:

  • Carapace: The upper shell, a living structure fused to the skin with over $50$ bones (mostly ribs and vertebrae).

  • Plastron: The lower shell, formed by the fusion of clavicles, interclavicles, and abdominal ribs.

  • Scutes: Keratin shields protecting the bone seams. Belly scutes can be used to estimate age (e.g., box turtles up to $15$ years).

• Vertebral Variations:

  • Snakes: Can have over $300$ vertebrae, most with ribs.

  • Crocodilians: $60$ to $70$ vertebrae; specialized sacral ribs anchor the column to the pelvic girdle.

  • Tail Autotomy: Many lizards have a cartilage plate in tail vertebrae creating a fracture plane, allowing the tail to break off to escape predators and regrow later.

The Reptilian Muscular System

• Jaw and Head Muscles:

  • Reptiles lack facial muscles.

  • Crocodilians: Possess weak jaw-opening muscles but powerful jaw-closing muscles capable of crushing bone.

  • Turtles: Have large muscle masses to retract the head into the shell.

• Locomotion and Trunk Muscles:

  • Snakes: Highly developed trunk muscles with multiple attachments (ribs, vertebrae, and skin) to facilitate movement without limbs.

  • Turtles: Have few trunk muscles due to the restrictive shell.

  • Pectoral Muscles: Dual function of maintaining stance and rotating bones for propulsion.

• Specific Turtle Muscles:

  • Triceps brachii: Extends and rotates the arm.

  • Biceps brachii: Flexes the forearm.

  • Serratus magnus: Extends and moves the arm backward.

  • Deltoid: Moves the arm forward.

  • Pectoralis major: Pulls the arm down.

  • Attrahens pelvim: Anchors the pelvic girdle.

  • Oblique/Transverse abdominis: Attached to the plastron; aid in respiration.

  • Triceps femoris adductor: Extends the thigh.

The Reptilian Respiratory System

• Turtle Respiration: The rigid shell prevents chest expansion. Turtles use two flank muscles to enlarge the body cavity for inhalation and two other muscles to press viscera against lungs for exhalation. Some breathe through the skin of the mouth or cloaca.

• Crocodilian Respiration:

  • Nares are on the dorsal surface and can be closed.

  • Velum palatinum (soft palate): Meets a tongue fold to allow breathing through nares while the mouth is open underwater.

  • Crocodilians are unique among reptiles for having a diaphragm.

• Lizard Respiration: Lack diaphragms; rely on rib movement. Advanced lizards have divided, sponge-like lung chambers.

• Snake Respiration:

  • Most have only the right lung, which may be half the body length.

  • The left lung, if present, is no more than $85\%$ of the right lung's size.

  • Tracheal lung: An extension of the lung into the dorsal trachea in some species.

  • Epiglottic cartilage: A rigid plate used by some snakes to produce a hiss.

The Reptilian Digestive System

• Tongue and Oral Cavity:

  • Turtles and crocodilians have fleshy tongues attached to the floor of the mouth.

  • Chameleons can project tongues longer than their body length.

  • Monitor lizards and snakes have forked, telescoping tongues for scent collection.

• Venom Systems: Venom glands are modified salivary glands. Delivery occurs via fangs (snakes) or grooved teeth (Gila monster, Mexican beaded lizard).

• Gastrointestinal Track:

  • Esophagus: In snakes, it is half the body length and lacks muscle; in sea turtles, it has posterior-pointing papillae to swallow slippery prey.

  • Stomach: Crocodilians have a gizzard-like anterior portion and swallow stones to grind food.

  • Intestines: Herbivorous turtles and lizards use the large intestine/colon for bacterial fermentation.

  • Cloaca: The common exit for digestive and urogenital systems. The coprodaeum is the distal portion for fecal elimination.

• Large Organs: The liver is typically two-lobed and chocolate brown; the pancreas produces both endocrine (hormones) and exocrine (enzymes) secretions.

The Reptilian Circulatory System

• Metabolism: Low hemoglobin levels result in a slower pace of activity compared to birds/mammals.

• Heart Anatomy:

  • Most reptiles (turtles, lizards, snakes) have a three-chambered heart (two atria, one ventricle).

  • Crocodilians are the only reptiles with a true four-chambered heart.

• Blood Mechanics: Crocodilians possess two aortae that can allow blood to bypass the lungs entirely during submersion via a small opening between the aortae.

• Renal-Portal System: Blood from the tail and hindlimbs passes through the kidneys before entering general circulation. This requires caution during medical injections to avoid nephrotoxicity or drug filtration.

• Lymphatics: Extensive system but lacks lymph nodes. Includes the thymus and spleen for lymphocyte production.

The Reptilian Nervous and Sensory Systems

• Brain Structure: Consists of the forebrain (olfactory lobes, cerebral hemispheres, diencephalon) and hindbrain (cerebellum, medulla oblongata).

• Pineal and Parietal Organs: The pineal organ regulates thermoregulation and hormones. Some lizards have a parietal eye (third eye) to sense light.

• Vision:

  • Most reptiles have a bony ossicle surrounding the eye.

  • Crocodilians have a nictitating membrane for underwater protection.

  • Snakes have a protective spectacle (fused eyelids) shed during ecdysis.

• Auditory System:

  • Crocodilians have a single middle-ear bone, the stapes.

  • Snakes lack a tympanic membrane/middle-ear cavity but use the columella bone to sense vibrations.

• Olfaction and Senses:

  • Jacobson’s Organ (Vomeronasal organ): Receives pheromones via the tongue.

  • Heat-sensing pits: Infrared receptors in certain snakes that allow navigation in darkness.

  • Cutaneous sense organs: Mechanoreceptors in the skin for pressure and pain.

The Reptilian Urogenital and Endocrine Systems

• Renal Physiology: Metanephric kidneys lack a loop of Henle, meaning reptiles cannot concentrate urine and instead produce insoluble waste to conserve water.

• Reproduction:

  • Internal fertilization: Turtles and crocodilians use a penis; lizards and snakes use paired hemipenes.

  • Parthenogenesis: Reproduction without fertilization (found in some lizards and the Asian Brahminy snake).

• Endocrine Glands:

  • Pituitary: Master gland regulating all others.

  • Parathyroid: Controls calcium levels ($H$-shaped in crocodilians).

  • Thyroid: Regulates growth and ecdysis.

  • Adrenal glands: Produce epinephrine and corticosteroids to control electrolyte/water balance.