Reptile Adaptations Notes

Reptile Adaptations

Background

  • Reptile adaptations emerged as responses to amphibians' increasingly terrestrial lifestyles.
  • Eventually, a complete independence from the need to return to water for breeding was achieved, marking the evolution of the first reptiles.
  • The evolution of the first true reptile is not as clearly understood as that of the first true amphibian.
  • Many intermediate forms existed, such as Casineria, which was highly adapted to terrestrial life but retained some amphibian characteristics (e.g., no claws but reptile-like scales).
  • Internal fertilization likely evolved last but is poorly preserved in fossils.

Early Reptiles

  • Hylonomus lyelli is the earliest known unequivocal 'true reptile' from the Permian period.
  • It probably resembled a lizard and possessed most of the main adaptations of modern reptiles.
  • Claws, scales, and (soft?) eggs had already evolved by this time.
  • The lineages that would lead to synapsids (and later, mammals) had already diverged.

Permian Rainforest Collapse

  • The collapse of rainforests in the Permian period increased aridity, favoring early amniotes like Hylonomus and Elginerpeton.
  • Key adaptations included:
    1. Extended embryo retention (EER)
    2. Skeletal structure
    3. Teeth
    4. Keratinized scales
    5. Claws
    6. Respiration
    7. Sensory organs

Internal Fertilisation

  • Amphibians use external fertilization (female lays eggs, male fertilizes).
  • In salamanders, the male drops a spermatophore on the ground for the female to pick up with her cloaca.
  • The development of intromittent organs allowed reptiles to fertilize internally, removing a tie to water.
  • This became the dominant mode of reproduction in all later tetrapods.

Amniotic Egg

  • The amniotic egg structure includes:
    • Embryo
    • Amnion
    • Amniotic fluid
    • Yolk sac (nutrients)
    • Yolk
    • Allantois
    • Chorion
    • Albumin
    • Shell
  • Contrast with amphibian and fish eggs, which have:
    • Jelly capsule
    • Perivitelline fluid
    • Vitelline membrane

Reptile-Egg Order

  • It was probably the reptile that came first.
  • Jiang et al. (2023) indicated hard-shelled eggs appeared later.
  • Viviparity existed far before hard-shelled eggs.
  • EER likely helped early reptile-like tetrapods breed away from water.

Skull Morphology

  • Diapsid: Reptiles have skulls with two temporal fenestrae (openings) - a supratemporal fenestra and an infratemporal fenestra.
  • Synapsid: Mammals have skulls with one temporal fenestra.
  • Anapsid: Absence of temporal fenestrae in the skull (ancestral state; seen in early reptiles and turtles, though the condition in turtles is secondarily derived).

Tortoise Anatomy

  • Skull: Anapsid skull.
  • Carapace: complete shell.
  • Plastron: complete; consists of keratinous scutes.
  • Marginal teeth lost; incipient beak present.
  • Numerous skeletal adaptations related to shell incorporation.

Autotomy

  • Many diverse lizard lineages have evolved brightly colored tails.
  • Attracts predator attacks to the easily lost sections of tail (tail loss).
  • Many will also wag this section of tail to further highlight the tasty (regrowable) snack!

Teeth Attachment

  • Acrodont: Teeth sit on top of the jaw.
  • Thecodont: Teeth are embedded in the jaw.
  • Pleurodont: Teeth are attached to the side of the jaw.
  • Tooth location is an important taxonomic character.

Respiration: Lungs

  • Lizards: Simple lung structure.
  • Varanus (Monitor Lizards): More complex lung structure, with air sacs.

Snake Lungs

  • Snakes typically have one functional lung (right lung).
  • The lung is divided into:
    • Gas exchange section (L)
    • Air sac section (AS)

Aquatic Respiration

  • Sea snakes can respire through their skin, releasing CO_2 into the water.
  • Some have re-evolved a gill (analogous structure).

Ectothermic Metabolism

  • Ectotherms: Body temperature equilibrates to the environmental temperature.
  • Endotherms: Maintain a constant body temperature.
  • At colder environmental temperatures, metabolic heat production increases in endotherms, but falls in ectotherms.
  • The metabolic rate of a lizard is slower than that of a mouse at all temperatures.

Circulation

  • Amphibians:
    • Three-chambered heart (two atria, one ventricle).
    • Pulmonary and skin circulation.
    • Systemic circulation.
  • Reptiles:
    • Mostly three-chambered heart (two atria, one ventricle with a partial septum).
    • Pulmonary circulation.
    • Systemic circulation.

Crocodilian Circulation

  • Foramen of Panizza (FP): Connection between the left and right aorta.
  • Cog-teeth valve (CTV): Structure within the heart that helps regulate blood flow.
  • Right aorta (RA)
  • Left aorta (LA)
  • Right ventricle (RV)
  • Left ventricle (LV)
  • When diving, crocs 'shunt' deoxygenated blood from the right ventricle into the bloodstream.
  • Helps to stabilize blood oxygen levels when submerged for long periods.
  • Also speeds up digestion! CO_2 goes to the stomach/intestine and is used to produce gastric acid (Farmer et al., 2008).

Skin Structure

  • Epidermis: Outer layer.
  • Dermis: Inner layer.
  • Scales: Formed from the epidermis and contain both alpha and beta keratin (only alpha keratin in amphibian skin); beta keratin is harder and also gave rise to feathers in birds.
  • Melanophores: Pigment-containing cells.
  • Osteoderm: Bony deposits in the dermis.
  • Flexible hinge: Allows for movement.

Shedding (Ecdysis)

  • When a reptile sheds its skin, it is not simply one layer of cells that is lost.
  • Resting phases are the parts of cycles between sheds.
  • Lakes of fluid form between the old and new layers to help lubricate the shedding process.
  • Snakes have a brille (spectacle) scale over the eye; the fluid gives them their blue eyes in the build-up to shed.

Scale Specializations

  • Keeled/Unkeeled: Presence or absence of a ridge on the scale.
  • Toe Pads: For adhesion, as seen in geckos.

Croc and Turtle Skin

  • Croc scute: Armor scale formed from the dermis (not epidermis like lizards!); bones underneath = Osteoderm.
  • Tortoise scute: Armor scale formed from the epidermis.

Chameleon Skin

  • Color change is governed by hormones, mood, and temperature.
  • Photonic crystals cause active color change in chameleons (Teyssier et al., 2015).

Claws

  • Formed of an unguis (dorsal side) and sub-unguis (ventral side).
  • Derived from the integument of reptiles in response to more locomotion over land and eventually became more useful in climbing, etc.

Nitrogenous Waste

  • Reptiles excrete nitrogen primarily through uric acid (urates - the white part of reptile feces).
  • Adaptation to terrestrial living.
  • Drink directly (unlike amphibians).

Venom

  • Aglyphous: No grooved fangs.
  • Opisthoglyphous: Rear-fanged (e.g., tree snake, cat snake).
  • Proteroglyphous: Front-fanged, fixed (e.g., cobra, taipan).
  • Solenoglyphous: Front-fanged, hinged (e.g., adder, rattlesnake).

Sensory Organs

  • Fovea: Areas of the eye with sharper focusing capabilities; absent in amphibians.
  • Jacobson's organ: More developed in reptiles, allows detection of high molecular weight compounds in the air (e.g., pheromones).

Reptile Adaptations: Take-Home Messages

  • Reptiles thrived and colonized increasingly arid habitats through a suite of key adaptations from ancestral reptiliomorphs and amphibians:
    • Skeletal shifts to more robust forms.
    • Specialized teeth for the wide variety of terrestrial food items they encountered.
    • More ‘advanced’ hearts than amphibians (especially crocodiles).
    • Keratinized skin with scales and claws allowed for further adaptations of specialist scale structures like toepads, rattles, horns, etc.
    • Adaptations to sensory organs to tune them to perceiving signals through air, rather than water.
    • Internal fertilization and the amniotic egg.
    • Shift to maximizing water retention through uricotelism and direct drinking.