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.