Vertebrate Zoology
Unit 4: The Reptiles
Overview of Amniotes
Amniotes include Reptiles, Crocodilians, Birds, and Mammals.
The Class Reptilia is not recognized by phylogenetic systematists.
Traditional taxonomy includes turtles, tuataras, lizards, snakes, crocodilians, and birds as a monophyletic group (Sauropsida).
There is ongoing disagreement among experts regarding relationships within these groups.
Cladistic Structure of Reptiles
Lissamphibia: frogs, toads, salamanders
Mammalia: mammals
Testudines: turtles
Lepidosaurs: lizards, snakes
Crocodylia: crocodilians
Aves: birds
Crocodylia and Aves are sister taxa within Archosauria.
Sauria (saurians) connects Mammalia & Sauropsida as sister taxa.
Amniota encompasses both Mammalia and Sauropsida.
Tetrapoda includes Lissamphibia and Amniota.
Classification Concerns
The cladistic view of Sauropsida is monophyletic (includes Aves), while traditional Reptilia is paraphyletic, excluding Aves.
Fossil Record & Evolutionary Adaptations
Superclass Tetrapoda includes Class Reptilia; fossil records are more complete for reptiles, dating back over 300 million years.
There is disagreement regarding the evolutionary relationships between reptiles.
The amniotic egg is noted as an evolutionary first, offering mechanical protection and moisture regulation with either leathery & flexible or calcified & rigid shells.
Extra Embryonic Membranes of the Amniotic Egg
Chorion: outer membrane surrounding the embryo for gas exchange and blood vessel formation.
Amnion: inner membrane that encases the embryo, forming an amniotic cavity filled with amniotic fluid.
Yolk Sac: encloses the yolk (vitellus), serving as a food supply.
Allantois: storage of nitrogenous wastes and acts as a respiratory structure.
Characteristics of Amniotic Egg
Components:
Albumin: protein supplying water and nutrition
Chorion: facilitates gas exchange
Amnion: protects the embryo within the amniotic fluid
Yolk Sac: stores nutrients for the developing embryo
Allantois: waste storage and gas exchange
Shell: protective layer surrounding the egg
Evolutionary Discussion of Reptiles
Amniotic eggs display no larval stage and little fossil evidence due to poor fossilization of eggs.
Early reptiles are debated to have been either aquatic with land-depositing eggs or terrestrial utilizing the amniotic egg for drought adaptation.
Extinct and extant reptiles were dominant during the Mesozoic era; they adapted to terrestrial, aquatic, and aerial environments, showcasing quadruped and biped forms, with diets ranging from carnivores to herbivores.
Reptilian Classification & Anatomical Features
Origin of mammals is traced back to the late Triassic.
Traditional classifications are based on the number and location of temporal fenestrae, which are openings in the skull accommodating jaw musculature:
Anapsida: no openings (e.g., turtles)
Diapsida: two openings (e.g., lizards, snakes, crocodiles, birds)
Synapsida: single opening (e.g., mammals & mammal-like reptiles)
Detailed Classifications of Reptilian Orders
Order Testudines (turtles): anapsid; originate in the late Permian.
Order Rhynchocephalia (tuataras): diapsid; identified by two pairs of fenestrae; located in New Zealand and considered the most primitive reptiles from the Triassic, noted for the parietal foramen (third eye).
Order Squamata (lizards and snakes): diapsid; characterized by two pairs of fenestrae; shows limb variation and more recent evolution.
Order Crocodilia (crocodiles and alligators): diapsid; known for size, with fossils showing lengths of 12m and over 13 tons.
Morphology of Reptilian Integuments
The integumentary system predominantly consists of scales formed from epidermis, providing protection and reducing water loss.
Turtles have shells covered by horny scutes; each species exhibits specific scales in number and arrangement.
Squamates maintain continuous body coverage of scales overlapping each other. The stratum corneum of the epidermis undergoes ecdysis (shedding):
Snakes shed as a single piece, lizards in pieces, and turtles may shed some scales.
Upon shedding, scales detach from the eyes, leading to a cloudy appearance, with frequency varying by species and age.
Integumentary Glands and Modifications
There are few integumentary glands producing strong-smelling substances for defense and sex recognition (pheromones).
Reptiles lack sweat glands.
Other epidermal modifications include keratinized claws, rattles, spines, and various horny structures:
Turtles shed claws; claws, nails, and hooves are worn down by birds and mammals.
Adhesion Mechanisms in Reptiles
Geckos and anoles use a dry adhesion system where scales possess keratinized setae, enabling them to walk vertically due to intermolecular interactions with surfaces.
Rattles in pit vipers result from modified stratum corneum, growing with each ecdysis.
Turtle Morphology
Turtle shell components include the dorsal carapace and ventral plastron, united by bony lateral bridges reflecting their ecological niches:
Terrestrial turtles possess high domed shells and elephant-like feet.
Flattened forelegs are seen in burrowing species; e.g., the pancake tortoise.
Aquatic species have streamlined bodies and softshells for faster swimming.
Head Retraction Mechanisms
Turtles can retract their heads into shells:
Suborder Cryptodira: retract heads vertically; prevalent in most turtles.
Suborder Pleurodira: retract heads horizontally; found in South America and Australia.
Coloration and Chromatophores
Chromatophores provide color changing abilities (e.g., chameleons) based on environmental stimuli, serving for camouflage, social status, and thermoregulation.
Snakes exhibit patterns—spots for camouflage and stripes to obscure speed.
Skeletal Adaptations
Skeletal structures have adapted for support and speed:
Skulls are higher, narrower, and more ossified, with reduced or fused bones.
Many turtles feature a partial secondary palate for feeding purposes.
Some snakes display independent movement of the upper jaw, promoting a stronger bite.
Vertebrae and Limbs
Snakes can have up to 500 vertebrae; the first cervical vertebrae (atlas and axis) are modified for head rotation.
Limbs are generally pentadactyl, enhancing support and speed. Aquatic and burrowing adaptations are evident in limb shapes:
Sea turtles have flattened, paddle-shaped front limbs.
Squamates demonstrate a unique evolutionary path with limb loss and redevelopment of undulatory movement, tracing back to lizards.
Snake Locomotion Techniques
Different types of movement seen in snakes include:
Horizontal undulation: serpentine motion using the body along a wavy track.
Concertina movement: S-shaped coils with stationary portions bent.
Rectilinear movement: the skin draws forward, utilizing scutes for traction.
Sidewinding locomotion: minimal contact with hot sands or unstable surfaces.
Cardiovascular and Respiratory Systems
Reptilian hearts exhibit varying chamber numbers:
3-chambered heart: found in turtles and squamates.
4-chambered heart: seen in crocodilians.
Blood cells are oval and nucleated.
Respiratory Adaptations
Snakes possess protruding tracheas when swallowing prey to avoid choking, with most being voiceless.
Turtles and lizards have vocal cords, but sounds are often inaudible; examples of audible sounds include:
Grunts from turtles,
Barks from geckos,
Squeaks from anoles.
Lung Structures in Reptiles
Reptilian lungs are more developed compared to those of amphibians:
Snakes may have paired or unpaired lungs:
Boas & pythons often have two functioning lungs (the left being smaller).
Many snakes and glass lizards have one rudimentary or absent lung.
Some species utilize specialized lungs, such as:
Cobra and hognose: possess diverticulums that allow neck inflation.
Sea snakes: utilize skin for gas exchange, with sealed nostrils allowing underwater accommodations.
Turtle Respiratory Mechanisms
Turtle gas exchange occurs via:
Buccopharyngeal respiration
Cloacal respiration
Cutaneous respiration in soft-shelled species (70% via shell) and other water turtles (e.g., musk, mud, snappers).
Lung function changes with dive depth:
Shallow divers primarily use lungs.
Deep divers experience lung collapse, thus obtaining oxygen primarily from blood.
Elevated blood volume enhances oxygen carrying capacity.
Digestive Traits
Many reptiles possess immovable, thick lips; turtles have keratinized jaws (beaks).
Teeth vary by group, with most being homodont and polyphyodont except turtles. Key classifications include:
Tuataras: monophyodont (one set of teeth).
Lizard and snake teeth types: acrodont, pleurodont.
Crocodilian teeth: thecodont (rooted in sockets).
Mandibular Structure in Snakes
Mandibles are flexible in snakes and certain lizards, connected by elastic ligaments.
Two halves of the lower jaw are supported by a folding strut, enlarging the throat region. Notably, it is essential to clarify:
These jaws do NOT unhinge; instead, the palate and jaws can move independently.
Recurved teeth on each side draw prey in through alternating movements.
Venomous Adaptations
Certain lizards (e.g., Gila monster, Mexican beaded) have grooves in teeth, enabling venom delivery from modified salivary glands that are neurotoxic. \
Venomous snakes display varying fang types:
Opisthoglyphs: rear-fanged with enlarged grooved teeth (notably weak venom, with notable exceptions like Boomslang).
Proteroglyphs: grooved fangs fit into lower jaw pockets; common in cobras and coral snakes.
Solenoglyphs: hollow, elongated hinged fangs found in vipers and pit vipers.
Venom glands are modified salivary glands that vary:
Elapidae: neurotoxic (cobras, sea snakes).
Viperidae & Crotalidae: primarily hemolytic venoms.
Envenomation Statistics
In North America, approximately 8000 envenomations occur annually, with few resulting in fatalities.
Tongue and Olfactory Adaptations
Turtle tongues: non-protrusible and attached to the mouth floor.
Squamate tongues: long, slender, forked, and protrusible. The African chameleon's tongue can exceed body length.
Tongue flicking allows sampling of environmental chemicals such as pheromones and scent trails.
Vomeronasal (Jacobson’s) organs: paired organs enhancing chemical detection, particularly in lizards and snakes.
Esophageal Adaptation
The esophagus in reptiles adapts with longitudinal folds permitting expansion; specific taxa may exhibit esophageal teeth aiding in specific dietary habits.
Stomach and Digestion
The stomach is generally elongate in lizards and snakes.
Nervous System and Senses
The hypothalamus regulates body temperature (poikilothermy) and thermoregulation through basking behavior.
Increased optic and auditory lobe size showcase evolutionary advancements, with an enlarged cerebellum contributing to motor control.
Sensory Receptors
Cutaneous receptors: respond to mechanoreception (pressure, tension, pain, temperature).
Infrared receptors: present in pit vipers and other species, aiding in prey detection (nocturnal small mammals).
Auditory and Visual Structures
Reptilian hearing varies, with snakes lacking external ear openings but receiving auditory stimuli through quadrate bones, while other reptiles maintain visible tympanic membranes.
Turtles demonstrate variable ear adaptations based on terrestrial or aquatic habitats.
Unique Adaptations in Eyes and Nose
Snake eyes feature transparent upper and lower lids fused in the closed position resembling a spectacle.
Nictitating membranes function as protective eyelids in turtles and some lizards, facilitating cleaning and lubrication.
Taste and Olfactory Sense
Generally poorer taste perceptions observed compared to fish and amphibians; taste buds largely line the pharynx.
Urogenital System Structure
Kidneys: elongated shapes are common in snakes and legless lizards; usually, the ureters feed into a cloaca.
Urinary bladders are present in tuataras, lizards, and turtles but absent in snakes and crocodilians.
- Turtles and some lizards possess large bladders acting as reservoirs for water storage.
Cloaca Structure
Tri-chambered:
Coprodeum: connects with the large intestine.
Urodeum: serves for excretory and genital processes.
Proctodeum: contains sphincter muscle.
Nitrogenous waste disposal varies:
Aquatic species excrete ammonia; terrestrial species excrete uric acid, with specialized glands in marine reptiles excreting excess salts through lacrimal or nasal glands.
Reproductive Strategies
Female reptiles typically have paired ovaries, producing usually less than 100 ova, with some squamates possessing spermatheca for sperm storage.
In many squamates and all crocodilians, only the right oviduct is functional.
Males possess paired testes with size fluctuations seasonally.
Internal fertilization is common, with various copulatory mechanisms:
Hemipenes in squamates: erectile structures deployed during mating.
Penis with a groove is observed in turtles and crocodilians.
Reproductive Stimuli and Patterns
Various environmental factors stimulate reproduction (temperature, photoperiod, precipitation).
In temperate regions, reproduction occurs 4-8 weeks post-hibernation, while tropical species breed throughout the year.
Visual and olfactory mechanisms aid mate selection; courtship behaviors are not well documented.
Mating Patterns
Aquatic turtles mate in water; terrestrial turtles may have varied behaviors including head bobbing, while lizards utilize colored displays and behaviors such as head bobbing and exhibiting crests.
Anoles present a displayed dewlap during courtship.
Snakes use tactile interaction for females and olfactory cues for males.
Most squamates are oviparous, but some, like boas and certain pit vipers, are viviparous, which poses mobility costs to females.
Sperm Storage in Females
Female snakes can store sperm for several months, leading to competition within females before fertilization.
Some lizards may mate with multiple males, often rejecting the sperm of relatives, while most reptiles exhibit a single reproductive cycle yearly.
Parthenogenesis
This method allows females to produce new individuals from unfertilized eggs.
Some reptiles can undertake facultative parthenogenesis as a backup reproductive strategy, characterized by faster population growth in those capable of exclusive reproduction through this method, often observed in lizards.
Development and Hatching
Eggs are typically buried in a variety of substrates such as sand or mud, depending on the species:
Turtle incubation: takes 8-16 weeks; females dig nests for protection against predators, with species laying from 4-5 to hundreds of eggs.
Sea Turtles
Sea turtles generally nest upstream from feeding grounds, utilizing currents to facilitate hatchling dispersal.
Factors influencing navigation include light, wave direction, and magnetism.
The “lost years” phenomenon indicates hatchlings may not resurface until they reach approximately 10 lbs, possibly utilizing algae beds for concealment.
Hatching Periods
Lizards take around 5-12 weeks to hatch, while snakes commonly hatch in 8-12 weeks.
Tuatara eggs may remain in the oviduct for 7 months before incubating for up to 16 months.
The presence of a caruncle, or egg tooth, is common in oviparous reptiles.
Temperature-Dependent Sex Determination
Three types exist:
Type A: males at high temperatures, females at low (e.g. crocodilians, some lizards).
Type B: females at high temperatures, males at low (certain turtle species).
Type C: females at both high and low temperatures, males at intermediate temperatures.
Parental Care and Longevity
Parental care in reptiles is rare, but certain snakes guard eggs, and pythons exhibit brooding behaviors to enhance temperature through muscle contractions.
Growth in reptiles is indeterminate, with sexual maturity correlated to size rather than age, which varies significantly among species:
Snakes: 2-3 years; Turtles: 3-5 years;
Longer-lived reptiles include Galapagos tortoises (up to 150 years), tuataras (77 years), and various snake species (e.g., cobras: 25 years).
Diverse Characteristics of Lizards
Over 4600 lizard species exist, ranging from 3 cm to 3 m (e.g., gecko to komodo dragon).
Habitats and lifestyles include:
Arboreal: exhibit laterally flattened forms and prehensile tails (e.g., Old World chameleons).
Carnivorous: such as monitor lizards and ambushing behaviors in the Komodo dragon.
Many large lizards (e.g., iguanas) fall into herbivore categories, and various strategies of foraging can be observed among species (e.g., motionless/inconspicuous movement for ambush).
Snake Diversity and Adaptations
With over 3400 species, snake lengths range from 10 cm to 10 m, with adaptations for diverse environments:
Burrowing Adaptations: short blunt heads and small eyes against long bodies in arboreal species.
Sea snakes possess flattened tails and nostrils with valves for aquatic respiration.
Prey Capture Methods in Snakes
Snakes utilize three primary methods for catching prey:
Seize and swallow (notably risky), seen in species like green snakes and water snakes.
Constriction: method used by boas, pythons, and certain colubrids; the snake tightens its loop as the prey exhales.
Venom delivery for immobilizing prey, a feature employed by multiple snake families.
Conclusion
This comprehensive overview of reptiles from their evolutionary adaptations to physiological traits demonstrates their ecological diversity and significance in the tree of life. Each class and order presents unique characteristics that enable their survival and reproduction across varied environments, showcasing their evolutionary success in terrestrial, aquatic, and aerial habitats.