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26-1
CHAPTER 26
Amniote Origins and Nonavian
Reptiles
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Diversity
Amniotic “Pond” Any animal with a shell-less egg remains tied to
water
Development of a shelled egg freed the reptilian groups to exploit land
Extraembryonic membranes from previous evolutionary aquatic stages are maintained
Allantois serves as a respiratory surface and a chamber to store nitrogenous wastes
Chorion allows O2 and CO2 to freely pass
Surrounding the organism is a porous, parchment-like or leathery shell
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Amniotes Amniota
Single, monophyletic lineage of Paleozoic tetrapods
Nonavian reptiles, birds, and mammals
Developing young enclosed by extraembryonic membrane called the amnion
Secretes fluid, amniotic fluid, in which embryo/fetus floats
Diversity
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Diversity Members of the paraphyletic class Reptilia
includes nearly 8000 species Approximately 340 occur in the U.S. and Canada
The Age of Reptiles lasted over 165 million years and included the dinosaurs
A mass extinction occurred at the end of the Mesozoic
Modern reptiles represent surviving lineages Tuatara is the sole survivor of a group that
otherwise disappeared 100 million years ago
Lizards and snakes radiated into diverse and abundant groups
Diversity
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Origin and Early Evolution of Amniotes
History Amniotes arose from amphibian-like
tetrapods, the anthracosaurs, during the Carboniferous
By the late Carboniferous, skulls of amniotes could be separated into groups based on three patterns of openings (fenestra) in the temporal region
Anapsids
Have a skull with no temporal opening behind the orbits
Modern turtles
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Diapsids Gave rise to all other traditional “reptiles” (except
turtles) and to birds
Skull has two temporal openings
One pair below the cheeks and another above
Lepidosaurs
Ichthyosaurs and living reptiles, including lizards and snakes
Archosaurs
Includes dinosaurs, living crocodilians, and birds
Sauropterygians
Includes extinct aquatic groups including the long-necked plesiosaurs
Origin and Early Evolution of Amniotes
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Synapsids Mammal-like reptiles with a single pair of
temporal openings low on the cheeks
These openings are associated with large muscles that elevate the lower jaw
Changes in jaw musculature Might reflect a shift from suction feeding in aquatic
vertebrates to terrestrial feeding
Required larger muscles to produce static pressure
Functional significance of the evolution of temporal openings in amniotes is not fully understood
Origin and Early Evolution of Amniotes
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Derived Characters of Amniotes
Amniotic egg
All amniotes have eggs with four extraembryonic membranes Amnion, allantois, chorion, and yolk sac
Amnion Encloses the embryo in fluid-filled space
Cushions the embryo and provides an aqueous medium for growth
Allantois Stores metabolic wastes
Origin and Early Evolution of Amniotes
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Chorion Surrounds embryo and all other extraembryonic
membranes
Lies just beneath shell
Highly vascularized
Respiratory surface
Allantois and chorion sometimes fuse to form respiratory structure: chorioallantoic membrane
Yolk sac Nutrient storage
Origin and Early Evolution of Amniotes
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Amniotic egg permitted development of a larger, faster-growing embryo
One hypothesis suggests that one step in the evolution of the amniotic egg was replacement of the jelly layer with a shell Provided better support and movement of oxygen
Shell could also be broken down to provide calcium for growing skeletal structures
All amniotes Lack gilled larvae and have internal fertilization
Eliminated the need for aquatic environments
Penis is the most common copulatory organ Derived from cloacal wall, appears to be an amniote
innovation
Origin and Early Evolution of Amniotes
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Rib ventilation of the lungs
Amniotes draw air into lungs by expanding the thoracic cavity using costal muscles or pulling the liver posterior
The shift from positive to negative ventilation probably influenced the evolution of amniotic limbs
Early tetrapods used the rib muscles to make lateral undulations producing a wriggling motion
Origin and Early Evolution of Amniotes
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Thicker and more waterproof skin
Amniote skin is thick and tends to be more keratinized and less permeable to water
Variety of structures composed of keratin such as scales, hair, feathers, and claws project from the skin
Keratin protects the skin from physical trauma, and lipids prevent water loss
Few amniotes use skin as a primary respiratory organ because keratin and lipids limit exchange of gases
Origin and Early Evolution of Amniotes
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Changes in Traditional Classification of Reptiles
Clade Archosauria Includes the birds, crocodilians, and the extinct
dinosaurs and pterosaurs
Archosaurs along with their sister group the lepidosaurs (lizards and snakes), and turtles form a monophyletic group that cladists call Reptilia
The term “nonavian reptiles” refers to a paraphyletic group that includes the living turtles, lizards, snakes, tuataras, and crocodilians, and a number of extinct groups including plesiosaurs, ichthyosaurs, pterosaurs, and dinosaurs
Origin and Early Evolution of Amniotes
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Characteristics of Nonavian Reptiles
Nonavian reptiles have better developed lungs than amphibians Nonavian lungs have more surface area than
amphibians’ lungs
Nonavian ventilation occurs by drawing air into the lungs rather than pushing air into the lungs
Nonavian reptiles expand the thoracic cavity thus expanding the rib cage in order to draw air in
Pulmonary respiration is supplemented by respiration in the cloaca or pharynx in many aquatic turtles and by cutaneous respiration in sea snakes and turtles
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Nonavian reptiles have tough, dry scaly skin that offers protection against desiccation and physical injury Amphibians’ thin, moist skin permits gas
exchange
Also makes them vulnerable to dehydration
Skin of nonavian reptiles has an epidermis of varying thickness and a thick, collagen-rich dermis
Dermis contains chromatophores
Color-bearing cells that give many lizards and snakes their colorful hues
Characteristics of Nonavian Reptiles
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Resistance to desiccation Provided primarily hydrophobic lipids in epidermis
Scales of nonavian reptiles Formed mostly of beta keratin and provide
protection against wear in terrestrial environments
In crocodilians, scales remain throughout life
In other nonavian reptiles such as lizards and snakes, new keratinized epidermis replaces old epidermis which is shed
Turtles have scutes
Platelike modified scales
Crocodiles and many lizards possess osteoderms
Bony plates located the dermis beneath scales
Characteristics of Nonavian Reptiles
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Amniotic egg of nonavian reptiles Permits rapid development of large young in
relatively dry environments
All egg shells of nonavian reptiles are impregnated with calcium
Turtles have eggs with rigid shells
All others are leathery
In some species of nonavian reptiles
Embryo develops in reproductive tract of female
Characteristics of Nonavian Reptiles
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Jaws of nonavian reptile Efficiently designed for applying crushing or
gripping force to prey
Larger jaw muscles have mechanical advantage over the jaws of fishes which are designed for suction feeding or for quick closure
Tongue is muscular and mobile Functions to move food in mouth for mastication
and swallowing
Characteristics of Nonavian Reptiles
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Nonavian reptiles have an efficient and versatile circulatory system and higher blood pressure than amphibians Right atrium receives deoxygenated blood and is
partitioned from the left atrium which receives oxygenated blood
Crocodilians have separated ventricles
In other nonavian reptiles, ventricle is incompletely partitioned into multiple chambers
All nonavian reptiles have two functionally separate circulations
Characteristics of Nonavian Reptiles
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Nonavian reptiles have efficient strategies for water conservation Amphibians secrete metabolic waste primarily as
ammonia
Ammonia is toxic
Must be removed in a dilute solution such as water
Nonavian reptiles secrete nitrogenous wastes as uric acid
Relatively non-toxic
Characteristics of Nonavian Reptiles
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All amniotes have metanephric kidneys
Nonavian reptiles cannot concentrate urine in the kidneys
Urinary bladder receives undiluted urine
Water and most salts
Resorbed in the bladder
“urine” voided as a semisolid mass of uric acid
Salt is removed by salt glands near the nose, eyes, or tongue
Characteristics of Nonavian Reptiles
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The nervous system of nonavian reptiles is more complex than that of amphibians Brain of nonavian reptiles small compared to
brain of other amniotes
However, cerebrum of all amniotes is relatively enlarged
Enlargement of cerebrum correlated with integration of sensory information and muscle control during locomotion
Nonavian reptiles have good vision
Snakes and many lizards use a highly sensitive sense of smell to find prey and mates Olfaction assisted by a Jacobson’s organ, a specialized
olfactory chamber in the roof of the mouth
Characteristics of Nonavian Reptiles
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Characteristics and Natural History of Reptilian Orders
Order Testudines (Chelonia) Fossils appear in the Upper Triassic, 200
million years ago
Shells consist of a dorsal carapace and a ventral plastron Outer horny layer of keratin and an inner layer of
bone
Bony layer is a fusion of ribs, vertebrae, and dermally-ossified elements
Unique among vertebrates, limbs and limb girdles are located deep to the ribs
Lack teeth and use tough, horny plates for gripping food
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Breathing Consequence of having a rigid shell
Turtle cannot expand chest to breathe
Solve problem by using abdominal and pectoral muscles as a “diaphragm”
Air is drawn in by contraction of the limb flank muscles, increasing abdominal cavity volume
Exhalation accomplished by drawing back the shoulder girdle to compress the viscera
These actions are visible as bellows-like movements at the turtle’s “limb pockets”
Movement of limbs while walking also helps ventilate the lungs
Some aquatic turtles gain sufficient oxygen by pumping water in and out of the mouth cavity
Characteristics and Natural History of Reptilian Orders
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Nervous System and Senses Brain is small, less than 1% percent of body weight
Cerebrum is larger than that of amphibians
Some turtles can learn a path through a maze
Have a middle and an inner ear but sound perception is poor
Make few sounds aside from those made during mating
Good sense of smell, acute vision, and color perception about equal to humans
Characteristics and Natural History of Reptilian Orders
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Reproduction and Development Oviparous
Fertilization is internal
Bury their eggs in the ground
Female lays her eggs in a nest and deserts them
In some turtle families nest temperature determines sex of hatchlings
Low temperatures produce males
High temperatures produce females
Characteristics and Natural History of Reptilian Orders
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Giant Turtles Buoyed by water, marine turtles may reach two
meters long and 725 kilograms in weight
Giant land tortoises, such as those on the Galápagos Islands, weigh several hundred kilograms
Low metabolic activity may explain their longevity, believed to exceed 150 years
Box Turtles Shell is an effective coat of armor
Plastron is hinged
Pulls the plastron up to fully enclose body
Characteristics and Natural History of Reptilian Orders
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Snapping Turtles Have a reduced shell that does not permit full
withdrawal of the body
Jaws are adequate defense
Entirely carnivorous and can eat fish, frogs, waterfowl, etc.
Aquatic but must come ashore to lay eggs
Characteristics and Natural History of Reptilian Orders
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Order Squamata: Lizards and Snakes
Most recent and diverse of diaspids
Account for up 95% of living nonavian reptiles
Lizards appeared in the fossil record in the Permian but did not radiate until the Cretaceous
Snakes appeared in the late Cretaceous from a group whose descendants include monitor lizards
Characteristics and Natural History of Reptilian Orders
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Diaspid skulls have lost dermal bone ventral and posterior to lower temporal opening
Allowed evolution in lizards of a mobile skull with movable joints, a kinetic skull
The quadrate, fused to the skull in other nonavian reptiles, has a joint at the dorsal end and articulates with the lower jaw
Joints in palate and across roof of the skull allow snout to be tilted up
Allows squamates to seize and manipulate prey and effectively close the jaw with force
Characteristics and Natural History of Reptilian Orders
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Exceptional skull mobility of snakes is considered a major factor in their diversification
Viviparity In reptiles, limited to squamates
Evolved at least 100 separate times
Associated with cold climates
Increasing the length of time eggs are kept in oviduct
Developing young respire through extraembryonic membranes
Young obtain nutrition from yolk sacs, via the mother, or a combination of both
Characteristics and Natural History of Reptilian Orders
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Suborder Sauria: Lizards Diverse group with terrestrial, burrowing,
aquatic, arboreal, and some aerial members
Geckos Small, agile, nocturnal forms
Adhesive toe pads allow them to walk on ceilings
Iguanids Include many New World lizards as well as the
marine iguana of the Galápagos
Chameleons Arboreal lizards of Africa and Madagascar
Many have an extendible tongue
Characteristics and Natural History of Reptilian Orders
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Some have degenerate limbs Glass lizards are nearly limbless
Movable eyelids whereas snakes have a transparent covering
Nocturnal geckos have retinas with only rods
Day-active lizards have both rods and cones
Have an external ear that snakes lack
Geckos use vocal signals to announce territory and drive away males
Some lizards survive well in hot and dry regions
Characteristics and Natural History of Reptilian Orders
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Conserve water by producing semisolid urine with a high content of crystalline uric acid
Some can store fat in tails to provide energy and metabolic water during drought
Gila monster and beaded lizard are the only lizards capable of a venomous bite
Lizards keep body temperature relatively constant by behavioral thermoregulation, although they are ectotherms Successful strategy in ecosystems with low
productivity and warm climates, such as tropical deserts and grasslands
Characteristics and Natural History of Reptilian Orders
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Amphisbaenians or “worm lizards” highly specialized for a fossorial (burrowing life) Until recently, were placed in a separate
suborder, Amphisbaenia because they appeared to be so different from other lizards
Morphological and molecular data show they are highly modified lizards
Have elongate, cylindrical bodies of nearly uniform diameter
Lack any trace of limbs
Eyes are usually hidden below skin and there are no external ear openings
Skull is conical or spade-shaped to assist in tunneling
Characteristics and Natural History of Reptilian Orders
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Skin is formed in independently-moving rings which can grip the soil
Creates a movement similar to earthworms
Found in South America and tropical Africa
In the U.S., one species occurs in Florida called the “graveyard snake”
Characteristics and Natural History of Reptilian Orders
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Suborder Serpentes: Snakes Limbless and have lost pectoral and pelvic
girdles (except in pythons)
The many vertebrae are shorter and wider than in other tetrapods, allowing undulation
Elevation of the neural spine gives the musculature more leverage
Feeding apparatus allows them to eat prey several times their own diameter Two halves of lower jaw are loosely joined,
allowing them to spread apart
Skull bones also loosely articulated so mouth can accommodate large prey
To allow breathing during the slow process of swallowing, the tracheal opening is extended
Characteristics and Natural History of Reptilian Orders
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Eyeballs have reduced mobility and a permanent corneal membrane for protection
Most have poor vision Arboreal snakes in tropical forests have highly
developed vision
Lack external ears and do not respond to most aerial sounds
Can feel vibrations at low frequencies, especially vibrations carried in the ground
Chemical senses rather than vision or hearing are main senses used to hunt prey
Characteristics and Natural History of Reptilian Orders
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Jacobson’s organs Pair of pits in the roof of the mouth
Lined with olfactory epithelium
Forked tongue picks up scent particles and conveys them to this organ
Skin is infolded between scales When stretched by a large meal, the skin is
unfolded
Characteristics and Natural History of Reptilian Orders
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Snake Locomotion Lateral undulation
S-shaped movement that pushes against rough ground and water
Concertina movement Extension of S-shaped loops to strike or to climb trees
Rectilinear movement Straight movement using minute lifting of consecutive
ribs
Sidewinding Sideways looping by desert vipers that “walks” them
across loose sand
Characteristics and Natural History of Reptilian Orders
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Pit vipers, such as rattlesnakes Have “pits” with nerve endings sensitive to heat
emitted by warm-bodied birds and mammals
Viper fangs are hollow and hinged to inject venom when snake strikes
Of an average of 8,000 snake bites each year in the U.S., only about 12 result in death
Nonvenomous snakes Kill prey by constriction or by biting and
swallowing
Characteristics and Natural History of Reptilian Orders
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Venomous Snakes Family Viperidae
Includes New World and Old World vipers with and without pits
Family Elapidae
Includes cobras, mambas, coral snakes, and kraits
Family Hydrophiidae
Includes the specialized sea snakes
Family Colubridae
Most are non-venomous
Several, including the African boomslang and African twig snake, are rear-fanged and bite can be fatal to humans
Characteristics and Natural History of Reptilian Orders
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Snakebite and Toxicity Saliva of harmless snakes contains limited toxins
Provided a basis for natural selection of venom
Most snake venoms are a complex combination of venom types
Neurotoxins act on the nervous system, causing blindness or inhibit respiration
Hemorrhagin type venoms break down blood vessels
Blood leaks into tissue spaces
Toxicity is measured by the median lethal dose in laboratory animals, called the LD50
Sea snakes and the Australian tiger snake have the most deadly venom per unit volume
Characteristics and Natural History of Reptilian Orders
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Large venomous snakes deliver more venom
In India and Burma, dense populations and poor footwear contribute to 200,000 bites per year
Characteristics and Natural History of Reptilian Orders
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World total for deaths from snakebite is about 50,000 to 60,000 each year Most deaths occur in India, Pakistan, Myanmar,
and nearby countries where poorly shod people frequently come into contact with venomous snakes or do not get immediate medical attention once bitten
Less than 20% of all snakes are venomous, although venomous species outnumber nonvenomous species by 4 to 1 in Australia
Characteristics and Natural History of Reptilian Orders
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Reproduction Most are oviparous and lay shelled eggs
Others, including pit vipers, are ovoviviparous
A few snakes are viviparous, having a primitive placenta to exchange nutrients with the young
Female snakes can store sperm and lay several clutches of fertile eggs long after a single mating
Characteristics and Natural History of Reptilian Orders
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Order Sphenodonta: The Tuatara
Only 2 living species in New Zealand represent this ancient lineage
Sphenodontids radiated modestly in the early Mesozoic but then declined
Once widespread across New Zealand, the 2 species are now restricted to small islands
Loss of the tuatara populations caused by human introduction of nonnative species which preyed upon the tuatara Tuatara are vulnerable because they have slow
growth and reproductive rates
Characteristics and Natural History of Reptilian Orders
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Lizard-like and live in burrows often shared
with petrels
Slow growing and may live to 77 years of age
Skull nearly identical to diapsid skulls of 200
million years ago
Well-developed median parietal eye buried
beneath skin
Sphenodon represents one of the slowest
rates of evolution known among vertebrates
Characteristics and Natural History of Reptilian Orders
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Order Crocodilia: Crocodiles and Alligators Modern crocodilians are the only surviving
reptiles of the archosaurian lineage
Clade gave rise to the Mesozoic radiation of dinosaurs and to birds
Modern crocodilians differ little from primitive crocodilians of the early Mesozoic
There are 3 families of modern crocodilians Alligators and caimans are found primarily in the
New World and have a broader snout
Crocodiles are widely distributed
One species of gavial occurs in India and Burma and has a very narrow snout
Characteristics and Natural History of Reptilian Orders
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All have long, well-reinforced skull and jaw musculature for a powerful bite
Theocodont dentition Teeth are set in sockets
Have a complete secondary palate, a feature only shared with mammals
Four-chambered heart
Alligators and crocodiles are oviparous Usually 20–50 eggs are laid in mass of vegetation
Unguarded nests are easily discovered and raided by predators
High nest temperatures produce males
Low temperatures produce females
Can result in females outnumbering males 5 to 1
Characteristics and Natural History of Reptilian Orders