amniote origins and nonavian reptiles - unicamdocenti.unicam.it/tmp/2604.pdf · nonavian reptiles...

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CHAPTER 26

Amniote Origins and Nonavian

Reptiles

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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”



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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



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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


amniote origins and nonavian reptiles - unicamdocenti.unicam.it/tmp/2604.pdf · nonavian reptiles...

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