The Reptiles

A Quick Review of Amphibia

• Advantages of the amphibious lifestyle– escape predators– exploit ephemeral water

bodies– utilize previously

unavailable -- and unexploited -- resources. Fill vacant niches

A Quick Review of AmphibiaThe transition to land

PROBLEM DESCRIPTION SOLUTION

Support Water provides muchmore buoyancy thanair

Modification andstrengthening of fins,pelvic and pectoralgirdles, vert. column

Locomotion Tail used forpropulsion in water,but won’t work in air

Use modified fins, butstill use undulatingmotion for walking

Respiration Gills don’t work in air– surface areaproblem

Lungs

Dessication Gills would lead toexcessive water loss

Lungs, eyelids

A Quick Review of AmphibiaThe transition to land

PROBLEM DESCRIPTION SOLUTION

Sensingvibration

Lateral line won’twork in air

Modifiedhyomandibulatransmits vibration tofluid-filled inner ear

Reproduction Fertilization,dessication problems

Reproduce in water

Prey capture Fish could usesuction to helpcapture prey; won’twork in air

Tongue

Amphibians were able to exploit some terrestrial resources, but…

Reproductive constraints, among other things,forces them to remain in the vicinityof water

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Early amphibians gave rise to a new group evolved from that was able to lay eggs on dry land

The Reptiles

Amniotic Egg (more later)• Developed in proto-

reptiles and is found in their descendents-birds and mammals– no larval stage; young

look like miniature adults

– provide “aquatic environment” withinthe egg

– THE MOST IMPORTANT DIFFERENCE BETWEEN AMPHIBIANS AND REPTILES

Fossilized eggs in nest

Class Reptilia

General Characteristics of Reptilia

• Tetrapod– means four-

limbed

– secondarily lost in snakes

General Characteristics of Reptilia• Skin made of scales derived from the

epidermis– ancestors of reptiles had heavy, bony dermal

scales– reptiles developed lighter, more flexible scales

made of keratin– some still retain bony plates in skin (e.g.

alligators)– skin dry, rather than moist. Modern reptiles

generally lose less water via skin than modern amphibians

General Characteristics of Reptilia• Heart with three

chambers• Amphibians have same

system;• Ventricles not

completely separated so there is some mixing of oxygenated and de-oxygenated blood

• Does this place any constraints on activity?

• Crocodilians have four chambered heart

General Characteristics of Reptilia• Ectothermy

– depend primarily on environmental sources of heat

– basking, seeking shade, etc.

– specialized body structures

– much debate about dinosaurs, though

• vascularization of bones

• ratio of fossils of predatory vs herbivorous dinosaurs

Dimetrodon (not a dinosaur)

General Characteristics of Reptilia• Amniotic Egg

– appeared app. 310 mya, about 50 million years after the first amphibians

– required the evolution of internal fertilization

– has three membranes not found in amphibian eggs: amnion, chorion, and allantois

Components of the Amniotic Egg• Shell

– may be leathery and flexible (as in lizards) or calcified (as in birds)

– provides mechanical protection while allowing for gas and water exchange

• Albumin (egg white)– protection against

mechanical damage– provides a reservoir of

water and protein

Albumin

Components of the Amniotic Egg• Yolk

– rich in lipids (fats); energy supply for the developing embryo

– eventually enclosed by the yolk sac, which is part of the developing gut

– by the end of development only a little yolk remains and this is absorbed before or shortly after hatching

• Every structure thus far is common to both amniotic and non-amniotic eggs

Albumin

Components of the Amniotic Egg

• Amnion– inner membrane

surrounding the embryo in a contained environment of water and salts

– acts as a water reservoir

– aids in osmoregulation

Albumin

Components of the Amniotic Egg• Chorion

– protective outer membrane

– surrounds all embryonic structures

• embryo• yolk sac• albumin• allantois• amnion

– vascularized for gas exchange

Albumin

Components of the Amniotic Egg• Allantois

– outgrowth of embryonic hindgut

– used to store nitrogenous wastes produced by the embryo

– vascularized; aids in respiration

– increased in size as embryo grows

Albumin

Possible Steps in the Evolution of the Amniotic Egg

• Development of a terrestrial habit by adult• Initiation of internal fertilization• Reduction in body size (many early amphibians were

large)– terrestrially laid, non-amniotic eggs > 10 mm diameter

cannot exchange enough oxygen for respiration, may also collapse under their own weight

– since egg size and adult body size are correlated, ancestors of the first amniotes were likely to have been small

• Reduction in the number of eggs produced, with increase in size and quantity of yolk in each egg

• Shortening and later elimination of the larval stage• Laying of small non-amniotic egg on land• Development of amniotic membranes

Advantages of Amniotic Eggs

• Storage of wastes– keeps toxic waste products away from the

developing embryo

• Improved gas exchange• Can be laid in terrestrial environments• Since amniotic eggs are not as sensitive

to the size constraints of terrestrial, non-amniotic eggs, they could allow for the evolution of larger body size

Disadvantages of Amniotic Eggs and Laying Eggs in a Terrestrial

Environment• Air is a far more variable environment than

water– temperature is more variable and changes more

rapidly– moisture content

• Must have internal fertilization• Usually requires more parental care than fish or

amphibians (e.g. alligators)• Relatively expensive to produce and since more

energy is invested in each egg, fewer eggs can be produced by any individual female

Example: American Alligator• Native to SE U.S.,

with large populations in FL, LA, and parts of GA and SC

• Large reptile that lays amniotic eggs– adult males average

11.2 feet in length– adult females

average 8.2 feet– but many get larger,

with records > 19 ft and 1,000 lbs

Alligator Nests• Lay eggs terrestrially• Nest is typically on the

edge of a lake or river• Constructed from

aquatic vegetation– keeps eggs above water

level– rotting vegetation keeps

eggs warm, moist

• Since amniotic eggs can “drown”, female must predict future water levels

• Usually 35 - 50 eggs are laid; take 65 days to hatch

Building

Eggs

Laying

Hatching

• Female guards nest until hatch

• As eggs begin to hatch, nestlings make grunting noises

• Female removes vegetation to release hatching young

• Female may carry young to water’s edge in mouth

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3

4

Early Life

• Female attends young for a year or more, protecting them from predators such as wading birds or other alligators

Pod of young alligators; female is nearby

Cannibalism: adult male eating a juvenile