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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
1 2
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
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