The Reptiles a Quick Review of Amphibia

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 Amphibia The transition to land PROBLEM DESCRIPTION SOLUTION Support Water provides much Modification and more buoyancy than strengthening of fins, air pelvic and pectoral girdles, vert. column Locomotion Tail used for Use modified fins, but propulsion in water, still use undulating but won’t work in air motion for walking Respiration Gills don’t work in air Lungs – surface area problem Dessication Gills would lead to Lungs, eyelids excessive water loss A Quick Review of Amphibia The transition to land PROBLEM DESCRIPTION SOLUTION Sensing Lateral line won’t Modified work in air hyomandibula vibration transmits vibration to fluid-filled inner ear Reproduction Fertilization, Reproduce in water dessication problems Prey capture Fish could use Tongue suction to help capture prey; won’t work in air Amphibians were able to exploit some terrestrial resources, but… XXXXXXXX XXXXXXXX XXXXXXXXXXX XXXXX Reproductive constraints, among other things, forces them to remain in the vicinity of water 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 Fossilized eggs in nest adults –provide “aquatic environment” within the egg – THE MOST IMPORTANT DIFFERENCE BETWEEN AMPHIBIANS AND REPTILES 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 Dimetrodon (not a dinosaur) – 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 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 • Albumin (egg white) – protection against mechanical damage – provides a reservoir of water and protein 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 Albumin yolk remains and this is absorbed before or shortly after hatching • Every structure thus far is common to both amniotic and non- amniotic eggs Components of the Amniotic Egg •Amnion – inner membrane surrounding the embryo in a contained environment of water and salts Albumin – acts as a water reservoir – aids in osmoregulation Components of the Amniotic Egg • Chorion – protective outer membrane – surrounds all embryonic structures • embryo • yolk sac Albumin • albumin • allantois • amnion – vascularized for gas exchange Components of the Amniotic Egg • Allantois – outgrowth of embryonic hindgut – used to store nitrogenous wastes produced by the embryo – vascularized; aids in Albumin respiration – increased in size as embryo grows 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 Building • 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 Laying “drown”, female must predict future water levels • Usually 35 - 50 eggs are laid; take 65 days to Eggs hatch Hatching 1 2 3 • Female guards nest until hatch • As eggs begin to hatch, nestlings make grunting noises •Female removes 4 vegetation to release hatching young • Female may carry young to water’s edge in mouth Early Life • Female attends young for a year or Pod of young alligators; more, protecting female is nearby them from predators such as wading birds or other alligators Cannibalism: adult male eating a juvenile.

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