Reptiles

The first group of vertebrates to become completely free of an aquatic stage in their life cycle. Originated from amphibians in the Carboniferous and by the Mesozoic were the dominant vertebrates. In a tropical environment the diversity of reptiles has been greater than any other group.

Reptiles During some period in their long phylogenetic history reptiles have come to occupy all possible environments. There are typical terrestrial quadrupeds, bipeds, limbless species, burrowing forms, plus those that invaded air, marine and f.w. habitats.

Amniotic egg Leathery or lime-impregnated shell provides protection from physical and chemical shock and desiccation, and is sufficiently porous to allow exchange of gasses (oxygen and carbon dioxide). Internal fertilization is required because egg is laid encased in the shell. There are four extra-embryonic membranes produced by the embryo

Amniotic egg The amnion (a membrane produced by the embryo) surrounds the developing embryo, enclosing it in a cavity filled with amniotic fluid, which is secreted mainly by cells of the amnion. Amniotic fluid prevents dessication.amnion The allantois is a sac-like development from the embryo's hind gut. It increases in size with the growth of the embryo, comes into contact with the chorion and, with it, serves both as a repository for nitrogenous metabolic waste and as a respiratory mech.allantois chorion

Amniotic egg The yolk sac also grows from the embryo's gut. It develops just below the embryo and is filled, in reptiles and birds, with a large mass of yolk.yolk sac The layer from which the amnion is formed gives rise to the chorion. This thin cellular membrane lies beneath the shell and encloses the whole complex: embryo, amnion, yolk sac, albumin and allantois.chorion

Amniotic egg Water-dwelling stage has not been eliminated, but is now spent within the protecting shell of an egg. Animal can now assume a land existence immediately upon hatching = first purely terrestrial vertebrate.

Taxonomic difficulties Once the amniotic egg appeared, three major amniote lines envolved.  The anapsids gave rise to the present turtles.  The diapsids gave rise to all of the other reptiles and the birds.  The synapsids gave rise to the mammals

Taxonomic difficulties The amniotes are believed to be a monophyletic group including all of the descendants of the original amniote ancestor. By separating birds from reptiles we have a paraphyletic grouping because the classification does not include all of the ancestors (the birds) of the diapsid ancestor. Birds, reptiles and mammals all belong in one class, the amniotes.

Taxonomic difficulties In spite of this, I will treat them as separate classes because we are used to thinking of them that way and because birds have diverged so much in developing their adaptations for flight that it is convenient to think of them as a separate taxon.

When and why did amniote egg evolve? Experts disagree. Romer says: Fossil evidence shows that the earliest reptiles were not fully terrestrial. They lived in swamps or spent much of their life in the water and were aquatic fish eaters, even though they had limbs capable of walking on land. This was not the result of a secondary return to the water, the earliest reptiles were not fully terrestrial even though they had the amniotic egg and strong walking limbs.

When and why did amniote egg evolve? Easy to see how amniote development could be selected for if habits were terrestrial (eliminates sole obstacle to fully terrestrial life), but what is advantage for organisms which were still aquatic or, at most, amphibious in adult life? To answer need to consider types of reproduction in modern amphibians.

When and why did amniote egg evolve? "Typical". Eggs laid in water and water dwelling tadpole stage in development of young. Very large proportion of modern amphibians don't follow pattern. Go to extremes to avoid laying eggs in water. Never get shelled egg, but in one or another modern form get, singly or together, other principal features of amniote type. Get large yolk, embryonic breathing organs, protective membranes analogous to an amnion. These amphibians have followed a path parallel to that followed many millions of years ago by ancestral amniotes. Is this evidence of some sort of "drive" to terrestrial life? No. These amphibians are as varied in their adult habits as are amphibians generally.

When and why did amniote egg evolve? There are two major advantages for an amphibian to getting eggs out of the water. Eggs and young in pond are tempting and vulnerable food supply for variety of animals including insects. Larvae are in competition for food with other small water dwellers. Solution: lay eggs in less obvious places or have them guarded or carried around by parents. Annual dry seasons in some areas cause ponds and pools to dry up. Solution: decrease or eliminate water stage and increase chance of survival of young.

When and why did amniote egg evolve? Romer concludes that amniote egg evolved to gain an immediate advantage for aquatic animals. It protects the most vulnerable stage in the life cycle. It did not evolve to satisfy some "urge" toward terrestrial existence. In modern world protecting eggs and young from predation is most important pressure. Drying of ponds probably was more important for Paleozoic reptilian ancestors. Fewer predators then, but more chance of desiccation. Conclusion supported by fossil records.

Ancestral reptile can be pictured as one of many amphibians in late Paleozoic streams. Basically water dwellers eating fishes and invertebrates. Little animal life on land to tempt them. One group developed amniote egg so young would be prevented from desiccation when water dried up. At first there was no trend to increase life on land. Slowly began (toward end of Paleozoic) to take advantage of new opportunities offered by this mode of development. Result = major reptilian radiation in Mesozoic. Age of reptiles = happy accident. Advanced reptiles took advantage of potentialities gained by an adaptation of immediate value to their amphibian ancestors.

Other adaptations Ectothermic, but regulate temp. very well Behaviorally Large body mass works to --> constant temp. Can control amt. of blood flow thru skin. Some can pant as do dogs to decrease temp. Color changes. Dark absorbs heat and light reflects it. Body temp. regulated by center in hypothalamus.

Other adaptations Lungs operated by negative pressure generated by rib movements. Positive pressure in amphibians. They swallow air. Heart almost completely 4 chambered Shift in nitrogenous waste excretion from ammonia and urea to uric acid (decrease water loss).

Other adaptations Dry horny skin with horny scales to decrease water loss. Small plates of bone under scales in some liards and crocs. therefore no longer breathe thru skin. Stronger skeleton than amphibians More complex skeleton. Some lizards bipedal. Begin tendency to increase importance of cerebrum. In some reptiles get neocortex (highly developed in mammals --> thinking brain.

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