THE WORLD OF LIFE with Douglas Drenkow - Higher Reptiles, Dinosaurs, Birds, etc.

6,000 Modern Reptiles (not including dinosaurs or birds). Please note that turtles are now considered sufficiently different from other reptiles to be placed in a separate group.

ENVIRONMENTS

Marine, freshwater, terrestrial, and aerial (Extinct pterosaurs truly flew). As you will read below, reptiles have many adaptations to life on dry land.

DESCRIPTION

The body of a reptile bears dry, horny scales, which keep the occasionally bony-plated skin, underneath, from drying out. There are typically four (sometimes no) legs, with claws on the toes. The mouth bears a tongue and jaws (capable of opening very wide in snakes), typically with bony teeth (continually replaced in the adults). The eyes typically bear lids and tear glands. There is typically a well-developed neck and a slender tail.

Plesiosaurs (extinct) had teeth, long necks, and legs modified as flippers (Their similarity to the flippers of seals, which as mammals are only distantly related, is an example of "convergent" evolution -- as always, form follows function).

Ichthyosaurs (extinct) looked like fish, with a toothed snout (Convergent evolution with dolphins).

Lizards have teeth and usually four legs (sometimes none, at least externally), eyelids, and openings for ears.

Snakes have no legs, a pair of jaws capable of opening very wide, a see-through scale instead of an eyelid, and no opening for an ear.

Dinosaurs (extinct) had an often enormous body, sharp and pointed to blunt and broad teeth (depending on the diet), and four legs. Some species used the typically thick tail as a counterweight while walking on the typically strong hind legs, as did "thecodonts" (the little reptilian ancestors of dinosaurs). Saurischian dinosaurss were "lizard-hipped"; ornithischians, "bird-hipped". Dinosaurs probably became extinct after the impact of a large comet or asteroid near the modern Yucatan Peninsula created enough smoke and dust in the upper atmosphere to block out sunlight for several years -- the photosynthesis of green plants worldwide would have been sharply reduced; and as the dominant plant-eating herbivores and plant-eater-eating carnivores of the day, dinosaurs would have suffered greatly. In addition, about this time (about 65 million years ago), Continental Drift (perhaps accelerated by the impact???) was moving land masses and changing world climates: After this time, "warm-blooded" birds and mammals -- better able to withstand the increasingly harsh winters over much of the continents -- became dominant; whereas "cold-blooded" reptiles -- except for small species or large aquatic species, insulated by water -- declined in importance (However, fossil evidence indicates that the dinosaurs themselves may well have been "warm-blooded", so there was probably more involved in their extinction than one single factor).

Pterosaurs (extinct) had teeth and a long fourth finger, to which was attached skin forming a wing.

Alligators and crocodiles have four legs (the hind larger than the fore) and an aquatic lifestyle.

FEEDING HABITS

Scavenging, herbivorous, and carnivorous.

MOTION

The claws on the toes help grip the rocks or limbs over which the reptile crawls or climbs. In addition, the African "chameleon" lizard has a "prehensile" (grasping) tail, for use in the treetops; and "gecko" lizards have pads on their toes that are so finely ridged that they practically stick to tree bark.

Legs, especially the hind legs, are typically strongly attached to the spine of a reptile, well adapted to walking or running on land; but legs have been lost in the evolution of some lizards and virtually all snakes (some, primitive pythons do have tiny, useless hind legs) -- snakes typically use their elaborate musculature to produce well-coordinated, rippling "undulations" of their body, whose scales and loops provide "traction" -- a grip on the ground or tree limb over which they travel. Such extinct reptiles are plesiosaurs and ichthyosaurs had legs modified as flippers (as do modern sea turtles).

DIGESTION

A few lizards (such as "the Gila monster", of the desert Southwest) and many snakes bear poison fangs; some snakes (such as boa constrictors) wrap themselves around their prey and kill it by "constriction" (squeezing and suffocating the prey); and almost all snakes have "extensible" jaws (able to be opened very wide, for swallowing prey large enough to sustain them between their typically infrequent feedings).

Dinosaurs must have been very efficient eaters: Reaching 75 feet or longer and weighing 45 tons or more, Apatosaurus (Brontosaurus) and its relatives were the largest plant-eaters (or any other animals) ever to walk the face of the Earth (probably feeding like elephants or giraffes); and standing over 35 feet tall, Tyrannosaurus rex was the largest meat-eating animal ever (probably using its powerful hind legs to run-down or jump-at and then hold-down its prey, which it then tore into with its six-inch-long, knifelike teeth).

RESPIRATION

In higher vertebrates, such as reptiles, gases are exchanged across the extensive infoldings of the lungs and are typically inhaled and exhaled through nostrils, which allows the animal to continuously eat while breathing (higher vertebrates typically have a relatively active lifestyle and need to spend a lot of time eating).

Movements of the ribcage, not the throat muscles (as in amphibians), efficiently draw air into and expel air out of the lungs of reptiles.

CIRCULATION

In most reptiles, the "ventricle" of the heart is partially divided in two; and in crocodilians (as in birds and mammals), the ventricle is completely divided in two, producing a heart with four chambers -- two "atria" (one for receiving blood from the veins, the other for receiving blood from the lungs) and two ventricles (one for pumping blood to the lungs, the other for pumping blood out into the arteries, serving the rest of the body) -- there is no inefficient mixing of "oxygenated" and "de-oxygenated" blood, as in the undivided heart of amphibians or lower vertebrates.

Modern reptiles are "ectothermic" (so-called "cold-blooded") -- their body temperature controlled by the environment: Many reptiles "sun" themselves in the cool of the morning, hide under rocks or pant during the heat of the afternoon, and hibernate overwinter (or "aestivate" oversummer). However, at least some of the dinosaurs may have been "endothermic" (so-called "warm-blooded"), like modern birds or mammals -- biochemical reactions release heat, carried by the blood throughout the tissues, resulting in a relatively high and constant temperature within the body (as insulated by a large size), thus helping to maintain a "high metabolic rate" (a relatively active lifestyle).

EXCRETION

Nitrogen-rich and other wastes (as in our urine) are cleansed from the blood of a reptile by the many tubules within a pair of kidneys and are excreted (through a urinary bladder) into a "cloaca" chamber, which also receives digestive wastes and opens as the anus.

Excess nitrogen in reptiles is typically excreted as "uric acid", which is relatively insoluble in water (an advantage within the egg).

Terrestrial vertebrates typically secrete less precious water from their kidneys than do aquatic species. The horny scales of reptiles also help prevent water loss.

COORDINATION

The growth, development, and behavior of reptiles are under genetic and hormonal control, influenced by the environment.

Reptiles typically have a well-developed nervous system, especially for well-coordinated walking and running. Snakes and other, often burrowing reptiles flick their tongue about, in order to pick-up the scent of prey or predators; and pit vipers have a sensory pit on each side of the head that senses the body heat of their small, warm-blooded prey (typically rodents).

Fossils indicate that at least some "ceratopsians" (dinosaurs related to Triceratops) were social animals, grazing in herds, like modern-day cattle.

REPRODUCTION

Reptiles reproduce sexually, with the sexes "separate" (an individual is either male or female). Terrestrial reptiles were the first vertebrates to "copulate" (mate with physical contact) -- on dry land, the sperms of the male cannot otherwise swim to the eggs of the female.

The fertilized eggs of reptiles are typically hard-shelled and laid in terrestrial environments -- the developing "embryo" within is bathed and cushioned by an "amniotic sac", fed from a "yolk sac", and "oxygenated" by an "allantois" membrane. Some snakes (like prehistoric, completely aquatic ichthyosaurs) bear developing young inside the body of the mother, who, thus, gives birth to "living" (already hatched) young.

Representatives

Vertebrates (Vertebrata)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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