43-1: Origin and Evolution of Reptiles

43-1: Origin and Evolution of Reptiles

History of Reptiles

• Reptiles are in the class Reptilia and appeared more than 300 million years ago.
• Reptiles arose from amphibians and are considered one of the most successful groups of terrestrial vertebrates.
• Evidence in fossil record when comparing anatomy.

• Early reptiles had access to an abundance of large insects.
• The Mesozoic Era is also referred to as the Age of Reptiles because nearly all large terrestrial vertebrates were reptiles at that time.
• This is when dinosaurs appeared and diversified.
• Fossils have been discovered on every continent, even Antarctica, which had a more mild climate.

Reptile Skin

• Although many reptiles live on land, there were several groups that were aquatic.
• Ichthyosaurs were sleek aquatic reptiles that resembled modern bottle nosed dolphins.
• Plesiosaurs had long, flexible necks and compact bodies.
• Pterosaurs evolved the ability to fly.

• Reptiles are covered in thick. dry, scaly skin to prevent water loss.
• The surface cells are filled with a protein called keratin.
• Proteins and lipids in the skin help make it watertight.
• This prevents against water loss, infections, injuries, and wear and tear.

Respiration and Excretion

The Amniotic Egg

• All reptiles, even aquatic ones, have lungs for gas exchange.
• The excretory system also helps reptiles conserve water.
• Terrestrial reptiles excrete nitrogenous waste in the form of uric acid.
• Much less toxic than ammonia or urea.
• Requires little water for dilution so reptiles only lose small amounts of water in their urine.

• Reptiles are considered the first fully terrestrial vertebrates because they do not depend on water to keep their skin moist or for reproduction.
• Reptiles have an amniotic egg, which encases the embryo in a secure, self-contained aquatic environment.
• Birds and mammals also have an amniotic egg.
• Amniotic eggs provide more protection for the embryo than the jelly-like eggs of amphibians.

Parts of the Egg

• The amnion is the thin membrane which holds the amniotic fluid and the embryo.
• The yolk sac contains a fat-rich yolk for the embryo.
• The allantois stores nitrogenous waste for the embryo.
• Also serves as the "lung" as it exchanges gases.
• The chorion surrounds all other membranes and helps protect the embryo.
• The albumen contains the protein and water for the embryo.
• This is the white of the egg.

Extinction of Dinosaurs

Modern Reptiles

• Representatives of the 4 modern orders of reptiles survived the mass extinction.
• There are over 6,000 species in these four orders.
• Reptiles are found in terrestrial and aquatic habitats on all continents except Antarctica.

Asteroid - Impact Hypothesis

• Dinosaurs became extinct about 65 million years ago at the end of the Cretaceous Period.
• Many species of aquatic and terrestrial organisms also became extinct.
• Asteroid-impact hypothesis suggests that a huge asteroid hit Earth, sending large amounts of dust into the atmosphere which greatly reduced the amount of sunlight reaching Earth's surface.

Pieces of Evidence

• According to this hypothesis, the dinosaurs would have become extinct very rapidly, perhaps even within a few months.
• Luis Alvarez, and his son Walter, first proposed this hypothesis in 1980.
• They had three main pieces of evidence to support their hypothesis.

1. They noted that sediments at the end of the Cretaceous period had an unusually high concentration of iridium.

• Iridium is rare in Earth's crust but is more abundant in asteroids.

2. Other scientists discovered quartz crystals that had been deformed by a powerful force, such as that resulting from the collision of a large asteroid with Earth.

3. A crater was found on the Yucatan Peninsula that dates to the end of the Cretaceous period.

• This would be a likely impact site.