Dinosaurs are believed to have laid amniotic eggs. These eggs are on of the most significant features in reptile evolution., as it allowed for life on land without the need for a water source in which to reproduce. Instead the egg provided the aquatic environment needed for the development of the embryo within the egg itself.
The first advantage for life on land was the development of a hard outer shell, which provided protection whilst remaining porous. This allowed the diffusion of oxygen into the egg and carbon dioxide out, allowing respiration to take place. The yolk sack provided the embryo with food and the albumin supplied water and nutrients, eliminating the need for a larval stage. Instead the embryo would develop directly into a miniature version of the adult.
Evolution of dinosaurs
Dinosaurs began to evolve after the Permian-Triassic extinction wiped out much of life on Earth, perhaps filling newly vacant ecological niches. They evolved into two separate classes, Saurischia and Ornithischia. The Saurischia were divided into two: Therapoda, including birds and the well- known dinosaur, Tyrannosaurus; and Sauropoda, including Diplodocus. Ornithischia includes the Iguanodon.
Characteristics of Saurischian dinosaurs
This group contains the well known carnivore Velocirapter and the herbivore Diplodocus.
- The primitive arrangement of the hip bones is similar to reptiles, in which the pubis points forward.
- They have long, S-shaped flexible necks, allowing rapid and precise movement.
- Hands consisted of only three digits. The digits where asymmetrical with the first digit similar to a thumb , allowing the hand to grasp and with the second digit being the longest of the three.
Characteristics of Ornithischian dinosaurs
- The arrangement of the hip bones are similar to birds, in which the pubis points backwards.
- The front teeth are small or absent, replaced at the front with a horny beak which later became broader , giving the name "duck-billed dinosaurs".
- Many where armored with bony plates, such as the stegosaurus. These were thought to be a defense mechanism but could have also acted as heat exchanges. The fossilised plates have tiny grooves, which may have housed blood vessels allowing heat to be given off or taken in.