The lineage of tetrapod vertebrates contains two main lines: one line branches into all extinct and extant amphibian groups, while the other line contains the remaining tetrapods (reptiles, birds, mammals). The branch point of these two lines is based on embryonic development. In the reptiles, birds and mammals, the embryo develops structures that can maintain it within a terrestrial environment, including a fluid filled sac (the amnion) that surrounds the embryo. Thus these tetrapods are called amniotes and are said to have an "amniotic egg".


Lineage of the tetrapods

Reptiles followed amphibians in the evolutionary timeline. They first appeared in the fossil record during the late Carboniferous Period.

figure 27


Examples of reptiles

A. turtle

B. lizard

C. snake

D. alligator.

Reptiles were the first group of vertebrates to be completely adapted to life on land.  They have a skin that is covered in scales to reduce water loss. They also produce uric acid as their principle waste product to conserve water.  Modern reptiles, like amphibians are ectotherms. Reptiles also feature internal fertilization during reproduction which allows them to reproduce on land. Amphibians typically shed eggs and sperm into the water and fertilization occurs externally.

figure 28

Reptile hatching from an egg

The first reptiles produced leathery amniotic eggs that could be laid on land.  Reptiles were the first amniotes. 

The amniotic egg
Inside the amniotic egg are a series of fluid-filled membranes (known as extraembryonic membranes) which permit the embryo to survive in a terrestrial environment: the amnion, allantois, yolk sac, and chorion. Surrounding and protecting the embryo is the amnion, filled with amniotic fluid, and providing the embryo with a stable fluid environment. The allantois performs provides for gas diffusion, and removal of wastes. Food for the developing embryo comes from the yolk sac, which reduces in size as the embryo matures. Surrounding all the other membranes is the chorion, providing an overall enclosure. It usually fuses to the allantois.  In most mammals, the allantois and chorion will contribute to the embryonic portion of the placenta which is a organ that is composed of maternal and embryonic tissue. Mammals also have an amnion and vestigial yolk sace and thus are also amniotes.


Types of vertebrate eggs

A. Non-amniotic egg with developing embryo, typical of fish and amphibians

B. Amniotic egg with several labeled extraembryonic membranes, found in reptiles and birds. In most mammals, some of these membranes will form the embryonic portion of the placenta.

Skull types
In the amniote line, skulls have become more dome-shaped, and additional "openings" have formed to provide space for large jaw muscles. These openings are called temporal fossae. Vertebrates skulls are classified as:

1. anapsid: no additional opening behind the orbit of the eye.
2. synapsid: one opening behind the orbit of the eye.
3. diapsid: two openings behind the orbit of the eye.

orbit of the eyefigure30

Three types of skull

Top to bottom:
anapsid, synapsid, diapsid

All modern reptiles have the diapsid skull type, except for turtles which have anapsid skulls.


Major amniotic lines

Skull type is one characteristic used to place those vertebrates descending from a reptilian ancestor into different branches of the phylogenetic tree. Mammals are derived from reptiles with a synapsid skull. Modern lizards, snakes, crocodilians, and birds are derived from a diapsid ancestor.

View the skulls below to familiarize yourself with reptilian skulls. Try to identify the temporal fossae (behind the orbits) on the tuatara, alligator, and Allosaurus skulls.


Tuatara skull

Tuataras are considered to be "living fossils" and have primitive skulls.

skull Alligator skull
skull Dinosaur skull (Allosaurus)

Gila monster skull

A lizard with a highly modified skull (temporal fossae not visible)


Turtle skull

Turtles are anapsids (no temporal fossae)