Amphibians, although terrestrial, are challenged by terrestrial environments because they must undergo a larval stage in water (although some complete this stage in eggs laid in a very moist environment).   The first amniotes were fully adapted to terrestrial life because they did not require water for development and could lay their eggs on land. 

TURTLE 1 turtle 2

The success of the amniotes in terrestrial environments caused the extinction of many of the Paleozoic amphibians.

     Anthracosaurs were amphibians which possessed a number of derived features which link them to the ancestry of reptiles including a more upright posture, wrist and ankle specializations, an atlas-axis complex (first two vertebrae), and stronger jaw muscles.  Unfortunately, the defining characteristic of amniotes is the set of extraembryonic membranes that support the embryo.  Since these membranes do not fossilize, it is difficult to distinguish between the fossils advanced amphibians and primitive reptiles.  

     Before a developing amniote embryo forms a brain or heart, kidneys or intestines, it wraps itself in a number of membranes.  These extraembryonic membranes are made of cells from the embryo but will not form any part of the infant at birth.  In the following models, note that the relative size of the embryo and these embryonic membranes early in development.

embryo embryo 2
     There are 4 extraembryonic membranes of amniotes, membranes which surround the embryo but do not form part of the future individual.    The amnion is filled with a fluid that absorbs shock and prevents temperature change.  This fluid has about the same concentration of salts as seawater. Interestingly, even the embryos of terrestrial embryos develop in water similar to the ancient seas.  

The yolk sac provides nourishment for the developing embryo in the egg (it still present in placental mammals despite the presence of the placenta that is the source of nourishment).  The allantois collects the wastes which accumulate during embryonic development in the egg (wastes of amphibian embryos can dissolve into the surrounding water). 

The photo to the right depicts the allantois of a chick; the photo bleow is of a cat fetus.

cat embryo
human embryo
The chorion is the layer inside the egg shell that performs gas exchange (and forms the fetal part of the placenta in placental mammals).


     The reptilian vertebral column was much more advanced and adapted for supporting an animal’s weight on land than that of primitive amphibians.   The vertebral column had gradually become a more significant support structure during early vertebrate evolution and had gradually replaced the notochord as the primary longitudinal support.  The first “vertebrae” consisted only of cartilaginous neural arches around the spinal chord, on top of the notochord.  Hagfish are not considered vertebrates because their cartilaginous arches are only present in the tail.

hagfish tail hagfish 2
     The notochord (not the vertebral column) was the primary structural support for vertebrates for the first couple hundred million years of their existence.  Jawless fish such as lampreys possess the first components of vertebrae, the neural arches.  In jawed fish, alternating elements developed around the notochord, called pleurocentra and intercentra.  Although intercentra were the more important of the two in some sarcopterygians and amphibians, the pleurocentra became the dominant part of the vertebral “body” in anthracosaurs and reptiles.  The pleurocentra constricted the notochord until it was no longer continuous and the vertebral column became the major longitudinal support. 
Amniote (and thus human) vertebrae today are composed of neural arches fused to a pleurocentrum (simply called the centrum).  The intercentra make little, if any, contribution in modern amniotes.  (The intercentra may be the source of small processes on ribs, each called a capitulum, and the intercentrum of the atlas is described below.)   The neural arches developed processes that articulate with neighboring vertebrae over time.

The human vertebral column human thoracic vertebra is pictured below.

vertebral column thoracic vertebra
embryonic vertebra
Human vertebrae still begin their development as several separate centers of bone which later fuse.
     The first two vertebrae in amniotes are special and are the only vertebrae given their own names:  the atlas and axis.  The atlas is the point at which the skull attaches to the vertebral column; the atlas lies between the skull and the axis.  Interestingly, during the course of evolution, the pleurocentrum of the atlas became attached to the axis (rather than the atlas) and the atlas retained its intercentrum.  In humans, the atlas rotates around this projection of the axis when we shake our heads “no”.
atlas axis atlas