At posterior end, below the neural arch, are pleurocentra.

This element gave rise to the centrum in amniotes.

At anterior end, below the notochord, is the intercentrum.

Ichthyostegids (labyrinthodont amphibians) had vertebrae nearly identical with Rhipidistians

persistant notochord

neural arch

large intercentrum around the notochord

small paired pleurocentra

In later Labyrinthodonts, there is a trend toward expansion of either the pleurocentrum or the intercentrum, resulting in a more solid centrum.

3 conditions are known:

1. intercentrum develops - extinct line

2. intercentrum and pleurocentrum develop, two centra formed

extinct line

3. pleurocentrum expanded and fused to form a complete ring

evolved into reptiles

present construction in birds and mammals

Lepospondyli - solid vertebra in which all three elements are fused on to a single, spool-shaped centrum

Lissamphibians - unknown type, can’t fit them into clear story

have a centrum

no evidence of intercentrum and pleurocentrum

ossify directly into centrum

no cartilagenous preformation

Along with the development of stronger centra, there was a development of processes off from the centra that interlocked with processes from adjacent centra, allowing flexibility but preventing twisting of the vertebral column.

In fish, the occipital processes (occipital condyle) on the back of the skull looks like the end of a typical amphicoelus centrum.

The cranio-vertrebral joint in fish is like any other intervertebral joint. The head is not any more mobile on the spinal column than any other two vertebrae are on each other. Fish can’t move its head independent of the body.

Amphibians are the first vertebrates to have a neck, even if it is short & stout.

Linked with this advance is a more flexible cranial-vertebral joint. In modern amphibians (Lissamphibians), there is a convex pair of occipital condyles on the back of the skull, which insert into a concave pair of articular facets on the first vertebrae, allowing the head to move up and down. The second vertebrae is unchanged or somewhat enlarged.

Reptiles have even greater flexibility in their cranio-vertebral joint than do the amphibians, allowing increased head movement.

Reptiles have a ball and socket type joint between the single occipital condyle and the first vertebrae. The first vertebra is now called the atlas. The second vertebra, called the axis, is enlarged and the joint between the atlas and axis is specialized.

The story of the evolution of the cranio-vertebral joint has been pieced together from evidence from embryology, paleontology and comparative anatomy.

Several ancestral vertebrae were incorporated into the chondrocranium. The cranio-vertebral joint is thus really a joint between two vertebrae.

One or more times during vertebrate evolution, one vertebra at the cranio-vertebral boundary became reduced in size and became partly fused to adjacent skeletal elements.

This was the proatlas. Pieces of the proatlas are found in the skulls of some extinct amphibians, in crocodiles and in the embryos of many tetrapods.

The next vertebra changed as well. This vertebra, the atlas, is formed from neural arch and intercentrum: forms almost a ring. The atlas is the only place in higher vertebrates where intercentrum occurs.

The pleurocentrum from the atlas is present but is now fused with the neural arch and pleurocentra of the second vertebra to form the axis.

Atlas: made up of neural arch and intercentrum

Axis: made up of neural arch and pleurocentra from atlas and axis

Similar condition in birds and mammals but with some modification

Improved joint between skull, atlas and axis maintains strength because elements are interlocked, while at the same time permitting side to side motion and rotation of head, greatly improving flexibility and rotation of skull with vertebral column.

This increased the movement of the head

Advantages:

increasing feeding opportunities

assisting in locomotion with the head raised above the ground improving the use of the sensory organs because head can be moved to reposition sense organs.

Reptiles with these modifications -> birds and mammals

In stem reptiles, the construction and proportions of limbs are essentially the same as that found in amphibians.

short legs

proximal elements of limbs are at right angles to the body

proximal elements move in horizontal plane

movement by thrusts of limbs and body undulations

As reptiles evolved and became better at moving in terrestrial environments, the appendicular skeleton changed from the amphibian condition.

The number of bones in the pectoral girdle was reduced in the evolutionary sequence from osteichthyes -> amphibia -> reptilia

With the exploitation of terrestrial habitats, the weight of the body is borne by the limbs. This led to improved anchorage of the hind limbs.

In fish, the pelvic fins bear no weight and the pelvic girdle is not attached to the axial skeleton.

In amphibians, the pelvic girdle is attached to one vertebra.

In the early reptiles, the pelvic girdle broadened its attachment to the vertebral column. Two vertebrae are used to support the pelvic girdle. In the bipedal dinosaurs, even more vertebrae were involved in pelvic attachment.

Other changes were evident.

The ischium and the pubis became elongated and directed downward. This gave greater leverage for muscles and permitted greater muscle range of movement.

In therapsids (ancestors of mammals) and archosaurs (dinosaurs, ancestors of birds), both quadrupedal and bipedal types, the limbs moved more directly beneath the body, giving more efficient support and locomotion.

Elbows rotated backwards and knees rotated forward. There were associated changes in the feet, so the feet were also positioned to point forward.

This improved locomotion, facilitated faster speeds and more efficient locomotion. These groups gave rise to birds and mammals, and this condition is present in these vertebrates.

In existing reptiles (eg turtles, crocodiles), aside from snakes, locomotion is close to that of the early reptiles. Exceptions are some of the fast moving lizards, some of whom move bipedally when running fast.