 <unk> ' vertebrae are divided into several segments . In living tetrapods , the main body of the vertebra is a single piece of bone called the centrum , but in <unk> , this region was divided into a <unk> and <unk> . Two types of vertebrae are recognized in <unk> : <unk> and <unk> vertebrae . In <unk> vertebrae , the <unk> are large and wedge @-@ shaped , and the <unk> are relatively small blocks that fit between them . Both elements support a spine @-@ like neural arch , and well @-@ developed interlocking projections called zygapophyses strengthen the connections between vertebrae . The strong backbone and strong limbs of many <unk> <unk> allowed them to be partially , and in some cases fully , terrestrial . In <unk> vertebrae , the <unk> have been lost entirely , with the <unk> enlarged as the main body of the vertebrae . This weaker type of backbone indicates the <unk> <unk> spent more time in water . 
