Nectridea: Difference between revisions

In many groups of early tetrapods, each [[vertebra]] is formed by three parts: an intercentrum at the front, a pleurocentrum at the back, and a plate-like neural spine jutting out from the top of the vertebrae which may be positioned between the two or fused to the pleurocentrum. In nectrideans, only the pleurocentra are present, and the neural spines are completely fused to them. Their vertebrae also possess an extra set of joints connecting the vertebrae (apophyses), just above the typical set (zygapophyses). Furthermore, their caudal (tail) vertebrae also possess haemal spines which jut out from the bottom of each vertebra. While most early tetrapods have slanted haemal spines which are positioned between the vertebrae, the haemal spines of nectrideans are completely vertical in orientation and fused to the pleurocentra, directly opposite the neural spines. Both the neural spines and haemal spines of the tail are characteristically fan-shaped. As a whole, nectrideans have rather short bodies and long and paddle-like tails, designed for swimming.<ref name=":1">{{Cite book|title=The rise of amphibians|last=Carroll|first=R. L.|publisher=The JohsnJohns Hopkins University Press|year=2009}}</ref><ref name=":0">{{Cite journal|last=Milner|first=Andrew R.|date=1993|title=The Paleozoic Relatives of Lissamphibians|jstor=1466948|journal=Herpetological Monographs|volume=7|pages=8–27|doi=10.2307/1466948}}</ref>