Westlothiana
The small size has made it a key fossil in the search for the earliest amniote, as amniote eggs are thought to have evolved in very small animals. There are derived (advanced) features which tie it in with the amniotes rather than amphibians. These are: unfused ankle bones, lack of labyrinthodont infolding of the dentine in teeth, a lack of an otic notch and a generally small skull.