While some prehistoric turtles had teeth, all extant species have horny beaks made of keratin. The shape and size of the beak can reflect the type of diet.
Chelonian synamorphies clearly define their clade but the phylogenetic relationship between turtles and other clades has changed many times as discussed here in the Tree of Life.
Testudines includes the Chelonia and some of their extinct relatives from the extentive turtle fossil record.
Chelonia (ke-lō′ni-a) is a group name given to extant animals commonly known as turtles, tortoises and terrapins. Alternative scientific names for the group include Chelonii, Testudinata, Testudines.
Chelonia is the Greek word for tortoise whereas testudo (meaning "moveable shelter") is Latin.
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