Turtles are reptiles of the order Chelonia /kɪˈloʊniə/ or Testudines /tɛˈstjuːdɪniːz/. They are characterized by a special bony or cartilaginous shell developed from their ribs that acts as a shield. Colloquially, the word “turtle” is generally restricted to fresh-water and sea-dwelling Testudines. Testudines includes both extant (living) and extinct species. Its earliest known members date from the Middle Jurassic. Turtles are one of the oldest reptile groups, more ancient than snakes or crocodilians. Of the 360 known extant species, some are highly endangered.