Sea turtles have been around for 110 million years and gave rise to 4 families of marine turtle. The earliest sea turtle ancester found this far is desmatocheys padillai, an early species that lived about 120 million years ago.  This turtle was found was 6.5 feet (2 meters long).  Desmatocheys Padillai is about 25 million years older the the oldest sea turtle.  tThe Cheloniidae contain six extant species within five genera: the flatback turtle (Natator depressus), the green turtle (Chelonia mydas), the hawks turtle (Eretmochelys imbricata), the loggerhead turtle (Caretta caretta), the Kemp's ridley turtle (Lepidochelys kempii) and the olive ridley turtle (Lepidochelys olivacea). These animals genus Chelonia are  Characterized by their bony shell, no other tetrapod has both its pectoral and pelvic girdles encased in bone. 

Sea Turtles 101 | National Geographic

https://www.youtube.com/watch?v=5Rmv3nliwCs