According to the latest available information, turtles been on earth for between 200-300 million years. That makes them very very ancient animals. In comparison, the evolution of humankind can be traced back to only 2.5 million years.
The oldest turtle fossil discovered to date was unearthed only as recently as 2007 in China’s Guizhou province. It was estimated to be 220 million years old, was an aquatic turtle and given the name Odontochelys semistestacea. The previous record-holder was a terrestrial turtle named Proganochelys, at least 10 million years more recent than Odontochelys semistestacea.
The latest find therefore suggests that the evolution of turtles occurred in water.
Odontochelys is very different from the modern turtle because it had a long, pointed snout lined with teeth. All modern-day turtles have sharp beaks and no teeth. Further, in Odontochelys only the plastron (lower shell) was fully formed while the carapace (top shell) was not.
A picture of Odontochelysis given above to let you have an idea of how prehistoric turtles look like.