"Their ventral body armor speaks to how turtles originated in a marine environment, protecting them from predators that could attack them from below"
The Field Museum's own Olivier Rieppel, curator of fossil reptiles, co-wrote a paper introducing the oldest turtle fossil ever found: Odontochelys semitestacea. Discovered in China, the fossil dates to the Triassic and reveals a creature that had both top and bottom teeth ('modern' turtles only have a toothless beak) and only a plastron - bottom shell.
But wait! Colleague Robert Reisz thinks this creature once had a top shell - and lost it due to evolving environmental conditions. So this specimen may not be as primitive as Rieppel thinks...
At any rate, at an estimated 220 million years old, this is the least evolved turtle ever found.