This is definitely not a school assignment I’m using because I have very limited time…
260 million years ago, Eunotosaurus. No shell, but wider ribs than a lizard.
Late Triassic period, 220 million years ago, one of the first proto-turtles, Henodus. This proto-turtle has a half shell.
200 million years ago, Testudines. This turtle has a mostly developed shell.
70 million years ago, Archelon.
Less than 5 million years ago, Stupendemys.
Modern turtle, all features fully formed.
That may or may not be a mine turtle.
A major feature of a turtle is the shell. Many people jumped to the conclusion that the turtle’s shell had evolved as a defence mechanism against predators, however the shell made it harder for the turtle to walk and to breathe, so it wasn’t much protection at all. The actual function of the shell was to make it easier for the turtle to burrow, and looking at their eyes which are built to spend a lot of time in the dark, it is theorized that as the shell developed, turtles began to spend more and more time underground.
The beak is another major feature. It is theorised that turtles once had teeth, and lost these through a mutated gene and natural selection, developing a beak.
Turtles evolve to adapt to their environment. For example, turtles that spend most of their time in the water have flatter shells and flipper-like feet, whereas turtles (or tortoises) that spend most of their time on land have rounded shells and pillar-like feet. They also develop different shaped mouths depending on the food they eat and the environment they live in.
Well that was fun and interesting…