He says the creature grew so big it needed stronger bones to support its back. Then, it started growing a shell and could not stop. Tortoises and turtles are both reptiles, putting them in the same grouping as snakes. But their armoured shell, made of a fused spine and ribs, sets them apart in a group known as "chelonians".
They are both descended from a small, extinct reptile, which had no armoured shell, a flexible spine and was only slightly smaller than modern turtles.
Michael Lee, at the University of Sydney, has studied the family tree of these animals, and found that at some stage in their evolution, millions of years ago, they grew much bigger - up to a metre long - and needed larger vertebrae on their back to support the stronger muscles needed to swim by flexing their spine. "Contrary to previous intuition, dermal armour probably did not function defensively," he says in the science magazine Nature, published today.
But as the muscles grew larger and stronger, bony additions, known as "osteoderms", were needed - until they began to cover the animals' backs and restrict the spine's ability to flex. Tortoises and turtles had increasingly to use their limbs as paddles, because they could not rely on the fish-like twisting of the spine to propel them through water or over land.
Professor Lee says this loss of agility and speed would have "locked them further into a herbivorous [plant-eating] niche."
This process soon became unstoppable in evolutionary terms. Over a few tens of millions of years, the bones of the spine and the osteoderms fused into a single piece, as did the ribs underneath, because they served to ward off predators. That left tortoises and turtles at almost the same size as their original ancestor - but with an entirely different appearance.