At some point — let's just say around 260 million years ago — Earth got turtles. They look strange in these modern, mammalian times when lots of things are squishy and unarmored. But during the Late Permian Epoch, the early turtles were dressed in all the latest fashions: short, sturdy legs, bony plates and a stiff, splayed, crawling strut.
Shortly after turtles arrived here, a fairly standard Earth thing happened: a mass extinction event. Although mass extinctions have happened with some regularity on our planet, this one was a doozy, and it wiped out almost all the life in the oceans and over two-thirds of the vertebrates on land. The things that survived had to have been pretty good at survival. Turns out, turtles were.
"Turtles have a really successful body form that hasn't changed all that much over time," says Lora Smith, a research scientist who specializes in herpetology at The Jones Center at Ichauway, an organization in Newton, Georgia, that promotes excellence in natural resource management and conservation, in an email interview. "They've retained the primitive shell, which is a really protective, safe body design. Also, turtles live in a lot of different habitats — they're aquatic and also terrestrial, so living in a lot of different habitats has allowed them to persist."
Tortoises ARE Turtles
All the animals alive today that protect themselves with a shell — basically just a modified rib cage — are in the order Testudines. Collectively, we call this group of animals "turtles," but individually, we might call them different things based on where they live and some morphological and physiological traits.
"They say that not all turtles are tortoises, but all tortoises are turtles," says Smith. "The turtles are organisms with a shell, which might be in water or might be on land. A tortoise is a type of turtle."
In general, both turtles and tortoises (as well as other reptiles) lay their eggs on land — it's what makes them different from amphibians, which need water for egg-laying and at least part of their life cycle.
Because tortoises are turtles, it's difficult to lay down hard and fast rules about what makes something tortoise-ish rather than turtle-ish. But in general, tortoises are always found on land, whereas turtles can be found in aquatic or marine habitats as well as land.
How To Tell Them Apart
"Turtles and tortoises look different because of where they live," says Smith. "A sea turtle is only found in the ocean — the females are the only ones that come on land, and it's just to lay eggs. They have four legs, but the front legs are almost like wings or paddles — they're not great for moving around on land at all because they're adapted for swimming quickly. Their shells have a low, flat profile for cutting through the water."
Compare that to a Galápagos tortoise (Chelonoidis nigra), whose body can weigh up to 920 pounds (417 kilograms), with stocky, elephantine legs, a high-domed shell and big scales on their exposed skin to protect them from predators. They wouldn't last long in the ocean — but luckily they don't have to.
"For the most part, there's not really one characteristic that tells you whether something is a tortoise or a turtle," says Smith. "But it's pretty clear if you see a little turtle on the side of the road and it has a sort of flattened shell profile, webbed feet in the back, smooth skin and some brighter colors, that's going to be a turtle. Tortoises have a heavier, more domed shell and subdued colors."
As usual, terminology is confusing. Box turtles, for instance, which are widespread in the United States and Central America, don't really swim or spend any time in the water, but they're still considered turtles rather than tortoises. And then there are terrapins, which is the name given to aquatic turtles in the U.K. In the U.S., aquatic turtles are just called "turtles," with the exception of the diamondback terrapin (Malaclemys terrapin), which lives in brackish water in tidal marshes in the eastern U.S.
Both tortoises and turtles have made themselves at home on this planet — we find both tortoises and turtles on every continent other than Antarctica with one exception: There are no tortoise species native to Australia.
"The greatest diversity of aquatic turtles are in Southeast Asia and in the Southeastern U.S.," says Smith. "The greatest tortoise biodiversity is in South Africa. There used to be more giant tortoises across the world, but now there are just remnants on the Galápagos and Aldabra [an atoll in the Seychelles] — places like that. Giant tortoises do occasionally disperse (or more likely drift) across oceans, basically by surviving for weeks/months bobbing around. In recent years, an Aldabran tortoise with barnacles on its shell washed up in east Africa."