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Sink or Swim

If you have been lucky enough to watch a turtle in a glass aquarium, you saw that turtles are strong swimmers, but sometimes they just hang out on the bottom. For you to do that, at the bottom of a pool, for instance, you would need two things: a way to breathe and something heavy to keep you down there. How does a turtle stay under, perhaps even napping?

painted turtle in aquarium floating just below the surface

Educational ambassador Diane demonstrates how turtles can float just below the surface.

In our last post about turtle physiology, we noted that, because of the shell, turtles have significantly more bone mass than other animals. That heavy bone sinks like a rock. But turtles can also float, so something must be making them buoyant.

We have seen turtles described as little submarines. That is a pretty accurate comparison. Turtles sink or swim like submarines dive or surface except, instead of ballast tanks, a turtle has a bladder.

To counteract the weight of their shells and float, turtles pump air into their large lungs. To submerge, they draw water into the urinary bladder through the cloaca. How deep a turtle stays depends on the ratio of air in the lungs to water in the bladder. There are variations in this depending on the species of turtle, but that is the general idea.

Having all that air in their lungs, plus slow metabolism, means a turtle can hold their breath a long time. Under normal summer conditions, it might be an hour before a turtle comes up for air. Napping, the turtle might stay even longer. And if the oxygen in the lungs gets used up before the turtle surfaces, they will switch to anaerobic metabolism and not need any oxygen at all. But that is a story for another time.

You can learn more about how turtles sink or swim in our educational outreach programs. We are always looking for new program hosts. Please contact us to bring a program to your school, library, or club meeting.

It’s All About the Shell

What makes a turtle a turtle? There are a few other things that differentiate turtles from their reptile relatives, but, really, it’s all about the shell.

A turtle’s shell is made up of expanded ribs that have fused together into bone plates. The ribs give the shell its shape and are fused into the bone of the carapace, or top shell. The carapace is a kind of bone called dermal bone because it is derived from the skin. It is not odd to have dermal bone; the human skull is also dermal bone. Turtles just have an exceptionally large amount of dermal bone. Bone mass makes up almost 40% of a turtle’s weight, compared to less than 15% for some crocodiles, a turtle’s closest cousins.

painted turtle in tub of water with strips of tape holding shell cracks togetherThe shell provides protection for the turtle’s heart, lungs, and other organs which is much better than that offered by your ribcage. By retracting their heads and limbs into their shells, turtles can protect those, too. The box turtle’s hinged plastron, or bottom shell, allows the shell to close completely.

The shell is not like a suit of armor over the turtle but, rather, a living part of the turtle. The turtle’s shell has nerves and blood vessels, and it grows and heals like any other broken bone. It can heal so well, in fact, that for many cracks we only need to realign the pieces and hold them in place until the turtle’s body takes care of the rest.

Unfortunately, there are instructions on the internet for repairing a turtle’s shell with glue or epoxy. Do not glue a turtle’s shell back together! If glue gets between the pieces of shell, they will not be able to heal back together. Without restored blood flow, some pieces of the shell may die. Please contact a veterinarian or wildlife rehabilitator if you find a turtle with a broken shell.

You can learn more about turtle shells and see the shells of our educational ambassadors up close in our educational outreach programs.

The Earliest Turtles

Thanks to good fossil records, we know that turtles are some of the oldest vertebrates on Earth. They made their appearance during the late Triassic period, about 250 million years ago. They are part of the reptile family that includes crocodilians (alligators and crocodiles), who are their closest relatives, the dinosaurs, who they lived alongside, and birds.

You read that correctly. Current studies show that genetically, birds are most closely related to crocodiles, turtles, and dinosaurs. But you have always suspected your Mr. Tweetie was hiding something, haven’t you?

fossil turtle skeleton head neck front of shell front legs

Image by Icewall42 from Pixabay

You might look at sea turtles and assume turtles have always been aquatic, but the earliest turtles lived on land. At first, they had expanded ribs, not the fully formed shell we associate with turtles today. The early turtles include:

  • Eunotosaurus africanus, 260 million years ago. Eunotosaurus looked like a modern-day lizard but had wide, flat ribs that rounded the torso into the shape of a turtle shell. Eunotosaurus fossils were found in southern Africa.
  • Pappochelys rosinae, 240 million years ago. Found in Germany, the fossils are like Eunotosaurus with wide, flat ribs.
  • Odontochelys semitestacea, 220 million years ago. Until Eunotosaurus and Pappochelys were discovered, Odontochelys, whose fossil was found in China, was thought to be the original turtle. While still lacking the carapace (upper shell), Odontochelys had a fully formed plastron (lower shell) and are believed to have spent at least part of the time in shallow water, the earliest evidence of turtles becoming aquatic.
  • Proganochelys quenstedti, 220 million years ago. Fossils indicate Proganochelys was the first “true turtle,” in that it had a fully formed carapace as well as a plastron. Proganochelys was believed to be only semi-aquatic and an herbivore, so more like a tortoise than today’s aquatic turtles. The shell of Proganochelys was about three feet long and it had a long tail, reminiscent of snapping turtles.
  • Meiolania, 20 million years ago. There are three distinct species in the genus Meiolania, and these are almost present-day tortoises, except for their horns and clubbed tails. They are the largest tortoises that have ever existed, with a carapace length over six feet. Most Meiolania fossils have been found in Australia and nearby South Pacific islands. Unfortunately, there is evidence Meiolania disappeared shortly after the arrival of humans, which may make them one of the first instances of human-caused extinction.
  • Stupendemys, 10 million years ago. Fossils found in South America tell us Stupendemys had a carapace exceeding six feet in length, and sometimes as long as ten feet, making it the largest freshwater turtle ever to have existed.
  • Hesperotestudo, 2 million years ago. Fossils of these giant tortoises are found mainly in North America and are like present-day gopher tortoises. Hesperotestudo existed alongside humans who hunted them for food, which most likely led to their extinction.

In the 250 million years that turtles have been on Earth, they have never been in as much danger of extinction as they are today. We hope that as you learn more about ancient turtles, you will also learn how to help turtles today.

Dancing Turtle’s education programs include fascinating facts about the earliest turtles to inspire conservation.