What is a Turtle?

Rocks with legs. Armored lizard guys. Got scales, sometimes lives in the water. You’ve got a good idea of what a turtle is.

But let’s do the whole chain, starting with turtles are alive:

Child's drawing of a turtle
Dara K of Hawaii drew this anatomically correct turtle in the second grade for NOAA.
  • Life Figure this out for yourself. You are entitled to the conversation about if you bang two rocks together to get little rocks, is this reproduction?
  • Cellular organisms There is an inside and an outside, with a fatty layer between the two. Inside has a bunch of floaty bits to make the cell work. There’s not much structure; it’s just a bag of bits.
  • Eukaryota The cells have a nucleus with the genetic material packed inside. Other complicated organelles (mini-organs) perform particular functions. You can think of eukaryotic cells as a big simple cell eating a bunch of other smaller simple cells, while the smaller ones stay intact. There’s a lot of structure.
  • Opisthokonta They ingest food like fungi instead of making it like plants.
  • Animals They ingest their food in a particular way. This is actually mostly chemistry, nothing to do with using forks or behavior. Think less morality, more ATP.
  • Bilateria They are split into two mirror halves. There is a left and a right side. A head is at one end and an anus at the other. 
  • Deuterostomia The anus forms before the mouth while changing from a ball to a tube. You also did this. Remember your butt showed up before your brain. This may explain a lot.
  • Chordates A central nerve runs from tail to head. 
  • Vertebrata A spine or version thereof protects the central nerve.
  • Gnathostomata The head has two pieces: a skull and a jaw.
  • Osteichthyes The bones are calcified.
  • Lobe-finned fishes The boney legs have hands/feet/paws/flippers. No fins. Legs.
  • Tetrapoda There are four of these legs.
  • Amniotes A nifty filtering layer called an amnion protects the embryo.
  • Reptiles The eggs have a waterproof shell. Reptiles are scaly. The reptiles, not the eggs. Eggs have a shell. So there’s a two-fer trait in this division.
  • Diapsida Not counting eye and ear holes, these are extra auxiliary holes, two extra holes form in their skull around the temple. These holes are called foramen, in case you want to be fancy. Or even fancier than just using “diapsid.”
  • Testudinata The auxiliary skull holes have sealed up, and the ribs have grown tougher to make a solid shell. This is now a turtle.
  • Testudines A modern turtle, not a weird prehistoric turtle.

Turtle vs Tortoise vs Terrapin?

A red eared slider sitting on a rock in an aquarium
A terrapin named Paul, sitting on a rock.

Technically, turtles are salt water based testudines with flippers, tortoises are land based testudines with elephant like legs, and terrapins are fresh water based testudines with a webbed foot. Sometimes terrapins experiment with alternative lifestyles, like the local Diamondback Terrapin that lives in coastal areas, sometimes salty, and the Box Turtles, which are actually terrapins (look at the legs) but walk around acting like tortoises. It’s all about the legs: flippers vs elephant feet vs webbed feet. As for the group, no one uses testudines outside of herpetologists, but in general, American English uses turtles for all of them, British English uses tortoise. No one uses terrapin for the whole group.

Basic Vocabulary

The shell has two parts. The back part of the shell is the carapace while the belly part of the shell is the plastron. They’re grown solid together, but they are separate sets of bones.

Turtles, along with birds, other reptiles and the monotremes (platypus and echidnas) have a single “out” called a cloaca. Eggs, sperm, feces, urine (or their equivalent-chemistry is a bit different than human pee) all depart from the same door.

Turtles can breathe through their butts. It’s called cloacal respiration. Seriously. They can shunt the blood supply away from their lungs to the lining of the colon then pull water through their anus. It doesn’t give a lot of oxygen, but it does give enough for a sedentary existence, allowing them to hibernate in pond bottoms.

Some Interesting Errata

Baby turtles are can be squish. It takes a bit for the shells to completely ossify. Also they are fine outside. They know what they’re doing. Don’t try and save them. You are welcome to move a turtle off the road, but always put it on the side it was trying to get to. Turtles do not change their minds.

Turtles don’t have teeth. They mostly have a beak like chicken. But some have pretty dang interesting tongues.

Thus ends your turtle primer. Next week, box turtles.

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