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On our Earth, what makes a turtle a turtle is its ribcage widened and fused to create a tough, protective shell. In comparison to other reptiles, alive or extinct, the turtle design has got to be the most conservative.

But is the shell the only defining feature of the turtle order (Testudines)?

Suppose that in a mass extinction, past or future, wiped out a great deal of species, leaving the few remaining turtles to radiate into new niches. Would all the new turtles retain their iconic shells, or would some other species abandon them to suit their new niches? If yes, how can we tell a turtle without its shell?

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    $\begingroup$ For a moment I thought you were referring to some sort of weird font... $\endgroup$
    – L.Dutch
    Sep 21 '18 at 19:25
  • $\begingroup$ The beak and feet of turtles is very telling, anatomically $\endgroup$ Sep 21 '18 at 20:27
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    $\begingroup$ What makes a dinosaur dinosaur? Would they survive without their iconic features after mass extinction? It is either naming or philosophy question to tell if chicken is a dinosaur or not. Exactly the same with your turtles. $\endgroup$
    – Mołot
    Sep 21 '18 at 21:09
  • $\begingroup$ @Mołot the defining feature of dinosaurs is a hole in the hip socket (perforated acetabulum), creating something more like a rod and socket joint (hinge) than a ball and socket joint. And yes chickens share this feature. $\endgroup$
    – John
    Sep 21 '18 at 21:24
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    $\begingroup$ This is in the VTC queue. I disagree that it's OT:NAW because asking how things evolve is worldbuilding. However, I'm VTCing as POB. How would you judge the best answer? Given enough time, almost anything can evolve into any condition. Humans no longer have their fur, claws, tails, etc., etc., Asking "what evironmental pressure could cause X" is possibly answerable, asking "what could X evolve into" never is. $\endgroup$ Sep 21 '18 at 23:22
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The Skull

There are several other features common to turtles most in the skull like beaks and being secondary anapsids, other features like the internal shoulder joint,and flattened ribs are related to the shell.

Early fossil turtles do not have shells but do have beaks and flattened ribs.

In fact being secondary anapsids (no holes in the skull behind the eye) is one of the major features of the turtle group, and has been a point of contentions for decades , before earlier fossils were found, about what their closest relatives are.

enter image description here

An extinction in which turtles are the only tetrapods to survive is basically impossible, turtles are too specialized.

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Most turtles' best bet in nature is their defensive shell and it's hard to imagine circumstances where it might be beneficial to get rid of it.

Snapping turtles just made their neck longer and evolved to have fake baits in their mouths and heavy bits; certain turtles got softer shells or smaller ones to aid mobility, but the shell is still very useful (for example, green sea turtles can't retract into the shell but use it as a shield against shark bites when angled right). The turtle's shell isn't just bones, it houses blood vessels, nerves, and important organs. As you said, it's like an extension of their rib cage. It's impossible to remove a turtle from its shell, and it'd be just as unlikely for them to bother backtracking so much that they revert to being shell-less.

However, if some mutation does occur and ends up being beneficial to them, they would still be recognizable by basic skeletal structure (jaws, legs, etc.), organs, and DNA checks.

Reasons as to why they might lose their shells might even be the reason they evolved to have them-- for digging. If, say, some apocalypse happened that wiped off most turtle habitations, they might choose to start burrowing underground and live there most of their lives to find more food and escape droughts, eliminating the need for such protection and making it more beneficial to revert to semi-shells. https://www.theatlantic.com/science/archive/2016/07/the-turtle-shell-first-evolved-for-digging-not-defence/491087/

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  • $\begingroup$ It is possible for the shell part to be lost, modern softshell turtles are most of the way there. but the rib structure that supports it is needed for how turtles breath. $\endgroup$
    – John
    Sep 23 '18 at 2:19
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what makes a turtle a turtle is its ribcage widened and fused to create a tough, protective shell

I'm not so sure about that.

how can we tell a turtle without its shell?

Comparative anatomy. What other creatures' organs does this specimen's organs look like?

https://en.wikipedia.org/wiki/Comparative_anatomy

the study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny1 (the evolution of species).

The science began in the classical era, continuing in Early Modern times with work by Pierre Belon who noted the similarities of the skeletons of birds and humans.

By Волков Владислав Петрович - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=37704829 enter image description here

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  • $\begingroup$ Care to clarify? $\endgroup$ Sep 21 '18 at 20:07
  • $\begingroup$ @JohnWDailey: Comparative anatomy. (For example, turtles are the only tetrapods with their shoulder girdles inside their rib cages.) Molecular phylogenetics. And some extant sea turtles (e.g. Dermochelys coriacea, the leatherback) lack bony carapaces. $\endgroup$
    – AlexP
    Sep 21 '18 at 20:18
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    $\begingroup$ You do realize deer are ungulates as well right? They are also both ruminants,, deer and Chevrotain are closely related. This is also a very poor representation of how comparative anatomy works, it is not just a vague guess because things kinda look alike.. $\endgroup$
    – John
    Sep 21 '18 at 21:00
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The turtles will evolve to fit all other niches

And eventually, they will stop being turtles.
I´m going to mention here the book "After Man, A Zoology of the future" (Dougal Dixon, 1981). enter image description here
Among all the topics mentioned in the book, I want to mention this one: In a few centuries, a new volcanic island emerges. Brand new with no life at all. After a few months, the first living organisms in the island are plants. And later,the first living animals arriving to the island are bats. (And here comes the answer to the turtle part). Dougal Dixon explains: since bats are the first animal species arriving to the island, they become the dominant specie, and evolve and start occuping many types of niches. For example, some loose their wings and adapt to a ground life:
enter image description here

Others bats change more dramatically and adapt to acuatic life:

enter image description here

The point is: If all the animals disappeared except the turtles, then the turtles will evolve to fill the empty niches, and in a few milenia, you will have plenty of animals that will have very little (if any) resemblance with the original turtles. You could expect to have predator turtles, turtles that run, turtles that climb trees, and (perhaps) even turtles flying. The only way a turtle could mantain its turtle style, is if the conditions in the new ecosystem give the original turtle an advantage versus transforming itself in other type of organism.

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