How large could a giant turtle get?

Question

Assuming a turtle or tortoise evolved adaptations to become large comparable to that of sauropods. How large could they become (in weight or potentially carapace size)? The turtle needs to be primarily terrestrial.

Adaptations:

• Hollow, honeycombed bones (including shell)

• More efficient lungs like that of birds

• Metabolisms comparable to those of sauropods.

Answer 1

Not bigger as they were (up to couple of meters in diameter). Or they will stop being turtles:

• Hollow, honeycombed bones (including shell) are the worst armor - they can be easily pierced or crushed. Yes, Apollo spacecraft was basically made of "honeycombed foil" and survived reentry, but could be pierced (and was once) with a screwdriver and crushed with a hammer. This makes the shell totally useless as a protection against predators. And predators of that time had knife-like teeth and jaws strength of 0.5-2 tons.
• More efficient lungs like that of birds - this would not help turtles. They would not be able to use "bird" lungs because they cannot breathe with their ribs (because their ribs are solid shell). Breathing is not the thing that dictates turtle size: marine turtles take a breath only once in 15 minutes (or more) - and still bigger than terrestrial ones.
• Metabolisms comparable to that of sauropods - that will totally remove turtles main evolutionary advantage, which allowed them to survive through 2 (two) major extinction events (no other group of species of large animals did). And having high volume and low surface, compared to sauropods turtles would have to reduce their size not to boil alive. Just look at stegosaurus - how hard it was trying to increase its body surface!

It all means that with these adaptations turtles would deevolve to something more crocodile-like, as they were at the start of their evolution. You do not need a shell, if you can run fast - it will only slow you down!

But if you look for giant "animal tanks", look at glyptodont (real giant armadillo). First homos were using their shells as homes! Both this, and the largest terrestrial turtle were about the same size (about 2 t weight and 2-3m length), and I don't think this is just a coincidence. It is the limit for biologically created shell to be strong enough to provide reasonable protection. The larger they get - more fragile they become (square-cube law).

Answer 2

The book and TV series "The Future is wild" postulated gigantic tortoise which would fit the bill easily: the Toraton

Toraton

The Toraton is an enormous tortoise from the Bengal Swamp in 100 million AD, in the documentary The Future is Wild.

In an environment such as the Bengal Swamp, where vegetation flourishes, the presence of large herbivores is no surprise. The largest of them by far is the Toraton.

The Toraton is by far the largest member of the ancient tortoise family, which dates back to the Pliocene epoch of the Neogene period. Since 220 million years or so before the human era, in the Late Triassic period, their order has thrived. Their basic shape and lifestyle were so successful from an early stage in their history that they hardly evolved. Now, 100 million years past humans, conditions have encouraged a huge increase in their size, enabling them to exploit the incredible mass of vegetation that now exists in this greenhouse environment.

The Toraton is a cold-blooded reptile, so it does not face the problem of overheating that would confront a similarly-sized mammal.

With an average body weight of 120 tons, this is the biggest terrestrial animal known ever to have walked Earth, more massive than even the greatest of the known dinosaurs, the sauropods, though some particularly large dinosaurs could have achieved a body weight similar to or even greater than that of the Toraton, possibly rivaling or even exceeding the Blue Whale, the heaviest known animal.

So given the proper environment, the humble Tortoise has the potential to reach astonishing sizes if it moves into the niches once occupied by giant herbivours like sauropods, elephants and so on.

The only other feasible way to become larger is likely to go the other way and become a completely aquatic sea turtle, and take the niche of baleen whales straining plankton from the ocean. Of course, what sort of predators exist in the ocean to justify a shell of that size?

Answer 3

I see no problem for sea turtle being as big as modern blue whale. They would only need to develop ovoviviparity as they can't go back onland to lay eggs. No big deal.