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As a whole, the reptiles have a problem. The majority of them have legs sprawled away from the body, so when they move, they would suffer a kind of problem called "Carrier's constraint", and what that means is that the reptiles flex their bodies sideways as they move, expanding one lung and deflating the other, preventing the animal from moving and breathing at the same time.

The one notable exception to this are the alligators and the crocodiles, who have a "bent knee" posture, encouraging a "high walk", a kind of movement more similar to how a mammal moves than how a lizard moves.

In this alternate scenario, two other types of reptiles position their legs for the high walks:

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Testudinidae, the tortoises...

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...and Varanidae, the monitor lizards. (The bonus is that they use their throat muscles to gulp air, thus improving stamina.)

Add the high-walk posture on these two groups. Would it be help the tortoises to support their heavy carapace shells and the monitor lizards to add to their already efficient predatory standards?

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  • $\begingroup$ It would help to know what evolutionary direction you're imagining these creatures taking, if you have one in mind. If not, it's ok, just a heads up. :) $\endgroup$
    – Atlas the Worldbuilder
    Nov 19, 2016 at 3:39

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Well, that depends on what you're expecting the turtles and varanids to do with their niche. But in general, it can definitely help.

While I see a lot of the ideas in The Future is Wild to be a bit...too far, the Toraton is nearly exactly what you're describing for tortoises. They shifted their gait to be fully straight-legged, as opposed to sprawling, and thus it allowed them to get much, much bigger, turning them into sauropod-esque behemoths.

Here to sing his top hit single!

As for monitor lizards, it can help, but based off their evolutionary history and biology, I'm skeptical. Personally, I could see varanids re-evolving into a new mosasaur lineage more than completely changing their gait. If you're really looking into active predation for varanids, then while it might be a bit of a stretch for varanids, a point of comparison I'd draw to are certain members of the early mammal lineage, like gorgonopsids and therocephalians. Similar body plan, similar diet, makes sense. enter image description here

All in all, it depends on what you're looking for in a reptile. Hope these help!

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  • $\begingroup$ Not expecting much for either, except that both live in the African and South American mainlands. A monitor bigger than a Komodo Dragon is going too far, and my tortoises are bigger than ours--one ton, maximum. $\endgroup$
    – JohnWDailey
    Nov 19, 2016 at 3:42
  • $\begingroup$ @JohnWDailey just to be clear, you are aware that monitor lizards have outgrown Komodo dragons in the past yes? If not i suggest looking up megalania prisca. Just not clear what your comment about "going to far" was meant to mean. $\endgroup$
    – Myrdden Wyllt
    Nov 19, 2016 at 18:51

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