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Certain anatomical features distinguish the alligators, crocodiles, gharials and caimen from the other reptiles.

  1. A four-chambered heart, something also found in birds and mammals
  2. A unidirectional respiratory system
  3. A semi-erect posture, midway between the legs of mammals and birds (which were straightly tucked underneath their bodies) and those of other reptiles (which splay away from the body, resulting in a respiratory issue called "carrier's constraint")

On an alternate Earth, ALL of Reptilia (including the legless snakes) have most if not all of the features listed above. (Specifically, each species of reptile has 15% of its body volume reserved for air sacs.) Is this sort of thing both biologically and evolutionarily feasible for all reptiles?

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    $\begingroup$ Sure, just have the ancestors of the other modern reptiles killed off in your alternate Earth's version of the K-T extinction. Then the crocodiles would likely have radiated to fill many of those empty ecological niches. (If mammals & birds didn't beat them to it, of course.) Don't know about turtles, though. $\endgroup$ – jamesqf Mar 9 '17 at 5:47
  • $\begingroup$ keep in mind amphibians will fill a lot of those niches. $\endgroup$ – John Mar 9 '17 at 14:09
  • $\begingroup$ this really depends on how earthlike you want your earth. lots of unique features will not re-evolve so the new reptiles will not produce copies of all the variable forms of reptiles. things like chameleons, turtles, and vipers probably won't re-evolve from crocodiles. Although snake breathing is already similar to crocodiles. $\endgroup$ – John Mar 9 '17 at 14:15
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When two different life forms live in the same place and one is clearly fitter than the other, the less fit tends to become extinct. Crocodilians are older than the dinosaurs - in fact, they are older than modern lizards and snakes. If the upright gait of crocodilians was fundamentally better in all respects than the sprawling gait of lizards, lizards would have been replaced long ago by tiny crocodiles.

The answer, as is the case for many seemingly "primitive" species outcompeting more modern ones, is energy cost. Holding the body upright requires the leg muscles to be constantly in use, which means the animal uses more energy and therefore requires more food and oxygen. Most reptiles have "low energy" lifestyles - they spend a lot of their time lying around and don't need to eat or breathe as much as comparatively-sized mammals.

The "breathing" part is most relevant here. While lizards are restricted by Carrier's constraint, it really isn't such a big issue for them, since the same posture that makes it hard for them to breathe also means they don't need to breathe as much. While it does mean that they can't run long distances to chase down prey, they don't really need to.

Crocodilians occupy a midway point in the high-energy and low-energy lifestyles, a unique niche which is strongly linked to their semi-aquatic lifestyle. They can put on bursts of speed that most lizards can't handle, and avoid the disadvantage of an upright (or partially upright) posture by using water to support their bodies. The crocodilian form is specialized for a water-dweller, and it is unlikely that it would be retained by a fully terrestrial animal.

In the past, there were periods where crocodilians actually did diverge into many different niches, and the forms they took closely resembled other species that occupied their niches at other points in history - terrestrial crocs either went fully upright, or fully sprawling. Over time, the high-energy terrestrial lifestyle was dominated by dinosaurs and later mammals, while the low-energy terrestrial lifestyle was dominated by lizards.

Unless your world is significantly different than Earth, there will probably be low-energy niches available. As a result, even if all species at some point in time have given up the sprawling gait, sooner or later upright species will return to the sprawling gait to fill those low-energy niches. It isn't fundamentally inferior to an upright one, just specialized for a different niche.

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If the original ancestor from which all other branches of Reptilia spring includes those traits, I don't see why not. Although as with earth's alligators etc. differing from other reptiles, there will probably be some branches on Alternate Earth that don't have all those traits.

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There are different branches of the reptile family tree. Crocodilians have semi-erect posture and a four-chamber heart because they belong to the same branch of the reptile family tree as dinosaurs and birds, which shared these features (and were warm-blooded, crocodilians appear to have later lost this trait).

There are other branches, with the main existing branches being the lizards (including snakes) and (maybe) the turtles (which may actually be a sub-branch of snakes and lizards, their origins are a bit murky).

There are also smaller branches like the tuatara, but they are rare enough that they could still exist without violating your scenario.

So the easiest way to have most reptiles be like crocodilians is to have the lizards and turtles never evolve to begin with, or die out early in their history due to some freak extinction event. If the rest of history went exactly the same way, the (non-bird) dinosaurs would have died out, as would have most of the members of other groups like tuataras, leaving the only cold-blooded land animals being crocodilians. They would then radiate after the dinosaurs died out to fill various niches currently filled by lizards and snakes (since legless lizards have evolved several times) and turtles (since there are extinct armored crocidilians and close relatives).

In the past crocodilians have had a much wider variety of forms than the underwater ambush predators that exist today, so we know this sort of radiation is possible for the group.

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