Crocodiles sun bath, a lot. Body size is mostly dependent on how readily available food is. Crocodiles are big because their diet can afford it, they used to be bigger because the animals they preyed upon were bigger. It's not unheard of for large animals to go extinct when their food source begins to diminish (e.g. titana boa, megalodons, etc.)

If it is warm blooded and it's gets to big it basically cooks itself because it can't get the heat away fast enough. Growing and heat regulation come hand in hand during evolution other wise it wouldn't survive. Inanutshell has two videos on YouTube about the effect of size on lifeforms. Like shrink an elephant to the size of a mouse ind it would freeze to death grow a mouse to the size of an elephant and it would practically boil alive. And I heard in a documentary that the megafauna back than might have been pseudo warm blooded.

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You're right that looking at only the size of an animal doesn't tell us much about its thermoregulation. Cold blooded animals (i.e. ectotherms) come in all sizes from massive sharks to small reptiles and of course insects. Warm blooded animals (i.e. endotherms) are a little more limited at the small end of the spectrum because of the scaling of surface area to volume (Y = aX^(2/3)) making heating proportionately more costly for smaller animals.

Endothermy for small mammals can be too costly because their high surface area to volume ratio causes proportionately more heat loss than if they were 10x larger. This is related to another point brought up here at the opposite end of the size spectrum. Large endothermic animals like elephants are more at risk of overheating because the dissipation of heat is limited by their surface area to volume ratio being much lower. All animals have metabolic rates that are appropriately scaled for their body size or they wouldn't be here!

I think it's also important to include here the spectrum of heterothermy to homeothermy as a concept. Heterotherms maintain a 'wide' range of body temperatures and include some endotherms (e.g. squirrels that decrease their body temperatures to hibernate) and a lot of ectotherms (e.g. reptiles living through day-night swings in ambient temperature in the dessert). Whereas, homeotherms maintain a more narrow range of body temperatures but also include both endotherms (e.g. humans, only experience very small changes in body temperature) and ectotherms (e.g. many fishes are restricted to narrow temperature bands). But what really is the difference in heterotherms and homeotherms? It comes down to the sum of all biochemical processes in the body. In a homeotherm, all of the enzymes, signalling, membranes etc. are optimized to function at a set temperature (i.e. specialized) whereas heterotherms have cell components that can continue working across a broad range of temperatures (i.e. generalist). This kind of segues into what I think is beneath your question which is why endotherm when ectotherm do job?

The clearest answer is that endothermy allows animals to occupy different thermal niches than ectotherms. If it's beneficial to spend a little extra energy maintaining a constant body temperature with specialized cell components that work faster (because warmer temperatures mean faster reactions) to occupy different niches than ectotherms. And on the flip side ectotherms may be better suited to handle disruptions in food availability over time because they are not burdened with the metabolic furnace bill.

There is of course a lot more to this, but it is a really interesting evolutionary question. My understanding is more from the physiology side of things, hence the answer.

edit: removed ~ in front of 2/3 because it looks like '-' in superscript

I don’t know what you mean by “megafauna back then may have been pseudo-warm blooded”, partly because I don’t know what “pseudo-warm blooded” means. For the most part, an animal is either endothermic, or it isn’t (unless you’re talking about a few special cases like bumblebees, which use movement to raise their temperature sometimes, but are overall still ectothermic, but this is only really a thing in a few small invertebrates to my knowledge). Dinosaurs are believed to have all been endothermic, although like their modern bird descendants, they probably did have lower body temperatures on average than most mammals. But if you’re talking about past mammalian megafauna (or even present to some degree), the suggestion that they may have not been truly endothermic seems pretty baseless, as the range of average body temperatures in living mammals doesn’t really correlate with size very much (in that there are both examples of large and small animals with relatively high temperatures, and large and small with relatively low temperatures), really, adaptations to different climates and other physiological factors are much more important, and larger animals with high body temperatures require a more energy-rich diet to sustain the metabolic requirements, but to my knowledge that’s the main limitation. And all living mammals are true endotherms that generate body heat metabolically, even if there is some variance there.

Whales are true endotherms, and as far as we can tell, blue whales are actually the biggest animals in general have ever gotten (at least in terms of mass). A much bigger constraint on size is gravity (which is why aquatic animals have always grown the largest), followed by pulmonary efficiency (essentially, it takes special physiology to ensure that oxygenated blood can reach all the tissues it needs to in a big animal, and again, gravity is also a factor here when an animal is terrestrial, which is why it’s thought that the complex bird-like respiratory systems of dinosaurs may have played a role in them getting so big on land, in addition to the higher oxygen content of the atmosphere during the Mesozoic).

While terrestrial mammals never got as big as dinosaurs, they did once get a lot bigger than African elephants do today, such as the gigantic rhinoceros Indricotherium, and there’s absolutely no reason to believe any of these animals weren’t true endotherms (we obviously can’t know from fossils exactly how their body temperature stacked up to their smaller living relatives, but again, it’s not likely to have been nearly as important factor as many other things).

EDIT: also, as someone pointed out above, getting really big is actually in some respects harder for ectotherms, as it takes longer to warm a very large body in the sun, so they need to spend far more time basking the bigger they get. This is part of why cold-blooded giant dinosaurs were initially criticized as a theory, because a cold-blooded Tyrannosaurus, for instance, would literally waste all its time soaking up heat from the sun rather than hunting, and would just be far too plodding and sluggish to be an efficient apex predator.

EDIT 2: my bad, it seems that there are also some fish with a physiological means of generating some of their own heat, to the point of sometimes being referred to as “endothermic” informally, but they do so through different means than birds/dinosaurs or mammals do, and this also still doesn’t relate to any past land megafauna.

Wouldn’t a larger body mass mean that you cooled off more slowly and in principle I don’t know if it would be much of a problem if you were a dinosaur to maintain a reasonable body temperature.

Body size is affected by a lot of factors. One of the biggest is if they have active aspiration and circulation or passive aspiration and circulation. The more passive the more they benefit from thin bodies with high surface area ratios to absorb oxygen.

I think it depends on a lot of factors. In general, I believe that adipose tissue (fat) retains heat better than a lot of other tissues, so I think it depends somewhat on an organism’s bodily composition. For ectotherms, they do need circulation to distribute warmth around to all their tissues, and a bigger body means that process naturally is going to take somewhat longer. This is part of why crocodiles (which in many cases are really big animals) are all in aquatic sit-and-wait predatory niches in their surviving forms, as they can be huge but still relatively sluggish and spend a lot of time basking, because they just need enough energy to strike out hard when the moment is right, they don’t need to pursue prey, etc. There used to be fully-terrestrial predatory lineages of crocodilians, but they were in general not quite as big and were built very differently, being generally lean and long-legged for pursuing land-based prey (they were probably a lot like big monitor lizards like Komodo dragons in terms of the niche they occupied and how they hunted, and they were probably similarly limited to being diurnal hunters in hot climates).

Like I know that, at least for big predatory dinosaurs, the assumption that they were ectothermic like living reptiles was long a source of confusion, to the point where (prior to the theory that dinosaurs were probably mostly or entirely warm-blooded) a number of paleontologists forwarded the idea that dinosaurs like T. rex were more likely scavengers than active hunters, because it was just too hard to imagine them being able to relentlessly pursue prey with that kind of bulk if they were cold-blooded.

Clearly, it isn’t a complete barrier against getting really big, as plenty of very large presumably cold-blooded creatures have existed over time (like I believe that most Mesozoic marine reptiles like ichthyosaurs and mosasaurs are assumed to have been ectotherms, since they came from older diapsid lineages that hadn’t evolved endothermy—in fact, mosasaurs seem to have literally been gigantic marine lizards, related to monitors—and they were still able to get pretty massive in some cases, although still not quite as big as the largest whales), but I also think it probably indicates something that the very biggest of animals to ever live all seem to have been warm-blooded, and that’s definitely not that endothermy itself is a barrier to great size, quite the opposite in fact.

Dinosaurs are mesotherms. They're neither endothermic nor ectothermic. That's what's meant by 'psuedo-warm blooded'. They maintain their own body temperature while simultaneously relying on external heat sources.

Endotherms rely on circulation as well, that's the whole way they distribute heat across the body. Our heat moves across the body all the time. It's why our hands and feet get cold, because our body moved the heat to the vital organ when outside temperatures are relatively colder. Adipose tissue is only reliable so long as you can actually get heat to the tissue.

Also weren't there terristrial crocodiles which were active chase predators such as boverisuchus?

Where did you the info that marine reptiles didn't regulate their body temperature? I just found a paper stating the marine reptiles probably controlled their body temperature.

https://www.science.org/doi/full/10.1126/science.1187443?casa_token=898ordxLwggAAAAA%3Ae0iFRf87IUS5PDaLeyVXbxlRbMcijkQzj_eey_0kwikLMYLA1x5cIdznkyARW3jNqUxGFjjYZqn_KCv_

That's true, I forgot about that. That's why insects and other arthropods were bigger when the earth had a higher amount of O2 in the atmosphere and shrank when the O2 levels dropped.

There were terrestrial predatory crocodiles, but they weren’t anywhere near the size of large predatory dinosaurs (nor even present day saltwater crocodiles, let alone the even larger aquatic crocodiles that existed in the Mesozoic).

Last I’d heard, most marine reptiles from the Mesozoic were not assumed to be warm-blooded because non-archosaur diapsids like that simply weren’t assumed to have evolved endothermy, but I haven’t really kept up on speculation about that, so it’s possible the consensus has shifted with at least some of them (as obviously dinosaurs themselves have only been widely-assumed to be endothermic relatively recently).