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So, I am writing a comic which will perhaps grow into an entire series about a group of aquatic heroes. One of those heroes is a monster size aquatic lizard. The story of how this lizard came about is complicated and is addressed in the comic but for simplicity's sake here, it was a lab error. Scientists were breeding lizards to try to figure out some genetic mysteries and it lead to there being a clutch of about 10 eggs that would develop into monster sized aquatic lizards.

This accidentally hatched lizard hybrid is now out in the wild and is colloquially referred to as "The Komodo of the Sea". Now in this alternate universe, the existence of this monster lizard is not leading to species extinction.

Now, I will ask in future questions about the specific features of this monster lizard and its habitat and life cycle, but for now I will be asking about the size of it. I already mentioned that it is The Komodo of the Sea. This already gives a clue about the size. It is at least as big as a Komodo Dragon.

enter image description here

Now that is a giant lizard. It weighs about twice as much as an average human and is about twice as long as well. Prehistorically, there was a relative of this giant lizard known as Megalania. It was about twice as long as a Komodo dragon and more than 3 times heavier. So there is no doubt that a monster size lizard could exist. And, given that this monster of a lizard is aquatic, it wouldn't be surprising if it could grow bigger than Megalania. No bigger than a blue whale obviously because at that size, if it gets bigger, it will likely collapse and not be able to breathe.

Sure, hollow bones would ease that weight but they are fragile and not ideal for an aquatic animal. In fact, I do believe the water pressure would cause the bones to become more dense. And I know that bone density contributes a significant amount to weight when you are talking about giant animals.

Is it possible that this aquatic lizard could grow up to 50 feet long, as long as a humpback whale given the inverse cube law and that the lizard is aquatic? How massive would it get at that length?

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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    $\begingroup$ We've had giant aquatic lizards in the past. Search for plesiosaurus and mosasaurus. They didn't need hollow bones to be viable. $\endgroup$ – Renan Mar 30 at 6:26
  • $\begingroup$ Many fishes don't even have bones in their skeletons, just cartilage. And they grow as big as a shark. $\endgroup$ – L.Dutch Mar 30 at 10:47
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    $\begingroup$ Hollow bones is not an advantage for a marine air breathing animal, the lungs already supply plenty of buoyancy, tetrapods that go back to the ocean usually end up with super dense bones like whales, penguins and prehistoric, marine reptiles. they need ballast to counteract the buoyancy of the lungs. $\endgroup$ – John Mar 30 at 13:58
  • $\begingroup$ Note also 50ft long is not even a third the length of a blue whale. $\endgroup$ – John Mar 30 at 14:37
  • $\begingroup$ Are you looking for something fully aquatic or semi-aquatic, does it need to come back out on land? $\endgroup$ – John Mar 30 at 14:52
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Nature did it first.

enter image description here

Mosasaurs were fully aquatic lizards, most closely related to modern monitors (such as the Komodo dragon), which swam Earth's seas for much of the Cretaceous period.

These were well-adapted to ocean life. They evolved viviparity so they didn't have to return to shore to lay eggs, and were likely warm-blooded to cope with the chilly ocean temperatures.

Lastly, the giant size. The largest mosasaur, Mosasaurus hoffmanii, was a whopping 17 metres long, a good five feet over the length you describe. So there you have it.

EDIT: As John pointed out, no hollow bones are needed. Floating to the top is as much a problem for aquatic animals as is sinking to the bottom, so mosasaurs made do with high lung capacity, and also (possibly) a sort of blubber.

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    $\begingroup$ You can add these animals have very dense bones not lighter ones, the lungs already supply to much buoyancy they don't need any more. $\endgroup$ – John Mar 30 at 13:59
  • $\begingroup$ You can also add the largest mososaurs are around 16 meters long, far bigger than the noe in the picture but far smaller than blue whale. you can find an acceptable size chart here, prehistoric-wildlife.com/species/m/mosasaurus.html $\endgroup$ – John Mar 30 at 14:03
  • $\begingroup$ @John Well, I did say that ("was a whopping 17 metres long"). I kinda just chose the pic because it was a nice, accurate reconstruction. The mosasaurs in the charts you gave are horribly shrink-wrapped, for example. Anyway, I changed it to one of M. hoffmanii. $\endgroup$ – SealBoi Mar 30 at 14:08
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    $\begingroup$ Yeah unfortunately the artists that favor nice images for size comparison go for the gaunt look, they tend to be afraid to add any excess flesh. $\endgroup$ – John Mar 30 at 14:21
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The Komodo dragon is a monitor lizard. They can swim. There are other monitor lizards which spend most of their time in the water; they get big too but not as big as Komodo dragon.

Australia (and maybe other areas below Wallace's line?) had a monitor lizard that was bigger than the Komodo dragon - Megalania.

https://en.wikipedia.org/wiki/Megalania monitors

Those are big! Supposedly their time overlapped with the first humans in Australia. Also probably Homo Floresienis rode them around. Hyah!

In his book In the Wake of the Sea Serpents, Heuvelmans makes reference to "reports out of New Guinea" of 20 foot long aquatic monitor lizards. Maybe some Megalania escaped extermination by hiding out on remote islands - oh, yeah; those are Komodo dragons. A giant water monitor could happen, and the water is a good place is hide from humans. I would put it in New Guinea if it were anywhere.

A giant water monitor begins to converge on the mosasaurs: seagoing lizards. A totally legitimate body plan and lifestyle.

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  • $\begingroup$ Megalania never made it to Flores, as far as we know; you see, the Wallace Line is but the western border of a whole region of transition, Wallacea; the eastern boundary being Weber's Line, which is also east of Flores. However, the island did and does have true Komodo dragons. $\endgroup$ – SealBoi Mar 30 at 13:56
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How lizardlike does it need to be, is it fully aquatic or does it need to come out on land?

Pliosaurs were the largest marine reptiles (by mass). Not quite blue whale sized but surprisingly close. Pliosaurus macromerus is estimated to be around 15 meters (49ft) and Pliosaurus funkei is estimated around 16 meters (52 ft). Larger pliosaurs may even exist, below is an image of the largest estimates. Mass estimates vary by quite a lot, 35-45 tons is a good range however. Longer marine reptiles (plesiosaurs) exist but most of that length is neck.

Note weight is not an issue in marine animals they want dense bones, hollow bones would be a huge disadvantage, most marine tetrapods with lungs develop super dense bones to ADD weight, they need ballast to compensate for the buoyancy generated by the air filled lungs. Everything from pliosaurs, to whales, to penguins have solid bones that are far denser than their terrestrial relatives.

enter image description here

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As long as it stays in the water the cube/square issues that limit the practical size of a giant creature don't really apply. Really the practical limitation is the difficulty in catching enough prey to feed such a large creature. Which is why the largest sea animal has to rely on straining vast amounts of seawater just to keep itself fed. But it will be very problematic if your monster ever tries to leave the water.

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    $\begingroup$ I did a calculation once (for something else entirely), and concluded that a 200,000 kilogram fish/reptile could sustain itself by eating only one bowhead whale every 10 years. $\endgroup$ – SealBoi Mar 30 at 10:15
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    $\begingroup$ @SealBoi How were you able to calculate that? Metabolism is way more complicated than just caloric intake and mass. $\endgroup$ – Caters Mar 31 at 0:51

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