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In my speculative evolution project, the primary group of animals was a clade of 8 limbed “vertebrates” that evolved from an aquatic boney fish that left the water. They left the water through a peculiar means, that being evolving flight before walking. Some clades landed, giving rise to the first group of land animals with 8 limbs.

One group that stayed in the air evolved connected limbs, in that their 4 sets of wings had merged into only one. After a mass extinction, some of these 2 limbed vertebrates landed and became a dominant clade, similar to how mammals filled most large niches after the K-PG mass extinction, or how dinosaurs did after the Triassic mass extinction.

The largest modern bipeds are ostriches get to around 130 kg, with the largest ever being spinosaurus aegyptiacus that could get to around 8150 kg. The problem I have with the spinosaurus is that the spinosaurus was most likely at least partially aquatic, allowing them grow much larger due to buoyancy, and my animal will not have that luxury.

And even if we go onto the next largest biped, we’ll begin to see a trend with them all being carnivorous. My animal would be herbivorous, and plants are less energy efficient than meat. The largest fully bipedal herbivores cap out at around 6 feet (im not counting hadrosaurids due to them being primarily quadrupedal).

The planet is 2/3rd the mass of earth with 2/3rd the atmosphere.

TLDR, my animal only has two limbs, what is the largest it can get?

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    $\begingroup$ Knowing how to fly before knowing how to walk of solid ground? That's going to make for some interesting crashes.. ahem, landings. $\endgroup$ Commented Feb 23 at 8:37
  • $\begingroup$ spinosaurus may have been partially aquatic but t-rex and carcharodontosaurus were not and got to similiar size. also look at Therizinosaurus cheloniformis for a large bipedal herbivore. $\endgroup$ Commented Feb 23 at 12:42
  • $\begingroup$ I think you're making an uncorrelated observation about bipedal carnivores. Argentinosaurus was 70-100 tons while Spinosaurus was only 7-10. If we say Argentinosaurus was only that big because it had four feet we're disingenuous. The higher energy density carnivorous diet may have (that's the problem with evolutionary theory) made larger sizes less advantageous. But in the end, the real reason is still the square-cube law, which (very indirectly) accounts for things like "a leg bone can only hold so much weight." The size, interconnection, and design of those bones are really important. $\endgroup$ Commented Feb 23 at 15:31
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    $\begingroup$ Which all points to an important aspect of worldbuilding: rationalization (because science won't tell us the max mass of your creature in kilograms). Argentinosaurus had leg (and foot!) bones that could individually carry 17.5-25 tons, which is 2.5X the weight of Spinosaurus. So, science-based, we know there is a leg structure that can handle a creature 2.5X the mass of Spinosaurus. But I wouldn't expect it to run very fast, or to jump at all. $\endgroup$ Commented Feb 23 at 15:36
  • $\begingroup$ The *Pachycephalosaurids were bipedal and herbivorous. About 4 m (13 f) long and 400-odd kg (~880 lb). $\endgroup$ Commented Feb 23 at 23:12

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Aepyornis maximus, the biggg elephant bird of Madagascar, probably weighed around 400 kg; different paleontologists have different opinions, with estimations ranging from 275 kg (600 lb) to 1,000 kg (2,200 lb). It went extinct at some point in the 9th or 10th century AD, soon after the terrible predator H. sapiens invaded the island of Madagascar. And yes, the elephant birds were herbivorous.

In the spirit of the question, what's nice about Aepyornis is that, just like the ostriches, the emus, the moas and so on, it descended from much smaller, flight-capable ancestors, probably similar to the extant tinamous of South America.

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Lets try a rough estimation (without warranty) using the Square Cube Law and some given references and see where we get.
Note: This is the first time I do this calculation and also the use of MathJax, so I hope everything is correct.

  • We have gravity of around $2/3$rd from earth
  • From @JBH we got the input that an Argentinosaurus had legs which could hold around 17.5-25 tons.
  • The animal has to be able to stand on one leg, at least for a short time when walking (so no multiplication here)
  • I borrow the information from @AlexP about the Aepyornis maximus with around 3m height and 1000 kg or conveniently around 1 ton. Lets assume an animal with comparable proportions

With 2/3rd of the gravity from earth, the legs should be able to hold around 26.2 to 37.5 tons. Lets choose 35 tons.
Next we use the Square Cube Law to guess how many times we can scale this down: $\sqrt[3]{\frac {35 t}{1 t}} = 3,27$
Now we can use this factor to scale up the size from the Aepyornis maximus
$3m * 3,27 = 9.81m$

I am kind of surprised that this calculation turns out to be so small, if we consider that an Argentinosaurus could reach a height of around 18 to 20 meters. Maybe I should have calculated with around 1.3 to 1.8 legs instead of 1.
Note: The same calculation with 2 legst instead of one produces a factor of 4.12, and thus a height of 12.36 meters

The Argentinosaurus had a better height-to-weight ratio than my scaled up bird. This kind of makes sense due to the 4 legs and animals probably don't need to carry the full unnecessary "square-cube" weight around and can become a bit skinnier the bigger they get.

So with the lower gravity but less legs, I guess something around 12 to 15 meters would make sense. The faster the bipedal has to run, the stronger the impact on the legs and thus the lighter and smaller it has to be.
As I said, no warranty, but I guess you can use it as a rough orientation and adopt the numbers or recalculate as you think seems plausible for your animal.

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    $\begingroup$ Note that the maximum load that the legs can hold is usually very much larger than what the animal weighs. For example, human legs can hold half a ton with no problems, but humans are very much lighter than that. The reason that the legs must allow the animal to walk and run. P.S. Like all the big sauropods, Argentinosaurus had elephant-like pillar legs. Birds are not sauropods and their legs are not at all arranged like vertical pillars. $\endgroup$ Commented Feb 23 at 23:31
  • $\begingroup$ What if we "cheat" by giving the animal a tail? $\endgroup$ Commented Feb 24 at 18:45
  • $\begingroup$ While a tail might help reduce support load during walking, it's unlikely to be contacting the ground during running, so its additional mass would likely be more of a hinderance than help, at least for load-bearing, although it can improve balance and thus speed. $\endgroup$ Commented Feb 25 at 1:27
  • $\begingroup$ @T.E.D. Nice idea. The animal could have evolutionary remnants of its fish-tail and also of its wings. Both can be used for stabilization and could be a better solution than the pure reduction in size. $\endgroup$ Commented Feb 25 at 14:00
  • $\begingroup$ I mean, I didn't come up with it from nowhere. Tyrannosaurs were all 2-legged with very long tails, and while they didn't use it for support, the even larger 4-legged Sauropods supposedly would rise up on theirs (along with their back legs), using it as kind of a 3rd leg while they reached for higher vegetation. Plus, well, Godzilla depictions usually show his tail on the ground. It seems like a dragged tail could take at least a little weight off the 2 legs. $\endgroup$ Commented Feb 25 at 14:32
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If it counts, large Hadrosaurs such Shantungosaurus and Edmontosaurus are believed to be facultative bipeds that could operate on two legs, despite usually using four. Size estimates for Shantungosaurus range as high as 18 tons, much larger than Spinosaurus.

There are also unpublished fossils from the Lower Elliot Formation that record a bipedal prosauropod giant of 10–15 tons:

"The morphology of the ulna suggests that, like many other large basal sauropodomorphs, the giant Elliot form was an obligate biped... The giant Elliot form represents the largest known non-sauropod sauropodomorph, one of the largest terrestrial animals prior to the Jurassic, and one of the largest bipedal animals of all time. It demonstrates that basal sauropodomorphs were able to achieve very large body size despite lacking most of the presumed key innovations of sauropods, such as quadrupedal stance, columnar limbs, and extensive skeletal pneumaticity."

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There's about a billion considerations here, but I'll take a stab at it. As a strict biped with no forelimbs at all, the problem you will run in to is how hard it is for it to get up and stand. The best real world comparison I can think of is the Carnotaurus sastrei, since its arms were tiny and your creature lacks even these arms. Yes, it was a carnivore. That said, there are a number of considerations.

Why isn't plant-eating an issue? Well as long as there's enough food to support a huge creature, size works to your advantage here. In general the larger a creature is, the more efficient its digestive system. Plant matter is generally very hard to digest, so a herbivore benefits greatly from having a long digestive tract. So as long as you have enough plants to eat, you can support massive sizes. In the modern world , most herbivores lack the size for such a long digestive tract, and have to ruminate or else spend the vast majority of their time grazing. Your creatures would not have this issue if they were large enough, with a long enough digestive tract. Taller herbivores can also reach more vegetation, and of course size is an advantage against predation.

Some other considerations:

  • It might need either a flexible neck or some kind of trunk/proboscis. Without something like this it would have a hard time lowering itself to drink water or eat low vegetation. Even if it only eats taller plants, herbivores benefit from being able to reach both high and low. Perhaps if your creature only eats ground plants through a proboscis it can avoid becoming too tall and reach a larger size.
  • Not related to diet but also worth noting: Having evolved from flying creatures, your creatures would probably have light, hollow bones, which is great for weight saving. Nice!
  • A slow metabolism would generate less heat and make your creature less likely to cook itself from the inside due to the lower relative surface area.
  • Large animals have longer periods of gestation. Elephants are pregnant for nearly two years. Your creature would fare better it had smaller, less developed young, in larger quantities.

The first problems arise with bipedalism. A biped is not as stable as a quadruped, and with only 2 limbs it will be especially hard for these creatures to stand up. A tail can partially mitigate stability problems, but getting up in general is going to be a problem for a large enough creature. A tall herbivore that can reach high vegetation is going to struggle even more with this.

So how big can it get? Barring any potential cardiovascular, or circulatory limitations, the limit is the point at which it is too heavy to get up. For a creature that is strictly bipedal, with no other limbs at all, this will stop you before anything else. The Carnotaurus reached 1.3–2.1 metric tons. The Tyrannosaurus had slightly larger forelimbs, but it could reach almost 9 tons. With 2/3 gravity and no forelimbs your creature will probably be somewhere in the middle.

Since there are too many variables to answer definitively, perhaps the best thing you can do is look at the size of your creature and decide if it can roll over and get its legs underneath it to stand. Try to imagine how it will be able to balance while it gets up. If it can't do either of these it's too big.

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  • $\begingroup$ not just ground plants, reaching water is important as well $\endgroup$ Commented Feb 28 at 2:23
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Not really an answer, more a general addition on the cost of size: Wild elephants spend approximately 16 to 18 hours per day foraging, that leaves 4 h for sleep. Thats very little.

The energy needed results in energy waste beeing a dangerous endavour. Like avoiding hills and behaving all in all like a flatland dweller on a super-earth. https://news.mongabay.com/2006/07/elephants-avoid-hills/

A elephants foot is very soft, the whole apparatus to move is desperatly avoiding any movement that will bruise this tissue or damage the bones. Now during mounting one another, elephants, get on two legs. Making them the official kings of 2 legs, make that 3 legs for a day.

2 to 7 tons on two legs. My estimate is towards half that to permanently move on two legs at max. 2-7 - gives you 1t to 3.5 tons as upper limit. Birds are bad examples here, due to there bones beeing originally adapted to weightlesserness of flight. Bones really got a job cut out for them- first as fish in zero-gravity, then as land-dwellers, then as birds, finally again as land dwellers. Pick a lane life!

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  • $\begingroup$ considering real living bipedal herbivores on earth made it to 5 tons, your mass estimate is off. Mammals are just poorly built to be large, so many adaptations for being small they can't undo. $\endgroup$ Commented Feb 28 at 2:29
  • $\begingroup$ 5 tons is the size limit for a quadruped elephant. If you go bipedal - that halfs it unless you have a better way for legs to catch all that weight? $\endgroup$ Commented Feb 28 at 8:39
  • $\begingroup$ 5 is the limit for a quadoped elephant and also the limit for a bipedal therizinosaur. when your reasoning runs counter to the real world your reasoning is flawed. like I said mammals are just poorly built for large size. less joint cartilage, heavy weaker bone, heavy low efficency respritory system, ball and socket hip, ect. $\endgroup$ Commented Feb 28 at 14:46

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