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I'm working on a species of fantasy creature based off the mythological Behemoth. It's only constraint is the laws of physics itself. Environment and evolutionary lineage won't be relevant due to the species' place in the story. It's a vertebrate land animal, and I want it to be larger than any land creature that's ever existed, but there's a problem:

The largest terrestrial animal known to history is the Argentinasaurus, which by some estimates weighed up to 100 tons. (Meanwhile, Elephants weight around just 6). Some have said this is the absolute size limit for terrestrial land animals.

If so, what changes would need to be made to make an organism's design exceed that mass? Like, 200-500 tons? So far the major issues with gigantism in vertebrate species I've found include-

  1. Issues with bone structure, with the weight soon becoming too much for the bones to handle.
  2. Too much stress on the heart and lungs
  3. The strength-to-weight ratio of Muscles declining the bigger an organism is, eventually making it unable to support just it's own weight.
  4. Issues with regulating body temperature.

So far, the solutions I've come up with simply involve adding more sets of hearts and lungs evenly distributed around the body and more solid bones than that of a dinosaur (which are hollow like birds) but less so than mammals.

The species I'm designing has 6 limbs, at least one set of heart and lungs for each major section of it's body, with extra limbs for support and even skeletal structures designed to reinforce certain muscles to have greater strength, like the jaws for example. The creature lives on a planet with a gravitational pull similar to earth's.

Would these be viable means of making a super-sized physics-based living organism? What else am I missing?

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    $\begingroup$ How strictly do you need to stick to the terrestrial vertebrate body plan? $\endgroup$ – Monty Wild Jun 3 at 2:02
  • $\begingroup$ @MontyWild Something that can still be recognized as vertebrate, but only then. A creature with extra limbs, ribcages, and other skeletal structures not found ANYWHERE in the animal kingdom. Anything that can accommodate the extra biological equipment needed to support the sheer size of this hypothetical creature. $\endgroup$ – TommyProductionsInc Jun 3 at 17:45
  • $\begingroup$ Terrestrial vertebrates are all tetrapods. Are you sticking with 4 limbs? $\endgroup$ – Spencer Jun 4 at 16:06
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    $\begingroup$ the limb bones of sauropods were not hollow. Vertebra and ribs were hollow, which reduces weight were you don't need it and actually ends up making the bones stronger. saying something had hallow bones does not mean ALL its bones are hollow. even in modern birds only some of the bones are hollow. $\endgroup$ – John Jun 4 at 16:59
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    $\begingroup$ @DavidHambling I call it the milli-phant $\endgroup$ – DKNguyen Jun 4 at 18:10
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The main problem with scaling up a quadrupedal vertebrate is that beyond a certain mass, the legs simply can't be made thick enough to both support its weight and also allow it to move. Before reaching that point, it would pass through a stage where it was still able to move, but only so slowly that it would be highly vulnerable to predators.

This problem can be resolved by segmentation. By adding extra pairs of legs down the length of the body, each leg would need to support a smaller fraction of the total body mass. For quadrupedal vertebrates, the upper mass limit appears to be around 100 metric tons, which equates to 25t per leg. However, such a creature would have moved quite slowly. However, by reducing the mass per leg, and increasing musculature, a segmented Behemoth would be able to move faster. It may be possible to have a mediportal gait at up to 20 tons per segment, 10 tons per leg, though 10 tons per segment 5 tons per leg would be better.

So, we could have a Behemoth that at adult size might have 30 segments, 60 legs, and weigh over 300 metric tons.

The next problem that must be overcome is that of circulation and respiration. A single heart would suffer great stresses circulating blood around even a mostly horizontal creature, however by having a heart in each body segment, this can be overcome. Likewise, by having a set of lungs in each body segment, the creature's respiratory needs can be scaled up without having to resort to an impractically wide trachea leading to a single set of lungs.

So far, we have a creature much as described by the OP. However, some elements are still missing. The most important is that of diet. However, it does not make sense to have such a massive body and to have a diet consisting primarily of high-quality food, such as other animals or fruit. A creature such as this seems built to eat grass or even whole trees.

With a body well in excess of 100 tons, a Behemoth need not have a particularly high basal metabolism, though an elongated body plan would give a higher surface area than its mass would suggest, requiring a somewhat higher energy input to maintain its body temperature. I'm assuming that it is homeothermic, since it isn't likely to evolve to that size if it wasn't.

So, given that the Behemoth is a herbivore, it then has the problem of gathering enough food. Given that it has a narrow body in comparison to its length, a single head would limit the rate at which it could collect food. Whether grazing or browsing, there's only so much food that can be gathered in a single bite... so either that is a limiting factor, or another solution is found. As the Behemoth evolved, its mouth might widen to be able to take in more food with a single bite, but while this works with grass to a degree, it wouldn't work very well when browsing upon woody plants. The solution is to make the lips more mobile, so that different sections of the mouth can approach the Behemoth's food in different ways. Evolving this approach to its logical extreme would mean that the Behemoth would have perhaps six to eight mouths on stalks, that could independently browse or graze upon whatever plant material was available. The mouths would most likely have a broad, beak-like appearance, with horny upper and lower lips so as to be able to snip or strip off grasses or leaves. Given that it might take a bite at a tree branch, then pull back without completely biting through a branch in order to strip off the leaves, the beak might take on a serrated appearance.

Internally, the individual mouths are likely to evolve ossifications and joints to support the structures and allow a greater bite force, and as they extend beyond the head like enlarged elephantine trunks, further ossifications within their musculature would allow greater strength and extensibility.

The food ingested by these mouths would not have been masticated. The original jaws and dentition might be adequate for some pre-processing of the food, but to process bulk vegetation, a gizzard is the most effective solution. Since a gizzard relies upon rocks to provide an abrasive surface, when the rocks wear out, they can readily be replaced with others found in the environment. With a gizzard, it is reasonable to expect that the original jaws and dentition would atrophy, and the former buccal cavity would effectively be a manifold that collects food and passes it further down the digestive system to the gizzard.

A herbivore as large as a Behemoth would not have the luxury of selectiveness as to the food it eats. This, then precludes it being a foregut fermenter like a cow: ruminants are sensitive to overly-rich food, which can cause life-threatening complications. Hindgut fermentation allows the digestive system to remove excess nutrients before delivering the indigestible components to the fermentation apparatus, guaranteeing that there will be no risk of overly-rich food causing runaway bacterial growth.

The elongated body would allow plenty of room for the digestive system without having to be especially wide to accommodate the volume of the gut.

As a hindgut fermenter, without the need to be terribly selective with its food, there is a high probability that a Behemoth would be an opportunistic predator and scavenger. If an animal of the right size crossed its path so that it could simply reach out and ingest it, or if presented with the carcasse of an animal, it might take the opportunity to do a bit of predation or scavenging.

The Behemoth would most likely follow the basic vertebrate paradigm in that its brain and senses of sight, hearing, taste and smell would be centred in the head. Being a herbivore, it would have eyes on the sides of its head so as to maximise its field of view. It would have sensitive, mobile ears. It might also have an excellent sense of smell, given that it has a lot of volume in which to put the necessary sensory apparatus.

In order to defend itself from predators, a Behemoth could charge and trample, however the great length of its flanks could present a significant vulnerable area, so it seems reasonable that a Behemoth would evolve the ability to kick sideways, potentially with some variation on the theme of claws or horns on each of its feet so as to increase the potential lethality of its kick. Naturally, it would be able to bite with its multiple mouths, the jagged beaks being able to inflict serious injuries on a large opponent, or simply ingest a smaller opponent.

It might also have the ability to body-slam, though this would be more a side-effect of being able to rear up to exert more force upon a tree in order to push it over.

On the matter of reproduction, a Behemoth would likely produce relatively small offspring, if for no other reason than its narrow body would not readily allow larger offspring. However, the length of the body would allow for multiple offspring relatively easily. So, we can expect litters of relatively immature offspring in fairly large numbers.

It is unlikely that a Behemoth would be terribly intelligent. Much of its brain would likely be dedicated to controlling its complicated body, however, such an animal would not have the need for particularly great intelligence. So long as it can locate food, mates and its progeny, and lash out at anything threatening, it needs little else.

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  • $\begingroup$ A single head would be more than enough if it has a gizzard, if the mouth is only used for eating and not processing. 100tons would be in the 3-5 pairs of limb range considering there were real sauropods that got close. with a longer body you can have a much larger gut which solves a lot of your food issues, by having much better extraction. $\endgroup$ – John Jun 4 at 16:47
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There were larger sauropods, but they are known only from a few bones. The current record-holders include Argentinosaurus, which may have weighed 100 tonnes. The problem with size is the ratio between the surface area and volume of a creature increases exponentialy as it get's bigger, thus a large arachnid body would be largely unable to support it's own weight.If an animal were isometrically scaled up by a considerable amount, its relative muscular strength would be severely reduced since its mass would increase by the cube of the scaling factor. As a result of this, cardiovascular and respiratory functions would be severely burdened. For a creature that massive to exist certain environmental circumstances would have to be in place.

  1. Lower gravity: Therefore the weight penalty wouldn't matter as much
  2. Stronger bones/reinforcement to withstand the weight of their own mass. A structure of materials far stronger than mineralized bones, organo-metallic compounds, carbon-carbon style bones?
  3. MORE oxygen in the atmosphere to fuel the metabolic requirements for large animal locomotion (This is apparent in the Carboniferious period on Earth, Arthropods were 10x the size they were average today, Scorpions the size of a dog, Dragonflies the size of a seagull

references:

  1. http://en.wikipedia.org/wiki/Square%E2%80%93cube_law
  2. https://dinosaurpivoting.boards.net/thread/895/important-scaling
  3. https://en.wikipedia.org/wiki/Largest_organisms#:~:text=The%20current%20record%2Dholders%20include,are%20known%20only%20from%20fragments
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    $\begingroup$ Higher metabolism make the problem of heat dissipation worse $\endgroup$ – L.Dutch - Reinstate Monica Jun 3 at 4:33
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    $\begingroup$ "The ratio between the surface area and volume of a creature increases exponentialy as it gets bigger": No it doesn't. The ratio between the surface area and the volume decreases linearly as the size increases. Scale the creature by a factor of 2, the ratio between surface area and volume decreases by a factor of 2. Scale the creature by a factor of 3, the ratio between surface area and volume decreases by a factor of 3. And no, you don't need more oxygen in the air, except in the case of insects with their very specific respiratory system. $\endgroup$ – AlexP Jun 3 at 8:20
  • $\begingroup$ Lower gravity would mean that the mass doesn't matter as much, but the weight still matters. $\endgroup$ – jdunlop Jun 4 at 18:12
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    $\begingroup$ Please always attribute your sources. Copypasting from other sources without attribution is plagiarism, and it is taken seriously here. $\endgroup$ – L.Dutch - Reinstate Monica Jun 14 at 3:28
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Social insect style: at maximum size dispense with movement.

1: Nonambulatory. At full weight the behemoth would lie on its belly, distributing weight across a broader surface.

2: Main bulk under lungs. Expanding the lungs means lifting the back. The belly and most weight is underneath and does not need to move to breathe.

3: Nonmetabolic tissue. Most of the weight is fat, which has minimal metabolic and circulatory needs.

A question becomes feeding. The B could have a long neck and snag things passing by, but things would learn to steer clear. It would quickly eat all plants within reach. You could have a head like a flamingo and filter feed in the ocean or some other resource that renewed itself from a larger territory.

Or you could have smaller and more mobile animals bring it food, which I think is the best way.

This now converges on a queen termite - nonmotile and fed by its smaller, nonreproductive offspring. That method could work for vertebrates. The mature full sized behemoth eats and has young and that is it. Its offspring defend and feed it. Periodically the queen gives birth to more mobile dispersing forms who can leave the huge territory it needs and start their own colonies.

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Short Answer:

So what you want is a "giant Behemoth".

https://en.wikipedia.org/wiki/The_Giant_Behemoth[1]

In my long answer I discuss modifying the planet of your story, if it is not Earth, to make large creatures more probable, discuss the size records of known creatures on Earth, and discuss how to modify your creatures to make them more mammoth.

Long Answer:

Part One of Ten: Being A Pedant.

I'm working on a species of fantasy creature based OFF the mythological Behemoth

I believe that correct grammar is to say "based OFF OF the mythological Behemoth" or "based ON the mythological Behemoth". And the second sounds better to me.

Part two of Ten: Making The Planet Suitable.

Planetary characteristics to consider include both the surface gravity and the escape velocity. The surface gravity determines how much creatures weigh and how strong their bones and muscles have to be. The escape velocity determines the planet's ability to retain an atmosphere. And of course the type of atmosphere a planet has depends on both what atmosphere it produces or acquires and its ability to retain an atmosphere.

https://en.wikipedia.org/wiki/Surface_gravity#:~:text=The%20surface%20gravity%2C%20g%2C%20of,including%20the%20effects%20of%20rotation.&text=Surface%20gravity%20is%20measured%20in,are%20meters%20per%20second%20squared.[2]

https://en.wikipedia.org/wiki/Escape_velocity#:~:text=In%20common%20usage%2C%20the%20initial,to%201.7%20km%2Fs).[3]

And I have noticed that surface gravity and escape velocity do not change equally. I have noticed that for planets and other bodies in the solar system with less mass than Earth their escape velocities and surface gravity do not have the same ratio compared to Earth's escape velocity and surface gravity. The escape velocity of a smaller world will be slightly larger, relative to Earth's, than the surface gravity.

This suggests to me that a planet with less mass than Earth should be a good setting for your giant behemoths.

How much can you change the mass of your fictional planet while still having a breathable atmosphere?

You should read Habitable planets for Man, Stephen H. Dole, 1964, 2007, which has a section on the size range of planets habitable for humans, and thus for other large land based vertebrates.

Part Three of Ten: A highly Oblate Planet?

Another factor to consider is making your planet rotate very fast and have an oblate shape. Surface gravity will be higher at the poles and lower at lower latitudes, being the lowest at the equator, where your giant behemoths might live.

The classic example of a highly oblate planet in science fiction is Mesklin in Mission of gravity by Hal Clement (1953). Mesklin is very large and rotates very fast, so the surface gravity is three g at the equator and hundreds of g at the poles.

https://en.wikipedia.org/wiki/Mesklin[4]

I am not sure about whether the atmosphere would have uniform density or be denser at the poles. I also wonder what the escape velocity would be at the equator of a rapidly spinning world and whether the atmosphere would escape from the equator.

Some people speculate that a large moon, like the Moon, is necessary for a planet to have large tides and since Earth life might have began in tidal zones, necessary for a planet to have life. Since the Moon has been receding and slowing down Earth's spin for billions of years before complex life appeared, an initially highly oblate planet might slow down its spin and loose most of its oblateness before it developed an oxygen rich atmosphere and large lifeforms.

But maybe advanced aliens came to a rapidly spinning new planet and terraformed it with an oxygen rich atmosphere and seeded it with advanced lifeforms from their planet, and over millions of years the giant behemoths evolved on that planet while it was still spinning rapidly.

Or maybe a large moon is not necessary for life on a planet.

Part Four: A Jinxed Planet?

Another interesting science fiction world is Jinx in Larry Niven's Known Space series.

Jinx, orbiting Sirius A, is a massive moon of a gas giant (called Primary), stretched by tidal forces into an egg shape and tidally locked. In the habitable areas it has high surface gravity near the limits of human extended tolerance. The points nearest to and farthest from Primary (called the "East" and "West" ends) lie elevated out of the atmosphere in vacuum. The atmosphere of the belt-region halfway between them is too dense and too hot to breathe, and is inhabited only by the Bandersnatchi. The zones between the vacuum areas and the high-density belt area have atmosphere breathable by humans. Jinx's "East" and "West" ends become a major in vacuo manufacturing area. Jinxian humans are short and squat, the strongest bipeds in Known Space. But they tend to die early, from heart and circulatory problems. There is a tourist industry which provides substantial useful interplanetary trade credits for the Bandersnatchi, who allow themselves to be hunted by humans under strict protocols.

https://en.wikipedia.org/wiki/Known_Space#Locations[5]

So a Jinx-like world might have the right surface gravity and atmosphere in some regions for giant behemoths to flourish.

Part Five of Ten: The Largest Known Dinosaurs.

The largest terrestrial animal known to history is the Argentinasaurus, which by some estimates weighed up to 100 tons. (Meanwhile, Elephants weight around just 6). Some have said this is the absolute size limit for terrestrial land animals.

If so, what changes would need to be made to make an organism's design exceed that mass? Like, 200-500 tons?

The weight of the largest dinosaurs is not estimated with great certainty.

For example, Wikipedia has list of the heaviest sauropodmorphs:

Argentinosaurus huinculensis: 50–100 t (55–110 short tons)[16][18][35][31]

Patagotitan mayorum: 55–77 t (61–85 short tons)[36]

Mamenchisaurus sinocanadorum: 50–80 t (55–88 short tons)[37]

Notocolossus gonzalezparejasi: 44.9–75.9 t (49.5–83.7 short tons)[6]

Alamosaurus sanjuanensis: 39.5–73 t (43.5–80.5 short tons)[16][18][38]

Apatosaurus ajax: 32.7–72.6 t (36.0–80.0 short tons)[39]

Sauroposeidon proteles: 40–60 t (44–66 short tons)[40][18][41]

Dreadnoughtus schrani: 22.1–59.3 t (24.4–65.4 short tons)[16][34]

Paralititan stromeri: 20–59 t (22–65 short tons)[18][42]

https://en.wikipedia.org/wiki/Dinosaur_size#Heaviest_sauropodomorphs[7]

Many people would disagree with the relative order of the various dinosaurs and with the weight estimates for various species. And of course new large sauropodmorphs are discovered all the time and such lists become more and more obsolete as time passes.

Part Six of Ten: The Largest Not Well Known Dinosaurs?

And of course that list omits the two most controversial and possibly largest sauropod dinosaurs ever discovered, known from now lost fossils.

There was Amphicoelias fragillimus, now reclassified as Maraapunisaurus fragillimus, which might possibly according to some interpretations been up to 60 meters (200 feet) long and weighed up to 150 tonnes (150 long tons, 170 short tons). Some estimates make it much smaller.

The third named Amphicoelias species, A. fragillimus, was known only from a single, incomplete 1.5 m (4.9 ft) tall neural arch, either last or second to last in the series of back vertebrae. Based only on an illustration published in 1878, this vertebra would have measured 2.7 meters (8.9 ft) tall in life.[3] However, it has been argued that the scale bar in the published description contained a typographical error, and the fossil vertebra was in fact only 1.38 meters (4.5 ft) tall.[8] In addition to this vertebra, Cope's field notes contain an entry for an "[i]mmense distal end of femur”, located only a few tens of meters away from the giant vertebra. It is likely that this undescribed leg bone belonged to the same individual animal as the neural spine, but it was never collected or described.[3] In 2018, A. fragillimus was given its own genus, Maraapunisaurus, and reclassified as a primitive rebbachisaurid.[9]

https://en.wikipedia.org/wiki/Amphicoelias#Previously_assigned_species[10]

While M. fragillimus as a sauropod would be relatively elongated, its enormous size still made it very massive. Weight is much more difficult to determine than length in sauropods, as the more complex equations needed are prone to greater margins of error based on smaller variations in the overall proportions of the animal. Carpenter in 2006 used Paul's 1994 estimate of the mass of Diplodocus carnegii (11.5 metric tons (11.3 long tons; 12.7 short tons)) to speculate that M. fragillimus could have weighed up to 122.4 metric tons (120.5 long tons; 134.9 short tons).[4] The heaviest blue whale on record weighed 173 metric tons (170 long tons; 191 short tons),[11] and the heaviest dinosaur known from reasonably good remains, Argentinosaurus, weighed 80 to 100 metric tons (79 to 98 long tons; 88 to 110 short tons), although if the size estimates can be validated, it could still be lighter than Bruhathkayosaurus, which has been estimated to have weighed 126 metric tons (124 long tons; 139 short tons), but is also known from highly fragmentary remains.[8] In 2019 Gregory S. Paul estimated Maraapunisaurus at 35-40 meters (115-131 feet) in length and 80-120 tonnes (88-132 short tons) in weight with a femoral length of 3-3.5 meters (10-11.5 ft) or more, larger than Carpenter's estimation .[9]

https://en.wikipedia.org/wiki/Maraapunisaurus#Size[12]

Possibly the most controversial sauropod fossil of all was Bruhathkayosaurus.

Bruhathkayosaurus (/bruːˌhæθkeɪoʊˈsɔːrəs/; meaning "huge-bodied lizard") is a genus of dinosaur found in the Kallemedu Formation of India. The fragmentary remains were originally described as a theropod but later publications listed it as a sauropod. Estimates by researchers exceed those of the titanosaur Argentinosaurus,2 as longer than 35 metres (115 ft) and weighing over 80 tons. All the estimates are based on the dimensions of the fossils described in Yadagiri and Ayyasami's 1987 paper, which announced the find.[3] In 2017 it was reported that the original fossils had disintegrated and no longer exist.[4]

No total body size estimates for Bruhathkayosaurus have been published, but paleontologists and researchers have posted tentative estimates on the Internet. In a post from June 2001, Mickey Mortimer estimated that Bruhathkayosaurus could have reached 40–44 m (131–144 ft) in length and might have weighed 175–220 tons, but in later posts retracted these estimates, reducing the estimated length of Bruhathkayosaurus to 28–34 m (92–112 ft), and declined to provide a new weight estimate, describing the older weight estimates as inaccurate.[13][13][14] In a May 2008 article for the weblog Sauropod Vertebra Picture of the Week, paleontologist Matt Wedel used a comparison with Argentinosaurus and calculated the weight of Bruhathkayosaurus at up to 126 metric tons (139 short tons).[15] In 2019 Paul suggested that the supposed tibia is probably a degraded femur, in which case its length was slightly greater than that of Dreadnoughtus (1.91 meters) and Futalognkosaurus (1.98 meters). So he estimated its mass between 30-55 tonnes (33-61 short tons) much lower than any previous estimation.[6]

https://en.wikipedia.org/wiki/Bruhathkayosaurus#Size_estimates[16]

So some tentative and probably inaccurate mass estimates of Bruhathkayosaurus put the upper limit of the mass range a little bit above the lower limit of your goal of 200 to 500 tons.

Part Seven of Ten: The Largest Living Land Mammals.

As for elephants, baby elephants are in the human mass range, while adult females average less massive than adult males.

The smallest living elephant species is the African forest elephant Loxodonta cyclotis.

Bulls reach a shoulder height of 2.4–3.0 m (7.9–9.8 ft). Females are smaller at about 1.8–2.4 m (5.9–7.9 ft) tall at the shoulder. They reach a weight of 2–4 tonnes (2.2–4.4 short tons).[10] Foot print size ranges from 12.5 to 35.3 cm (4.9 to 13.9 in).[15]

https://en.wikipedia.org/wiki/African_forest_elephant#Size[11]

The middle sized elephant species is the Asian elephant Elephas maximus.

On average, males are about 2.75 m (9.0 ft) tall at the shoulder and 4 t (4.4 short tons) in weight, while females are smaller at about 2.4 m (7.9 ft) at the shoulder and 2.7 t (3.0 short tons) in weight.[16][17][18] Length of body and head including trunk is 5.5–6.5 m (18–21 ft) with the tail being 1.2–1.5 m (3.9–4.9 ft) long.[5] The largest bull elephant ever recorded was shot by the Maharajah of Susang in the Garo Hills of Assam, India in 1924, it weighed an estimated 7 t (7.7 short tons), stood 3.43 m (11.3 ft) tall at the shoulder and was 8.06 m (26.4 ft) long from head to tail.[16][19][20] There are reports of larger individuals as tall as 3.7 m (12 ft).[15]

https://en.wikipedia.org/wiki/Asian_elephant#Size[8]

The largest living elephant species is the African bush or savanna elephant, Loxodonta africana.

The African bush elephant is the largest and heaviest land animal on Earth, being up to 3.96 m (13.0 ft) tall at the shoulder and an estimated weight of up to 10.4 t (11.5 short tons).[16][17] On average, males are about 3.20 m (10.5 ft) tall at the shoulder and weigh 6.00 t (6.61 short tons), while females are much smaller at about 2.60 m (8.53 ft) tall at the shoulder and 3.00 t (3.31 short tons) in weight.[16][18][19][20] Elephants attain their maximum stature when they complete the fusion of long-bone epiphyses, occurring in males around the age of 40 and females around the age of 25.[6]

https://en.wikipedia.org/wiki/African_bush_elephant#Size[9]

Since the average mass of male African bush elephants is about the six tons you mentioned for elephants, it is not surprising that many males are more massive, some much more.

Part Eight of Ten: The Largest Extinct Land Mammals:

The largest land mammal extant today is the African bush elephant. The largest extinct land mammal known was long considered to be Paraceratherium orgosensis, a rhinoceros relative thought to have stood up to 4.8 m (15.7 ft) tall, measured over 7.4 m (24.3 ft) long and may have weighed about 17 tonnes.[66][67] In 2015, a study suggested that one example of the proboscidean Palaeoloxodon namadicus may have been the largest land mammal ever, based on extensive research of fragmentary leg bone fossils from one individual, with a maximum estimated size of 22 tonnes.[68][66]

https://en.wikipedia.org/wiki/Largest_organisms#Mammals_(Mammalia)[13]

https://en.wikipedia.org/wiki/Paraceratherium[15]

https://en.wikipedia.org/wiki/Palaeoloxodon_namadicus[14]

So known extinct land mammals may have weighed two, three, or four times as much as your 6 ton elephants.

Part Nine of Ten: Vertebrates With More Limbs.

One way to make giant behemoths larger would be to give them more limbs.

the Op asks for a vertebrate body plan, not a tetrapod body plan. So maybe the giant behemoths could be vertebrate hexapods, octopods, decapods, etc. Their torsos might have the same spacing between pairs of legs as giant sauropod dinosaurs had, but with more pairs of legs, thus doubling or more the possible mass of the torso.

If more than four limbs are permissible, the giant behemoths could even have tens or hundreds of pairs of legs and be built like vertebrate caterpillars, centipedes, or millipedes, with a section of torso between each pair of legs with the mass of an elephant or brontosaurus torso.

And possibly such a multi legged creature could be like a giant hippo or crocodile, mostly living in lakes, rivers, or oceans, but sometimes walking on land, and mostly feeding on plants but sometimes getting vital nutrients by eating smaller creatures.

Part Ten of Ten: Smaller Creatures Linked Together To Make A Larger One:

If that is not permissible, possible the species could be contain individuals the sizes of elephants or sauropod dinosaurs, who occasionally join with other individuals to form a hive creature, a really giant behemoth.

Such a larger creature formed by the linking of smaller creatures was in "The Planet of Doubt" by Stanley G. Weinbaum, Astounding Stories October, 1935. The linked creatures were based on actual behavior of a species on Earth.

...When they receive a signal from the ship, they hurry back to find it under attack by an immensely long black creature made up of dozens of connected segments. They manage to fight it off and return to the ship. Burlingame decides that the creature is similar to the larval Thaumetopoeidae, which forms processions when it travels from its nest. She hypothesizes that the individual segments link nervous systems so that they all act in unison. As for the shapes in the fog, Burlingame thinks they are analogous to honeyguides, and that they lead the segment-creature to its prey.

https://en.wikipedia.org/wiki/The_Planet_of_Doubt[6]

http://www.isfdb.org/cgi-bin/title.cgi?47540[18]

http://gutenberg.net.au/ebooks14/1401921h.html[17]

I think that the linking of nervous systems is not found in any Earth animals, but would tend to make the linked super behemoth more or an individual instead of a herd.

And possibly when the creatures link up they might pass partially digested food from the anus of one creatures to the mouth of the one behind it, which would extract some nutrients and pass it on to the creature behind it. That sounds disgusting to me, but it would make them a bit more like a creature instead of a herd.

And it seems to me that as many behemoths as could live in the area as a herd of separate behemoths could link together from time to time to form a more or less hive giant behemoth.

[3]: https://en.wikipedia.org/wiki/Escape_velocity#:~:text=In%20common%20usage%2C%20the%20initial,to%201.7%20km%2Fs). [4]: https://en.wikipedia.org/wiki/Mesklin [5]: https://en.wikipedia.org/wiki/Known_Space#Locations [6]: https://en.wikipedia.org/wiki/The_Planet_of_Doubt [7]: https://en.wikipedia.org/wiki/Dinosaur_size#Heaviest_sauropodomorphs [8]: https://en.wikipedia.org/wiki/Asian_elephant#Size [9]: https://en.wikipedia.org/wiki/African_bush_elephant#Size [10]: https://en.wikipedia.org/wiki/Amphicoelias#Previously_assigned_species [11]: https://en.wikipedia.org/wiki/African_forest_elephant#Size [12]: https://en.wikipedia.org/wiki/Maraapunisaurus#Size [13]: https://en.wikipedia.org/wiki/Largest_organisms#Mammals_(Mammalia) [14]: https://en.wikipedia.org/wiki/Palaeoloxodon_namadicus [15]: https://en.wikipedia.org/wiki/Paraceratherium [16]: https://en.wikipedia.org/wiki/Bruhathkayosaurus#Size_estimates [17]: http://gutenberg.net.au/ebooks14/1401921h.html [18]: http://www.isfdb.org/cgi-bin/title.cgi?47540

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    $\begingroup$ Hint: if you want to be taken seriously as a pedant, do not spell it "pendant" :-) $\endgroup$ – KerrAvon2055 Jun 4 at 2:30
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the dinosaurs and birds (which are also dinosaurs) are quite adept for being scaled up so it's likely your Behemoth has a body plan similar to one. but even still a sauropod over 100 tons to exist in modern day earth seems unlikely so for a creature that massive to exist certain environmental circumstances would have to be in place.

Lower gravity

would allow creatures to grow to larger sizes which is why the ocean has some of the largest animals alive

More food

even if a creature could grow to be as large as an island if it is unable to find enough food to sustain in's self it won't which is one of the main reasons why land carnivores often don't grow to be as large as herbivores

More oxygen

likely a side effect of more vegetation increased amounts of oxygen would fuel the metabolic requirements for large animal locomotion

Redundant organs

multiple hearts, pairs of lungs, possibly even proto-brains would all be beneficial to a creature larger than a blue whale helping to carry oxygen though out the body and making it's reaction time less abysmal

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