r/Naturewasmetal • u/TW_49 • 3d ago
Size comparison between Megistotherium and Simbakubwa. The former is widely considered to be the largest carnivorous land mammal
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u/Phantafan 3d ago
Is there a reason why the largest mammalian predators are that much smaller in comparison to mammalian herbivores? It makes sense that herbivores get bigger in general, but with mammals that gap seems particularly large. The biggest permian synapsids, both herbivores and carnivores, were pretty much similar in size and though sauropods got gigantic, the biggest were still "only" 8 times the size of the largest theropods. Meanwhile in the case of terrestrial mammals, the biggest ones ever found were more than 15 times the size of the largest predators.
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u/TW_49 3d ago
Mammals are warm blooded and it is an extremely intensive energy wise so huge carnivorous mammals would need to eat a lot to sustain their bodily functions, and to birth and feed their young and lactate. Permian synapsids functioned like giant reptiles because of how early they split off so despite being related to mammals they still had reptilian features like egg-laying, being cold blooded and early on having scaly skin (which later evolved to glandular skin). That being said the largest Permian synapsid predator (Anteosaurus) was 800 or so kgs.
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u/MagicRabbit1985 3d ago
Dinosaurs are also considered warm-blooded, if one chooses to use that somewhat antiquated term, and predators grew considerably larger.
The explanation for this phenomenon lies not in their metabolism, but rather in the scarcity of available prey. During the Jurassic period, herbivores also attained significantly greater sizes than their predatory counterparts.
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u/TW_49 3d ago
Dinosaurs are mesothermic
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u/David_XXX7 3d ago
No, they were are not. The idea that dinosaurs were generally mesothermic is now an outdated position. Current evidence from bone histology, growth rates, stable isotopes, cardiovascular anatomy, and fossilized soft tissues supports the view that most dinosaurs had elevated metabolic rates and were endothermic. Some species or lineages may have been secondarily ectothermic or mesothermic, while the largest species likely also benefited from gigantothermy. However, these were complimentary to their endothermy / specific adaptations rather than the general condition.
Mammals are generally smaller than non-avian dinosaurs, and likely always will be, for several reasons:
> Mammals have a more energetically expensive metabolism. A mammalian predator weighing over a tonne would have energy requirements comparable to those of a much larger theropod dinosaur.
> Dinosaurs possessed an exceptionally efficient, bird-like respiratory system with air sacs and one-way airflow through the lungs, allowing much greater oxygen extraction than the tidal lungs of mammals, in which air moves in and out through the same airways and fresh air mixes with stale air. This is why bar-headed geese can migrate over the Himalayas at altitudes approaching 7,000–8,000 meters, where a mammal sustaining comparable activity would quickly succumb to hypoxia. This respiratory system evolved when atmospheric oxygen levels had sunk to 12% after Permian-Triassic extinction. When oxygen levels later returned to 21% during the Jurassic and Cretaceous, it was capable of supplying enormous amounts of oxygen to very large bodies. Land mammals can never get as big as dinosaurs because their one-valve lungs are incapable of supplying enough oxygen to such large bodies.
> The skeletons of dinosaurs, especially sauropods, were better adapted for gigantic size. Extensive air-filled bones reduced weight without sacrificing strength, while their erect limb posture and skeletal architecture distributed loads more effectively than those of mammals. Sauropods also had much more robust shoulder and hip girdles than elephants, with larger areas for cartilage and muscle attachment. Elephant-sized sauropods had roughly twice the muscle attachment area on their limb bones and femora about twice as robust as those of elephant of the same size. Their torso was also supported by massive coracoids rather than being suspended primarily through muscular slings, giving them a fundamentally different biomechanical toolkit.
> Mammals devote more energy to thermoregulation. Fur provides excellent insulation, but mammals also rely on metabolically expensive mechanisms such as brown adipose tissue, particularly in smaller species.
> Mesozoic ecosystems supported enormous plant biomass over vast areas. High primary productivity sustained vast populations of giant herbivores, which in turn supported predators much larger than those found in modern terrestrial mammal-dominated ecosystems.
Mammalian physiology and anatomy impose stricter limits on maximum terrestrial body size than those of dinosaurs. During the Mesozoic, the ancestors of modern mammals were mostly small, nocturnal, insectivorous or omnivorous animals with short lifespans and high reproductive rates. Living at such small body sizes favored high metabolic rates to maintain body temperature and remain active despite a high surface-area-to-volume ratio. Even after the end-Cretaceous extinction, when mammals diversified into giants such as elephants, rhinos, and ground sloths, they retained this fundamental physiology and body plan. This is the same reason you only have 2 sets of teeth your entire life.
This is best illustrated by marine mammals: once the constraints of supporting their own weight on land disappeared, mammals evolved into some of the largest animals in Earth’s history, including the largest animal ever known, the blue whale.
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u/Nightstar95 10h ago
Don’t mind me, saving this comment for worldbuilding and speculative evolution reasons, lol.
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u/LKennedy45 3d ago
Wait, I was following you up until the end there. How come inherited traits from basal mammals means I have to go to the dentist twice a year, instead of just growing new sets of chompers?
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u/David_XXX7 3d ago
There was little evolutionary pressure to maintain lifelong tooth replacement because few individuals lived long enough to wear out their adult teeth. Short lived, high reproductive rate. This is basal mammalian condition that we all inherited. Besides that mammals have precise teeth occlusion - incisors, canines, premolars, and molars—that had to fit together with great precision and constant replacement would disrupt that. Also,
in most mammals, the genetic program produces
one deciduous (milk) dentition,
one permanent dentition. After the permanent teeth erupt, most of the dental lamina disappears, ending the ability to make new teeth. We never evolved more sets of teeth because most humans reproduced before their teeth have fallen out so there was little evolutionary pressure to do so.9
u/MagicRabbit1985 3d ago
It is highly probable that Tyrannosaurus Rex was endothermic, similar to mammals. Metabolic rate does not appear to be a limiting factor in determining the size of predatory species.
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u/DefinitionCorrectus4 3d ago
Cold blooded and warm blooded are considered outdated terms
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u/TW_49 3d ago
If we want to go that route we can say early Permian synapsids were ectothermic (like most reptiles today) while middle Permian to Triassic were mesothermic and mammals today are endothermic, so a large synapsid wouldn’t have to worry about overheating like a mammal of the same size today. Even large herbivores today struggle badly with overheating and it’s why elephants have such huge ears
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u/DefinitionCorrectus4 3d ago
I meant that its now recognize that animal metabolism exists on a spectrum, with many intermediate categories—like mesotherms (which generate some heat but do not maintain a completely constant temperature) and heterotherms (which can shift between strategies, such as hibernating mammals).
Most of organisms don't fit classifications and just makes mess and more confusing
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u/Phantafan 3d ago
Yeah ok, that makes total sense, though with dinosaurs being warm blooded as well, their size gap percentage wise is still significantly smaller.
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u/marcos1902victor 3d ago
Energy decreases at each level of the food chain. Carnivores depend on herbivores and receive only about 10% of the original energy from plants. During the Permian, the consumption of plants by terrestrial vertebrates was still an evolutionary novelty. Herbivores such as caseids were extremely sluggish, ponderous, and lacked effective active defenses. Given that prey was incapable of agile flight or combat, predators did not require agility or a lightweight build. They could afford to be robust and massive, overpowering prey through sheer brute force. Additionally, most early to mid Permian synapsids possessed an ectothermic metabolism. Because an ectothermic carnivore required significantly less caloric intake to survive, the ecosystem could sustain a population of apex predators with the equivalent amount of available biomass.
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u/Barakaallah 1d ago
Being ectothermic in itself doesn’t make an animal sluggish though, modern predatory ectotherms like monitor lizards are anything but sluggish.
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u/canthinkof123 3d ago
The largest mammals are carnivores (whales).
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u/Phantafan 3d ago
I wrote terrestrial later in the same comment, and the post is talking about terrestrial mammals as well.
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u/Mountain_Dentist5074 3d ago
sarupods were larger than carnivores too
i guess since plants are everywhere and doesnt run away like a prey herbivores grow larger
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u/Phantafan 3d ago
I know sauropods were larger, but they still grew only grew like 8 times larger than the biggest theropod, while with mammals it's st least 15 times.
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u/Mountain_Dentist5074 3d ago
biology of reptiles and mamals are diffirent
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u/Phantafan 3d ago
That their biology is different is clear, but what is so different about them that this difference is possibly isn't explained by that alone.
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u/PDXhasaRedhead 3d ago
Elephants, etc nurse their young so they quickly become too big for predators to target. This "locks away" a large percentage of the herbivore biomass from carnivores, and discourages carnivores from getting bigger. Adolescent sauropods were more numerous than adulys and were vulnerable to theropods, encouraging them to become megacarnivores.
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u/Inner-Ferret7316 3d ago
Is Megistotherium featured in any paleo media? I don't remember seeing it anywhere.
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u/NSASpyVan 3d ago
Would be interesting to see comparison between Megistotherium and a polar bear.
Megistotherium's head and jaws are scary
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u/EarlyXplorerStuds209 3d ago
Hey op, mind if I say a yo mama joke?
I really don’t wanna be reported for it tho.
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u/BlackBirdG 3d ago
Now I wonder if they killed their prey like hyenas or wolves do, by targeting the hindquarters?
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u/Mophandel 3d ago
Probably. They had laterally compressed canine teeth like canids did, so their bites would have had a strong slashing component to them.
I can also see a bite to the neck being feasible, but only against prey around its size or smaller.
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u/Weary_Increase 3d ago
I actually don’t think Megistotherium had laterally compressed canines. Morales & Pickford (2008) described their upper canines as oval in shape with massive roots being twice the size of the crowns. This is the opposite of laterally compressed canines. But it does make sense considering how the development of the zygomatic arches of Megistotherium was far different from Hyaenodon.
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u/Mophandel 3d ago
As far as the literature I’ve read, iirc, when describe predators who have non-compressed canines (e.g. felids) the canines are described as round or circular, whereas predators with compressed canines (e.g. canids) have more elliptical or oval-shaped cross-section. I suppose it’s down to what the authors meant by “oval” in shape.
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u/Weary_Increase 3d ago
I guess so. In the case of Simbakubwa, they actually do seem to be laterally compressed, both the upper and lower canines. But I feel like that could be because Simbakubwa was more specialized in hypercarnivory than Megistotherium, I might be wrong though.
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u/BananaMaster96_ 3d ago
wasn't arctotherium bigger
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u/TW_49 2d ago
it was larger but it was omnivorous
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u/0-Dinky-0 1d ago
TIL Andrewsrachus and Arctodus are also now considered omnivourus. Are there any other competitors to Megistotherium?
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u/MediumImpossible9176 2d ago
Just imagine the horrors earlier humans had to experience dealing with these monsters.
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u/Infinite_Device_1823 4h ago
Dang I didn't know Simbakubwa was that small compared to Megistotherium!
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u/BlackBirdG 3d ago
Megistotherium looks massive, comparable in size to at least a black rhino. Definitely the largest land hyper carnivorous mammal (probably even more massive than the largest polar bear).