Why do dinosaurs have big heads and tiny arms?

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Submitted ⁨⁨1⁩ ⁨year⁩ ago⁩ by ⁨kucuva@lemmy.world⁩ to ⁨[deleted]⁩

What evolutional benefit is that?

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girl@lemm.ee ⁨1⁩ ⁨year⁩ ago
I’m not sure of the answer, but generally not everything has to have an evolutionary benefit. As long as it isn’t detrimental to a species reproducing, it will continue to exist in the population.

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- driving_crooner@lemmy.eco.br ⁨1⁩ ⁨year⁩ ago
  People think of evolution as it’s needed straight A+ to pass, when C is enough.
  
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  - NOT_RICK@lemmy.world ⁨1⁩ ⁨year⁩ ago
    So instead of “C’s get Degrees” it’s “C’s make Babies”
    
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  - FuglyDuck@lemmy.world ⁨1⁩ ⁨year⁩ ago
    “D=Diploma, baby”-T-rexes, probably.
    
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  - dreadgoat@kbin.social ⁨1⁩ ⁨year⁩ ago
    Also what's the definition of "passing?" The dinos we are talking about are extinct, they didn't "pass" for long. A+ creatures things like alligators, ants, and crabs. On average a given species survives around a million years before going extinct. How long do you have to exist before you're considered a successful species?
    
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Lando_@lemmy.world ⁨1⁩ ⁨year⁩ ago
Someone will likely chime in with a more complete answer but the short answer is large therapods had big heads because they needed big strong heads for killing big strong prey.

If a stronger bite results in more successful kills, that creates a selective pressure towards the individuals with stronger bites. Weak bite dinosaurs die. Strong bite dinosaurs live.

To get a stronger bite, you need a lot of features. (Ex. more muscles, different structural elements) and this generally leads to heads getting bigger because big heads have more places for muscles and will then bite harder. Plus, bigger heads can bite bigger things (like, bigger necks).

The opposite is true for the tiny arms. The arms are not tiny because it’s beneficial for them to be that small. They’re tiny because there’s no reason for them to be big.

If you have two individuals. They both have big powerful legs, and big powerful heads but one also has thick ole arms. If those arms don’t provide any advantage to the individual, then they just cost energy, and put that individual at a disadvantage because they spent energy on arms while the other guy did just as well without them.

There is a much better, much more science-y answer to this too but I hope this helps in a more basic sense.

And I bet there are some cool YouTube videos and such on this exact thing if you want to do further research.

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- Aganim@lemmy.world ⁨1⁩ ⁨year⁩ ago
  
  The opposite is true for the tiny arms. The arms are not tiny because it’s beneficial for them to be that small. They’re tiny because there’s no reason for them to be big.
  
  It is theorised that it might actually have been an advantage to have smaller arms, so there may have been some selective pressure:
  
  “What if several adult tyrannosaurs converged on a carcass? You have a bunch of massive skulls, with incredibly powerful jaws and teeth, ripping and chomping down flesh and bone right next to you. What if your friend there thinks you’re getting a little too close? They might warn you away by severing your arm”
  
  phys.org/…/2022-04-rex-short-arms-lowered-frenzie…
  
  In case of larger species with a tremendous bite force getting bitten in the arm would result in very bad, and probably very short, times.
  
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- unnecessarygoat@lemmy.world ⁨1⁩ ⁨year⁩ ago
  interestingly, the opposite happened in south america, where megaraptors evolved long arms with more flexible hands for hunting prey
  
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MrPoopyButthole@lemm.ee ⁨1⁩ ⁨year⁩ ago
Keeps them from masturbating, allows them to transmute their sexual energy into raw power when not actively engaging in sex.

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- LemmyFeed@lemmy.world ⁨1⁩ ⁨year⁩ ago
  The dinos invented nofap
  
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  - Death_Equity@lemmy.world ⁨1⁩ ⁨year⁩ ago
    Man, you are gonna look so foolish when that fossil is found.
    
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loaExMachina@sh.itjust.works ⁨1⁩ ⁨year⁩ ago
There was an evolutionary trend among carnivorous theropods that the larger they grew the less they used their arms to grab the prey: If you can either inflict a deadly bite or grab the whole neck in your jaw, grabbing with claws becomes unnecessary. Two lineages took it particularly far: in abelisaurids, such as carnitaurus, the arms became vestigial and in some cases completely disappeared.

In a separate theropod lineage, T-Rexes, while much bigger than any abelisaurid and with a bigger head, did not have vestigial arms. Their arms, tho tiny, still had bones that locked into each-other and seemed to have muscle attachments. There’s been several theories as to why: This certainly meant that T-Rexes still used their arms, but how? The main theory is that juvenile T-Rexes had a different morphology as their adult counterpart. They already hunted, but much smaller preys, and with a different technique, so they would’ve still used their arms. On top of that, there’s the idea that adult T-Rexes might’ve slept on their bellies and their arms would’ve been useful to bush themselves back up.

Another reason for T-Rex’s large heads, in addition to their big jaws being terribly efficient, is that it may help with placing their center of gravity over the knees, which is useful to keep balance while walking or running.

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- flambonkscious@sh.itjust.works ⁨1⁩ ⁨year⁩ ago
  Whaaat? As if a trex could push itself anywhere with it’s arms, that’s ridiculous!
  
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  - Buddahriffic@lemmy.world ⁨1⁩ ⁨year⁩ ago
    We only know what their bones look like. Maybe they were swole af from all those T-Rex push-ups.
    
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  - loaExMachina@sh.itjust.works ⁨1⁩ ⁨year⁩ ago
    I know how unlikely this sounds, but I didn’t make it up, it’s an actual theory.
    
    zenodo.org/record/3674749
    
    scientificamerican.com/…/if-t-rex-fell-how-did-it…
    
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teft@startrek.website ⁨1⁩ ⁨year⁩ ago
They probably didn’t need their arms for how they hunted. Same as modern birds. Think of how an eagle kills and then eats its prey.

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- deranger@sh.itjust.works ⁨1⁩ ⁨year⁩ ago
  Birds use their arms extensively though, they’re wings
  
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  - PM_Your_Nudes_Please@lemmy.world ⁨1⁩ ⁨year⁩ ago
    Go look at an ostrich skeleton, and you’ll quickly realize that the arms on a t-rex skeleton have been posed backwards from what they should be. They’ve traditionally been posed based on the assumption that the skeletons were related to lizards. In reality, they’re avian, (we’ve found plenty of evidence that they had feathers, for instance,) and the tiny arms easily could’ve been wings like an ostrich. An ostrich skeleton also has the distinctive tiny arms like a t-rex, but they’re rotated 180 degrees to work as wings instead.
    
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  - teft@startrek.website ⁨1⁩ ⁨year⁩ ago
    I’m speaking more to them grabbing the prey with their legs and just holding it to death. They also rip and peck with their beak as they hold the unfortunate snack. The same would probably be true for dinos with short arms minus the beak thing but they would use teeth instead.
    
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Today@lemmy.world ⁨1⁩ ⁨year⁩ ago
There were some articles last year that said they think T-Rexes had small arms so they don’t get bitten off when competing for food with other T-Rexes.

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- havokdj@lemmy.world ⁨1⁩ ⁨year⁩ ago
  Before anyone goes and asks why they had arms to begin with, remember that whales tend to have very tiny legs on the inside of their bodies.
  
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  - Uncle_Bagel@midwest.social ⁨1⁩ ⁨year⁩ ago
    Their vestigial hip bones serve as an anchor point for their genital muscles. Those have to anchor somewhere or the whale can’t reproduce
    
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  - atlasraven31@lemm.ee ⁨1⁩ ⁨year⁩ ago
    Just imagine a whale walking around on tiny legs.
    
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incendiaryperihelion@lemm.ee ⁨1⁩ ⁨year⁩ ago
arms wont just “go away” when an organism evolves to use them less, and eventually would become vestigial. However I think that this question relies on a sweeping generalization that is not backed by evidence, only supported by media and the way we usually see dinos depicted

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FlyingSquid@lemmy.world ⁨1⁩ ⁨year⁩ ago
Sauropods had tiny heads and huge legs. So it varies. Evolution is an amazing thing.

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bl4ckblooc@lemmy.world ⁨1⁩ ⁨year⁩ ago
Ah man I thought this was on dad jokes for a minute.

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anolemmi@lemmi.social ⁨1⁩ ⁨year⁩ ago
Not an expert here, but I think you’re looking at this particular example backwards. Evolution isn’t a “smart” process that picks and chooses favorable traits, it’s more simply “this animal with these traits was able to survive and mate, so these traits get passed on”

So start with what was before the dinosaurs, to my recollection that would be some kind of amphibian, living in both water and land. Probably has four equal-length limbs and walks on all fours.

Whatever exactly happened between A and B idk, but eventually you get to the dinosaurs that walked on two legs. And that there is the answer. They used their legs more, their arms became less important to their survival techniques, and so over time they shrunk.

Survival for those guys was “run fast + big jaws and sharp teeth”. Long tails for balance while running, not much use for the arms. They didn’t shrink because it was an evolutionary advantage, they shrunk because they weren’t doing the heavy lifting. Terrible pun intended.

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- bdesk@kbin.social ⁨1⁩ ⁨year⁩ ago
  Evolution is the process of turning things into crabs. The little T-rex arms were just part of this process.
  
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antonim@lemmy.dbzer0.com ⁨1⁩ ⁨year⁩ ago
Which dinosaurs? Predators usually had relatively large heads because big head > big jaw > kill better and bite off more meat. But herbivores usually did not, as they could just focus on plants (instead they could develop longer necks to reach them in various places); some species such as Stegosaurus had rather famously tiny heads.

Tiny arms are associated with Tyrannosaurus and similar large theropods, but lots of other dinosaurs had relatively large arms, such as Dromaeosauridae (“raptors”). Of course discussing “arms” of various gigantic four-legged sauropods is pointless…

Basically there’s too much variety among dinosaurs to answer your question directly.

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- Norgur@kbin.social ⁨1⁩ ⁨year⁩ ago
  Can we acknowledge the super polite way the second linked article calls something bullshit?
  
  Gregory S. Paul, in his 1988 book Predatory Dinosaurs of the World, also considered Deinonychus antirrhopus a species of Velociraptor, and so rechristened the species Velociraptor antirrhopus.[34] This taxonomic opinion has not been widely followed
  
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  - antonim@lemmy.dbzer0.com ⁨1⁩ ⁨year⁩ ago
    Idk, I’ve read some relatively popularly-oriented stuff on dino paleontology and classification, and some of those areas are shockingly shaky, so I don’t think this is meant to be some special “diss” at the researcher. E.g. when I was a kid I knew very well what Troodon was, it was shown in all those dino encyclopedias and “documentary” films (Discovery Channel’s Dinosaur Planet), noted for having the largest brain-to-body ratio, but it turns out that the whole species was reconstructed based on vague fossil fragments (like many other species, mind you) and tenuous connections between them (i.e. without good reasons to assume they belong to the same animal), and the current consensus is that the species literally did not exist at all. Having your taxonomic reclassification rejected seems pretty negligible compared to erasing a whole damn species…
    
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- unnecessarygoat@lemmy.world ⁨1⁩ ⁨year⁩ ago
  i think its quite obvious they’re talking about large theropods like Tyrannosaurus
  
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ImplyingImplications@lemmy.ca ⁨1⁩ ⁨year⁩ ago
So just because a living thing has a certain feature doesn’t mean that feature is advantageous. It simply means it doesn’t hinder reproduction. Dinosaurs in particular are also difficult to study since we really only have bones to go off of. Animals alive today often look nothing like their bone structure suggests so we might be thinking of dinosaurs all wrong.

From some googling, it seems scientists are guessing as the heads became larger, the arms became smaller. Their giant heads are thought to provide bone crushing jaw strength. Maybe their giant heads used up so much energy that maintaining complex limbs was too much. Another thought is that they ate in packs and limbs would get in the way while pack eating.

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- Delighted@ttrpg.network ⁨1⁩ ⁨year⁩ ago
  The tail plumage of peacocks for example is the opposite of advantageous in everyday life. But since it’s sexually selected for (peahens like a showoff with a big tail-fan as a sign of fitness), it’s advantageous for reproduction.
  
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tallwookie@lemmy.world ⁨1⁩ ⁨year⁩ ago
arms were evolving to become vestigial

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match@pawb.social ⁨1⁩ ⁨year⁩ ago
Arms cost calories. If you don’t need it, better to have less investment in it

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RIP_Cheems@lemmy.world ⁨1⁩ ⁨year⁩ ago
The best answer comes down to two factors; oxygen and food. Oxygen levels were much higher 45 million years ago than they are today, and exposure to high amounts of oxygen have shown to drastically increase the productivity of the human body, so why not the same for animals. Second comes down to food. The more food, the more of an animal. The bigger the prey, the bigger the preditor needs to be or they need to be in greater numbers. When creatures like brachiosaure exist, it would understandable be difficult to kill it when small, thus animals that were larger would have better chances, or conversely animals that worked together. Of course, this is somewhat speculation and there could be other factors involved, but this is the simplest answer that makes sense.

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Mothra@mander.xyz ⁨1⁩ ⁨year⁩ ago
I’m sure someone else can chime in with a better reply.

First, not all dinosaurs have large heads and tiny arms. But you are right that some were like that, like famous Trex for example.

The large head is so that these dinosaurs can have a large mouth. A large mouth means they can attack larger prey with it. That’s beneficial if you are big, because you can eat with one kill as opposed to spending the whole day trying to find enough small prey. It’s also beneficial if most prey available is large.

The small arms are simply because they didn’t need them to hunt or defend themselves, or to move around. The mouth and legs alone were enough as these dinosaurs evolved, and so the advantage of having small arms is that you don’t waste body resources and energy in growing and maintaining unecessary limbs.

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shortypig@lemm.ee ⁨1⁩ ⁨year⁩ ago
Start with long arms and walk on all fours. Evolve to walk bipedal and have shorter arms. The dinosaurs you’re thinking of are just in the process of evolving and might have eventually evolved to have no arms unless those little shorties are advantage somehow. (Holding on during mating, evolving towards wing development) Some modern snakes still have little nubbins where their legs used to be.

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