What evolutional benefit is that?
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.
People think of evolution as it’s needed straight A+ to pass, when C is enough.
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?
T. rex was around for 20 million years or so, I’d say they passed for long enough to be considered successful. Despite the tiny arms.
100+ million years qualifies as right in the middle of “for long” in my book. The fact that an asteroid or comet of biblical proportions wiped them out has nothing to do with evolutionary effectiveness. Most of the animals that did survive either A) lived in water or B) lived underground.
I don’t think science really categorizes species based on how successful they were. “Passing” in this sense refers to the individuals in the species who were able to reproduce, not the population as a whole. Most dinosaurs “passed” until ecological conditions killed them off, they didn’t die because they failed to adapt. A lot of the species that survived mass extinction events were just lucky, rather than having some ideal set of characteristics that allowed them to survive.
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.
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”
https://phys.org/news/2022-04-rex-short-arms-lowered-frenzies.html
In case of larger species with a tremendous bite force getting bitten in the arm would probably result in very bad, and likely very short, times.
Keeps them from masturbating, allows them to transmute their sexual energy into raw power when not actively engaging in sex.
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.
Whaaat? As if a trex could push itself anywhere with it’s arms, that’s ridiculous!
I know how unlikely this sounds, but I didn’t make it up, it’s an actual theory.
https://zenodo.org/record/3674749
https://www.scientificamerican.com/article/if-t-rex-fell-how-did-it/
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.
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.
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.
Their arms are small, but beyond that there’s basically nothing similar between them and an ostrich’s wing. The muscular anchor points are not similar at all to winged creatures, who require significant musculo-skeletal connection to the breastbone even in mostly vestigial wings. You can see this in the ostrich skeleton as the large “blob” of bone in the middle of the rib cage. There is nothing similar in the T-Rex. Even more of a problem with this theory is that the T-Rex’s popularity is in large part due to the fact that we’ve discovered a fairly large number of T-Rex fossils in good condition and not substantially disturbed… It’s why we have famous models like “Sue” and “Black Beauty” that make such good displays in natural history museums. Unless you’re proposing that a dozen different skeletons from several different regions with different ages all had bones shift after death to end up in the same position…
Our knowledge of what dinosaurs looked like is not perfect, but we’ve also come a very long way from the Magdeburg Unicorn or horned Iguanodons of the 1800s. Paleontology has largely moved past “puzzle piece” biology, where things are just haphazardly thrown together because they kinda look like they fit. There’s comparison to other species - not just reptiles- to see what are comparable modern equivalents or to other contemporary animals. There’s kinematics and musculature considered. Unless some fossil discovery is made that completely upends the evidence we have now, at least in the case of skeletal articulation of well-known and well-studied species like T-Rex, we can be reasonably confident that we’ve got it pretty close when it comes to what their skeletons looked like.
