Today, all land (mammal) predators that I can think of (dog sized or larger, except for humans) are quadrupedal. Especially the largest- the big cats and bears. And the vast majority of land mammals of any type are quadrupedal, especially the largest- elephants, rhinos, hippos, bovines, etc.
But there were many bipedal dinosaurs- the hadrosaurs (I think), iguanodon, ornithomimus, and all the large predatory dinosaurs I can think of (Allosaurus, T Rex, the raptors) were bipedal. Why is this? Was there something about dinosaur physiology that makes being bipedal, especially for a predator, advantageous? Is there something about mammal physiology that makes being quadrupedal advantageous?
So the bipedal ancestor might explain part of it, but if there were plenty of dinos like the ceratopsians that evolved to be quadrupedal, herd-living, and grazing, then why didn’t a quadrupedal, social predator like wolves or lions evolve?
Coupled with the bipedal ancestor:
My guess is that grass had not yet appeared and so there wasn’t as much advantage to being down on four legs with your face on the ground. There was still advantage to being bipedal to reach bushes and low trees for additional foraging.
Some modern lizards can go bipedal. Look up the Jesus Christ Lizard. Dinosaurs had the heavy tail which makes bipedalism easier by acting as a counter weight. Dinos employed T form bipedalism, where the head and torso balances over the central legs. This is different from I form bipedalism used by humans.
The way I see it, a T. rex is basically a giant mouth, that can walk around. Having greater mobility of that mouth is a good thing. With its bipedal stance, Rexy can effectively use its entire body as “neck”, and thus has more flexibility to put its mouth wherever it wants.
T form would be using the tail as a counterweight while in motion, which marsupials seem to do. The roos do appear to use the tail as a third leg, but I don’t know if it’s actually a support. Of course cartoon kangaroos can stand on their tail alone and kick you with their legs.
It is generally adventageous to be bipedal if you are a predator, but our current environment has some element that rewards quadrupedalism.
It is generally pretty much equally adventageous to be bipedal and quadrupedal.
It is generally adventageous to be quadrupedal if you are a predator, but the dinosaur environment had some element that rewarded bipedalism.
If none of these are true, then I would expect either bipedalism or quadrupedalism to take over given the length of the time frames in question, even if the ancestor had the least adventagous mode.
doesn’t seem likely to me, so I would guess some combination of 1 and 3.
A guess would be, if you are a solitary hunter, it’s an advantage to be bipedal. Dinosaurs had smaller brains, so therefore were to a higher degree solitary hunters, and therefore bipedal.
That requires some qualification. The earliest known dinosaurs were bipedal, and their immediate ancestors may have been bipedal, but the archosaurs from which they in turn are believed to be descended were not typically bipedal.
I don’t know how much this has to do with bipedalism, but it’s worth noting that there are two main groupings of dinosaurs, based on their pelvic structure: Saurischia (“lizard-hipped”) and Ornithischia (“bird-hipped”). Bizarrely, the birds themselves are not grouped with the “bird-hipped” side of the tree.
The bipedal predators with which we’re familiar (Tyrannosaurs, Allosaurs, Raptors) are all saurischians, but the huge quadrupedal Sauropods (“Brontosaurus,” Brachiosaurs, and the like) are ALSO saurischians. And on the other branch, which were, as far as I am aware, herbivores, there are some quadrupdal groups (the Ankylosaurs and Triceratops’s clan), some bipedal groups (Pachycephalosaurs and the early and less-well-known Heterodontosauridae), and several groups containing “facultative bipeds” – animals that could use both forms of locomotion as necessary (famously, the Hadrosaurs [duckbills]).
Seeing that both types of pelvis produced bipeds and quadrupeds, and there were highly successful bipedal herbivores, I’m reluctant to associate bipedalism with either branch or with carnivorous diet.
It may be what Stephen Jay Gould called “contingency” (basically, “luck”) – animals that were tending toward bipedalism may have been successful for other reasons and just passed bipedalism on to their descendants.
This is pretty much conjecture, if not outright prejudice left over from Victorian attitudes toward “lower” animals.
Although dino behavior is very poorly understood (for obvious reasons), there’s a considerable school of thought and some direct evidence indicating that some of the predators were pack hunters. I’ve specifically read assertions that Tyrannosaurs and raptors are (now) believed to have been. And there’s direct evidence that some dinosaurs nurtured their young in nesting colonies, presumably with complex social behavior.
Although they did have small brains for their body size, any connection between brain size and complex behavior is also pretty thin.
Does flexibility of the spine have anything to do with it? Mammals have flexible spines, especially in the neck, as an aid to grooming their fur. Most mammals when they run take advantage of this by using a “bounding” motion where the spine acts like a spring, absorbing and releasing energy. By contrast, most reptiles seem to have little vertical flex in their spines, and run either with a horizontal snakelike flex or a stiff bodied lunge propelled mostly by their hind limbs.
could it have anything to do with strenght:weight ratio (and force of gravity?) i don’t know of any dino weighing more than 15 tonnes that’s bipedal. anything heavier than that is diplodocene. smaller dinos looked like they were as agile as ostriches.
We have no idea if the larger dinos were bipedal or not. There are clear indications that they were quadripedal, but even the largest may have been able to rear up on their hind legs. Maybe. I don’t know the flavor of the month on that theory.