None! All known quadrupedal dinosaurs were obligatory or facultative herbivores.
Spinosaurids have, at various times, been suggested to have been quadrupedal; I don’t find this likely, however, as their hands don’t show any weight-bearing adaptations and I can see no reason why they would need to be.
The ankylosaur Liaoningosaurus has been suggested to have been piscivorous due to apparent preserved fish in the gut and supposed aquatic adaptations. I have my doubt about this; I don’t see why the fish bones in the body couldn’t have been from either scavenging fish or that they were simply washed in. Some of the supposed swimming adaptions have either been shown to be inaccurate (the “belly armour” was probably preserved skin) or may not be piscivorous adaptions (lack of fusion, odd proportions may be due to juvenile specimens).
#this is so whack#i wonder why that is
The ancestor of all dinosaurs was a biped. So, why did quadrupedal dinosaur evolve quadrupedality?
For sauropods, the answer is guts. Plants are way harder to digest and are less nutritious than meat, so herbivores need to be able to have massive guts in order to hold large amounts of plants and break them down over long periods of time. (This may also be the case in silesaurids, a group closely related to dinosaurs)
Ornithischians got around this by changing their hip configuration so that their guts could go back further, and indeed many ornithischians were bipedal. So why did some become quadrupedal? In thyreophorans, they evolved heavy bony armour, likely to prevent predation from theropods. In ceratopsians, their heads became very large, in part due to increased space for anchoring jaw muscles (for chewing plants) and and defence against predators. Ornithopods always retained some bipedal capabilities, and only started walking quadrupedally when they got really big, and the weight of their guts couldn’t all be compensated for by moving it backwards.
If we look at it this way, it seems that dinosaurs only evolved quadrupedality when they had to due to the effects of being an herbivore.
This is probably due in large part to the evolutionary history of dinosaur ancestors. The earliest tetrapods were superficially lizardlike animals; they walked with a sprawled posture.
(A typical sprawled-stance tetrapod. Image by Deror Avi)
This is a very energetically efficient posture, as the animal doesn’t have to waste energy holding itself up. There is a big downside though – animals with this posture waggle their bodies back and forth when they run, meaning that their lungs collapse and they can’t breathe while running. The ancestors of dinosaurs solved this problem by adopting an erect stance, which allowed them to breathe while they ran.
(Coelophysis, an early dinosaur. Image in public domain)
The transition from sprawled posture to erect (yes, yes, you’re very mature), however, had the side effect of effectively locking the hands in a position where the palms face each other. This was really always the case – in sprawled-stance animals, the palms are in this position as well. The arms were just swung out to the side so that the palms were flat down.
Humans have the luxury of being able to rotate our forearm bones past each other, so we tend to assume this is the case for everything. It’s not. We can only rotate our wrists because of a special cylindrical joint between the two bones in our forearm that lets one turn while the other stays still. Dinosaurs didn’t have this.
In dinosaur lineages that took to all fours, they couldn’t simply turn their wrists and go; they were locked in position. Some groups (namely sauropods and thyreophorans) evolved twisted forearms that were locked with palms facing backwards (or “down”); others (namely ceratopsians and ornithopods) kept the palms facing inwards and just walked with their hands turned like that. (We don’t really know much about the hands of silesaurids, but i suspect they did something similar to this as well)
So, why didn’t theropods ever evolve quadrupedality? Because of their evolutionary history, dinosaurs were kind of up a blind alley when it came to re-evolving quadrupedality. The lineages that did take to four legs likely did so as side-effects of their herbivory, and there wasn’t really any factor that was strong enough to push (known) theropods into quadrupedality.
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