To add just one thing to Blake’s excellent analysis- energy conservation. If you’re a fast, active lifestyle endotherm, energy efficiency with regards to movement is vital. Because mammals evolved a vertically flexible spine, they can run with a bounding gait that in conjunction with the muscles and tendons of the spine alternately store and tap energy as movement against and with gravity. For a bipedal archosaur, the mechanism is different. With a vertically stiff, springy tail flexing horizontally, energy is tapped and released as the archosaur shifts its center of gravity alternately between hind legs.
ETA: the difference between the mammalian spine and the reptilian spine shows up in another place. Cetaceans flex their tails vertically, while icthyosaurs flexed theirs horizontally like fish do.
Well, all birds are bipedal when they’re not flying. Raptors hunt with their feet/talons, but there are plenty of predatory birds that don’t hunt with their feet.
Most, if not all, dinosaurs descended from bipedal ancestors. However, the origin of dinosaur bipedality remains a big mystery. To quote Gregory Paul from his 1988 book, Predatory Dinosaurs of the World, “The reason predatory dinosaurs became bipedal is not at all clear. […] The only thing that can be said in the end is that bipedalism was a serendipitously crucial adaptation…”
I believe that bipedal hopping behavior in the very first dinosaurs in the Mid-to-Late Triassic (roughly Carnian time) led directly to obligatory bipedality, which characterized most primitive and many advanced dinosaurs, as I argued here.
Bipedal hopping would have enabled dinosaurs to move very quickly without being warm-blooded because hopping relies on elastic storage-and-release rather than metabolism, and it may help explain how the cold-blooded dinosaurs suppressed the increasingly warm-blooded mammal-like reptiles and mammals.
When I was reading this, I was wondering if differing oxygen levels could contribute to differently favoring quadrupedalism or bipedalism. I recall hearing that Oxygen levels were higher in the past, although Wikipedia says that’s still being debated:
How would it contribute to bipedalism in particular? I can see oxygen level changes contributing to the extinction, but how would it have favored bipedalism in the first place? And in asking that question, I guess one answer might be that bipedalism was a more efficient way for dinosaurs to evolve that great size, though I’m not sure how. But the higher oxygen levels may have enabled the evolution of larger animals, and dinosaurs got there first somehow by being bipedal.
Very generally, yes, although the jury is still out.
I would also point out that the paper linked about hopping dinosaurs is far from mainstream. Do a Google search on the title, and you get exactly 2 hits, both on the same website. Plus, it was written in 1989, and there have been many further studies on locomotion in dinosaurs, none of which, to my knowledge, has confirmed a hopping origin for bipedality, or even hopping locomotion in later species. For example, from this 2005 paper:
The issue is which dinosaurs were warm-blooded and also when did dinosaur endothermy evolve. It is certainly possible, maybe likely, that advanced dinosaurs in the Cretacous were warm-blooded, but that doesn’t mean that primitive dinosaurs in the Triassic were also warm-blooded, and it’s the primitive Triassic dinosaurs that are at issue here. Primitive dinosaurs and crocodiles were very closely related and possibly shared a common thecodont ancestor. Modern crocodiles are not warm-blooded, so it stands to reason that their Triassic ancestors, and by inference, primitive dinosaurs, weren’t either.
Interestingly, Robert Bakker, in advancing his ideas about dinosaur endothermy back in the 1980s, suggested that endothermy actually evolved in thecodonts, which allowed them and their dinosaur descendants to suppress the warm-blooded mammal-like reptiles. (I don’t know if he still believes that.) But if thecodonts were warm-blooded, then why aren’t crocodiles?
The idea that warm-blooded animals are inherently superior to cold-blooded animals, which was Bakker’s basic thesis, is bogus.
To quote Carl Sagan (and many others) “absence of evidence is not evidence of absence.” The weight of evidence can never prove any theory – it’s one of the fallacies of induction. But more to the point, footprints are made on a wet substrate, such as mud or sand in a floodplain or stream bank, which is a small (and unrepresentative) fraction of the total exposed landscape. In other words, the rarity of hopping footprints may simply be due to an unrepresentative sample – another fallacy of induction. It is entirely possible and not unreasonable to suppose that dinosaurs walked (alternating footfalls) on wet soil and shifted into high gear (hopping) only in the dryer uplands, where the footing was more secure but where footprints were not preserved.
That begs the question: why did obligatory bipedality evolve? It is well known that many theocodonts were normally quadrupedal when standing still or walking but that they shifted to a bipedal gait at higher speeds – because bipedality was more efficient. (Modern crocodiles and some lizards do the same thing.) In other words, they were facultatively bipedal. But primitive dinosaurs were obligatory bipeds.
Now, I’m sure someone will suggest that obligatory bipedality allowed the dinosaurs to free their forelimbs for other uses, and that is certainly true, but that still fails to explain how or why dinosaurs solved the “stability” problem, i.e. how they could maintain their balance on two legs when not moving at all. In Bipedal Hopping and the Origin of Dinosaurs, I argued that in perfecting the bipedal hopping gait, the dinosaurs serendipitously solved the “stability” problem, which enabled them to remain bipedal even when they were standing still. I am not aware of any other hypothesis that can explain that.
And, to quote Isaac Newton, “We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances. Therefore, to the same natural effects we must, so far as possible, assign the same causes.” One could simply attribute the rarity of hopping ichnofossils to the absence of hopping in dinosaurs, especially given the lack of otherwise supporting evidence for that mode of locomotion.
The Newton quote is off-point. Obviously, alternating footprints means alternating footfalls and therefore a symmetrical walking gait. No argument there. Newton also said hypotheses non fingo when it came to the cause of gravity, and especially “action at a distance,” an absurd notion, which even Sir Isaac acknowledged, but upon which his theory of universal gravity is based.
As far as the rarity (or even absence) of asymmetrical (hopping) footprints, then you are stating a hypothesis – dinosaurs did not hop – from which you deduce an empirical and testable prediction – therefore, hopping dinosaur footprints will never be found. That’s great! But, following Popper, if we find a single set of hopping footprints, i.e. counter-evidence, then your hypothesis will be falsified. Unfortunately, IMO, footprints can never be used to test my hypothesis that (primitive) dinosaurs hopped because the absence of asymmetrical footprints does not falsify it. Therefore, we must look to other sources of evidence, esp. fossilized bones and skeleton. My hypothesis predicts that in order to become efficient hoppers and for dinosaurs to maintain their stability when hopping (as opposed to running), dinosaurs needed to restrict leg motion to the parasagittal plane with no lateral deviations, and that’s exactly what their pelvis and upper leg elements reveal.
Incidentally, there is also a paucity (or absence) of running trackways of large theropods. Here’s what mainstream paleontologist Gregory Paul said about that in his book on predatory dinosaurs: “It cannot be proven that large theropods were slow by their trackways because, as I’ve explained, this would be another case of unfair use of negative evidence. Someday the prints of a 5-tonne theropod running at high speed may be found and settle the issue once and for all. Until and unless that happens, we must turn to the design and stressing of the theropod skeleton for evidence of their speed.”
False. What I am arguing against is when you said this, in post #44:
No. Footprints of hopping dinosaurs have not been found (aside perhaps from those of birds…). The footprints in the paper you linked to, from 1984, have since been interpreted as belonging to a turtle swimming in shallow water.
Some dinosaurs, notably small ones, may well have hopped. They certainly do so today.
Again, no. That is not my hypothesis. My statement is that you are incorrect about footprints of hopping dinosaurs having been found, and that no accepted biomechanical studies have supported hopping locomotion in all but perhaps the smallest theropods. The video you linked to showing a hopping hadrosaur was part of a study performed by scientists at Manchester University; here is the associated paper (.pdf). The punchline of the paper is this:
(bolding mine)
And the closing statements from that paper:
Hopping may well have been possible in some groups, but the evidence thus far, from both ichnofossils and biomechanical analyses, does not point to such as common among bipedal dinosaurs, and thus cannot at this time sufficiently explain the evolution of bipedality.