Why were so many dinosaurs (especially predators) bipedal?

[Darwin's Finch]

Since this thread has been raised from the recently-dead, it's as probably as good a time as any to mention this recent paper: Associated evolution of bipedality and cursoriality among Triassic archosaurs: a phylogenetically controlled evaluation

This result indicates that, among Triassic archosaurs, bipeds could run faster than quadrupeds. Bipedalism is probably an adaptation for cursoriality among archosaurs, which may explain why bipedalism evolved convergently in the crocodilian and bird lineages. This result also indicates that the means of acquiring cursoriality may differ between archosaurs and mammals.

[Hoyasaur]

Originally Posted by Darwin's Finch

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:

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.

[Hoyasaur]

Originally Posted by Darwin's Finch

Since this thread has been raised from the recently-dead, it's as probably as good a time as any to mention this recent paper: Associated evolution of bipedality and cursoriality among Triassic archosaurs: a phylogenetically controlled evaluation

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.

[Darwin's Finch]

Originally Posted by Hoyasaur

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.

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.

[Hoyasaur]

Originally Posted by Darwin's Finch

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."

Couldn't have said it better myself.

[Darwin's Finch]

Originally Posted by Hoyasaur

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!

False. What I am arguing against is when you said this, in post #44:

Incidentally, footprints of hopping dinosaurs have been found - Discovery of Trackways of Hopping Dinosaurs...

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.

But, following Popper, if we find a single set of hopping footprints, i.e. counter-evidence, then your hypothesis will be falsified.

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:

The simulator has clearly been able to find a large range of gaits that are both physiologically and anatomically possible given the constraints of the model. The fastest gait is the bipedal hop, followed by the quadrupedal gallop and finally the bipedal run. The question then becomes which of these gaits would the animal have chosen? Bipedal hopping would seem to be the obvious answer but this interpretation of the findings would be very bold. It is true that hopping has been described in dinosaurs based on trackway evidence, and an ichniospecies has even been proposed based on a putatively hopping gait: Saltosauropus latus (Bernier et al. 1984). However the current interpretation of these ostensibly hopping tracks is that they are swimming traces probably
produced by a large turtle (Lockley 2007). The largest hopping mammals are probably the Pleistocene megafaunal species of macropodid Procoptodon goliah with a mean estimated body mass of 232 kg (Helgen et al. 2006) so a 715 kg hopper is not an impossibility but identification of hopping based on morphological features is not straightforward. Whilst kangaroos are highly anatomically specialised, other hopping animals are less obvious: Otolemur garnettii and Galago crassicaudatus are morphologically very similar bushbabies and yet one is a habitual hopper when on the ground whereas the other employs a bounding gallop (Oxnard et al. 1990). However, what is more likely is that the simulation is able to tell us that our reconstruction is incorrect: features of the model that are insufficiently constrained have allowed it to develop a highly effective hopping gait that would not be available for the animal itself.

(bolding mine)

And the closing statements from that paper:

This paper demonstrates what can be achieved with the current level of computer simulation technology. The simulations do demonstrate a wide range of possible locomotor modes and shows (a) that 10 independent repeats is probably
insufficient for a quadrupedal gait analysis and (b) that there are major gaps in our understanding of gait choice that are highlighted by this process. Whilst bipedality is currently the most likely option, a high-speed quadrupedal hadrosaur should not be ruled out as a serious locomotor possibility for this group of dinosaurs.

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.

[Hoyasaur]

Originally Posted by Darwin's Finch

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.

That is your opinion; it is not mine.