God is watching?
That sucks. Please try again. I’d be interested in reading it.
The immediate ancestors of the dinosaurs either were themselves, or were closely related to, a bipedal Triassic archosaur family, the Rythrosuchidae. As a result, the earliest dinosaurs of both major lineages were bipedal. Following a principle that says that creatures tend to retain characteristics until and unless there’s an evolutionary advantage in a change that is within their physical capabilities, dinosaurs remained bipedal if they:
[ul][li]Were among the early forms,[/li][li]Belonged to a 'conservative/ lineage, or[/li][li]Belonged to a lineage for which retaining bipedalism was advantageous.[/ul][/li]
The coelurosaurs, smaller pack hunters, and carnosaurs, the bigger carnivores, all remained bipedal. Evidently no advantage to quadrupedalism ever showed up that was sufficient to outweigh the cost of losing the forelimbs as weapons.
The therazinosaurs (AKA segnosaurs), dinosaurian parallels to the ground sloths and chalicotheres, seem to have, like them, been partly bipedal and partly quadrupedal, using bipedal stance to feed and fight and quadrupedal stance for locomotion.
By the time the big sauropods show up in the fossil record, they’re already committed to a quadrupedal stance. But the Horner/Bakker school of paleontologist believe that one or more sauropod lineages may have been able to rear on hindlegs and tail.
The prosauropods, herbivorous Triassic forms (and a misnomer since they were a parallel line to, not the ancestors of, the sauropods), are particularly interesting with regard to this question. The earliest forms are lightly built bipedal animals. Plateosaurus, one of the earliest large dinosaurs, appears to have been principally bipedal but occasionally dropping to all fours. And the later and larger melanorosaurs were obligate quadrupeds.
The ornithopods are an assemblage, probably not cladistically valid, of bipedal ornithischian dinosaurs, all herbivorous. Even here, however, the trend occurs – the hadrosaurs, a late and large-sized group, appear to have been bipedal in fast locomotion, but to have fed mostly in a quadrupedal stance.
The stegosaurs, extinct outside India by the Cretaceous, were predominantly quadrupedal but evidently could rear, probably to feed. Likewise the nodosaurs and ankylosaurs, low-sling, heavily armored plant-eaters, were obligate quadrupeds.
Another group whose evolution is very interesting for the bipedality question are the pachycephalosaurs and ceratopsians. These appear to be closely related sister groups. The pachycephalosaurs were bipeds, with a very thick skull often ornamented with bosses and hornlike projections. It’s speculated that they used them for butting. The ceratopsians (e.g. Triceratops) on the other hand are Late Cretaceous forms which were mostly quadrupeds, but the earliest, smallest forms (e.g., Psittacosaurus or Montanoceratops) were bipedal. The weight of the beak and head shield appears to have outweighted any advantage to bipedality and led them to evolve in a quadrupedal stance.
The way I heard it, being quadrupedal is better for quick sprints, whereas being bipedal is more energy-efficient.
(heh – just noticed the parallel between this and 2 versus 4 wheel drive cars)
So perhaps dinosaurs had to travel very far to find their prey?
Could be bollocks, just throwing out another theory.
My WAG is that the dinosaurs had developed a way of running in a horizontal position with two legs, balancing with tail, that was superior to anything that time and as long as they hung around there was no slot for a gepard type predator unless it was able to evolve in a minute. When the fast dinosaur predators died, it opened the way for mammals to gradually increase speed.
The ancestors of dinosaurs, dinosauromorphs such as Lagerpeton and Dromomeron, were already bipedal (or at least, partially so), and, in general, unless there are selection pressures to change, lineages will tend to retain the traits of their ancestors. Thus, the default condition in dinosaurs is that of bipedality. Several groups (e.g., sauropods, ceratopsians, thyreophorans, etc.) did eventually evolve quadrupedality, but even those, for the most part, still retain some bipedal traits of their ancestors - notably, shorter forelimbs than hindlimbs are common in most dinosaurian groups (a notable exception being the brachiosaurids). For the most part, trends (within Dinosauria) towards quadrupedality appear to be related to a vegetarian lifestyle.
Now, the real question is why did the dinosauromorphs go bipedal in the first place? Unfortunately, we don’t know the answer at this time. While speculation abounds, about the best that can be reliably stated is that there were selective pressures toward using their forelimbs for non-locomotory purposes (or against using them for locomotion). What those pressures were remains unknown (what with behaviors being notoriously difficult to ascertain from the fossil record).
Sorry about that. Please consider writing it again, I would really like to read your thoughts about this.
Yeah, it was conjecture. I was just throwing a guess out there, I don’t know much about dinosaurs.
I don’t think the connection between brain size and complex behavior is thin though.
That’s why whales hunt us from their flying cars.
Whales are pretty smart. Also brain size relative to body size may be a factor.
More speculation: I saw a TV documentary on some desert dwelling critters that mentioned a small lizard that is normally quadripedal when still, but rears up on it’s hind legs and sprints when the sands get too hot. Apparently, being bipedal was more advantageous in the heat because there was less body parts touching the sand, and standing up taller and running helped with convective cooling.
Perhaps the protodino’s developed bipedalism for similar reasons.
Trouble is we don’t have much info on how the ancestors of dinos evolved. One theory is they descended from non-aquatic crocodillians with longer legs. This would have set up the scenario for bipedal running in the manner of some lizards. I don’t know if the croc theory has stood up over time, but clearly for a lizard or croc like animal, bipedal motion would have allowed greater speed over distance, especially if they couldn’t use their spines in a galloping motion as Lumpy noted.
Whale brains are larger in proportion to their body size than ours are.
Depends what proportion you are using. I assume you are using straight up division.
Also there can be a connection between the two factors, even if there isn’t in every single instance.
Also, whales do have very complex social systems.
[tangent]
On a raw brain mass to body mass ratio, perhaps. In terms of the much more useful and predictive encephalization quotient (EQ), however, humans are the top of the heap. Generally in mammals, brain size grows as the 3/4 power of body mass. EQ measures the deviation in brain size of a species from the “expected” brain size for a mammal that size. Humans have an EQ of 7.5 (i.e., our brains are 7.5 times as large as they “need” to be) while bottlenose dolphins are the top cetaceans 4.8. Most whales are even lower on the scale.
[/tangent]
I was hoping to see this post eventually. I think it would be very interesting, and the conclusions of this thread were not satisfactory final, in my opinion. Maybe other posters can help me pressure him into rewriting this post?
So…you’ve seen them too? I thought I was going nuts!
Oh, Father!
Heh, I’d forgotten about this thread.
Not physiology, but there is something about dinosaur anatomy that makes being bipedal advantageous, especially for a predator.
Your basic reptile walks by holding its upper limbs out at right angles from the body, with the forelimbs at right angles to that, facing down. The whole body is literally slung between the limbs, like a hammock slung between four tree trunks. In contrast the mammal posture has the body supported directly over the limbs, with limbs acting directly as pillars.
The reptilian posture is very stable. It is also very energy efficient since the animal can simply rest the belly on the ground and requires no energy at all while resting. Furthermore the high stability means that the animals don’t need much brain power to walk, meaning more energy savings and freeing up brain space for sensory activities.
However, there are a couple of major, related, downsides to the basic reptilian posture.
In order to increase speed, your basic reptile has only two options: throw the spine into a series of “S” shapes to make the stride longer and pivot just the upper limb to further enhance the stride length. Pivoting the lower limb with a reptilian posture doesn’t increase speed in any significant way because bone can’t get sufficiently long to avoid the belly scraping the ground without the torque generated becoming too great. In contrast the typical mammal can increase speed by throwing the spine into series of vertical arcs, but it is also able to pivot the entire limb to increase stride length and it is able to increase limb length to increase stride.
The “S” shaped stride and pivoting upper limbs is not a problem in itself for a small animal, but it means that the muscle activity is all going into producing movement a long way away from the central axis, and the central axis is what actually has to be moved forward, so the “S” shaped running motion distributes a lot of force over a very big area. The result is that acceleration is very, very slow compared to the mammalian “arc” shaped stride and pivoting of the entire limb under the body. In a sprint a mouse will beat a lizard every time. The reptilian stride is slow to get started. It isn’t necessarily any slower for small animals, but it has a drastically reduced acceleration.
The other problem with the reptilian posture is scaling. Bone is a very strong load-bearing material end-on, but much less strong when the load is placed on the side. In the same way, a human can easily balance on a set of stilts made out of 4 foot lengths of two-inch pine, but if you laid the pine between two bricks and stood on it, it would snap like the twig that it is. The reptilian posture means that the animal’s weight is supported by upper limb bones laid between the upright lower limbs.
Because the reptilian body is slung between the legs, rather than being supported over them, there is an absolute size limit for reptiles. Once an animal gets too big, the upper limb bones simply can’t support the weight and they snap. This is made worse by the torque generated by the “S” shaped stride associated with the posture. The faster a reptile wants to move, the smaller it has to remain. If an animal with a reptilian posture wants to run it can’t get much bigger then human sized. Any bigger than that and it will tear its own legs bones to splinters as soon as it picks up speed.
So we have two problems with the basic reptilian gait: slow acceleration and an upper size limit.
Many reptiles have overcome the slow acceleration by adopting facultative bipedalism. IOW they run on just the hind limbs when they need rapid acceleration. This facultative bipedalism has the effect of pulling the upper portion of the hind limbs more under the body, much more like the mammalian posture. The animal loses the advantage of being able to use the spine to increase stride and muscle power, thus the maximum speed for bipedal runners is usually less than for comparable quadrupeds, but this is more than made up for by the increases ground clearance and the ability of the entire limb to be rotated to generate speed, thus effectively doubling the stride length. So bipedal reptiles have much faster acceleration at the expense of reduced total speed.
As a fortuitous side effect, the bipedal gait also neatly circumvents the problem of scaling. With the limbs pulled under the body during running, the reptile can get much larger without breaking bones.
The ancestors of the dinosaurs adopted facultative bipedalism very early on. In fact it seems likely that the ancestors of all archosaurs was a facultative biped. In addition to the fossil evidence there are hints of this in the anatomy and behaviour of the living crocodilians, several of which, astonishingly, retain facultative bipedal movement. Having adopted facultative bipedalism for the acceleration advantage, the archosaurs were thus freed from the size constraints of the reptilian posture. They were able to become larger by maintain the bipedal posture for lower and lower speeds.
This then produced a feedback loop. Larger animals needed to be better at maintaining the bipedal posture for longer periods, and those animals which were able to maintain a bipedal posture for longer periods could grow larger, which in turn required longer maintenance of the posture and so forth. In short order all the dinos were bipedal all the time, and they needed to be. They had reached sizes at which any attempt to revert to the quadrupedal reptilian posture would have shattered their leg bones even standing still. So the dinos were able to become fast because they were able to become bipedal, and they were able to become large because they had become bipedal.
But that also meant that they were locked into a bipedal stance if they wanted to remain fast. Because they arrived at an upright posture via a fast bipedal posture, they couldn’t go back. They could never become fast quadrupeds again. To many of the changes in their hips and spines were locked in place to enable them to adopt the strategies mammals use to be fast quadripeds. So dinos had a choice: the could remain fast and walk on two legs, or the could support their weight on all legs and lose the ability to run. While many did adopt a quadripedal posture, they did it at the expence of sacrificing speed. IOW only those animals that could sacrifice speed could re-evolve a quadripedal stance. The armoured dinos could afford to do it, the aquatics could and the animals too big to be bothered by predators. The others were stuck with running on two legs.
Which brings us to the answer to the question: so many dinos were bipeds because they had to be bipedal to be fast. If you wanted to be anything but bipedal you did it at the expense of losing the ability to run.
Mammals have reached an upright posture from the other direction. We evolved as small quadripedal animals in order to run fast, rather than evolving it as large bipedal animals to support our weight. As such we retained many more options, and the fastest mammals are able to utilize the benefits of upright posture for acceleration together with arcing the spine for added top speed. As such it’s advantageous for mammals to be quadripedal if they want to be fast, hence the reason for so few bipedal mammals.
Macropods do use the tail as a counterweight while in motion, but no moreso than cats or rats. A tailless kangaroo an hop almost as well as a normal kangaroo. It is at low speeds that lack of a tail becomes a problem.
The tail is primarily a fifth leg, used when the animal is grazing. It is also used a single leg in fighting. When the animal is hopping the tail never touches the ground.
Real kangaroos do that too. Trust me on this, I have the scars to prove it.
