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rearing sauropods



In a message dated 12/31/01 11:34:13 AM Pacific Standard Time,
Buckaroobwana@aol.com writes:


<< Is there any advantage to horizontal feeding with such a long neck? >>

       Yes. A much greater area could be foraged with a minimum of energy
expended. An enormous swath could be cut by a sauropod with a thirty-foot
horizontal neck. DV

The idea looks pretty practical but a survey of modern biodiversity doesn't seem to support it. We don't see turkeys with three foot necks, or sheep with six-foot necks, or rhinos with fifteen foot necks ( we do see extant rearing browsers in the form of the gerenuk, admittedly several orders of magnitude smaller but its a viable strategy at least at one scale).
Perhaps this is because an extra fifteen to twenty feet of neck is anything but free- it takes a large amount of protein and energy to build it in the first place. Once built, it takes a fair amount of energy to maintain all that muscle and bone and tissue. Every bit of protein or energy spent on a longer neck means protein and energy that could go somewhere else- tail, limbs, overall size, eggs, tougher hide, food stores, etc, etc. Not to mention the added vulnerability- neck is one of the most vulnerable spots in an animal since the nervous, circulatory, and respiratory systems all run through in close proximity. In the end, the added cost of all this additional neck may just outweigh the cost of taking a few additional steps.
Perhaps the greatest problem with the "vacuum cleaner" hypothesis is the predictions it makes when compared against observations of diplodocid neck structure. You would predict to see a great deal of lateral flexibility to sweep the neck back and forth across the feeding area to get as much vegetation as possible. Vertical flexibility, however, should be relatively poorly developed- the neck would only need to reach from ground level to a few meters above in search of low-growing vegetation and shrubbery. This is the precise opposite of what you see in Diplodocus according to Stevens et al. where vertical flexibility is considerable but there is relatively little horizontal flexibility. It fits a little better what you see in Apatosaurus, but Apatosaurus has a proportionately shorter neck than Diplodocus. This suggests that whatever the purpose of the really freakin' long neck seen in some sauropods was, at least in the diplodocids it wasn't to forage on the ground. My suspicion is that mamenchisaurus and the other chinese longnecks are going to look like Diplodocus but I guess we'll have to wait for that study to be done.


And another argument:
Simple, you DON"T rear up! Rearing sauropods IMHO is a myth.

Regarding elephants, it seems to me that they do more to support Bakker's rearing hypothesis than refute it. For one thing, the fact remains that elephants can and do rear up, if only often enough to perpetuate the species, despite the fact that they aren't particularly well-designed for rearing. The first problem is that the forelimbs, not the hindlimbs, usually bear most of the weight of the animal in elephants (as in most mammals). Second, the center of gravity isn't located particularly near the hips. This means that elephants have to bend their knees to rear, in order to put their feet below the CG and remain stable. This subjects the femora to bending stresses, where the breaking point will be lower than if the limbs are loaded axially and function as columns.
Sauropods on the other hand, were much better suited to rearing (particularly diplodocids), as they suffered neither of these design problems:
First, the hindlimbs bore much more of the weight of the animal, and so accordingly were much more robust than forelimbs, the reverse of your typical mammalian situation (check out d'Arcy Thompson's _On Growth and Form_ with respect to this observation. Check him out if you're in any way interested in biomechanics or morphometrics, too). You've also got to consider that sauropods had to walk, so the amount of load being placed on a single hindlimb with the other foot off the ground may well have exceeded 50% of the animals weight. So putting all the weight on the hindlimbs, or the hindlimbs with the tail, probably didn't even come close to the breaking stress of the femora. This varies from sauropod to sauropod of course. Another thing to consider is that the stresses involved are going to depend in part on the weight of the animals- some of these guys may have been a lot skinnier than typically reconstructed, and some, for all their length, are mostly air-filled neck and whiplash tail.
Second, the center of gravity is much nearer the acetabulum than in elephants, because the front of the sauropod is filled with air, the forelimbs are pretty small, and the rear is pretty much solid, with a huge, muscular tail extending behind. Little if any bending at the knees would have been necessary in a bipedal/tripodal sauropod; instead the femora may have functioned as columns loaded largely in compression. This is to an extent true of all sauropods compared to elephants but especially so in Diplodocus, Barosaurus etc.
As for high neural spines, having the highest spines over the hips implies again that most of the load of the body was suspended from the hips (sort of like the pier of a suspension bridge being the highest point) rather than the forelimbs (high neural spines are generally over the shoulders in mammals, e.g. bison). The unusual height of the neural spines in some sauropods may also suggest that there were some unusually high tensile loads being supported by the ligaments and muscles attaching to the neural spines, since the greater the height of a beam (in this case the spine) the greater its bending resistance. Again, if this doesn't make sense check out d'Arcy Thompson for a more extensive discussion of this stuff (he even refers to diplodocids and stegosaurs in particular) and a really readable intro to beam theory, tension, compression, and all that stuff. Basically, most of the body was already suspended from the hips, so suspending the whole darn thing is hardly a stretch.
Finally, consider theropods. The elephant argument seems to be at odds with the mere existence of some of these creatures- you've got animals that may have ranged to ~4-5 tons (t. rex), and Spinosaurus (who knows how much but it must have been obscene), and finally a giant that weighed by some estimates as much as 8 tons (giganotosaurus, and they keep digging up bigger ones), and these creatures clearly must have spent substantial parts of their lives supported by just a *single* limb while moving. The maximum possible body size for an animal rearing onto two elephantine, columnar limbs logically must be quite a few tons higher yet- how much higher I wouldn't know but offhand I'm going to guess maybe even twice as much or more, perhaps near or even above 15-20 tons?


NL