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Re: Elasmosaur Necks and some other stuff




On Monday, July 15, 2002, at 03:02 AM, Dino Rampage wrote:

The article in PT # 53 about elasmosaurs left me a little confused. So elasmosaur necks were quite inflexible after all.

Haven't seen the article but a couple of things come to mind about the conclusions.
-if the necks were inflexible, what could one possibly gain by making them so long? Reach alone makes little sense- anything the head could reach, the body could also reach in a couple strokes (unless these guys fed by reaching into holes and burrows, or reaching out of the water into the trees... ). The only advantage one can see to having a neck like that is to get the head somewhere *faster* than the whole body could- that somehow the neck could be used to accelerate the head to strike at prey faster than the body could accelerate. This demands flexibility, however.
-at least some of the fossils of plesiosaurs show a fair amount of bend in the neck, particularly in the distal regions.
-why so many vertebrae? Every added vertebra adds not just length, but another joint. All else being equal, more joints = more flexibility. If elasmosaurs had relatively stiff necks, why not relatively few, long vertebrae(like in e.g. the tails of Archaeopteryx, basal dromaeosaurids, and Rhamphorhynchus)? (incidentally Tanystropheus has that structure).

Regardless these things are somewhat baffling... one thing you might suppose is that the head could be held above the water and then strike from above but there's little that indicates that the neck was directed upwards, and it has in the pasted been argued that the neck was more flexible laterally than dorsoventrally, and nothing about the animals recalls herons or cranes, which use the head and neck to rapidly strike down at prey (Quetzalcoatlus *does* look a heck of a lot like a really freakin' huge crane to me, tho). So presumably they struck underwater instead but one wonders how exactly they'd do this.
One thing I suppose is that the neck might have been used like a giant whip, propigating a wave down its length till the relatively small distal segments reach a high speed and make a rapid sideways snap at a fish. This might be relatively easy to model if it weren't for water resistance, which would probably greatly complicate things. On the other hand, elasmosaur necks are so large that water resistance would be relatively less important as a consideration. Or maybe the neck would strike something like a snake (this might also impose less water resistance than swinging the neck sideways)? But then the intervertebral articulations are pretty flat rather than ball-and-socket like in snakes (which I suppose is one reason to think that they weren't that flexible). Maybe looking at how sea snakes, eels and morays strike and how they are built would be useful.


on another topic...
>>Does anyone think that ceratopsians used their frills as thermoregulatory devices? In the animals with open frill fenestrae, the flesh would certainly act as a better conductor of heat than bone.<<
Well part of the issue is it's not necessarily an either/or question. It seems extremely likely that the frills are sexual display devices given that they are so extremely variable when the postcrania are so similar, this seems consistent with what you see in e.g. ungulate horns or bird of paradise feathers. *however* this doesn't rule out the possibility that they are also thermoregulatory, I think ungulates use their horns or antlers (when in velvet) to shed heat because they are pretty highly vascularized. I think it's pretty clear that sexual selection was an operating selective pressure, but its quite possible that both were operating. If you could show a latitudinal trend you might have some evidence for a thermoregulatory pressure (assuming that the size of sexual display structures doesn't follow a similar trend). How these structures relate to size would also be of interest. I think Romer showed that the sail area of pelycosaurs correlated to body mass rather than length, although then again if the amount of energy put into sexual display correlated directly to body mass maybe you'd expect the same thing. What we'd need to do is to create a couple of models for each hypothesis- thermoregulation and display- and see whether what we observe in ceratopsians can reject either model (I'm fairly sure it wouldn't reject the display model).


>>Furthermore, the work that I have done (that sounds cocky, doesn't it?) suggests that the postcrania of known _Dilophosaurus_ specimens don't show notable dimorphism <<

Syntarsus/Megapnosaurus/Coelophysis rhodesiensis shows dimorphism, as does Coelophysis bauri (pers. obs. of RTMP block), and _Ceratosaurus_(pers. obs. of BYU specimens), so in all probability Dilophosaurus showed the same sort of sexual dimorphism. Question- what about more primitive things like herrerasaurids? Is there any evidence that the trochanteric shelf for example is variably present in any species, or was it invariably present in these guys? (this would be sorta interesting in that supposedly sexual dimorphism is a derived feature of the ceratosaurs but could it be a symplesiomorphy?).
Anyways, consider if you're flipping coins- the odds of getting all heads or all tails on six flips in a row is 2/64, which is about 3% of the time and therefore within a .05 confidence interval which is generally agreed to be statistically significant by scientists. In other words to state that Dilophosaurus does not display male-female dimorphism you'd need to dig up at least six specimens which did not display dimorphism, to reject the null hypothesis with 95% confidence that you had dug up either all males or all females of a dimorphic population. We don't have six Dilophosaurus so we can't say with confidence that we don't have a dimorphic population. This model assumes of course that we are equally likely to recover male and female morphs of a hypothetical dimorphic population, which is not necessarily the case. Of course all you'd need are two specimens to show that they were dimorphic. We do have enough Allosaurus to be sure that these guys are not dimorphic, at least not in the glaringly obvious ceratosaur-coelophysoid way. I'm not sure if we have enough Herrerasaurus to be sure.


Another thing is that some of the fusions observed in theropods don't seem to have been tightly correlated to size, necessarily. For example one of the RTMP Coelophysis is pretty big but the synsacrum and tibiotarsus are absent; its a gracile individual. Much smaller robust individuals in the same block, however, show full fusion. So the question is, do some of these fusions relate to some degree to sex, rather than just maturity? Madsen has argued that fusion is not correlated tightly to size in Allosaurus, one of the few instances where we have a good sample size, and implies that something like this might be the case; we probably need to follow this up at some point. This might also really complicate attempts to gauge maturity by looking at arch-centrum fusion (I dunno, but if a monster like Stromer's Spinosaurus doesn't show arch-centrum fusion, maybe that is not a reliable indicator of something reaching or approaching adult size... or was the adult of Tokyo-stomping proportions?). One of the articles in, I think, _The Complete Dinosaur_ did talk about diffuse idiopathic skeletal hyperostosis (more or less random bone growth around joints and stuff, and damn thats a mouthful) being more common in males than females (in H. sapiens), certainly loss of bone mineralization is more a problem in females than in males in our species. I really don't know anything about this stuff but you might expect males to exhibit more fusion since (a) they don't need loose pelvic girdles to pass eggs/young, and (b) they aren't always losing calcium by producing eggshells or live young.