Re: Elasmosaur Necks and some other stuff

[longrich@alumni.princeton.edu]

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.