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Re: The New Paper That Time Forgot



--- On Sun, 10/12/08, David Marjanovic <david.marjanovic@gmx.at> wrote:

> P. Martin Sander & Marcus Clauss: Sauropod Gigantism.
> How did sauropod 
> dinosaurs reach body sizes that remain unsurpassed in
> land-living animals?, 
> Science 322, 200f. (10 October 2008)
> 
> Being a two-page "Perspectives" paper, it has no
> abstract and is already so 
> concise that I cannot summarize it unless I do it in one
> sentence. So, here 
> are a few quotes:

++++++++++++++++++++++++++++++++++++++++++++++++++++

Being a two page perspective piece, you'd think Science could afford to give 
that one out for free. 

No matter though.

_______________________________________________

> "Large body size in endothermic animals is associated
> with a major problem 
> of dissipating excess body heat. A long neck also means
> that a large volume 
> of air must be moved in the windpipe during ventilation
> before fresh air 
> reaches the lung. These problems appear to have been solved
> by an 
> evolutionary innovation shared by sauropods and theropods
> [...] and their 
> descendants, the birds: a highly heterogeneous avian-style
> respiratory 
> system (11) with cross-current gas exchange [uh, no,
> counter-current --  
> cross-current is what crocodiles have!] 

++++++++++++++++++++++++++++++++++++++++++++++

Actually birds are cross-current. The blood passes by the alveoli at a right 
angle. It's not as effective as counter-current, but it is still "better" than 
concurrent gas exchange. Especially when coupled with a unidirectional flow of 
air. 

My question in regard to all of this is do we actually know that any dinosaur 
had a cross-current unidirectional system in place? I can accept that the most 
bird like dinosaurs (maniraptors, deinonychosaurs etc) probably had the full 
suite of avian respiration in place, but I'm less inclined to say the same 
thing for dinos further from the avian side of the family. 

We have evidence of air sacs, but what evidence do we have for unidirectional 
flow of air? Surely one likely came before the other. As there are other 
animals today (certain snakes and insects) that have air sacs, but no flow 
through system of gas exchange, I'm inclined to think that air sacs came first; 
flow through second.


As for the rest of the article, it would appear that Sander and Clauss relied 
heavily on the Erickson et al 2001 study, for their assumptions about sauropod 
BMR; yet work by Lehman and Woodward (2008) has shown that there are flaws with 
the Erickson growth curve for sauropods. Bone histology data suggests that 
sauropods like _Apatosaurus_ (used in the Erickson et al study) would have 
taken closer to 80 years to reach adult size. 

The strangest thing about this is that Sander and Clauss did read the paper, as 
they cited it in their work ( []'s mine ):


"In addition, one of the studies (23) [Gilooly et al from PLoS Biol] suffers 
from poor bone histologic constraints on sauropod growth rates, as does a study 
(24) [Lehman and Woodward] arguing against fast growth in
sauropods. Compared to other dinosaurs, the long bones of sauropods rarely 
preserve growth marks, probably because bone tissue was deposited too rapidly 
to record them (15) [Sander 2000, Paleobiol]. Histologic growth rate studies 
using skeletal elements other than long bones may provide more reliable 
estimates."



The authours argued that Lehman and Woodward had based their data strictly off 
of long bone studies, but they didn't. Lehman and Woodward used the exact same 
specimens that Erickson et al had used. Sander and Clauss obviously had no 
problem with those specimens, as Erickson et al was frequently cited, so I see 
no reason for them to complain about the Lehman and Woodward study either.

The authours also stated that gastric mills were not present in sauropods. Has 
that been confirmed now? Are sauropods officially considered not to have 
gizzards anymore?

One last quibble I had was from here:

"In addition, the vulnerability of a long neck (10) makes its
evolution unlikely without protection from predation by superior body size."

An interesting thought, but if true would we not expect sauropod hatchlings and 
juveniles to be short necked? At the very least, we might expect early 
sauropods to have had short necks. Not to mention the existence of 
_Tanystropheus_, which was big, but hardly of "superior" size to contemporary 
predators. Then there were the smaller critters like _Hyphalosaurus_. I'm just 
not sure how well the large size argument holds up for sauropod necks.


Jason