Re: The Very Very Latest Paper From 2006!!!

[dinoboygraphics@aol.com]

LAGs are not produced by temperature changes, they are produced in 
variations in the rate of bone deposition.  Temperature is only 
indirectly involved, and so (to be frank) is thermoregulation.  Of 
course in extremes of either winter or summer food procurement may 
become difficult, and at that point you see LAGs in all types of 
animals.

The idea that the LAGs in dinosaur bone in any way indicate a reduced 
or "intermediate" (whatever that is) metabolic strategy is one of those 
false truisms in dinosaur paleontology taht won't seem to go away.  A 
LAG represents absolutely nothing more than an alteration in the rate 
of growth.  Of course, with extant ectotherms taking much longer to 
grow up (and hence many more changes in climate) they tend to accrue 
more LAGs than equally sized extant mammals.  The rate of bone 
deposition in between annual LAGs in dinosaurs shows that they are 
growing at a rate funadmentally different from that of extant, wild 
ectotherms.

I'm not claiming that all dinosaurs had a resting metabolism equal to 
that of the mean for extant placental mammals (and certainly not the 
mean of extant dinosaurs!), but there is little defensible support for 
an "extant croc-squamate" level of resting metabolic rate, and 
elevating it just a little higher than that and you already ARE in the 
extant avian-mammalian range.  So it seems most likely that extinct 
dinosaurs linneages fall into the lower to mid avian-mammalian range 
(with on group obviously shading into the extant avian range).

Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333

www.skeletaldrawing.com

-----Original Message-----
From: david.marjanovic@gmx.at
To: dinosaur@usc.edu
Sent: Fri, 9 Mar 2007 7:53 AM
Subject: Re: The Very Very Latest Paper From 2006!!!

> Restating-- Large animals have low surface mass ratios, and therefore 
lose > heat more slowly on a mass specific basis than smaller animals. 
It follows > that direct ambient temperature effects on growth are more 
pronounced in > small animals more than large, and LAGs would therefore 
be less defined in > larger animals, and would disappear entirely at 
some size.Â
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If any temperature has an influence on bone growth, then it's the 
temperature of the bone itself. Small mammals work harder tthan large 
ones at maintaining their body temperature constant and high -- but 
despite the higher cost they still succeed. Therefore we don't have any 
reason to expect more LAGs in small than in large mammals.Â
Â
However, despite this expectation, the largest mammals apparently 
_have_ fewer LAGs than smaller ones. So this must have another reason; 
I guess the speed of growth.Â
Â
> Due to the obvious seasonal effects on food intake of herbivorous 
animals,Â
Â
Good idea, but doesn't explain why elephants have fewer LAGs.Â
Â
> Also-- To change the subject slightly, would endotherms show a > 
proximal/distal limb bone LAG definition gradient?Â
Â
I'd expect that endotherms with long, slim legs living in a seasonal 
climate would show more LAGs in distal bones. That's because such legs 
have inbuilt heat exchangers: the distal parts, which contain tendons 
but more or less no muscles, approach ambient temperature, so I'd 
expect them to stop growing in winter, while the proximal parts don't 
even stick out from the body wall and approach body core temperature.Â
Â
However, this expectation, too, is wrong AFAIK. Â


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