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



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
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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
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Due to the obvious seasonal effects on food intake of herbivorous
animals,Â
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Good idea, but doesn't explain why elephants have fewer LAGs.Â
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Also-- To change the subject slightly, would endotherms show a >
proximal/distal limb bone LAG definition gradient?Â
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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
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However, this expectation, too, is wrong AFAIK. Â



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