Re: Fw: synapsids are reptiles

["Steve Brusatte" <dinoland@lycos.com>]

On Sun, 7 Apr 2002 20:41:00   
 arisi wrote:
>Steve Brusatte 3 April wrote:
>"Everything I've read has stated what Dr. Holtz posted back  in 1995: that 
>Dimetrodon (and other early synapsids) likely had poor color vision (along 
>with glandular skin etc.)
>
>At the reliable site www.ucmp.berkeley.edu/synapsids/pelycosaurs.html
>I have read the following: 
>"It is believed that pelycosaurs, like their living mammal relatives, were 
>endothermic, which means that they maintained a constant internal body 
>temperature. This is another characteristic that sets pelycosaurs apart from 
>reptiles".
>It would be interesting to know who really believe it and on which evidence. 

Yeah, I'm also interested in what evidence was used to come up with this 
statement.  True, many of the more derived sphenacodonts were becoming 
increasingly mammal-like, but it is difficult to say that they were 
endothermic.  In addition, I do not know of any evidence for respiratory 
turbinates in pelycosaurs (there is some evidence, IIRC, for RTs in cynodonts 
and therocephalians).

>Perhaps the Dimetrodon sail? Maybe the sail favorites the temperature control 
>in an ectothermic body, but endothermy implicate high metabolic rate and is 
>totally another thing. Not to mention the other pelycosaurs that are 
>completely without sail. It is true that some pelycosaurs have slender limbs 
>but the position is always sprawling.

Yep, the majority of pelycosaurs did not possess sails.  For example, the 
caseid _Cotylorhynchus_ (a really neat looking animal with a snout that 
"hooked" down; many of you may recognize it from paleo textbooks) had what has 
been reconstructed as a large hump.  I'm not certain how the evidence for this 
hump differs from the fossilized evidence used to reconstruct spines and sails 
on _Dimetrodon_, though.  

Like I've mentioned onlist before, there was a nice paper written that 
describes the rugose neural spines of _Edaphosaurus_ as having a heat-dumping 
function.  It is interesting to think of _Edaphosaurus_ as a possibly 
endothermic animal that used its sail to dump heat (like the ears of 
elephants), but this doesn't necessarily have to be the case.  As a herbivore, 
_Edaphosaurus_, regardless of its thermoregulatory behaviors, may have needed 
to dump excess heat generated by its hindgut fermentation mechanism.  This need 
to dump herbivory-caused heat may be responsible for the differences between 
the sails of _Dimetrodon_ and _Edaphosaurus_ (smooth spines vs. blistery and 
rugose ones, with the blistery ones apparently being able to shed heat better 
in a wind tunnel).  

AFAIK, the majority of pelycosaur fossils have been found in North America and 
Asia (Russia), areas that were near the paleoequator during the Permian.  This 
_may_ mean that the distribution of pelycosaurs was limited to warmer areas, 
although, because this relies on the absence of fossils to test, the idea is 
difficult to support conclusively.  

So, I wouldn't necessarily say that pelycosaurs were endotherms.  There isn't 
enough evidence...yet.

Steve


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