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Re: Comments on _Longisquama_ in _Science_.

Eric Lurio wrote:

"Several species of mackerel shark are (endothermic) and so are the 
billfish. That's when I began to doubt a lot of characterizations like 
"sharks are primitive and haven't changed in hundreds of millions of 
years." The Great White is the most advanced thing with gills."

Endothermy in tuna, billfish and lamnid sharks (e.g. great whites, 
porbeagles and makos) evolved independently (Block, B.A., and J.R. 
Finnerty 1994 Environmental Biology of Fishes 40: 283-302). Tuna and 
lamnid sharks are systemic endotherms (the whole body temperature is 
raised), while billfish heat only the eyes and brain by the action of 
specialized "heater" eye muscles. Thus endothermy in billfish and great 
whites is functionally quite different.


Kendall Clements