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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.