Re: arachnid evolution (posted for P. Manning)

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From: "P.L.Manning" <P.L.Manning@sheffield.ac.uk>
To: paleonet-owner@nhm.ac.uk
Date: Fri, 20 Oct 1995 10:15:26 +0100
Subject: Re: arachnid evolution
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}."Arachnids which came first book-lungs or book-gills"
are there any discussions or relevant reviews on the above topic?{

Yes ! Lots of discussions have been versed on this subject. The best
review papers are by Andy Jeram, Paul Selden, Jason Dunlop and a
minor attempt by myself !
Andy and Paul have been publishing on the subject since circa. 1989
in the Trans. Roy. Soc. Edin. Earth Sciences. While Jason and I have
written a recent article in Palaeontology (Vol. 38, pt 2) in which we
describe the 'Respiratory organs of eurypterids'. In this last
article we did mention the evolutionary  implications of book-gills
and book-lungs.
I am quite 'sure' that the book-gill developed first. In time the book-gill
 may have evolved possible struts between
individual lamellae (as seen in modern arachnod book-lungs)
to support the respiratory surafceswhen out of water. This could
have been the pre-cursor for the 'true' book-lung, allowing a more
amphibious mode of life. It is interesting to watch modern Limulids
'cavorting' on the beaches during certain times of year, with their
'primitive' book-gills allowing some degree of (forced) aerial respiration.
According to Prof. Bill Shear, he has observed some species of modern
Xiphosuran surviving for several hours out of water !! However the
level of oxygen in the blood may allow a far degree of cavorting
time!
It is interesting, albeit crual, to observe living terrestrial
scorpions submerged in water. Their book-lungs seem to allow a
degree of tolerance to submergence, however the spiracle to the lung
may well be reduced or closed in such conditions !
I have been fortunate enough to look at the Rhynie Chert arachnids,
 whose book-lungs have been preserved in 3-D.
These specimens are certainly worth a peek at if you get the chance.
The best person to speak on these beasties is Dr. Jason
Dunlop at the University of Manchester, Department of Geology.
Sorry to have rattled on for so long !!
Phil Manning