phaea did not evolve from flat oys- 
ters — H.B. Stenzel’s impressive mono- 
graph ( Treatise on Invertebrate Paleon- 
tology, Geological Society of America 
and University of Kansas Press, part N, 
vol. 3, 1971) has proved that the lin- 
eages of flat oysters (genus Ostrea) and 
coiled oysters (genus Gryphaea ) have 
been separate from their first appear- 
ance, that Gryphaea is a bit older than 
Ostrea, and that coiled ancestors for 
Trueman’s Gryphaea were living in 
Greenland before Ostrea ever appeared 
in England. 
For his second claim — that Gry- 
phaea did not increase in coiling during 
its evolution — I have reexamined all 
the published data and concluded that 
while Gryphaea did increase in body 
size, its degree of adult coiling re- 
mained constant. Ironically, this means 
that later Gryphaea were actually more 
loosely coiled than earlier specimens of 
the same body size, for coiling increases 
with growth, and if a larger adult de- 
scendant will reach the same intensity 
of coiling as its smaller adult ancestor, 
then the descendant’s shell must be 
more loosely coiled when it is still a ju- 
venile of the same size as its ancestral 
adult. (I have reprinted all major pa- 
pers from the great Gryphaea debate in 
a volume entitled: The Evolution of 
Gryphaea, Arno Press, 1980.) 
If Trueman’s edifice toppled so easi- 
ly, why was his “fact” accepted so 
readily in the first place? One might 
suspect that the overcoiling of Gry- 
phaea had received lengthy and com- 
plex documentation, however unreli- 
able it proved to be, and that Trueman 
simply snowed potential critics. Not at 
all. The original evidence was flimsy al- 
most beyond belief, and this, to me, is 
the most curious aspect of the entire 
tale. In his 1922 paper, Trueman 
showed how most large adult Gryphaea 
avoid calamitous overcoiling by de- 
creasing the tightness of their spiral late 
in growth. 
Trueman’s claim for overcoiling was 
based on a single specimen, the “type” 
(or name-bearing) specimen of the spe- 
cies Gryphaea incurva. I examined this 
specimen at the British Museum in 
1971. At first glance, the coiled valve 
does seem to press hard upon the flat 
valve. But the specimen was recovered 
from very fine-grained muds of the 
same color as the shell itself. A bit of 
probing revealed — and X-ray photo- 
graphs later confirmed — that the sup- 
posed “lock” upon the flat valve had 
not been formed by the coiled valve it- 
self, but by mud that wedged its way 
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