MARGINALIA
A Palaeontological Puzzle Solved?
Keith Thomson
Something New Out of Achanarras
Palaeospondylus gunni, so named by Ramsay Heatland Traquair
of Edinburgh in 1890 (after its discoverers and prominent backbone),
is abundant at Achanarras Quarry but is found nowhere else except
for two nearby localities where a very few additional specimens have
turned up. Not only is it minuscule in size, but its structure,
insofar as one can make it out, seems unlike that of any known
creature, living or fossil. It has a strange basket-like apparatus
on its snout but no teeth, a well-developed vertebral column but no
fins. In a 1992 Marginalia essay in which I reviewed the
sadly inconclusive story of this little fossil, my own meager
contribution was an allometric study of body proportions showing
that it was a larval form. But its anatomy and taxonomic position
were unresolved.
Oddly enough, the possibility that Palaeospondylus is a
larva had been mooted early in the case. Huxley, in famously having
nothing to do with it, dismissed it as a "baby
Coccosteus" (a placoderm arthrodire, one of those
weird extinct forms with a large head articulated onto massive neck
armor). Dean himself noted the change of head-to-body proportions
with size so typical of immature forms of vertebrates. J. W. Dawson
(principal of McGill University) thought it might be a
"primitive tadpole." Professor Graham Kerr of Glasgow
suggested that it was a larval lungfish. However, interpretation as
a larva was not taken seriously again until 1980, when Peter Forey
and Brian Gardiner at the Natural History Museum in London repeated
the suggestion. Now a new study shows that Palaeospondylus
is definitely a larval lungfish, and we can even be pretty sure that
the corresponding adult was the famous Dipterus
valenciennesi, equally abundant at Achanarras. It is
instructive to examine why it has taken so long to get this far.
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