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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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