VARIATION IN THE GENUS SPINATRYPA (BRACHIOPODA, ATRYPIDINA): TERATOLOGY, HYPERTROPHISM OR SPECIATION?

The brachiopod genus Spinatrypa Stainbrook, 1951, is abundant in Upper Devonian rocks of the Appalachian basin. It is easily recognizable and is biostratigraphically important as its disappearance is taken as a marker for the Late Devonian Frasnian-Famennian extinction event. Two species are common, Spinatrypa spinosa (Hall, 1843) and Spinatrypa hystrix (Hall, 1843). Recent investigations have revealed unusual variations in S. hystrix close to the Frasnian-Famennian boundary. The variations include an increase in overall size, a change from a circular to a distinctly triangular valve shape, and the presence of a distinct fold and sulcus not seen in typical forms.

This morphology has so far only been seen within a small interval of the Blizzard Member of the Foreknobs Formation. The Blizzard Member is known for the abundance and diversity of its fossils, however these forms appear to be unique. The unusual morphology could be the result of one of three factors. First, they could represent teratological specimens (biological monstrosities) of S. hystrix. Second, they could represent hypertrophic forms of S. hystrix (results of excessive growth sometimes seen in optimal ecological conditions). Third, they could represent a distinct new species directly descended from S. hystrix. The first two scenarios would provide little biostratigraphic information since the variation is very localized. The third possibility is more intriguing. It would imply a very short-lived species (probably less than 100,000 years) which itself became extinct before the other atrypids. The apparent endemic nature of the new morphology may restrict its biostratigraphic usefulness, but it may also signify the last speciation of Spinatrypa in the Appalachian basin prior to the termination of the lineage during the Frasnian-Famennian extinction.