Punctal density in the Ordovician orthide brachiopod Paucicrura rogata : anatomical and paleoenvironmental variation

Abstract

In the orthide brachiopod Paucicrura rogata, from the Upper Ordovician of New York, the density of punctae (small perforations in the shell) increases from about 350 to about 450 per mm2 of shell in the transition from shallow-water limestones to black graptolitic shales down the outer slope of the Taconic Trench. Punctal density is significantly correlated with water depth, as measured by distance downslope along individual volcanic ash layers, and by reciprocal averaging score for macroinvertebrate fossil assemblages (p < O.01). Because punctal density increases with temperature in living brachiopods, the results suggest that water temperature increased with depth in the Taconic basin, corroborating similar conclusions based on Paucicrura's oxygen-isotopic composition. The increase in punctal density with increasing water depth may also reflect reduced shell growth rates of P. rogata in oxygen-deficient bottom waters, causing a reduction in spacing between punctae. Ontogenetic decrease of punctal density with increasing shell size reflects differential rates of puncta formation at the shell margin. □Brachiopoda, Orthida, Ordovician, Taconic Basin, paleoceanography, paleoecology, punctae, caeca.