Brachiopod spines have been interpreted as a mechanism that enables the organism to rest on soft substrates by distributing shell mass over a greater area. This hypothesis is based on an arrangement in which spines are long, strongly curved, few in number, and generally restricted to the pedicle valve. However, other spine geometries also are prevalent, and the assumption that all arrangements functioned similarly may be unwarranted. Spinatrypa and Praewaagenoconcha, an atrypide and a productide, respectively, are genera of spinose brachiopods from the Upper Devonian Java Formation of New York. Although these genera belong to different subclasses, they have a similar spine geometry consisting of hundreds of thin, short, sub-perpendicular (to shell surface) spines, arranged concentrically on both valves.
Population densities of articulate brachiopods were obtained from six localities of the Java Formation along an offshore-onshore gradient. Individuals were counted relative to surface area of the bedding planes. There are moderate, but highly significant, correlations between grain size and abundance of each spinose taxon, and the greatest mean densities of both Praewaagenoconcha and Spinatrypa occur in coarse sandstones. Although both genera have broad environmental ranges, peak mean abundances of Praewaagenoconcha and Spinatrypa are in platform margin and outer platform facies. All of these distributions are statistically distinguishable (χ 2 test, p < 0.0001) from the distribution of non-spinose taxa within the Java Formation.
These results are inconsistent with the hypothesis that all spine geometries served as soft substrate supports. Alternative functional hypotheses for this spine geometry include (1) anchorage in high energy habitats, and/or (2) protection from predation.