A GEOMETRIC MORPHOMETRIC STUDY OF SILURIAN AND DEVONIAN BRACHIOPOD SUBFAMILIES OF THE ATRYPIDA ORDER (ATRYPINAE, VARIATRYPINAE AND SPINATRYPINAE) FROM EASTERN NORTH AMERICA

Copper’s phylogeny and taxonomy of atrypides has not been tested in terms of morphometric shape. Here, we investigated external shell shape variation in brachiopod morphology at the subfamily and generic level using geometric morphometrics. We measured valve shape in 1600 atrypide individuals from Silurian-Devonian time intervals from 8 EE subunits (Clinton, Lockport, Keyser, Helderberg, Schoharie, Onondaga, Hamilton and Genesee) from 14 geographic localities in eastern North America. The following representatives of the Atrypida were included in the morphometric analyses: Atrypa, Gotatrypa, Kyrtatrypa, Oglupes, Joviatrypa, Endrea, Dihelictera (Atrypinae); Pseudoatrypa (Variatrypinae) and Spinatrypa (Spinatrypinae).

In this study, we used 8 external shell landmarks to analyze shape. Multivariate and discriminant function analysis were used to determine shape differences between genera and subfamilies along successive time planes while Euclidean based cluster analysis was used to calculate pairwise distances between genera.

Using these data, 4 hypotheses were tested. (1) If individuals in the Atrypinae, Variatrypinae and Spinatrypinae subfamilies are morphologically distinct, then we would expect these subfamilies to show significant differences between them. MANOVA and DFA confirmed shape differences between these subfamilies (p<0.01); (2) If shell shape varies from one taxon to another, then we would expect morphometric differences between these taxa. MANOVA showed significant shape differences between taxa within the Atrypinae subfamily (p<0.01); (3) If genera are correctly referred to subfamilies, then we expect greater morphological distance between taxa from two subfamilies than from within one subfamily. Procrustes analysis shows greater morphological distance between taxa from distinct subfamilies than from taxa within subfamilies; (4) If atrypide samples from 6 time intervals have evolved phenotypically, then we would expect significant morphological differences between them. MANOVA showed significant shape differences between different time horizons (p<0.01) suggesting samples are morphologically distinct between time intervals. Thus, morphometrics supports Copper’s atrypide phylogenetic patterns and taxonomic arrangement.