Cupulocrinus angustatus (Meek and Worthen, 1870), a cladid crinoid from the Upper Ordovician Maquoketa Formation of the northern midcontinent of the United States

Abstract

Cupulocrinus angustatus (Meek and Worthen, 1870) is common and widely distributed in the Maquoketa Formation of the northern midcontinent of the United States, and specimens are known from the Isotelus and Vogdesia Zones of the Elgin, the Clermont, Fort Atkinson, and Brainard Members. Cluster significance tests indicate that crinoids from all stratigraphic horizons are conspecific. The most numerous primibrachs are located in the A and B rays, whereas the C ray exhibits the fewest plates. The largest and smallest numbers of secundibrachs occur in the B and C rays, respectively. The number of brachs is independent of stratigraphic position and the size of the crinoids. Correlation coefficients for the numbers of brachs demonstrate that the arms are divided into two overlapping and covarying levels: the proximal arms from the primibrachs to tertibrachs, and distal arms ranging from the tertibrachs to quintibrachs. Growth of the aboral cup is generally isometric or roughly so. Conversely, the width:height ratios of brachs typically increase in progressively larger individuals. Similarly, most deposition of calcite on the columnals affects their width rather than height. The correlations for the aboral cup and its plates generally exceed those of brachs and stem plates. The contrasts in allometry and integration and coordination between the aboral cup versus the brachs and column are attributed to differences in basic geometry and developmental constraints. Similar patterns are seen in other Paleozoic and perhaps all or most crinoids.