Published online by Cambridge University Press: 20 May 2016
A distinctive spiral unilaminate cyclostome bryozoan, recently discovered in the Eocene Castle Hayne Limestone Formation of eastern North Carolina, is described as Crisidmonea archimediformis new species. Colonies have a thickened spiral axis bearing systems of bifurcating branches. When worn and stripped of their branches, the resistant axes closely resemble the familiar screws of the Carboniferous to Permian fenestrate bryozoan genus Archimedes. However, the Eocene screws are shorter and become more loosely coiled with growth, corresponding to a helicospiral rather than a helical form. The paleoenvironment inhabited by C. archimediformis colonies is inferred to have been on the shallow (< 30 m depth) margin of an open marine, waveswept embayment. Autozooid distribution and the gradient of increasing apertural size from branch reverse to frontal sides in C. archimediformis resemble those of the living cyclostome Exidmonea atlantica and suggest colonial water currents that flowed upwards and inwards through the spiral colony. Polyascosoecia, a genus closely related to Crisidmonea, is shown to be an objective junior synonym of Reteporidea, and the new genus Polyascoeciella (type species Idmonea foraminosa Reuss) is introduced for species with exterior-walled gonozooids that were previously assigned to Polyascosoecia.