Nicholsonella Ulrich, 1890

Ernst, Andrej & Carrera, Marcelo G., 2022, A cool-water bryozoan association from the La Pola Formation (Sandbian, Ordovician) of Argentine Precordillera, Geodiversitas 44 (20), pp. 563-601 : 582

Genus Nicholsonella Ulrich, 1890

TYPE SPECIES. — Nicholsonella ponderosa Ulrich, 1890 by original designation,Trentonian, Sandbian, Upper Ordovician, North America.

DIAGNOSIS. — Encrusting, frondose, ramose, less commonly massive colonies. Apertures rounded and irregularly petaloid. Walls structureless, very thin, irregularly thickened in different part of colonies. Diaphragms usually abundant in whole colony, more rarely – only in the exozone. Mesozooecia abundant, containing frequent diaphragms, sometimes beaded, irregularly closed by calcitic material on the colony surface. Acanthostyles usually small, abundant, short, restricted to the outermost exozone.

OCCURRENCE. — Lower Ordovician to Lower Silurian of North America and Siberia.

COMPARISON

Nicholsonella Ulrich, 1890 and some other genera (e.g. Dianulites ) are unique among trepostome bryozoans by their re-crystallized walls which suggest a diagenetically unstable aragonitic ( McKinney 1971) or high Mg calcite composition ( Taylor & Wilson 1999).