Arthroclema Billings, 1865

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 : 586

Genus Arthroclema Billings, 1865

TYPE SPECIES. — Arthroclema pulchellum Billings, 1865 by original designation. Middle Ordovician, Trenton Limestone; Ottawa, Ontario, Canada .

DIAGNOSIS. — Colonies branched, with well defined axial stem and alternate secondary and tertiary branches. Sinuous or straight longitudinal ridges separating apertural rows often present. Axial region formed by well defined linear axis. Autozooecia attenuated to weakly inflated at their bases, having subtriangular cross-section in endozone, becoming elliptical in exozone, orientated at angles of 30-90° to colony surface. Diaphragms rare to absent. Exozonal wall material well developed, with narrow zooecial boundaries. Metazooecia absent.Paurostyles scattered to common, usually developed on ridges.

OCCURRENCE. — Middle to Upper Ordovician; North America, Europe.

COMPARISON

Arthroclema Billings, 1865 is similar to Helopora Hall in Silliman, Silliman & Dana, 1851, but differs from it in lacking metazooecia with diaphragms and acanthostyles. Arthroclema differs from Ulrichostylus Bassler, 1952 in wall structure with distinct zooecial boundaries.