Arthroclema sp. A

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

Arthroclema sp. A

( Fig. 14 A-C; Appendix 1)

MATERIAL EXAMINED. — CEGH-UNC 27525 a, CEGH-UNC 27528 b, CEGH-UNC 27529 b, CEGH-UNC 27530 e, f,CEGH-UNC 27533 c, CEGH-UNC 27538 c, CEGH-UNC 27539 a, e, f.

OCCURRENCE. — La Pola Formation, Upper Ordovician, Sandbian; La Pola creek section near Albardon village, San Juan Province, Argentine Precordillera, western Argentina.

DESCRIPTION

Ramose colonies with well defined median axis, articulated and consisting of cylindrical segments. Segments apparently flexibly connected, 0.47-1.03 mm in diameter, with 0.10- 0.26 mm wide exozones and 0.27-0.54 mm wide endozones. Autozooecia moderate in size, budding from the median axis at angles of 45-56°, bending gently to branch surface, triangular in cross-section in endozone, becoming oval in exozone. Diaphragms in autozooecia few to absent. Autozooecial apertures narrow, oval, arranged regularly in alternating rows on branch surface. Walls in endozone hyaline, 0.01-0.02 mm thick, continuing in exozone into the peristomes. Fine longitudinal striation between apertures present. Extrazooecial skeleton finely laminated, having well defined zooecial boundaries. Paurostyles abundant, irregularly arranged between autozooecial apertures, arising in the outermost exozone.

COMPARISON

Arthroclema sp. A is similar to Arthroclema striatum Ulrich, 1890 from the Middle Ordovician of United States, and to A. pulchellum Billings, 1865 the Middle Ordovician of Canada. It differs from the latter in larger autozooecial apertures (0.07- 0.10 mm vs 0.06-0.07 mm in A. pulchellum ; aperture width for A. pulchellum measured from Blake [1983: fig. 272g]).

Figures

FIG. 14. — A-C, Arthroclema sp.A:A, branch oblique thin section,CEGH-UNC 27530 e;B, C, tangential thin section showing autozooecial apertures and paurostyles, CEGH-UNC 27529 b; D-H, Trigonodictya elegans (Ulrich, 1893): D, E, tangential thin section showing autozooecial apertures, CEGH-UNC 27530 c; F-H, branch transverse thin section showing autozooecial chambers and mesotheca with rods,CEGH-UNC 27542 b.Scale bars: A, B, E, G, 0.5 mm; C, H, 0.2 mm;D, F, 1 mm.