Homotrypa vacua McKinney, 1971

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 : 574

Homotrypa vacua McKinney, 1971

( Fig. 7 D-I; Appendix 1)

Homotrypa vacua McKinney, 1971: 238-241 , pl. 50, figs 2-7. — Pushkin 1987: 186.

MATERIAL EXAMINED. — CEGH-UNC 27507 c, CEGH-UNC 27525 a, CEGH-UNC 27529 a, CEGH-UNC 27530 a, b, d-g, CEGH-UNC 27533 d, CEGH-UNC 27541 a, CEGH-UNC 27545 a.

OCCURRENCE. — Lower Chickamauga Group, Upper Ordovician, Sandbian; Alabama, United States. La Pola Formation, Upper Ordovician, Sandbian; La Pola creek section near Albardon village, San Juan Province, Argentine Precordillera, western Argentina.

DESCRIPTION

Ramose colonies, branch diameter 1.45-2.38 mm. Endozone 0.60-1.45 mm wide, exozone 0.25-0.75 mm wide, distinct. Secondary overgrowths not observed. Autozooecia long in the endozone, having larger diameters than in exozone, bending gently and intersecting branch surface at low angles. Autozooecial apertures rounded-polygonal. Autozooecial diaphragms rare to absent in the endozone, concentrated mostly in transitional region between endo- and exozone, common to abundant in outer exozone.Cystiphragms occurring throughout the exozone, occupying about the half of autozooecial diameter. Mesozooecia locally 3-5 surrounding each autozooecial aperture, otherwise rare; small, short, restricted to the outermost part of exozone, containing densely spaced diaphragms. Acanthostyles common, 2-5 surrounding each autozooecial aperture, moderately large, restricted to exozone. Autozooecial walls straight, displaying granular microstructure, 0.003 -0.005 mm thick in endozone; finely laminated with indistinct medial line, 0.023 -0.055 mm thick in exozone. Indistinct maculae consisting of macrozooecia.

COMPARISON

Homotrypa vacua McKinney, 1971 differs from H. mundula ( Ulrich, 1893) in having smaller colonies (branch diameter 1.45-2.38 mm vs 3.6-4.0 in H. mundula , measurements from Karklins [1984: 134]). Homotrypa vacua differs from H. subramosa Ulrich, 1886 in having thin branched colonies instead subramose and encrusting ones as well as in smaller autozooecial apertures (average aperture width 0.11 mm vs 0.16 mm in H. subramosa ).

Figures

FIG. 7. — A-C, Homotrypa subramosa Ulrich, 1886, CEGH-UNC 27537 b: A, B, longitudinal thin section showing autozooecial chambers with cystiphragms; C, tangential thin section showing autozooecial apertures,mesozooecia,and acanthostyles;D-I, Homotrypa vacuaMcKinney,1971: D, E, branch longitudinal thin section showing autozooecial chambers with cystiphragms, CEGH-UNC 27545 a; G, H, tangential thin section showing autozooecial apertures, mesozooecia, and acanthostyles, CEGH-UNC 27545 a; I, branch transverse section, CEGH-UNC 27507 b. Scale bars: A, D, E, G, I, 1 mm; B, C, F, H, 0.5 mm.