Stomatopora dichotomoides (d’Orbigny, 1850)

Hara, Urszula & Taylor, Paul D., 2009, Cyclostome bryozoans from the Kimmeridgian (Upper Jurassic) of Poland, Geodiversitas 31 (3), pp. 555-575 : 559-561

Stomatopora dichotomoides (d’Orbigny, 1850) ( Fig. 3 )

Alecto dichotomoides d’Orbigny, 1850: 288 .

Stomatopora dichotomoides – Haime 1854: 163, 164, pl. 6, fig. 2a-c. — Gregory 1896b: 50, pl. 1, fig. 3. — Illies 1963: 73, pl. 8, figs 1, 2, pl. 9, figs 1-3. — Walter 1970: 39, pl. 1, fig. 9.

MATERIAL EXAMINED. — Wierzbica Quarry: MUZ PIG 1719.II.1; NHM BZ 5511, BZ 5512(1). — Skorków Lumachelle, Małogoszcz Quarry: NHM BZ 5518(1), BZ 5519(2), BZ 5520(2).

OCCURRENCE. — Lower Kimmeridgian, Wierzbica and Małogoszcz quarries, Holy Cross Mts, Poland. Recorded by Walter (1970) from the Upper Aalenian to Upper Bathonian of France, England and Germany (see also Illies 1963), and by Taylor (2008) fromthe Upper Bathonian/Lower Callovian of Poland.

MEASUREMENTS. — Transverseaperturaldiameter = 0.08-0.10 mm; transverse peristomial diameter = 0.16- 0.20 mm; frontal wall length = 0.62-0.80 mm; frontal wall width (proximal) = 0.20 mm; frontal wall width (distal) = 029- 0.32 mm.

DESCRIPTION

Colony encrusting withuniserial branches bifurcating dichotomously; internodes comprising one to four zooids. Ancestrula short (0.4 mm), aperture 0.05 mm in diameter, protoecium of low profile, 0.24 mm wide with scattered pseudopores. Single budded zooid before first branch bifurcation at c. 140°, subsequent bifurcations at lower angles, those in late astogeny as little as 60°, the daughter zooids remaining conjoined for half of more of their lengths before separating.

Zooids monomorphic, elongate, subcylindrical. Apertures subcircular, set atop short preserved peristomes. Pseudopores drop-shaped, pointed distally, about 10 µm wide.

REMARKS

The genus Stomatopora remains problematical because of the loss of the type material of the type species, S. dichotoma , and the failure of Walter’s (1970) neotype to match the generally applied concept of the genus as a strictly uniserial encruster (see Pitt & Taylor 1990; Taylor & Wilson 1999). Unfortunately, Stomatopora lacks the basal gonozooids that are so useful in the taxonomy of other Mesozoic cyclostomes.

Species-level taxonomy of Jurassic Stomatopora was advanced greatly by the detailed studies of zooidal budding undertaken by Illies (1963). Nevertheless, it is becoming evident from SEM study of pristine specimens that some apparent “species” may in fact comprise more than one species differing in the detailed morphology of their pseudopores. Th e Polish Kimmeridgian specimens assigned here to S. dichotomoides have drop-shaped pseudopores ( Fig. 3F ), whereas otherwise identical specimens fromthe British Bathonian (e.g., NHM D13428) often possess long, slit-shaped pseudopores. Unfortunately, pseudopore morphology is unknown in the French Bajocian type material of S. dichotomoides .

As here interpreted, S. dichotomoides is a species common in the Middle Jurassic that develops more or less regular colonies of roughly circular outline. The radially spreadingform is achieved bya pattern of astogenetic decrease in branch bifurcation angle ( Gardiner & Taylor 1982). One or two zooids are typically present ineach internode inmost colonies of Middle Jurassic S. dichotomoides but someof the Polish Kimmeridgian colonies can have up to four zooids in some internodes ( Fig. 3D ).

Figures

FIG. 3. — Stomatopora dichotomoides (d’Orbigny, 1850), Lower Kimmeridgian, Wierzbica Quarry, NE margin of the Holy Cross Mts, Poland:A, B, NHM BZ 5511;A, intergrown colonies;B, ancestrula (protoecium arrowed) and first budded zooid;C-F, NHM BZ 5511(1); C, low angled bifurcations from late astogeny; D, long internode; E, single zooid; F, pseudopores in the same zooid. Scale bars: A, C, D, 1 mm; B, E, 200 μm; F, 100 μm.