Dimorphomicropora Ducasse & Vigneaux, 1960

Taylor, Paul D. & Villier, Loïc, 2022, Cretaceous microporid cheilostome bryozoans from the Campanian historical stratotype of southwest France, Geodiversitas 44 (18), pp. 515-525 : 521

Genus Dimorphomicropora Ducasse & Vigneaux, 1960

TYPE SPECIES. — Dimorphomicropora voigti Ducasse & Vigneaux, 1960 , by original designation.

OTHER SPECIES. — Dimorphomicropora crestulata ( Ducasse, 1958) (described below); Dimorphomicropora rugica ( Marsson, 1887) , Lower Maastrichtian, Rügen, Germany, and Upper Maastrichtian of Maastricht, Netherlands ( Voigt 1975); Dimorphomicropora transversa (d’Orbigny, 1851) , Lower Maastrichtian, Néhou, Contentin, France.

REVISED DIAGNOSIS. — Microporid with erect colonies, narrow cylindrical branches lacking complete bifurcations, colonies probably articulated; autozooids lacking a gymnocyst, cryptocyst depressed centrally, pierced by 2 large opesiules; orifice subcircular to dummy shaped with a raised rim; no spines or tubercles; ovicells clustered resulting in branch dilation, ooecia helmet-shaped, continuous with cryptocysts of the distal zooids, orifices of fertile zooids enlarged; mandibulate polymorphs in some species, infrequent, larger than autozooids but with similar frontal wall and opesiules, orifice semielliptical, longer than wide.

REMARKS

The name Dimorphomicropora refers to the zooidal dimorphism evident in the type species of this genus. In addition to normal autozooids (“Cellules A” of Ducasse & Vigneaux 1960), there are occasional polymorphic zooids (“Cellules B” of Ducasse & Vigneaux 1960) with an enlarged orifice and presumably an enlarged operculum or mandible. These mandibulate polymorphs resemble the B-zooids found in Steginoporella . Like the B-zooids of Steginoporella , it seems possible that they had the capacity to feed.

The existence of mandibulate polymorphs in Dimorphomicropora has led to the suggestion that this Cretaceous genus belongs to the Cenozoic family Steginoporellidae . However, Ostrovsky (2013: 329) believed Dimorphomicropora to be probably an unrelated homeomorph of Steginoporella and excluded it from his superorder Thalamoporellina to which Steginoporella is assigned. Ducasse & Vigneaux (1960) hypothesised that D. voigti might be an evolutionary link between Micropora rugica [since assigned to Dimorphomicropora by Voigt 1975] and Steginoporellidae . At the time of their publication, this was consistent with the stratigraphy because the type locality (Meschers, Charente-Maritime) of D. voigti was considered to be of middle Maastrichtian age, whereas D. rugica was known only from the early Maastrichtian. However, the re-dating of Meschers as Campanian makes the order of occurrence of these two species incongruent with this evolutionary hypothesis.

Voigt (1975) misunderstood the nature of the dimorphism described in Dimorphomicropora by Ducasse & Vigneaux (1960), interpreting it to refer to the contrast between nonbrooding and brooding ovicellate autozooids. However, the original description of Dimorphomicropora makes no mention of brooding zooids or ovicells. The type specimen of D. crustulenta ( Ducasse, 1958) is ovicellate ( Fig. 4A ), as is a poorly preserved specimen of D. voigti ( Fig. 3H ).

Figures

FIG. 4. — Dimorphomicropora crestulata (Ducasse, 1958): A, holotype (Ducasse 1958: pl. 7, fig. 3), UB C.B.917 Ducasse Collection, probably Barbezieux Fm (foram biozone CVI), RN 730, close to police station, Mirambeau, Charente-Maritime, dilated branch with ovicellate zooids; B, C, NHMUK PI BZ 8271, Aubeterre Fm,cliff north of town,Meschers,Charente-Maritime;B, well-preserved branch;C, detail of autozooids;D, NHMUK PI BZ 8272, Aubeterre Fm,cliff north of town, Meschers, branch. Scale bars: A, 500 μm; B, C, D, 200 μm.

FIG. 3. — Dimorphomicropora voigti Ducasse & Vigneaux,1960: A, lectotype, UB C.B.993 (Ducasse & Vigneaux 1960: fig. 1), probably Aubeterre Fm, Meschers, Charente-Maritime, branch with large polymorphic zooid near base; B, E, F, NHMUK PI BZ 8767, Aubeterre Fm, Pointe de Suzac, Meschers; B, branch; E, autozooids near base of branch; F, detail of an orifice; C, G, NHMUK PI BZ 8768, Aubeterre Fm, Pointe de Suzac, Meschers; C, branch with a large polymorphic zooid near the centre; G, detail of polymorph; D, paralectotype, UB C.B.994 (Ducasse & Vigneaux 1960: fig. 2), probably Aubeterre Fm, Meschers, Charente- Maritime; H, NHMUK PI BZ 8769, Aubeterre Fm, Pointe de Suzac, Meschers, Charente-Maritime, dilated branch with a cluster of ovicellate zooids. Scale bars: A, B, C, D, H, 500 μm; E, 200 μm; F, G, 100 μm.