Dibunostoma reversum ( Harmer, 1926 )

Dick, Matthew H., Ngai, Nguyen Danh & Doan, Hung Dinh, 2020, Taxonomy and diversity of coelobite bryozoans from drift coral cobbles on Co To Island, northern Vietnam, Zootaxa 4747 (2), pp. 201-252 : 205-209

Dibunostoma reversum ( Harmer, 1926)

( Fig. 3 )

Thalamoporella rozieri form falcifera: Thornely 1905, p. 112.

Calpensia reversa Harmer, 1926, p. 309 , pl. 20, figs 13–16.

Dibunostoma reversum: Cheetham 1963, p. 53 , text-fig. 27a; Gordon et al. 2007, p. 49, fig. 2B.

Thalamotreptos reversus: Soule et al. 1991a, p. 460 , pl. 4, fig 5.

Dibunostoma reversa: Badve & Sonar 1997, p. 36 , pl. 1, fig. 2, pl. 4, fig. 1.

? Thalamoporella transversa Guha & Krishna, 2004, p. 29 , figs 26–27.

Not Dibunostoma reversum: Yang et al. 2018, p. 495 , fig. 1.

Material examined. VNMN-0213 (CT-26), VNMN-0273 (CT-49), VNMN-0274 (CT-50), VNMN-0275 (CT-51), on SEM stubs; VNMN-0214, five fragments.

Measurements. AzL, 0.55–0.79 (0.654 ± 0.073); AzW, 0.30–0.41 (0.356 ± 0.033); OrL, 0.082 –0.107 (0.094 ± 0.007); OrW, 0.10–0.13 (0.114 ± 0.011) (n = 15, 1). AvL × AvW, 0.192 × 0.189, 0.225 × 0.175 (average = 0.209 × 0.182); AvRL × AvRW, 0.211 × 0.063, 0.197 × 0.065 (average = 0.204 × 0.064) (n = 2, 2).

Description. Colony encrusting, sheet-like, mostly unilaminar but self-overgrowth occurring in some parts of colony.

Zooids rectangular, ovoid, spindle-shaped or irregularly polygonal; distinct, delineated by thin incision in raised marginal gymnocysts. Cryptocyst perforated by small, infundibular pseudopores proximal to level of opesiules; surface between pseudopores faintly pustulose, irregular. Frontal wall scarcely depressed below level of marginal rim proximally, markedly depressed proximal to opesia-orifice in vicinity of opesiules; rising sharply, sometimes nearly vertically, from opesiules to orifice. Smooth gymnocystal calcification surrounding orifice laterally and distally; within gymnocystal region and lateral to orifice on each side is pronounced, smooth, conical lateral-oral tubercle. Opesia-orifice D-shaped, on average slightly broader than long; with low, narrow, smooth, rim raised around periphery laterally and distally; proximal margin straight, without rim. Narrow shelf around curvature of distal orificial wall, just inside orifice. Oral spines lacking. Opesiules irregularly circular or irregular, variable in size, their combined area sometimes less than area of orifice, sometimes nearly twice as great; set in from lateral margins.

Avicularia interzooidal; uncommon, with only two found among hundreds of zooids in six colony fragments; ratio of average avicularium length (n = 2) to average autozooid length (n = 15), 0.32; chamber nearly square or long-rectangular, surrounded by same raised marginal rim as autozooids, imperforate; rostrum raised, smooth, with proximal end at center of chamber, mandibular end acutely long-triangular, of variable width, directed across proximolateral corner of chamber toward base of sister zooid, crossing and abutting distal orificial margin. Kenozooids common, ranging in size from much smaller than to as large as autozooids; surface typically smooth, set below raised marginal rim, with small, circular, central opening, sometimes additionally with a few tiny pseudopores. Autozooidal orifice can similarly have closure plate with circular central opening; orifices in this condition can give rise to two or three distal zooids, which in turn continue to bud distally, producing wedge-shaped zone of self-overgrowth ( Fig. 3A, C ), forming frontal layer.

Ooecia not detected.

Remarks. The generic history of this species is convoluted. Harmer (1926) originally described it in Calpensia Jullien, 1888 , but had to amend the diagnosis of that genus to include opesiules descending to the basal wall, paired lateral-oral tubercles, and avicularia. Supporting placement in Calpensiidae , he found no trace of the internal spicules that are one of the defining features of Thalamoporellidae . Cheetham (1963) erected the new genus Dibunostoma in Calpensiidae (now synonymized with Microporidae ) to accommodate Calpensia -like species having vicarious avicularia, lateral-oral tubercles, large opesiules, and a straight proximal margin of the orifice. Cheetham noted that Dibunostoma is intermediate between Calpensia and Thalamoporella , but differs from the latter in having a straight proximal orificial margin and in lacking polypide tubes. Soule et al. (1991a) reexamined Thornely’s (1905) specimen ( Thalamoporella rozieri form falcifera), upon which Harmer (1926) had based his original description of Calpensia reversa , and detected compass spicules, which disqualified this species from Calpensia and Microporidae . Furthermore, they noted that several species of Thalamoporella have a straight proximal orificial margin. In addition, Harmer’s (1926) illustrations of C. reversa clearly show the basal insertions of calcified structures (presumably contributing to a polypide tube) descending from the opesial openings. It is thus now clear that Dibunostoma belongs in Thalamoporellidae rather than Calpensiidae (= Microporidae ).

Soule et al. (1991a) established the genus Thalamotreptos for thalamoporellids having one or both opesiules extending to the basal wall, the avicularian rostrum pointing proximally or proximolaterally and occupying most of the frontal surface of the chamber, and compasses as the only spicules present. However, it was not clear what distinguishes Thalamotrepos from Dibunostoma , and the former is now treated as a junior synonym of the latter. Dibunostoma , as it now stands, is characterized by a D-shaped orifice with a straight proximal margin; lateral-oral tubercles; basal insertions of one or both opesiules; the presence of only compass spicules; and vicarious (interzooidal) avicularia with the mandible pointing proximally or proximolaterally. Dibunostoma shares the first four of these features with some or many Thalamoporella species, which leaves only the nature of the avicularium defining the genus. Whether this is enough to ultimately justify a genus distinct from Thalamoporella is an open question (see Soule et al. 1992, p. 7). Interestingly, ooecia are unknown in Dibunostoma verrilli ( Soule & Soule, 1970) , D. expansa (Levinson, 1909) , and D. reversum , the living species presently known (three other species are known only from the fossil record), and this might prove to be a diagnostic generic character.

Guha & Krishna (2004) reported Thalamoporella transversa from the early Miocene (Aquitanian); that species is so similar to D. reversum that if it had been found in a Recent fauna, the two might well be considered conspecific. Zooid measurements are more similar between Miocene T. transversa (average ZL × ZW, 0.635 × 0.378 mm) and Recent D. reversum from Co To (0.654 × 0.356 mm) than between either of these and Recent D. reversum from Bangladesh (ZL range, 0.45–0.62 mm; ZW range, 0.23–0.36; Gordon et al. 2007). The orifice in T. transversa is as broad as long, whereas it is broader than long in D. reversum at Co To. Gordon et al. (2007) did not adequately describe the orifice of D. reversum from Bangladesh, but it appears to be more like that in T. transversa than that in D. reversum from Co To. Thalamoporella transversa differs from Holocene–Recent D. reversum in having many (but not all) autozooids narrowing distally, appearing somewhat flask-shaped, and in having the mandible of the avicularium oriented perpendicular to the long axis of the next-proximal autozooid, rather than pointing proximolaterally. The large, autozooid-sized kenozooids with a small, circular opening, which were common in the Co To material and occurred in Holocene material from western India ( Badve & Sonar 1997), were not reported in T. transversa ( Guha & Krishna 2004) , nor did Gordon et al. (2007) report them in Recent D. reversum from Bangladesh.

While T. transversa and D. reversum may represent a single, non-branching phyletic lineage from Miocene to Recent, we suggest retaining different names for the Miocene and Holocene–Recent material, based on the slight differences between them and the epistemological difficulties inherent in demonstrating conspecificity between Miocene and Recent populations. However, T. transversa clearly belongs in Dibunostoma rather than Thalamoporella , and we propose a nomenclatural change to Dibunostoma transversum . Finally. we note that Badve & Sonar (1997), in their summary of the distribution of D. reversum but not in a corresponding table, mistakenly attributed a geological time range for D. reversum from Eocene to Recent, based on Cheetham (1963). However, Cheetham illustrated Recent D. reversum but Eocene D. purii Cheetham, 1963 , when he erected Dibunostoma to accommodate those two temporally disjunct species.

Yang et al. (2018) reported Dibunostoma reversum from Jeju Island, in the East China Sea off southern Korea. However, their material differs from other reports of both living and fossil D. reversum in several ways. All measurements in the Korean material are markedly larger (e.g. average AzL = 0.895 mm, AzW = 0.472 mm), and the peristomial region distal to the orifice is conspicuously expanded and flared. The interzooidal avicularia are different; the rostrum occupies the entire frontal surface of the chamber; the opesial opening is enlarged; and the mandible is short and wide, forming an equilateral triangle directed perpendicularly toward the orifice and indenting the contour of the orificial rim, rather than being markedly elongate and extending laterally along the distal margin of the rim. We thus conclude that the Korean material is not conspecific with D. reversum and requires description as a new species.

Distribution. Dibunostoma reversum was originally described from Sri Lanka ( Ceylon) ( Harmer 1926), and Gordon et al. (2007) reported it from coastal Bangladesh. Co To is the third known locality for Recent material, extending the known range from the western Indian Ocean eastward to Vietnam. In the fossil record, it is known from Holocene deposits along the western coast of India ( Badve & Sonar 1997).

Figures

FIGURE 3. Dibunostoma reversum (Harmer, 1926). A. Colony, showing autozooids, kenozooids (arrowhead), and autozooid (arrow) with orificial closure plate giving rise to overgrowth layer, VNMN-0213. B. Autozooids, showing shape of orifice, VNMN-0213. C. Overgrowth layer arising from autozooid with orificial closure plate, VNMN-0213. D. Interzooidal avicularium, one of two daughter zooids at column branch point, VNMN-0273. E. Autozooids, with one vicarious avicularium, VNMN- 0274. F. Kenozooids at colony margin, VNMN-0275. Scale bars: A, 1 mm; B, C, 500 µm; D, 200 µm; E, F, 250 µm.