Dibunostoma transversum ( Guha & Gopikrishna, 2004 ) Sonar & Pawar & Wayal, 2022

Sonar, Mohan A., Pawar, Ravi V. & Wayal, Dyaneshwar V., 2022, Fossil Thalamoporellidae (Bryozoa) from Paleogene-Neogene sediments of western Kachchh, Gujarat, India, Zootaxa 5104 (2), pp. 251-274 : 253

Dibunostoma transversum ( Guha & Gopikrishna, 2004) n. comb.

( Fig. 4 , Table 1 )

Thalamoporella transversa Guha & Gopikrishna, 2004: 29 , figs 26, 27.

Material examined. GIS/B 0082–0110. Ochreous impure limestone horizon on the left bank of the Barkhan river section, Kharinadi Formation , lower Miocene (Aquitanian), 23°20’08’’ N, 68°45’40’’ E, elevation 12 m, 13 January 2011, DST project, New Delhi .

Description. Colony unilaminar, encrusting molluscan shell. Autozooids elongate-hexagonal, broader in the middle, tapering distally as well as proximally, arranged quincuncially, each bordered by distinct, thin, raised, smooth autozooidal boundary ( Fig. 4A ). Orifice transversely D-shaped, distally arched, straight or weakly concave proximally. Adoral tubercles large, conspicuous, located on either side of the orifice ( Fig. 4B ). Two small, rounded, equal opesiules placed away from the lateral walls of the autozooids. Cryptocyst well developed, flat, shallow, granular with moderately coarse perforations. Avicularia interzooidal, rare, small, tear-drop shaped, rostrum indistinct, at bifurcation of autozooids, directed obliquely proximally toward orifice of the preceding autozooid; avicularian cryptocyst proximally developed and granular ( Fig. 4C ). Ovicells not observed.

Remarks. Proximally or proximolaterally directed avicularia is an important character of Dibunostoma , allowing inclusion of the present species in this genus. Dick et al. (2020) noted that D. transversum is so similar to Dibunostoma reversum ( Harmer, 1926) that, had it been found in a Recent fauna, the two species might be considered conspecific. They did note, however, that D. transversum differs from Holocene–Recent D. reversum in having many (but not all) autozooids narrowing distally, appearing somewhat flask-shaped, with the avicularian rostrum oriented perpendicular to the long axis of the next-proximal autozooid, rather than pointing proximolaterally. Dick et al. (2020) also noted that putative D. reversum identified from South Korea by Yang et al. (2018) was misidentified.

Figures

FIGURE 4. Dibunostoma transversum (Guha & Gopikrishna, 2004) n. comb. (GIS/B 0082). A. General view of the colony showing autozooids, orifices and avicularia. B. Enlargement of ‘vase’ shaped autozooids, orifices and avicularia (arrowed). C. Detail of an avicularium (arrowed), cryptocyst and orifices. Scale bars: A, 500 µm; B, 200 µm; C, 100 µm.