Trochosodon praecox, Bock & Cook, 2004

Bock, Philip E. & Cook, Patricia L., 2004, A review of Australian Conescharellinidae (Bryozoa: Cheilostomata), Memoirs of Museum Victoria 61 (2), pp. 135-182 : 170-172

Trochosodon praecox sp. nov.

Figures 25A–F

Trochosodon sp. 2 .— Cook and Lagaaij, 1976, pl. 1 figs 5, 6. Trochosodon sp. —Cook, 1981, pl. C fig. 4.

Holotype. BMNH 2003.11 .27.1 (specimen figured by Cook and Lagaaij, 1976 and Cook, 1981), Challenger stn 185, Cape York, Queensland, Australia, 279 m.

Paratypes. BMNH 2003.11 .27.4 (as above), 1969.1.2.1, 1976.1.6.2

(part); NMV F99020–F99022. (67 colonies in total).

Etymology. praecox (L.) – precocious, immature, referring to the production of ovicells at the earliest astogenetic stages in these minute colonies.

Diagnosis and description. Colonies minute, domed, with mamillate calcification, that forms a raised mound adapically and covers the antapical surface. Primary orifice obscure, with a rounded sinus. Peristomes elongated; tubular and marginally prominent. Root pores adapical, rare, rounded. Lateral peristomial avicularia paired, very small, rostrum rounded. Ovicells developed on zooids of the second and third whorls, symmetrical, globular, very well calcified, opening into the peristome through a foramen. Ectooecium narrow, marginal; entooecium with frontal and marginal pores. Antapical surface granular and mamillate.

Colony diameter 0.50–0.80 mm, height 0.25–0.50 mm, number of whorls 2, number of zooids per whorl 3.

Remarks. T. praecox is known from more than 60 specimens, retrieved from one of the unstudied sediment samples from the Challenger collection, stored in the the Natural History Museum Mineralogy Department. Busk (1884) reported no bryozoan specimens from stn 185 from Cape York. Like T. optatus , T. praecox has ovicells that open into a tubular peristome but are symmetrical in development. They resemble those of C. africana Cook, 1966 (also Cook, 1981), that also has ovicells that differ from the ovicells observed in most other species in their relatively robust calcification. Of the 68 specimens examined, 17 have at least one completely developed ovicell. Some of the smallest colonies have two or three ovicells, developed on second or third astogenetic zooid generations. Although the specimens are all worn, scanning electron microscopy has revealed details of ovicell development. Zooids apparently develop an elongated peristome, with a foramen on its adapical surface. This is in contact with an adapical pore ( Fig. 25E ) at its edge. An ectooecial and an entooecial lamina then grow together, one “below” the other, from the adapical pore in the adapical direction ( Fig. 25C ).The two laminae then curve in an antapical direction, forming a capsule. The fusion of the laminae with the lateral and antapical edges of the foramen finally closes it, forming the complete ovicell ( Fig. 25B ). It should be noted that this does not produce a peristomial ovicell, the walls of which are expansions of the frontal shield. The ovicells of T. praecox comprise a separate development of ectooecium and entooecium. Other characteristics that are distinct enough for colonies to be recognised from additional, better preserved material, should it ever become available, are also revealed by scanning electron microscopy. The minute size of the colonies of T. praecox is comparable to those of accompanying foraminiferans; Cook (1981) noted the close similarity in appearance among them.

The occurrence of reproductive precocity in interstitial bryozoans with very small colonies has been described and discussed by Winston and Håkansson (1986: 43).

Figures

Figure 25. T. praecox sp. nov., Cape York. A, NMV F99020, paratype, detail of orifices and avicularia. with two complete ovicells and one peristomial foramen (arrowed), scale = 200 µm. B–C, NMV F99022, paratype, B, marginal zooids of colony with complete ovicell, avicularia arrowed, scale = 200 µm. C, detail of ovicells, showing ectooecial margin and central foramina of entooecium (arrowed); note avicularia, scale = 100 µm. D–E, NMV F99021, paratype. D, young colony with one peristomial foramen, root pores arrowed, scale = 200 µm. E, detail of foramen and adapical pore (arrowed), scale = 100 µm. F, NMV P99022, paratype. Colony with one complete and two developing ovicells (arrowed), scale = 200 µm.