Argyrotheca cf. angulata Zezina, 1987

Bitner, Maria Aleksandra & Logan, Alan, 2016, Recent Brachiopoda from the Mozambique-Madagascar area, western Indian Ocean, Zoosystema 38 (1), pp. 5-41 : 26-27

Argyrotheca cf. angulata Zezina, 1987

( Fig. 14 A-J)

cf. Argyrotheca angulata Zezina, 1987: 558 , 559, fig. 2.

MATERIAL EXAMINED. — North-West Madagascar. MIRIKY, stn 3199, 2 bivalved specimens.

DEPTH RANGE. — 527- 652 m.

MEASUREMENTS. — Length 2.9, width 3.8 mm, thickness 1.7 mm; length 2.5 mm, width 3.9 mm, thickness 1.6 mm.

REMARKS

There are two specimens of Argyrotheca in the MIRIKY collection from locality DW 3199 that externally resemble A. angulata as described by Zezina (1987) from the Mozambique Channel. However, while both our specimens resemble the single specimen of Zezina’s in being markedly wider than long and having similar ornament, they show a very distinctive internal feature of ventro-laterally directed expansions or “prongs” emanating from the dorsal median septum ( Fig. 14 B-E) that were not described in A. angulata . These prongs are comparable to those described as “extravagant internal thickenings” in A. hewatti from the Gulf of Mexico by Cooper (1977: 111). Similar but smaller-scale outgrowths are known for A. cuneata from the Mediterranean (see Álvarez et al. 2008a, b for list of authors and discussion of this feature). These outgrowths, which are formed of secondary fibres ( Fig. 14E ), do not seem to be muscle platforms and at present their function is unknown. Because of the small number of specimens at our disposal we are reluctant to describe our form as a new species at this stage.

Superfamily PLATIDIOIDEA Thomson, 1927 Family PLATIDIIDAE Thomson, 1927 Subfamily PHANEROPORINAE Zezina, 1981

Figures

FIG. 14. — A-J, Argyrotheca cf. angulata Zezina, 1987, North-West Madagascar, West of Cap d’Ambre, MIRIKY, stn DW 3199, 527-652 m: A-E, complete specimen (MNHN IB-2013-80), dorsal view (A); B-E, inner,oblique, tilted anterior views of dorsal valve, and enlargement (E) of septum and lateral outgrowths; F, inner view of ventral valve; G-J, complete specimen (MNHN IB-2013-81), dorsal view (G) and enlargement of umbonal part to show details of the beak; I, J, inner view of dorsal valve and enlargement (J) of tilted anterior view to show the prong; K, L, Argyrotheca jacksoni Cooper, 1973, South Madagascar, South Cap Sainte Marie, ATIMO VATAE, stn CP 3615, 284-286 m: K, inner of view of ventral valve (MNHN IB-2013-82); L, dorsal view of complete specimen (MNHN IB-2013-83). All SEMs. Scale bars: A-C, F-I, 1 mm; D, K, L, 500 µm; E, J, 200 µm.