Terebratula sp.

Bitner, Maria Aleksandra & Moissette, Pierre, 2003, Pliocene brachiopods from north-western Africa, Geodiversitas 25 (3), pp. 463-479 : 469-470

Terebratula sp. ( Figs 4 A-L; 5C, D; Table 4)

MATERIAL EXAMINED. — Eight complete specimens from Morocco: LP-MNHN B.38316 (see also Table 1).

OCCURRENCE. — The genus Terebratula is very common in the Late Tertiary of the Mediterranean.

DESCRIPTION

The shell is of small size (max. length 19.8 mm), with a subcircular to oval outline and maximum width about midvalve. The valves are ventribiconvex and smooth, marked only by numerous growth lines. The beak is erected, truncated by a large, circular, permesothyrid foramen. The symphytium is only slightly visible. The lateral commissure is curved ventrally. The anterior commissure is sulciplicate. The internal structures are unknown.

The ultrastructural analysis shows that the shell consists of two layers. The primary layer, up to 40 µm thick, is composed of acicular crystallites. The secondary layer is made up of fibres subparallel to the shell surface ( Fig. 5C, D ).

REMARKS

The unknown internal morphology prevents determination to the specific level. The specimens display externally many characters typical of the genus Terebratula , but are smaller than T. terebratula described above. The latter species has also a lesser degree of anterior folding. The shell built of two layers is characteristic of the genus Terebratula ( Taddei Ruggiero 1983; Gaetani & Saccà 1985a).

In the Pliocene assemblage from the Murge area, southern Italy, Taddei Ruggiero (1996) reported the occurrence of similar specimens with adult forms reaching only 10-15 mm length which she determined as Terebratula sp. The absence of detailed description and/or illustrations does not allow comparison with the specimens under study. Cooper (1983) created a new genus and species, Maltaia maltensis , for the specimens from the Miocene of Malta which resemble Terebratula but are smaller and strongly biplicate (see also Davidson 1864). The investigated specimens differ, however, considerably from Maltaia maltensis in shell outline, as well as in smaller size and gentle anterior folding.

Figures

FIG. 4. — A-L, Terebratula sp., ventral, dorsal, lateral and anterior views of complete specimens, Had Mramer, Morocco (LP-MNHN B.38316); M-O, Gryphus sp.; M, N, dorsal and lateral views of complete specimen, Mustapha inférieur (Algiers), Algeria (LP-MNHN B.38671); O, dorsal view of complete specimen, Aïn Benian, Algeria (LP-MNHN B.38666). Scale bars: A-L, 1 cm; M-O, 5 mm.

FIG. 5. — A, B, Gryphus sp. (LP-MNHN B.38671), SEM micrographs; A, transverse section of the entire shell showing the acicular primary layer (top) underlain by the secondary layer of anvil-shaped fibres, passing into the prismatic tertiary layer; B, section of the shell showing two puncta filled with pyrites, fibres near the canals bent outward; C, D, Terebratula sp. (LP-MNHN B.38316), SEM micrographs; C, transverse section of the shell showing the boundary of the acicular primary and fibrous secondary layers; D, section of the secondary layer showing fibres in oblique and longitudinal section, visible (left) modification of fibres near a punctum. Scale bars: 20 µm.