Argentiproductus margaritaceus ( Phillips, 1836 )

Sun, Yuanlin & Baliński, Andrzej, 2008, Silicified Mississippian brachiopods from Muhua, southern China: Lingulids, craniids, strophomenids, productids, orthotetids, and orthids, Acta Palaeontologica Polonica 53 (3), pp. 485-524 : 506

Argentiproductus margaritaceus ( Phillips, 1836)

Figs. 14–16 .

1836 Producta margaritacea Ph. ; Phillips 1836: 215, pl. 8: 8.

1861 Productus margaritacea Phillips ; Davidson 1861: 159, pl. 44: 5–8.

1960 Argentiproductus margaritacea (Phillips) , Muir−Wood and Cooper 1960: 182, pl. 123: 11–16 (not 17, 17a).

1966 Productina margaritacea (Phillips) ; Brunton 1966: 209–213, pl. 8: 1–19; pl. 15: 1–8.

1971 Productina margaritacea (Phillips) ; Roberts 1971: 94–96, pl. 17: 11–28; text−fig. 22.

1993 Argentiproductus margaritaceus ( Phillips, 1836) ; Brunton et al. 1993: 103–104, figs. 3–15.

1999 Argentiproductus sp. ; Baliński 1999: 441, fig. 3J.

1999 Productoid gen et sp. indet. 1; Baliński 1999: 441–442, fig. 3I–L.

Material.—Two shells, 19 ventral and 7 dorsal valves; in addition about 20 juvenile specimens most likely representing this species.

Remarks.—Specimens from the Muhua Formation are virtually identical with Argentiproductus margaritaceus described from the Late Viséan of northern Wales and Northern Ireland by Muir−Wood and Cooper (1960), Brunton (1966), and Brunton et al. (1993). The species has been reported also from the early Carboniferous of Europe (e.g., de Koninck 1847; Żakowa 1985), Russia (e.g., Bublichenko 1976), Kazakhstan ( Litvinovich et al. 1969), Algeria ( Pareyn 1962), and Australia ( Roberts 1971) (see also Brunton et al. 1993).

Among the material from Muhua there are several juvenile specimens found in samples Mu−42, MH 1, and M2−8. They reveal perfectly preserved juvenile shell, probably formed immediately after settlement of a larva. The ventral part of the shell shows a median swelling which protrudes posteriorly beyond the hinge margin. This swelling possesses a median, posteriorly directed, and more or less cylindrical structure, which was mentioned or described as “pedicle sheath” in productoids by Brunton (1965, 1966), Brunton and Cocks (1996), and Baliński (1999) (see also Bassett et al. 2008). On the ventral exterior the median swelling bears a deep longitudinal groove ( Fig. 15A, D) which was interpreted by Brunton (1966: 181–182) as an impression left by the anchoring object or as a result of differential growth rate reflecting a juvenile development of the muscles. The position and structure of the groove observed on the studied specimens from Muhua suggest, however, that at the moment of settlement it functioned as an open slit through which the pedicle epithelium was probably attached to the substrate ( Fig. 16A ). Later during growth the slit was filled by shell material ( Fig. 15A 4 ).

After settlement of a larva and its attachment to substrate by the pedicle epithelium the anchoring of a shell was almost immediately aided by a pair of symmetrically disposed clasping spines ( Figs. 15A, D, 16A ). The shape and extent of the spines were adjusted to the object which served as a substrate or attachment site ( Fig. 15A–D). This mode of the attachment of Argentiproductus margaritaceus larvae seems to be more functionally reasonable and advantageous than the attachment by the tip of cylindrical, thread−like pedicle ( Fig. 16B ) which is most common among pediculate living, as well as fossil, brachiopods. Latter during growth the pedicle atrophied and the slit in the ventral umbo was filled by the shell material ( Fig. 15A, D). Eventually, when the shell became big and heavy the clasping spines were not able to hold the shell above the substrate, they broke away and the shell settled on the sea floor ( Muir−Wood and Cooper 1960).

Stratigraphic and geographic range.—The species was revealed in samples GB, GT, MH 1, M2−8, and Mu−42. Outside the study area it is known from the Viséan of Europe ( Belgium, Poland), Kirghizia, Algerian Sahara, and northwest Australia (see Brunton et al. 1993).

Figures

Fig. 14. Productoid brachiopod Argentiproductus margaritaceus (Phillips, 1836) from the Muhua Formation. A. Small ventral valve, PKUM02−0278, in exterior (A1), interior (A2), lateral (A3), and posterior (A4) views. B. Shell, PKUM02−0279, in ventral (B1), dorsal (B2), lateral (B3), posterior (B4) views. C–F. Incomplete ventral valves, PKUM02−0280 (C), PKUM02−0281 (D), PKUM02−0282 (E), and PKUM02−0283 (F), in exterior (C1, D1, E1, F1) and interior (C2, D2, E2, F2) views showing well developed ear baffles. G. Incomplete dorsal valve, PKUM02−0284, in exterior (G1) and interior (G2) views. H. Large, almost complete shell, PKUM02−0285, in dorsal (H1), ventral (H2), lateral (H3), and posterior (H4) views; the ribbing in the middle of the ventral valve resembles ornamentation of Productina. Samples GB–GT.

Fig. 16. A, B. Two hypothetical mode of attachment of juvenile Argentiproductus margaritaceus (Phillips, 1836). The extent of attachment (here marked with white areas) by the epithelium emerging through longitudinal median slit in the umbonal region of the ventral valve (A) seems more probable and functionally efficient than the attachment by the tip of a cylindrical pedicle (B). Note also that the function of clasping spines in A is clear whereas in B reminds unresolved.

Fig. 4. Different results of the silicification of brachiopod valves from the Muhua Formation. A. Subradial lamellae of silica arising along fibres of the secondary shell layer as seen on the external surface of shell. B. Net−like pattern formed by silica on the internal surface of the terebratulid valve reflecting the mosaic of the terminal faces of fibres (see text for further explanation). C. Punctate shell microstructure preserved on a silicified terebratulid valve. D. Spiriferoid ventral valve silicified with “spongy” silica; general view (D1) and enlargement (D2).