Lamellispina spinosa, Sun & Baliński, 2008

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 : 516-518

Lamellispina spinosa sp. nov.

Figs. 24 , 25 .

1999 Schuchertella sp. ; Baliński 1999: 441, fig. 3H.

Etymology: After the presence of spinous projections on the shell exterior.

Holotype: Dorsal valve PKUM02−0327 figured in Fig. 24B .

Type locality: Muhua section, between villages of Muhua and Gedongguan (Guizhou province, South China).

Type horizon: Muhua Formation, correlated with the middle Tournaisian Siphonodella crenulata Zone.

Diagnosis.—Shell biconvex, wider than long, with long, straight hinge line developing short ears; ventral valve with asymmetrical umbo, and subtriangular, frequently irregular in outline, apsacline interarea; delthyrium covered by pseudodeltidium; dorsal valve weakly convex with linear to low−triangular interarea; chilidium semicircular with median groove; shell ornamentation costellate to parvicostellate, costellae very strong, extending marginally from concentric growth lamellae as semi−tubular projections.

Material.—More than 120 loose valves, predominantly juvenile; almost three−quarters of them are dorsal valves.

Description.—Shell biconvex, rounded−rectangular to rounded−quadrate in outline, hinge line straight equals or slightly less the greatest width of the shell, postero−laterally developing short pointed ears. Ventral valve subconical, variable in lateral profile, generally convex to slightly concave, especially in its posterior region; interarea apsacline, subtriangular, frequently irregular in outline, nearly flat to slightly convex or concave; umbo asymmetrical and deformed by attachment cicatrix; delthyrium completely covered by strongly convex pseudodeltidium which in large specimens is massive and thick and frequently bears clearly marked, imbricate growth lamellae; perideltidium and monticulus absent. Dorsal valve is usually weakly convex, but some gerontic specimens might be very deep; small umbo is well marked, slightly swollen and protruding a little behind hinge margin ( Fig. 25B ), but partly shadowed by overhanging chilidium; interarea linear to low−triangular; chilidium well developed, semicircular, thickened, with median groove.

Interior of the ventral valve with strong teeth and dental ridges, the later being well developed only distally, while proximally they are buried under a shell thickening; no dental plates; muscle scars poorly differentiated, presumably occupying a large surface in the posterior region of the valve, possibly including the whole umbonal cavity; in the most posterior part of the umbonal cavity a low and very short median ridge or thickening may be observed infrequently in some thick−shelled specimens. Dorsal valve interiors have divergent socket ridges at about 90 ° (total range: 65–114 °), in some they are expanded antero−ventrally to form distinct plates or brachiophore bases ( Fig. 25C, D ); when well preserved the ridges are ankylosed to the cardinal process but commonly damage to the thin proximal ends of the inner socket ridges results in gaps between them and the cardinal process; cardinal process low but wide, bilobed, without an internal median node; myophore grooves posteriorly directed; adductor scars more clearly impressed that the scars in the ventral valve, located between and anterior to the socket ridges, mostly flabellate but occasionally with clear dendroid pattern ( Fig. 24D ), bordered laterally and anteriorly by a low thickened ridge, divided by a low median myophragm which is highest anteriorly.

Shell ornamentation is dominated by very distinct, imbricate concentric lamellae and costellate to parvicostellate ribbing. Costellae are very strong, high, with rounded crests, separated by deep interspaces of varying width; increasing by intercalation and extending marginally from lamellose growth lines as spinous, semi−tubular projections. These projections are inclined at a low angle away from the valve surface, and being concave on their inner surfaces to overlap onto the rib anteriorly, thus forming the continuous costellate appearance ( Figs. 24E, G ; 25D ). On well preserved specimens the concentric lamellae show a clear tendency to form short frilly extensions ( Fig. 24E, G ).

Remarks.—The main characteristic of this new orthotetoid and its comparison with other representatives of the group have been discussed in details in generic description above.

The shape of the socket ridges and their relationship to the cardinal process merits some additional comments. In the majority of dorsal valves the ridges are not ankylosed to the cardinal process but this condition seems to be caused by partial damage of the very thin proximal (i.e., adaxial) walls of the sockets. In some rare well preserved specimens these ridges are more completely preserved and ankylosed to the cardinal process ( Fig. 25E ). This type of breakage may have resulted from post−mortem disarticulation. Similar damage to the sockets can be observed in some disarticulated Recent brachiopods (C. Howard C. Brunton, personal communication 2007).

Stratigraphic and geographic range.—This is one of the most common brachiopod species in the fauna of Muhua. It was found mostly in sample MH1, but a few specimens come also from samples D2, D4, M2/3, M2/4, DPS2, and Mu−42.

Figures

Fig. 24. Lamellispina spinosa gen. et sp. nov. from the Muhua Formation. A. Small dorsal valve, PKUM02−0326, in exterior (A1) and interior (A2) views. B. Dorsal valve, PKUM02−0327 (holotype), in exterior (B1), interior (B2), and posterior (B3) views. C. Ventral valve, PKUM02−0328, in interior view. D. Dorsal valve, PKUM02−0329, in interior view (D1) and enlarged view showing cardinalia and dendritic adductor scars (D2). E, G. SEM micrographs of exterior of dorsal valves, PKUM02−0330 (E) and PKUM02−0331 (G), in dorsal (E1, G1), oblique lateral (E2, G2), and anterior (E3) views. F. Ventral valve, PKUM02−0332, in exterior (F1) and interior (F2) views. All from sample MH1.

Fig. 25. Orthotetoid brachiopod of Lamellispina spinosa gen. et sp. nov. from the Muhua Formation, sample MH1. A. Dorsal valve, PKUM02−0333, in posterior view showing cardinalia. B. Juvenile dorsal valve, PKUM02−0334, in exterior view. C. Interior of dorsal valve, PKUM02−0335, in ventral (C1), oblique lateral (C2), enlarged ventral (C3), enlarged oblique (C4), and ventrolateral (C5) views illustrating disposition of socket ridges, cardinal process and adductor scars. D. Dorsal valve, PKUM02−0336, in interior (D1), enlarged interior (D2; notice slightly damaged socket ridges), ventrolateral (D3), oblique anterior (D4, D5; notice spinous, half−tube−like projections at the valve margin) views. E. Fragment of a large dorsal valve, PKUM02−0337, showing well preserved cardinalia. All SEM micrographs. Scale bars 1 mm.