Dignomia munsterii (d’Orbigny, 1842)

Emig, Christian C. & Herrera, Zarela, 2006, Dignomia munsterii (Brachiopoda, Lingulata) from the Ordovician of Bolivia, with redescription of the genus, Geodiversitas 28 (2), pp. 227-237 : 233-235

Dignomia munsterii (d’Orbigny, 1842)

Lingula submarginata d’Orbigny, 1850 (pro Lingula marginata d’Orbigny, 1842: 28 , pl. 2 fig. 5; non Phillips, 1836): 14.

Lingula lineata Hoek in Steinman & Hoek, 1912: 232.

Dignomia boliviana Emig in Gagnier et al., 1996: 338, fig. 6.

MATERIAL EXAMINED. — See Chapter Material.

DIAGNOSIS ( FIGS 3-5 ). — The outline of the shell is an elongate oval. Two large medio-lateral grooves, longitudinally striated, extend from the umbonal region to the anterior margin of the valves; a median groove is narrow. On the outer surface of the dorsal valve is a median groove that occupies its whole length. On the ventral valve, the groove extends from the anterior margin to about the half to two-thirds of the length.

Anterior margin straight, without indentation at the level of the grooves. Lateral margins subparallel; width of the shell largest at or above the middle; surface smooth with numerous weak, occasionally well marked, growth lines. Umbonal region rounded on dorsal valve, pointed on ventral valve. Small proparea slightly curved with a weak pedicle groove, continuous with internal valve side. The ventral lophophoral cavity occupies approximately 36% of the length of the valve. Mantle canals curved. A pair of narrow subparallel, V-shaped, grooves extend from the anterior adductor scars to the unpaired posterior adductor scars; three long oblique internal muscles; a V-shaped perimial line.

No internal observation of dorsal valve.

DESCRIPTION

Our specimens were cited under Dignomia boliviana nov. sp. by Emig in Gagnier et al. (1996) but never described completely. The d’Orbigny (1842) description of two species, Lingula submarginata (see also below) and L. munsterii , both of which are obviously Dignomia (see above) and are similar in aspect to our specimens collected in the same area. That was corroborated by the study of the syntypes of L. munsterii . Therefore we consider our specimens should be named Dignomia munsterii , the betterdescribed of d’Orbigny’s two species. However, as the name marginata was preoccupied by Lingula marginata Phillips, 1836 a species from the Carboniferous of England, d’Orbigny (1850) proposed its replacement by Lingula submarginata .

Shell and internal features

The length of the shell of our specimens ranges from 12 to 30 mm (mean 19.1 ± 5.5 mm; n = 12), and the width from 5.9 to 14 mm (mean 9.3 ± 2.4 mm; n = 12). The W/L ratio lies between 0.45 and 0.55 (mean 0.49 ± 0.02) ( Fig. 4B ). D’Orbigny (1842) measured two specimens. The W/L ratio is 10/20 = 0.5 for the specimen referred to Lingula submarginata , and for L. munsterii the W/L ratio is 8/18 = 0.44. The general shape of the shell of Dignomia appears similar to those of the species of Lingula with an elongate oval shell (W/L = 0.38-0.53; mean 0.43 ± 0.3-0.48 ± 0.3 mm derived from the geographic populations; n = 155) ( Fig. 4 ).

The two medio-lateral grooves are visible on the inner faces of both valves as ridges (or septa), sometimes well developed especially near the anterior margin, at the site where the central exhalant pseudosiphon and the two lateral inhalant ones separate ( Figs 3 ; 5 ). The anterior margin of the valves is straight and flat. These ridges may have a morpho-functional role related to the disposition of the lophophore in the lophophoral cavity. On the outer side, longitudinal ribs exist in the grooves and commonly extend laterally ( Figs 3 ; 5 ).

Ventral valve. The internal umbonal region differs from that of the lingulides in the presence of small propareas and the absence of a true pedicle groove ( Fig. 5 ). The existence of a pair of V-shaped grooves – the impressions of the pedicle nerves (Biernat & Emig 1993) – on the inner side of the ventral valve in Dignomia ( Fig. 5 ) can be considered as general in the superfamily Linguloidea since Cambrian times (Biernat & Emig 1993; Emig 2002, 2003). These grooves extend from the anterior adductor muscles until the level of the posterior adductor muscle where they join.

The disposition of the mantle canals in Dignomia could be seen in only one ventral valve ( Fig. 5 ). The distance from the distal tips of the mantle canal to the anterior margin of the valve represents 17% of the total length of the valve. The length of the lophophoral cavity, i.e. the distance between the distal limits of the anterior adductor muscle scars to the anterior margin of the valve is 36% of the total length of the valve. Both measurements are congruent with those seen in Lingula , respectively 17-22% (mean 20 ± 1.7%; n = 44) and 35-46% (mean 41 ± 4.5%) (Biernat & Emig 1993; Emig & Bitner 2005a). No vascula media could be found ( Fig. 5 ).

The disposition of the body muscles on the ventral side is similar to that in the family Lingulidae (see Emig 1982, 2003; Emig & Bitner 2005a, b): the oblique muscles have a V-shaped arrangement and a perimial line, the trace of the attachment of the body wall to the valves (a line surrounding the body muscles); the internal oblique muscles are represented by long scars ( Fig. 5 ). Such a disposition is similar to that described by Egorov & Popov (1990) in the lingulide Semilingula . The trace of the posterior adductor muscle could not be followed completely in Dignomia but it seems to be unpaired ( Fig. 5 ) and similar in shape to that of Lingularia .

Dorsal valve. Muscle arrangement and disposition of the mantle canal could not be observed on dorsal valve.

Figures

FIG. 3. — Dignomia munsterii: A, B, ventral valves; A, internal face; B, C, external faces; D-F, dorsal valves; D, internal face; E, F, external faces. Scale bars: 2 mm.

FIG. 4. — A, the shell shape of Dignomia munsterii compared to those of D. alveata, D. subalveata and D. lepta, and to Lingula anatina; B, regression curve of D. munsterii which is similar to the populations of Lingula anatina from New Caledonia: W = -0.229 + 0.435L (R2 = 0.98; n = 165).

FIG. 5. — Arrangement of the musculature and disposition of the two main anterior mantle canals on the ventral face of Dignomia munsterii; the pair of V-shaped narrow grooves extending posteriorly from the anterior adductor scars correspond to the impression of the pedicle nerves. Umbonal region of the internal side of a ventral valve.