Dignomia Hall, 1871

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 : 231-233

Genus Dignomia Hall, 1871

TYPE SPECIES. — Lingula alveata Hall, 1863 .

TYPE LEVEL. — Dignomia ranges from Middle Ordovician to the Middle Devonian.

GEOGRAPHICAL DISTRIBUTION. — North and South America.

EMENDED DIAGNOSIS ( FIGS 3-5 ). — Shell thin, elongate oval in shape; externally, two diverging medio-lateral grooves (internally ridges) extend from the umbonal region to the anterior margin, and a narrow median groove (a septum internally) extends from the umbonal region to the anterior margin of the dorsal valve but in the ventral valve only over the anterior half of the shell. Asymmetrical muscle system with three internal oblique muscles; unpaired posterior adductor muscle; no vascula media observed. (Note: these three characters are prerequisite to a diagnosis of the family Lingulidae – see Emig 2003).

REMARKS

Hall (1863), in the original description of Lingula alveata (only two valves were described), indicates that the valves “have a somewhat elevated or thickened border extending from the beak, within which is a distinct groove nearly parallel with the margin and reaching half of the length of the shell”, that is concurrent with our observation and could describe a ventral valve. The other valve (a cast) “preserves the mark of a thin septum, which extends from just beneath the beak three-fourths the length of the shell”: this valve is dorsal, and is probably the dorsal valve represented by Rowell (1965). The specimens mentioned were found in the shales of the Hamilton Group (Ludlowville, Cayuga county, New York) and in sandstone near Fultonham (Schoharie county, New York), both dated Middle Devonian. Hall (1871) created the genus Dignomia but gave no diagnosis; only the cast (see above) was figured on plate 13, figure 3. The genus has been recorded “in the Lower Silurian and Devonian, and probably in the Middle Silurian” ( Hall 1863; Cleland 1903). The description of two dorsal valves of Dignomia alveata by Hoare & Steller (1969) agrees with the genus characteristics; their specimens came from the Silica Formation (Middle Devonian; Ohio and Michigan).

In South America, d’Orbigny (1842) described from the Cochabamba and Chuquisaca regions of Bolivia two species: Lingula submarginata and L. munsterii , both of which are assigned here to the genus Dignomia . In his diagnosis of L. submarginata , d’Orbigny states: “Marquée en long de très légères stries, plus prononcées en approchant du bord. On remarque à partir du sommet, vers le bord, deux sillons assez prononcés, qui laissent entr’eux une surface bombée, formant feston sur le bord”. The shell length is 20 mm and the width 10 mm. The second species, L. munsterii , has a similar description: “Ornée en long de stries fines” and “sur le milieu de la longueur, on remarque trois sillons qui partent du sommet et vont en s’écartant vers le bord; l’un est au milieu, les deux autres sont aux côtés d’une partie légèrement saillante”. The shell length is 18 mm and the width 8 mm. According to d’Orbigny (1842), the specimens of both species were collected together in the same locality, that is “au sommet de la côte de Tacopaya [now: Zudáñez], entre Valle Grande et Chuquisaca [other name: Sucre]” ( Fig. 1 ). The syntypes of L. munsterii have been studied: they belong with no doubt to Dignomia . D’Orbigny (1842) also recorded L. munsterii east of Cochabamba, “près de Palta-cueva, au sommet de la Cordillère de Cochabamba, sur le chemin de Yuracares ( Bolivia), à la hauteur absolue de près de 5000 mètres” ( Fig. 1 ). Kayser (1897) described a broad Dignomia , that he named D. subalveata with generic characteristics like those of the diagnosis above ( Fig. 4 ). The specimens were collected in the “ Lingula -Schichten” ( Lingula - layers) from Jachal ( Argentina). Branisa (1965) stated that Lingula munsterii is clearly identified by the large size of the shell and by its ornamentation, i.e. a central groove and two lateral ones as shown in the figure 4 of his plate 2: the three specimens represented are similar to our specimens (see Fig. 4 ). Branisa’s description of Lingula lepta Clarke , 1912 (= Dignomia subalveata (Kayser, 1897)) was confirmed by Castellaro (1966), collected in San Juan ( Bolivia; Lower Devonian). Recently, Toro et al. (1994) reported Lingula sp. cf. munsterii with Dignomia -like characteristics in the area of Llavini (Cochabamba, Bolivia) in the Anzaldo Formation (Ordovician) ( Fig. 1 ).

Rowell (1965) diagnosed Dignomia as follows: “Relatively large, wide, thin-shelled forms, long conspicuous median ridge in both valves, with 2 low, broadly diverging ridges that extend anterolaterally from beak”. This description is confirmed by our present observations, but Rowell’s figure 158:4 does not represent the interior of a ventral valve, but an external view of a dorsal valve. Nevertheless, Holmer & Popov (2000) in the second edition of the Treatise have curiously simplified this original diagnosis to: “Shell subtriangular to suboval; ventral interior with two widely divergent ridges posteriorly (may be impression of pedicle nerve); dorsal interior with long median ridge. All other characters inadequately known”; the figure 37:5a used by Holmer & Popov to illustrate Dignomia is that in Rowell (1965) with the identical caption, but the valve represented on figure 37:5b to illustrate Dignomia is not Dignomia but probably a Barroisella (compare it with fig. 9:1b in Holmer & Popov 2000). After an examination by A. Balinski and C. C. Emig during a recent visit in Warsaw ( Poland) the two fragments referred to Dignomia sp. by Balinski (1995) are assigned to Barroisella Hall & Clarke, 1892.

Finally, the genus Dignomia appears to have been well recognized and defined by many authors. The characteristics of the shell defined in the emended diagnosis are sufficiently well described to permit the recognition with all certainty that Dignomia belongs to the order Lingulida . Furthermore, this genus has three characteristics defined in the diagnosis of the family Lingulidae (see Emig 2003): an asymmetrical muscle system with three internal oblique muscles, an unpaired posterior adductor muscle and the absence of vascula media ( Fig. 5 ). On this basis we consider that Dignomia belongs to the family Lingulidae as did Rowell (1965). However, this genus was classed as incertae sedis in the family “Uncertain” by Holmer & Popov (2000) in the second edition of the Treatise of Invertebrate Paleontology.

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.

FIG. 1. — Maps of Bolivia showing the distribution of the Ordovician (from Pareja & Ballon 1978).