Retichonetes obscurus, Cooper and Dutro, 1982

Day, Jed & Over, D. Jeffrey, 2002, Post-extinction survivor fauna from the lowermost Famennian of eastern North America, Acta Palaeontologica Polonica 47 (2), pp. 189-202 : 197

Retichonetes aff. S. obscurus Cooper and Dutro, 1982

Fig. 4A–D .

Retichonetes obscurus ; Cooper and Dutro 1982: 58, pl. 11: 19–25.

Material.—Figured specimens NYSM 15704, NYSM 15705, NYSM 15718; unfigured specimens NYSM 15708, NYSM 15709, and NYSM 15710; and approximately 2,000 unnumbered complete and partial shells on surfaces of parts and counterparts of split slabs.

Remarks.—This small chonetid (4–8 mm in width) is the numerically dominant species in the fauna of the Hanover shell bed. This form was initially interpreted as a new genus by Day (1997). However, careful study of numerous specimens indicates that the chonetid in the Hanover Shale shell bed is a species of Retichonetes Muir−Wood (1962) close to R. obscurus Cooper and Dutro (1982) described from the Ready Pay and Box members of the Percha Formation of southern New Mexico. Cooper and Dutro (1982: fig. 7) show the Percha as possibly spanning the interval of the Upper Pa. crepida Zone to Po. styriacus Zone (lower–upper Famennian). The Po. styriacus Zone as shown in fig. 7 of Cooper and Dutro (1982) is equivalent to the Lower and Upper Pa. postera and Lower Pa. expansa zones of the current Famennian conodont zonation (see revisions by Ziegler and Sandberg 1984: 183–184, figs. 1–4). Retichonetes obscurus from New Mexico is similar to the Hanover specimens in overall shape and size and numbers of costellae. The Hanover form differs by the presence of well preserved concentric growth lines on the rounded costellae and in the interspaces, and having a better developed dorsal septum than that shown for R. obscurus ( Cooper and Dutro 1982: pl. 11: 24).

Another similar form Retichonetes sp. is illustrated by Schwimmer and Feldmann (1991: figs. 7.1–7.8) from the Famennian Chagrin Shale of northeast Ohio. Their shells are similar to the Hanover material in overall shape, size, and in the high angle (60 to 90°) that the oblique orthomorph spines arise from the posterior margin of the ventral valve. They do not mention the reticulated ornament typical of species of Retichonetes ( Muir−Wood 1965: H428, fig. 286). Reticulated ornament is seen on shells of R. aff. R. obscurus with well preserved shell material, produced where fine concentric growth lines cross the radial costellae. Spines along the ventral cardinal margins in both the Hanover specimens and R. sp. from the Chagrin Shale of Ohio (Schwimmer and Feldmann 1991) arise at high angles as is typical for the genus (see Muir−Wood 1962: 62, 1965: figs. 285, 286).

Figures

Fig. 4. Brachiopods from the shell bed in the Hanover Shale Member of the Java Formation. A–D. Retichonetes aff. S. obscurus Cooper and Dutro, 1982. A. Internal mold of dorsal valve, NYSM 15704 with low medial myophragm, simple sockets flanking the pit near base of cardinal process, × 5.4. B. Upper exteriorviewofdorsalvalve,NYSM15710,×4.8. C.Internalmoldofdorsalvalve,NYSM15705,×7. D.Ventralvalveshowingcostellaewithcrushedand distorted ventral valve of Ambocoelia cf. A. gregaria in upper left, NYSM 15718, × 3.3. E. Tylothyris mesacostalis (Hall, 1867), view of dorsal valve showing centric lamellose ornament and central groove on fold, NYSM 15711, × 2.3. F, G. Praewaagenoconcha speciosa (Hall, 1867). F. Internal mold of dorsal valve with impression of medial myophragm and rugae along the postero−lateral margin, NYSM 15706, × 1.7. G. External mold of dorsal valve showing radial spine bases with some exfoliated shell material, NYSM 15695, × 1.5. H, I. Cyrtospirifer hornellensis Greiner, 1957. H. Exfoliated dorsal valve showing extended postero−lateral extremity, NYSM 15692. I. Ventral valve, NYSM 15691; both × 1.8.J. Ambocoelia cf. A. gregaria Hall, 1867, upper view of flattened dorsal valve showing dorsal groove, NYSM 15719 × 3.5.