Brachiopoda : OBOLELLIDA

Order Obolellida

Sous-embranchement (subphylum) des Rhynchonelliformea
Classe des Obolellata
Ordre des Obolellida
Super-famille des Obolelloidea

A small extinct order of articulated brachiopods that ranges in age from Early to Middle Cambrian and includes the earliest known calcitic brachiopods. Members of this group were presumably epifaunal and sessile.

Order OBOLELLIDA Rowell, 1965
Morphology: Obolellids have a biconvex, calcite, impunctuate (lacking holes) shell with an elongated oval shape and a laminar secondary layer. The order includes forms that have primitive articulation of the ventral and dorsal valves—consisting of paired ventral denticles (hinge teeth), where the pedicle (fleshy stalk) attaches to the shell, and dorsal sockets along the internal posterior margin (hinge line)—and forms that lack denticles, such as the genus Obolella. The ventral valve (formerly named pedicle valve) has a well-defined, low and relatively short flat shelf (interarea) at the posterior. The pedicle opening (delthyrium), located between the valves, is either uncovered, as in Obolella, or closed off by convex plates (pseudodeltidium). The mantle canal system lacks bifurcations (baculate type) in both valves (see Fig. a & b). The ventral and dorsal valves contain main mantle canals known as vascula lateralia. The dorsal valve (formerly named brachial valve) also contained a secondary mantle canal, called the vascula media. The muscle arrangement is very similar to that of other articulated brachiopods (see Fig. a & b): in the dorsal valve the pair of anterior and of posterior adductor muscles form a muscle field while the single pair of oblique diductor muscles is attached at the bottom of the pedicle opening (notothyrial cavity). In the ventral valve the pair of diductor muscles are located between the pairs of anterior adductor and of the posterior adductor muscles.

Order OBOLELLIDA Rowell, 1965

Morphology:

Obolellids have a biconvex, calcite, impunctuate (lacking holes) shell with an elongated oval shape and a laminar secondary layer. The order includes forms that have primitive articulation of the ventral and dorsal valves—consisting of paired ventral denticles (hinge teeth), where the pedicle (fleshy stalk) attaches to the shell, and dorsal sockets along the internal posterior margin (hinge line)—and forms that lack denticles, such as the genus Obolella. The ventral valve (formerly named pedicle valve) has a well-defined, low and relatively short flat shelf (interarea) at the posterior. The pedicle opening (delthyrium), located between the valves, is either uncovered, as in Obolella, or closed off by convex plates (pseudodeltidium).

The mantle canal system lacks bifurcations (baculate type) in both valves (see Fig. a & b). The ventral and dorsal valves contain main mantle canals known as vascula lateralia. The dorsal valve (formerly named brachial valve) also contained a secondary mantle canal, called the vascula media.

The muscle arrangement is very similar to that of other articulated brachiopods (see Fig. a & b): in the dorsal valve the pair of anterior and of posterior adductor muscles form a muscle field while the single pair of oblique diductor muscles is attached at the bottom of the pedicle opening (notothyrial cavity). In the ventral valve the pair of diductor muscles are located between the pairs of anterior adductor and of the posterior adductor muscles.

REFERENCES

Emig C. C., 2005. Obolellida. McGraw-Hill Encyclopedia of Science & Technology, 10th Edition, p. 257 and http://www.accessscience.com/Encyclopedia/ - in press -

Kaesler R. L. (ed.), 2000. Treatise on Invertebrate Paleontology. Part H. Brachiopoda. Geological Society of America and University of Kansas. Boulder, Colorado, and Lawrence, Kansas, vol. 2, 423 pp. (p. 200-206: Popov & Holmer)

Popov, L. E., L. E. Holmer, & M. G. Bassett, 1996. Radiation of the earliest calcareous brachiopods. In Paul Copper & Jisuo Jin, eds., Brachiopods, Proceedings of the Third International Brachiopod Congress Sudbury, Ontario, Canada, 2–5 September 1995. A. A. Balkema. Rotterdam, Brookfield. p. 209–213, 1 fig., 1 table.

Gorjansky, V. Iu., & L. E. Popov., 1986. On the origin and systematic position of the calcareous-shelled inarticulate brachiopods. Lethaia 19, 233–240, 3 fig.

Gorjansky V. Iu. & L. E. Popov, 1985. Morphology, systematic position and origin of the inarticulate brachiopods with calcareous shells [in Russian]. Paleontologicheskii Zhurnal, 1985 (3), 3–14, 5 fig., 1 pl.

Pelman Yu. L., 1977. Early and Middle Cambrian inarticulate brachiopods of the Siberian Platform [in Russian]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie, 316, 168 p., 22 fig., 24 pl.

Rowell A. J., 1965. Inarticulata. In R. C. Moore, ed., Treatise on Invertebrate Paleontology. Part H, Brachiopoda. The Geological Society of America & The University of Kansas Press. New York & Lawrence. p. 260–296, fig. 158–186.

Rowell A. J., 1962. The genera of the brachiopod superfamilies Obolellacea and Siphonotretacea. Journal of Paleontology, 36, 136–152, 2 pl.

Superfamily OBOLELLOIDEA Walcott & Schuchert, 1908
Family OBOLELLIDAE Walcott & Schuchert, 1908
Genus Obolella Genus Bicia Genus Brevipelta Genus Ivshinella Genus Magnicanalis
Family TREMATOBOLIDAE Popov & Holmer, 2000
Genus Trematobolus Genus Alisinia Genus Sibiria

Obolella (a) dorsal valve interior, and (c) ventral valve interior, with schematic illustration of musculature and mantle canals
(Reprinted with permission from R. L. Kaesler, ed., Treatise on Invertebrate Paleontology, vol. 2, 2000).