| Palaeos: | |
Rhynchonelliformea |
| Brachiopoda | Obolellida |
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The Obolellida constitute a short-lived order, apparently becoming extinct in the middle Cambrian, whose relationship to the other "inarticulates" is obscure. They previously lumped with the Siphonotretacea, then given their own order. A. D. Wright suggests they represent a lineage distinct from other brachiopods, arising independently from a general brachiophorate ancestor and developing the ability to secrete a calcareous shell. In the most recent classification they are considered a distinct class related to the articulate line.
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The Obolellids have a foliated, impunctate, biconvex (left) shell. As with the lingulids there is no hinge, in this respect they are typically inarticulate. Denticles on either side of a narrow delthyrium or ventral platform serve to articulate the valves. Examination of the shell structure of Trematobolus revealed a coarsely crystalline primary layer succeeded by a laminar secondary layer. The laminae are interpreted as being separated by protein sheets with a similar growth pattern to the modern Crania. |
Although the obolellids have a general inarticulate external appearance and muscle scar distribution; the group is somewhat unusual in the very variable disposition of the pedicle, which may emerge from a grooved interarea, or a foramen situated either apically or anterior to the beak. In the typical Early Cambrian genus Obolella (above) the valves low and oval, nearly equal; and opening for the pedicle is tiny, and located near apex of the pedicle valve.
The group is entirely limited to the early to middle Cambrian period,
being representatives of the first evolutionary wave of invertebrates (the Tommotian
biota)
Naukatida
Naukatoidea (Early Cambrian-Middle Cambrian)
Classification from 
Classification
des Brachiopoda (Lophophorata)
A. D. Wright, "Brachiopod Radiation", in Systematics Association Special
Volume No.12, The Origin of Major Invertebrate Groups, ed. by M.
R. House, 1979, pp.235-252, Academic Press, London and New York
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