Derbyia crassa

Overview

Derbyia crassa was a brachiopod with a nearly rectangular shell. One valve is almost flat or slightly concave, while the other is more convex. The meeting of two differently curved valves is characteristic.^[1]

Key facts

The ventral and dorsal illustrations of NU-B1221 show the same specimen from different directions. They compare both sides of one fossil with the valves joined. Two illustrations do not mean two fossils.^[1]

Time and place

Material from the Nagaiwa and Sakamotozawa areas of the South Kitakami Belt in present-day northeastern Japan has been studied. It belongs to an Early Permian Sakmarian context. The Permian as a whole spans approximately 300–250 million years ago.^[1][2]

The time range describes source context, not the complete duration of this species.

Size, form and skeleton

The outline is broadly rectangular, and the hinge is slightly shorter than the maximum width. The ventral valve is nearly flat or slightly concave, and the dorsal valve is more convex. Fine radial ridges and ventral folds are also described.^[1]

A specimen from Early Permian seas in what is now northeastern Japan had a shell about 2.4 cm long. At about 2.6 cm wide, it was slightly wider than long. Closely spaced radial markings spread across the small shell.^[3]

Habitat and reproduction

How did two differently shaped valves surround the body together? Articulated specimens show the meeting of a flatter side and a convex side. They invite consideration of one valve’s outline alongside the volume of both valves together.^[1]

Fossil discovery

A 2011 study described brachiopods from the Sakamotozawa area species by species. Articulated specimens such as NU-B1221 and NU-B1222 were treated as Derbyia crassa. Descriptions from different directions are connected with specimens to understand the shell.^[1]

References

1. Shintani (2011) — Derbyia shells from northeastern Japan
2. International Commission on Stratigraphy (2024) — Geological time chart
3. Shintani (2011), Orthotetoids from the Lower Permian (Sakmarian) of the Nagaiwa-Sakamotozawa area, South Kitakami Belt, northeast Japan
4. International Commission on Zoological Nomenclature — Type specimen and name of Terebratula
5. Brachiopod classification literature — Neotype of Terebratula

Research questions

What new features appear when a shell is turned over? Its valves of differing convexity, hinge, and surface ridges can be compared in sequence. This illustrates why one specimen is examined from several directions.^[1]

Observation

Move your gaze from the nearly rectangular outline to the rear hinge and front margin. Distinguishing the convexity of the two valves from radial ridges reveals overall organization and surface features together.^[1]