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Ventral valve
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Oxycolpella wreyi
Order: Brachiopod
Genus: Oxycolpella Species: Oxycolpella wreyi Location found: 88 Valley, Wakefield |
The Triassic in New Zealand is characterised by a number of different brachiopod species, all belonging to the order of Athyrids. Unfortunately, the differences between them is not always clear and there is consequently some debate over how to name the brachiopods - and about which species belongs to which genus. In general, however, brachiopods from the the earliest stages (Etalian and Kaihikuan) have been attributed to the genus Athyris. Those from the succeeding Oretian and Otamitan stages are attributed to Oxycolpella, and those from the Warepan and Otapirian to Clavigera.
This broad sequence was usefully laid out by Campbell in the 1980s. Unfortunately, it was only ever produced as a hand-drawn sketch, and was only published some years later as part of a review of the evolution of Pazleozoic and Mesozoic brachiopods, by Alvarez (2003). A simplified version of the sequence is shown on this pop-up page. Detailed description (from Trechmann 1917) It is rather wider than long, the ventral valve is rather inflated, the dorsal less so. The ventral beak is comparatively small; the area is narrow and faintly striate parallel to the hinge-margin; the delthyrium is triangular and deeply sunken, and only slightly hidden by the dorsal beak. The apex is perforated by a pedicular foramen of moderate size. The ventral valve has a broad, shallow, median sulcus which dies away about half way to the beak, and two faint lateral sulci which only appear near the anterior lateral margins. The dorsal valve bears near its anterior margin a broad median fold, which is bordered by steep slopes . . . The growth-lines are well marked. Further Reading: Alvarez, F. 2003 Convergence in the evolution of Palaeozoic and Mesozoic brachiopods. Journal of the Royal Society of New Zealand 33(1), 189–211. Trechmann, C.T. 1917 The Trias of New Zealand. Quarterly Journal of the Geological Society 291, 165-246 (+ 10 plates). |
Dorsal valve
The species shown here is believed to be Oxycolpella wreyi. It is characterised by a rounded shape, often slightly wider than long. The dorsal valve is considerably smaller than the ventral, so that it nests neatly within it. The beak on the ventral valve is smaller than on many other Athyrid species, and is somewhat masked by the thick shell that accumulates around it. The beak becomes more distinct, however, when the shell is lacking, and can be seen to be divided (see image above left). The ventral valve also has a broad but clear sulcus, matched towards the outer margin of the dorsal valve by a distinct fold. Growth lines are usually well-marked, giving the shell a somewhat wrinkled appearance in profile.
Good descriptions and photographs of the species are shown by Trechmann (1917), under the name Spirigera wreyi (see panel, left). In the Triassic sequence, O. wreyi first appeared in the Oretian, replacing Athyris kaihikuana which was dominant in the previous stage. It persisted into the Otamitan, but seems to have been gradually supplanted by O. manzavinioides. Distinction between these two Oxycolpella species, however, is not always easy. The gallery of specimens, here, therefore shows specimens ascribed to both species, plus ones where the species is uncertain (names, therefore, only to genus level). |