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Dorsal valve
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Oxycolpella manzavinioides
Order: Brachiopod
Genus: Oxycolpella Species: Oxycolpella manzavinioides Location found: 88 Valley, Wakefield |
According to the sequence sketched out by Campbell (and later published by Alvarez 2003), the Otamitan stage of the late Triassic sustained two species of Athyrids: Oxycolpella wreyi and Oxycolpella manzavinioides. The distinction between them isn't easy to make (see image, right), but clues are provided by a number of features, as outlined below.
Oxycolpella manzavinioides is semi-circular, and wider than it is long (thus contrasting somewhat with the more circular outline of O. wreyi). The dorsal valve is smaller than the ventral. The beak is very small (and smaller than in O. wreyi) and the gap in the ventral valve that allows the valves to open (the pedicle passage) is minute. The ventral valve has a sulcus, but this is shallow and faint compared to O wreyi. The fold on the dorsal valve is correspondingly weak. Growth lines are well-marked and overlapping, and as in O. wreyi give the shell a somewhat wrinkled appearance in profile. 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). |
Two Oxycolpella species, both showing the ventral aspect:
O. manzavinioides (left); O. wreyi (right) Detailed description (from Trechmann 1917) Shell wider than long; valves only slightly convex near the beaks, the ventral rather more so than the dorsal, becoming flattened towards the margins. Near the anterior margin there is a broad but feeble ventral sulcus, and a low and broad rounded dorsal ridge bounded by wide, feebly-marked, lateral sulci. The area is small, and less than the width of the shell; the beak is very small and pointed, and projects but very slightly over the hingeline. The pedicle-passage is minute. The growth-interruptions are prominent, foliaceous, irregular, and widely spaced, and the interspaces are marked with very faint, regular, parallel, concentric stria. |