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Ventral valve
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Mentzeliopsis spinosa
Order: Brachiopod
Genus: Mentzeliopsis Species: Mentzeliopsis spinosa Location found: 88 Valley, Wakefield |
Mentzeliopsis spinosa is a distinctive and relatively easily recognisable fossil amongst the Triassic brachiopods of New Zealand. It is characterised not only by the pimple-like remnants of the spines that give it its name, but also by its rather butterfly-like shape, its strong beak, distinct but not deep ventral sulcus, slightly flared ribs and clear, quite widely spaced growth lines. The growth lines sometimes overlap the younger shell, giving it a wrinkled appearance. The dorsal valve is generally less convex than the ventral valve, and has a weak central fold that matches the sulcus on the opposite valve. The lip of the shells (i.e. the commisure) thus has a slight wave at the front where the ventral sulcus dorsal meet the margin (i.e. it is plicate).
A more detailed description from Trechmann (1917) is given in the panel to the right. In the Nelson area, M. spinosa has often been found along with Alipunctifera kaihikuana in coarse sandy or gritty deposits (as in the example shown above). Reflecting this depositional environment, the animal must have been rather robust and, although the shell is often lost, its cast seems to survive fossilisation well. Because it occurs in gritty sediments, it's also often relatively easy to extract. For all these reasons it is probably over-represented in the collection - hence the large gallery of specimens here. All the finds to date have been in rocks of Kaihikuan age, but GNS's Fossil Record Electronic Database (FRED) shows that it has also been reported from older, Etalian stage deposits. Further Reading: Trechmann, C.T. 1917 The Trias of New Zealand. Quarterly Journal of the Geological Society 291, 165-246 (+ 10 plates). |
Dorsal valve. Note the pimply apperance of the surface, showing the spines
Detailed description (from Trechmann 1917) The shell is slightly wider than it is long; the area is rather concave, less than the greatest width of the shell, and striated parallel to the hinge-margin. The ventral valve is inflated and arched; the beak is prominent, tapers rapidly, and is rather bent. over the area. The delthyrium is triangular and deeply sunken. The ventral valve has a feeble, wide, median sulcus and very feeble lateral dorsal folds. The dorsal valve is much flatter than the ventral, and has a broad, rapidly-widening, triangular, slightly-raised fold, on each side of which are about five very feeble, narrow, lateral folds. The growth-lines are rather widely spaced and foliaceous, and are sometimes produced into tubular spines. The latter also occupy the spaces between the growth-lines, but are not very closely set. They attain a length of 3 or 4 millimetres. |