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Dorsal valve
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Athyris kaihikuana
Order: Brachiopod
Genus: Athyris Species: Athyris kaihikuana Location found: 88 Valley, Wakefield |
Although at their peak during the Palaeozoic, the Athyrids survived into the Triassic and managed to persist in limited numbers into the early Jurassic. Throughout the Triassic they evolved, and the progression of species is a useful stage-indicator throughout the period. There is some evidence that, as the diversity declined, the Athyrids became more specialised, and restricted to particular habitats. If so, they may also indicate something about environmental conditions. The changes, however, are somewhat subtle, and not always easy to define with certainty. There have also been several name changes over time which can add to the confusion. Early writers, for example, used the name Spirigera for the Triassic genera.
Athyris kaihikuana (previously Spirigera kaihikuana) is the oldest member of the family to be recognised in the Triassic sequence, and is restricted, as its name suggests, to the Kaihikuan stage. It is thus a good marker for this stage. The shell is generally smooth, save for growth-rings, with a prominent beak on the ventral valve and a smaller, down-turned beak on the dorsal valve. On the dorsal valve, good specimens show a low and faint fold running down from the beak; on ventral valves a shallow sulcus is visible. The thick shell that develops on either side of the ventral beak makes for considerable differences in shape between shelled and unshelled specimens - and can easily confuse. Note the differences between the shell-less specimen shown to the right, and the shelled fragment of the ventral beak in the lowermost image below. Partly because of this variability in form, not all specimens can be confidently assigned to A. kaihikuana; where in doubt, I have named them only to genus level. Further indication of the variations in the forms that Athyris might represent is given in the gallery of Athyris kaihikuana specimens from the collection. |
Ventral valve
Detailed description (from Trechmann 1917) Shell rather inflated, the ventral valve slightly more than the dorsal. Outline elongate-oval or subtriangular, often widening out towards the anterior margin, which is gently rounded. The ventral beak is prominent, and tapers gradually. The delthyrial cavity is more or less entirely concealed by the dorsal beak. There is a faint broad fold on the anterior part of the dorsal valve, and a very faint corresponding median depression on the ventral valve: consequently, the junction of the valves lies almost in one plane. The ventral apical region is much thickened at the sides with shelly material, but a passage for the pedicle-muscle always remains. |
Ventral (left) and dorsal (right) valves of a conjoined specimen. Note the pedicle opening (white disk) at the end of the beak in the right-hand image.
Five specimens of Athyris ?kaihikuana showing the range of forms depending on the valve and the state of the covering shell.
Top valve = dorsal; all others = ventral. Note the sulcus on the ventral valves, the narrow split in the beak and the thickness of the shell, where present.
Top valve = dorsal; all others = ventral. Note the sulcus on the ventral valves, the narrow split in the beak and the thickness of the shell, where present.
Further Reading:
Trechmann, C.T. 1917 The Trias of New Zealand. Quarterly Journal of the Geological Society 291, 165-246 (+ 10 plates).
Trechmann, C.T. 1917 The Trias of New Zealand. Quarterly Journal of the Geological Society 291, 165-246 (+ 10 plates).