Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 43j): holotype, Castlecliff, Wanganui (Tainui Shellbed ?), Castlecliffian (TM4903, GNS; a seventh valve)
(Pl. 43l): posterior valve, Tainui Shellbed, Castlecliff, Castlecliffian (GNS; coll. Mrs E. Wolffe)
Beu & Maxwell (1990): Chapter 16; p. 333; pl. 43 j, l.
Synonymy: Cryptoconchus marwicki Bucknill 1928, p. 163
Description: Moderately large, nearly equidimensional valves, each to about 12.0 mm wide and 9.7 mm long (dimensions of holotype). Articulamentum enormously expanded into wide, smooth, antero-laterally extended, strongly convex laminae; in median valves each side deeply cut by a single slit at posterior quarter of length; in posterior valve, 5 closely spaced, deep slits around posterior end, dividing off 4 thick, square-sided, peg-like insertion plates; anterior valve not seen. Tegmentum very limited in area, forming narrowly triangular, finely sculptured pleural area on each side of long, narrow, smooth, very narrowly triangular jugum.
Comparison: Cryptoconchus marwicki is very similar to C. porosus (Recent; and Haweran, Te Piki, Cape Runaway = C. mucronatus Grant-Mackie & Chapman-Smith, 1971), but has a wider and longer jugum and slightly larger pleural areas; presumably C. marwicki was ancestral to C. porosus. C. porosus is the common living intertidal to shallow subtidal "butterfly chiton", a curious yellow-green to dark brown, slug-like animal up to 80 mm long, with only a smooth "skin" (the girdle) showing exteriorly, and with large knobs at each of the girdle spicule-tufts, but with a row of small slits along the dorsal mid-line revealing the tiny tegmental areas. We may assume, then, that C. marwicki was a similar large, slug-like chiton living in shallow water on rocky substrates. No earlier species are known in New Zealand, but it seems very likely that Cryptoconchus is the ultimate development of the common evolutionary trend in Notoplax towards mantle expansion and consequent reduction of the tegmentum and expansion of the articulamentum. In C. porosus, the tegmentum has been lost almost completely; the final step of complete tegmentum loss and completely internal valves has been taken by the huge western North American and North Pacific chiton Cryptochiton stelleri (Middendorf) (reaching 250 mm in length). Cryptoconchus appears to be an endemic New Zealand development from Notoplax, and is not closely related to Cryptochiton.
Distribution: Castlecliffian; Castlecliff, Wanganui (type), probably from Tainui Shellbed (provenance of all of the few recently collected valves); uncommon.
Cite this publication as: "A.G. Beu and J.I. Raine (2009). Revised
descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990). GNS
Science miscellaneous series no. 27."
© GNS Science, 2009
(Included with a PDF facsimile file copy of New Zealand Geological Survey Paleontological Bulletin 58 in CD version from: Publications Officer, GNS Science, P.O. Box 30368 Lower Hutt, New Zealand)