Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 31k): GS7176, M34/f7302, Middle Waipara River opposite "The Deans", North Canterbury, Waipipian or Mangapanian (TM5028, GNS)
(Pl. 31l): GS7176, M34/f7302, Middle Waipara River opposite "The Deans", North Canterbury, Waipipian or Mangapanian (TM4902, posterior valve, specimen collected by J. A. Thomson and identified by Thomson 1920 and Suter 1921)
Beu & Maxwell (1990): Chapter 14; p. 274; pl. 31 k,l.
Synonymy: Ischnochiton maorianus Thomson 1920, p. 365 (not of Iredale, 1914); Chiton quoyi Suter 1921, p. 46 (not of Deshayes, 1836); Amaurochiton n. sp. aff. glaucus Fleming in Wilson 1963, p. 60; Chiton n. sp., Beu & Maxwell 1990, p. 274, pl. 31k, l.
Description: Shell short and wide, large for fossil chitons (posterior valve 10.7 mm wide, 6.7 mm long; large incomplete median valve 17.8 mm wide, 7.4 mm long), highly arched. Sutural laminae small, thick at base, tapering rapidly to thin edge; insertion plates thick, short, with many narrow, shallow slits between the major slits ("pectinate"). Anterior valve slightly incomplete; bearing 9 slits in anterior edge; tegmental sculpture of about 60 high, wide, closely spaced, weakly nodulous radial costae. Median valve with 1short slit on each side; tegmental sculpture of high, wide, widely spaced, weakly nodulous costae, up to 13 radial ones on lateral areas and 65 longitudinal ones on pleural areas of large specimen. Posterior valve semicircular, steeply arched, with central mucro, 13- 15 slits around posterior margin, and 55 coarse tegmental costae spaced as on other valves.
Comparison: The new species is similar (and was perhaps ancestral) to the living Amaurochiton glaucus (Gray, 1828), but differs in its more highly arched dorsum, its narrower overall shape, and its much coarser sculpture; the unnamed species has 55 radial costae around the posterior end of the posterior valve, whereas large C. glaucus collected on the intertidal rocky shore at Island Bay, Wellington have 80-105 low, narrow, smooth, widely spaced radial costae around the posterior end of posterior valves. Thomson's (1920, p. 365) identification as Ischnochiton maorianus is understandable, as the sculpture resembles that of I. maorianus, but the dorsum is more highly arched and the insertion plates are much thicker and pectinate, not thin and smooth as in Ischnochiton. Amaurochiton glaucus is an abundant living chiton (known fossil from a few "raised beach" deposits, at Mahia and Dunedin, apparently all Holocene) of rocky shores, and the near-shore molluscan fauna at the Middle Waipara site (including Crassostrea, Limnoperna huttoni, Circomphalus katherinae, and Cominella kereruensis ) supports the interpretation that the unnamed species was intertidal as well.
Beu & Maxwell (1990) adopted broader genera for chitons (Polyplacophora) than were used in the single major monograph of New Zealand chitons, by Iredale & Hull (1929a-1932). However, most subsequent writers on Australian chitons have continued to use very narrow, "Iredalean" genera, and we now adopt an intermediate classification. Notoplax is used here as a genus for all the New Zealand acanthochitonid chitons with a wide girdle and short spicule tufts, with major spicules much less obvious than those of Acanthochitona, awaiting a molecular phylogeny of these poorly understood groups.
Distribution: Waipipian (or perhaps Mangapanian), Middle Waipara River at end of ridge opposite "The Deans", North Canterbury, fairly common, with Acanthochitona (Notoplax) n. sp. C (above).
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)