Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 30l): GS3209, N34/f6272, Double Corner Shellbeds, Lower Waipara Gorge, North Canterbury, Waiauan (GNS)
Beu & Maxwell (1990): Chapter 13; p. 268; pl. 30 l.
Synonymy: Austrotoma obsoleta Finlay 1926b, p. 253
Classification: Turridae: Pseudotominae
Description: Very small for genus (to 28 mm high), with very short spire and long last whorl, tapered relatively little to short, widely open siphonal canal. Axial sculpture of very weak costae below the shoulder, fading out before last whorl of almost all specimens. Spiral sculpture of 2 narrow, closely spaced cords on very prominent, wide subsutural fold, followed below by smooth, deeply concave sutural ramp; whorl sides bear 4 (on spire) or about 12-14 (on last whorl) very prominent, wide, flat-topped cords, with each interspace slightly wider than 1 cord, and with weak secondary cords in a few interspaces of some specimens. Columella unusually well excavated in central area, followed below by straight section deflected to left. Siphonal canal with moderately deep notch, forming coarsely arcuate-sculptured fasciole, margined by prominent, narrow, smooth cord. Protoconch not seen; presumably as in other Austrotoma species, conical, of 4-5 whorls, first 2-3 whorls smooth, then developing flat-topped spiral cords, and finally developing narrow axial costellae over last half-whorl.
Comparison: Austrotoma obsoleta is easily recognised by its small size, its strongly concave sutural ramp, its low spire, its prominent, wide spiral cords, and its lack of axial sculpture on the last whorl. A very similar species with narrower spiral cords and with axial costae continuing onto the last whorl occurs in Tongaporutuan shallow-water sands in North Canterbury (common in Kaiwara River near the former Ethelton School), where it occurs with A. hurupiensis . A. hurupiensis is the only species common in early Tongaporutuan shallow facies ("Hurupi fauna") in South Wairarapa; it is a much larger species with a taller spire than A. obsoleta, and with a wide, elevated, smooth band below the shoulder angle.
Distribution: Waiauan; Double Corner Shellbeds, Lower Waipara Gorge, North Canterbury (type), and common in other North Canterbury localities with diverse soft-bottom, shallow-water Mollusca; also rare in Waiauan sandstone in southern Wairarapa.
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)