Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 6a): GS5502, J38/f6601, Opuha River, South Canterbury, below gorge, Bortonian (GNS)
(Pl. 6b): GS5502, J38/f6601, Opuha River, South Canterbury, below gorge, Bortonian (GNS)
Beu & Maxwell (1990): Chapter 7; p. 105; pl. 6 a, b.
Synonymy: Struthiolaria tuberculata concinna Suter 1917, p. 9
Type species of Monalaria Marwick, 1924
Description: Rather small for family (height 30-40 mm). Protoconch not known; teleoconch of 5-6 whorls, first 3 or so whorls strongly convex, later ones shouldered at about 0.6 whorl height on spire, last whorl excavated, with 2 (or in some shells, 3) narrow keels on base. Axial sculpture absent from early whorls (apart from fine growth lines), appearing on 3rd or 4th whorl as opisthocline, strongly opisthocyrt costae, reaching almost from suture to suture at first but eventually becoming restricted to shoulder angle where they form prominent, well spaced, sharp tubercles; 10-11 on last whorl. Basal keels with much weaker, more numerous tubercles. Adult shells typically with 1 or 2 previous varices retained on later whorls. Spiral sculpture commencing as 6-8 fine threads, which become relatively prominent on 3rd or 4th whorl. Interstitial threads appearing at about this stage, gradually increasing in strength while primaries decrease, so that last whorl is covered with numerous fine subequal threads. Aperture ovate, inner lip with moderately thick callus which does not obscure the straight, subvertical columella. Outer lip similar to that of Conchothyra australis (PI. 2g, h), i.e. almost straight from suture to shoulder angle and strongly convex below, with a broad, shallow anterior canal. Outer lip callus-development variable, apparently stronger in specimens from shallow-water facies than from deep-water ones.
Comparison: Monalaria concinna is one of the most distinctive Middle Eocene molluscs from New Zealand, and is readily distinguished from other struthiolariids by its mid-spire sculpture of strongly opisthocyrt costae and relatively strong spiral cords, its axial sculpture of sharp tubercles on later whorls, and its strongly sinuous outer lip, with deep anterior and posterior sinuses. Two other species of Monalaria, originally recognised as "subspecies", have been described. M. gracilis (Dannevirke Series?, poorly dated, Kaitangata) is smaller and more slender than M. concinna and has smaller, more closely spaced tubercles on the shoulder angle. M. filata (Mangaorapan, Otaio Gorge) is more like M. concinna in shape but has apertural callus developing at a much smaller size (at about 15 mm shell height). Stilwell (2002) described a fourth species, M. poliveta, from Kauru Formation (Mangaoparan-Heretaungan?) in the bed of Waihao River upstream from "Corrody" Station and downstream from McCulloch's Bridge. M. poliveta has much smaller and more numerous peripheral tubercles and a shallower anterior sinus than all other species, and is not far removed from Perissodonta, and in view of the poor knowledge of the age of other pre-Bortonian species, is possibly the earliest species of Monalaria. Stilwell (2002) also considered M. gracilis and M. filata to be synonyms, but in view of the long anterior canal and deep anterior sinus he illustrated on some GNS specimens from Otaio Gorge, this synonymy may not be correct. Early species of Monaralia are too poorly known for the evolutionary history of the genus - and so, the early history of the Struthiolariidae - to be clear at present.
Distribution: Bortonian, Waihao Greensand, Waihao Downs (type) and recorded from numerous localities in North Otago, South and mid-Canterbury, Westland and Northland. Moderately common in shallow-water, sandy bottom assemblages, rarer in deeper-water faunules (e.g. Hampden Beach).
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)