Camptonectes robusta Quilty, 1983
QUILTY, P. G. 1983. Bajocian bivalves from Ellsworth Land, Antarctica. New Zealand Journal of Geology and Geophysics, 26 (4): 395-418, figs. 1-60. [p. 403, figs. 15, 16]
1983 Camptonectes (Camptonectes) robusta Quilty, 1983
P. G. Quilty, 1983, figures 15-24.
|
«MATERIAL: Several specimens from localities 6 and 10. All but I are distorted. All are internal or external moulds, the best with very well preserved external sculpture. Locality 6: UTGD87247c. Locality 10: UTGD87322e (holotype), 87283a, 87330a, 87337d, 87341d (counterpart of 87337d). Holotype and paratypes are all from locality 10.
DIAGNOSIS: Shell higher than long; well-marked midline consistently at 110° to hinge line throughout ontogeny; sculpture more robust than usual in this genus. Anterior auricle very small.
DESCRIPTION (Based mainly on specimen UTGD87322e): Shell equivalve, inequilateral. Length 27 mm, height 28 mm, thickness to plane of commissure 4 mm. Hinge line 11 mm with umbo 3 mm behind anterior end. Angle between hinge line and midline 110º and seems consistent throughout ontogeny. Anterior auricle small, separated from body of valve by well-marked auricular sulcus which has posterior edge almost vertical to plane of commissure. Angle between hinge line and anterior margin of anterior auricle 110º. Angle between hinge line and anterior auricular sulcus 125º. Anterior auricle sculptured only with radial ribs, perpendicular to the auricular sulcus and thus directed anterodorsally. Posterior auricle larger, separated from body of valve by more marked auricular sulcus which makes an angle of 135º with hinge line. Posterior auricular sulcus well marked in 2 ways: (1) the anterior edge is angled at its maximum distance from the plane of commissure; (2) there is a change of sculpture. On body of valve at sulcus, pattern is radial only and perpendicular to sulcus. On the auricle, both radial and concentric patterns are preserved, and boundary between 2 patterns is a distinct line. Angle between hinge line and posterior margin 90º and concentric pattern perpendicular to hinge line, except near the auricular sulcus where it veers posteroventrally and dies out. Concentric pattern of continuous fine ribs. Radial pattern discontinuous and exists only between 1 concentric rib and next. It is weaker than the concentric.
Anterior auricular sulcus meets shell margin one-third of shell height from the hinge line, dorsally of the most anterior point, itself almost one-half shell height from the hinge line. Posterior auricular sulcus longer, meeting posterior extremity, which is two-thirds of shell height from hinge line. Main sculpture on shell body of diverging radial ribs, crossed by very weak growth lines. Ribs almost perpendicular to margin, but seldom actually at 90º. Ribs arise mainly in region of midline, which is well marked, and elsewhere by both dichotomy and intercalation. Dichotomy is particularly marked in the ventral area just anterior of the midline. Ribs least closely approximated in most anterior part of valve. Ribs very numerous, usually about 25 per centimetre. Internal moulds smooth. The specimen described is better preserved than most other forms in the fauna. Other specimens may be larger (length to 65 mm on UTGD87337d and 87341d) or smaller (21 mm on UTGD87330a). DISCUSSION: Several similar species have been examined at the New Zealand Geological Survey.
Marwick (1953) recorded C. cf. C. laminatus (Sowerby) and specimens have been examined from the Heterian at Captain King's, Kawhia Harbour. That species has much more delicate sculpture, though both species are about the same size. The Antarctic material is considerably better preserved than the New Zealand. C. (C). lens (WMC7453 from Middle Bajocian Dogger 8 of Talheim Wlirttemburg, West Germany) has similar size and amount of inflation. C. lens is more equilateral and has more delicate sculpture. Hayami's (1957) species of Camptonectes are represented in the New Zealand Geological Survey's WMC collection. The only similar one is C. (C.) cf. auritus, but the disc of that species is abraded smooth and the posterior sculpture preserved is much finer than on the Antarctic species, which is significantly smaller and has both radial and concentric sculpture on the posterior auricle. Camptonectes arcuatus (Sowerby) is different from the new species in being virtually equilateral. Camptonectes greenoughi was described by Skwarko (1974) from Bajocian sediments in Western Australia and had been recorded earlier by Coleman & Skwarko (1967) also from Western Australia. It is almost equilateral, has sculpture missing from the central parts of the valve, and less-regular more delicate sculpture of the margin of the valve. It also has a much more prominent anterior auricle. C. platessiformis White from Utah (Imlay 1964) is similar, but has a more circular disc. Details of the auricles are identical and the ribs are similar, but less regular, on C. platessiformis. ETYMOLOGY: From Latin robustus (strength) referring to the very strong appearance of the shell.
AGE: Middle-Late Bajocian.»
PATRICK GERARD QUILTY, 1983
|