GNS Science

Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)

New Zealand Cenozoic Mollusca

Aethocola (Neocola) beta (Finlay, 1926)


(Pl. 25c): GS10344, D45/f8483, Calamity Point, Clifden, Altonian (GNS)

Beu & Maxwell (1990): Chapter 11; p. 226; pl. 25 c.

Synonymy: Austrofusus (Neocola) beta Finlay, 1926b, p. 234; Beu & Maxwell 1990, p. 226, pl. 25c

Type species of Neocola Finlay, 1926

Classification: Buccinulidae

Description: Very similar to A (Neocola) alpha, but tending to differ in the following ways — (1) in having a more prominent peripheral angulation, particularly on the last whorl, thereby giving the shell an angulate rather than a rounded outline, (2) in having more prominent nodules on the basal subangulation, and (3) in having 3 basal spiral cords noticeably stronger than other spiral sculpture.

Comparison: Finlay (1926b, p. 234) regarded the three species Aethocola (Neocola) alpha, A. (Neocola) beta and A. (Neocola) gamma as members of a simple evolutionary lineage leading from A. alpha through A. beta to A. gamma, a view that seems to be well supported by subsequent study of large suites of specimens collected from throughout most of the Clifden section. However, these collections show that, rather than a simple succession of well defined species with equally well defined stratigraphic ranges, each population passes gradually into the succeeding one, so that the boundaries between the species are blurred, i.e., the A. alpha-beta-gamma lineage appears to be a good example of "phyletic gradualism" (Eldredge and Gould 1972) or gradual anagenetic evolution. The main morphological changes are: 1) an increase in the prominence of the peripheral angulation (more strongly projecting in A. beta than in A. alpha, much sharper in A. gamma than in A. beta) and a similar change in the prominence of the basal angulation (rounded in A. alpha, slightly more marked in A. beta, sharply angled in A. gamma); 2) an increase in the strength of the basal nodules from A. alpha to A. beta. (The basal nodules on A. gamma are on the whole no stronger than on A. beta, but are more prominent because of the development of prickly sculpture where they are crossed by the basal cords); 3) a change from the rounded peripheral nodules of A. alpha and A. beta to the sharp, prickly tubercles of A. gamma, related no doubt to the increase in the sharpness of the peripheral keel; and 4) an increase in the relative strength of the basal cords from A. alpha to A. gamma. These changes are statistical in nature and involve an increase in the proportion of individuals with "advanced" (or derived) characters between successive populations, so that in adequate samples it is possible to find specimens that closely match those in younger and in older populations. This means that it is usually difficult confidently to assign individual shells to one or other of the species recognised in this lineage, and there is some doubt whether these species should be separated.

Specimens of Aethocola (Neocola) are uncommon in the units above the Slip Point Siltstone at Clifden, but those from the Lill Sand (Lillburnian) closely resemble typical A. gamma in shape and sculpture, although some are much larger than any Slip Point shell. However, the very few specimens collected from the basal Nissen Shellbed (Waiauan) differ from typical Slip Point shells in having a much more rounded peripheral keel and less strongly differentiated basal spirals, and therefore more closely resemble A. beta. The significance of this is unclear, but it may indicate that A. beta persisted outside the Clifden area during the Clifdenian and Lillburnian, only to return during the Waiauan, possibly in response to warming or cooling conditions. It is unfortunate that no other sections have been discovered where all three of the Clifden species occur in sequence, so at present there are no direct, independent checks on their inferred stratigraphic and evolutionary relationships. Nonetheless, these relationships are not contradicted by the scattered records of the nominal Clifden species from other parts of New Zealand. Thus, A. alpha is present in the Altonian of Tommy's Creek, Upper Waipara River, North Canterbury; upper Takaka River, northwest Nelson, and lower Tengawai River, South Canterbury (although otherwise similar specimens from Target Gully and Ardgowan Shellbeds, Oamaru (Altonian) differ in having distinct axial costae on the last whorl). Altonian specimens from Weka Creek, North Canterbury, however, are more like typical A. beta in sculptural development, suggesting that they are from a younger horizon than those from Tommy's Creek, a conclusion consistent with local stratigraphy but requiring confirmation from microfossil evidence. Good collections of Aethocola specimens have been made from several horizons of Altonian and Clifdenian age in the Oparara River and its tributaries, and from Little Wanganui River, West Nelson. Specimens from late Altonian horizons resemble those from the Long Beach Shellbed and the lower part of the Slip Point Siltstone (i.e. late Altonian-early Clifdenian), whereas Clifdenian specimens are very similar to Neocola from the middle part of the Slip Point Siltstone. Although we have seen no large or well preserved lots of A. (Neocola) from Tongaporutuan rocks, the available specimens from Hurupi Stream and nearby localities in Palliser Bay (type locality of A. (Neocola) marwicki) show no marked differences from Clifden specimens of A. gamma.

The Aethocola (Neocola) form occurring in great profusion in Altonian siltstone at Parengarenga Harbour, however, does not fit comfortably into the above scheme. It resembles A. beta and A. gamma in having strongly differentiated basal spiral cords, but differs from both of these species in lacking definite nodules on the basal subangulation. The absence of basal nodules also differentiates this form from A. alpha, some specimens of which do have such nodules. In addition, the Parengarenga species differs from all three Clifden species in its larger size (height often greater than 35 mm), its more elongate shape, and its longer siphonal canal, and in having a fasciole that is more like the one on A. flexuosa (see below). It evidently belongs in a different group from the Clifden species, and possibly evolved from A. flexuosa.

Finlay (1926b, p. 233) proposed Neocola for species of Aethocola that differ from typical members of the genus in being smaller and more solid, and in having a smaller aperture, a less concave columella, and a more prominent fasciole. The earliest known species appears to be that previously known as A. zitteli (Whaingaroan, The Cliffs, Nelson). Material from this locality is very poorly preserved but seems to have the diagnostic characters of the subgenus, although as noted under Aethocola (Zelandiella) subnodosa, the name A. zitteli is based on a specimen from Starborough Formation (Waipipian) in the Awatere Valley. The youngest record of the subgenus is the Opoitian A. ngatuturaensis (Pl. 37l); we are not aware of Kapitean records. Where present, Miocene species of A. (Neocola) often occur in large numbers and at most localities greatly outnumber specimens of other species of Aethocola.

Distribution: Altonian; Calamity Point Sandstone, Clifden (type); Long Beach Shellbed, Clifden; Weka Pass; New Chum Creek, Oparara, Karamea district.

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
ISBN 978-0-478-19705-1
ISSN 1177-2441
(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)


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