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Larcopyle polyacantha polyacantha (Campbell and Clark, 1944)

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Larcopyle polyacantha:
Description. The shell is sub-cylindrical to elliptical and varies in size from 150 µm to 250 µm (the original description gives the length of the shell as 180 µm). Most specimens have clusters of short to medium length, subparallel to weakly radial spines on one or both poles. The outer wall is perforated by small- to
medium-sized, irregularly arranged pores, frames are absent in most specimens. The inner structure is made up of weakly increasing, single or double spirals, although in some specimens the internal structure is so spongy that the spirals are nearly undetectable. In some subspecies the shell shows characteristic deviations from a simple ellipsoidal shape, which is caused by the development of caps at the poles. A pylome is obvious in most specimens.
Occurrence. Early Miocene–Early Pliocene.
Remarks. Larcopyle polyacantha was first described by Campbell & Clark (1944) from North Pacific (California coast) sediments and, to some degree, can be considered a ‘garbage bag’ type of taxon. The various forms in this group are related closely to each other and (as also noted by Campbell & Clark) show a great range of morphological variation. Differences between forms are subtle and in poorly preserved individuals a precise assignment may not be possible. In this study the majority of the morphotypes that exist in this group have not been differentiated, particularly in the early Neogene. However, there is a formal division of Larcopyle polyacantha into three subspecies – L. polyacantha polyacantha, L. p. titan and L. p. amplissima. It has been decided to distinguish the L. p. titan and L. p. amplissima morphotypes as they provide useful stratigraphic information. Larcopyle polyacantha titan, under the name Prunopyle titan, has been used extensively for biostratigraphy in late Neogene Antarctic sediments for many years. P. titan is a species that was also described originally by Campbell & Clark (1944) from California. True P. titan morphotypes, however, are seen in Antarctic sediments only in the early Neogene, where they are generally rare. This form, in the new combination Larcopyle titan, is re-illustrated from the original type series material. There are many different morphotypes within this group, some of which appear to be restricted to the Early or Middle Miocene in the Antarctic. There are transitional specimens between L. p. polyacantha and the other named subspecies (L. p. titan and L. p. amplissima) which occur throughout the stratigraphic intervals in which these latter subspecies are found.
In the early Miocene, there are gradational specimens between L. p. polyacantha and several of the other species described in this study, including L. nebulum and L. hayesi (variety irregularis). There also appear to be some regional differences: for example, specimens that correspond to Campbell & Clark’s (1944) figured specimen number 7 (pl. 7) from California have not been seen in this Antarctic material. The specimens illustrated by Nigrini & Lombari (1984) are somewhat larger, and have a thinner cortical shell, than is typical of most of the Antarctic material studied here, but otherwise fall within the range of variability for this species group.
Lazarus et al











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