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Acrosphaera? labrata Lazarus, 1992

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Single spherical or elliptical shell, lackingspines of any sort, with fairly small, irregularly shaped, irregularly distributed pores. Many pores are surrounded by low, smooth, thick rims. Some specimens have more angular pores, and low straight ridges on the surface.

Comments:
Acrosphaera? labrata most resembles the primarily tropical to subtropical species Acrosphaera murrayana (Haeckel), but it has much thicker rims around the pores and lacks the corona of short spines typical of A. murrayana (Nigrini and Lombari, 1984). Most authors (Srelkov and Reshetnyak, 1971; Nigrini and Moore, 1979; Takahashi, 1981; Nigrini and Lombari, 1984) have included within A. murrayana specimens with coronal spines and radial tubes of varying lengths. Goll (1980) referred to specimens with coronal spines, but without even a short tube as A. murrayana, and specimens with a short tube as A. trepanata. The Antarctic material referred to as A. murrayana in this report would, under Gollīs criteria, be A. trepanata. Rare specimens of typical low-latitude A. murrayana morphotypes, with fairly thin, low, raised rims an short coronal spines, were seen in middle late Miocene sample 120-747B-4H-CC, near the base of the stratigraphic range of A.? labrata.
Early specimens of A.? labrata have very poorly developed rims and look much like specimens of A. murrayana without coronal spines. Thus, A.? labrata may have evolved in the late Miocene Antarctic from subantarctic populations of A. murrayana, although present observations are not sufficient to prove this conjecture. A.? labrata is questionably referred to the genus Acrosphaera because of this inferred evolutionary relationship to A. murrayana, although the lack of external spines in A.? labrata does not fit with the formal definition of the genus. The relative rarity of A.? labrata in northern- most Site 747- indicates a preference for more polar, antarctic-water-mass conditions for this species. The presence definition of A.? labrata is rather broad, including forms with rounded and rimmed pores and those with much more angular pores, forms with reduced rim development, and those with ridged surfaces. These latter morphotypes may well belong to another species, but more detailed work will be needed to determine if this is so.
Lazarus 1992


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