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Phormostichoartus crustula Caulet , 1979

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Phormostichoartus crustula (Caulet)
Plate 6, figures 10-14

Description: Shell three-segmented, very heavy and thick-walled. Cephalis subspherical with a few irregularly scattered subspherical pores and a short vertical tube lying along the proximal part of the thorax. No apical horn. Collar stricture indistinct. Thorax annular, slightly longer than the cephalis, with 4 transverse rows of subcircular pores of uniform size. Lumbar stricture not well marked on small specimens of this species, but distinguished by the change on contour between thorax and abdomen in larger specimens. Abdomen cylindrical (small form) or inflated (large form) with 7 to 15 transverse rows of hexagonally framed, quincuncially arranged, circular pores similar to those on the thorax. Mouth constricted. Peristome is a smooth, poreless band.

Dimensions ( based on 22 specimens): length of cephalis and thorax 37-50 µm; of abdomen 43-110 µm. Maximum breadth of thorax 43-62 µm; of abdomen 50-96 µm.

Distinguishing characters: Both the collar and lumbar strictures are poorly defined in this species, giving it a globose appearance. Pores are quincuncially and rows are not so widely spaced as those of Siphocampe.

Remarks: In our upwelling assemblages we have found not only the typical form (pl. 6, fig. 11) of this species as originally described by Caulet (1979), but also a much larger, but morphologically idencical form (pl.6, fing. 10). The large form has been observed only in the Peru margin material where its LAD is at 0.4 Ma, The smaller form was observed in material form our Pacific and Indian Ocean sites. In the Peru and Somalian material its LAD is younger than 0.2 Ma, but is has never been observed in Holocene sediments ( Caulet 1979, table 7 herein). Whereas in the Oman material its last consistent occurrence is in the S. pentas Zone with only sporadic, possibly reworked, occurrences in younger sediments. P. crustala appears to have evolved in the late Miocene from a form with a longer, more robust vertical tube and a wider peristome (pl 6, fig. 14).
Nigrini and Caulet 1992











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