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Aphetocyrtis rossi Sanfilippo and Caulet, 1998

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Description: Three-segmented, very variable form with a somewhat flattened cephalis. Cephalis small, poreless, or with very small, subcircular pores. Apical spine included in the cephalic wall, rarely protruding externally as a small apical horn. Distinct mitral arches on the inner wall of the cephalis give it a flattened appearance. Cephalic wall thicker in Early Oligocene forms giving the cephalis the appearance of being sunken into the wall or the upper part of the thorax. Thorax campanulate in Late Eocene forms, hemispherical in Early Oligocene specimens. Pores quincuncially arranged. Lumbar stricture marked by an internal ring, moderately to well expressed externally. The thoracic pores continue across the lumbar stricture. Size and shape of the abdomen are very variable from cylindrical to wavy, and its width is as broad as the thoracic width in Middle to Late Eocene forms. In early forms the abdominal segment is commonly wavy, rarely closed by a lattice plate, and the pores are generally of same size as the thoracic ones. In forms possessing a wavy abdomen, the pores frequently are very irregular in size and arrangement distally from where the change in contour occurs. In Early Oligocene forms the length of the abdomen increases substantially, and its width is generally greater than that of the thorax. The abdominal pores are larger than the thoracic ones. During the acme of their abundance the abdominal segment is very variable and increases in size. The termination may be closed by a lattice plate with a small short central tube formed by three or four coalescing spines, or open with the pores on the distal third of the abdomen irregular in size and arrangement. Occasionally the entire test may be covered by a spongy meshwork. After the acme, the size of the abdomen is strongly reduced and the pores become more regular in size and arrangement.

Etymology: This Antarctic form was first figured by Chen (1975b) from deep sea material recovered on DSDP Leg 28 Site 274 in the Ross Sea. hence the name rossi.

Distinguishing characters: A. rossi is distinguished from its descendant A. catalexis by not having a small apical horn with holes or dimples at the base. The general outline of A. rossi is similar to that of L. (A.) aspera; but the cephalis of L. (A.) aspera is more spherical, bearing a stronger apical horn and small
secondary horns. L. (A.) nomas differs from A. rossi by having a shorter abdomen, a longer cephalothorax, and by having an apical spine that is partially free in the cephalis.

Variability: Some large specimens have a secondary layer of spongy material covering the shell.

Distribution: Rare occurrences in a single sample at the Eocene-Oligocene boundary at Site 94. Common in all Antarctic sites; first occurrence in the Eucyrtidium spinosum Zone (36 to 37 Ma); last occurrence in the Lychnocanoma conica Zone (23.9 to 28.4 Ma). The acme of this species ranges between 31 and 33.5 Ma.

Phylogeny: A. rossi evolved from A. gnomabax by reduction of the size of the apical horn, disappearance of the small holes or dimples at the base of the apical horn, and the development of strong mitral arches. A. rossi gradually evolves into A. catalexis in the Middle Late Oligocene.
Sanfilippo and Caulet 1998


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