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Aphetocyrtis gnomabax Sanfilippo and Caulet, 1998

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Description: Three-segmented cylindrical shell with a variable abdomen. Apical spine partially free within the thick walled cephalis protruding outside as a short apical horn with small holes or arches at its base. Paired auxiliary spines attach the apical spine to the cephalic wall. Well differentiated mitral arches depart from the apical spine a quarter of the distance down from the top of the cephalis and join the cephalic wall before reaching the primary laterals. Median bar inclined towards the thoracic cavity giving off a vertical spine directed upward and protruding outside as a small spine. Dorsal and primary lateral spines are prolonged in the upper part of the thoracic wall as weak ribs, sometimes protruding outside as short thorns. Cephalic wall relatively thick with numerous small circular pores. Collar stricture well marked externally and commonly obscuring the horizontal secondary laterals. Thorax and abdomen with subcircular pores quincuncially arranged and variable in size. Commonly the abdominal pores are one to one and a half times the size of the thoracic ones. Thoracic and abdominal segments are variable in width, length and robustness. Early Oligocene forms have a conical thorax (due to the extended primary lateral spines being included in the wall), while later ones have a more subspherical thorax (shorter or absent primary lateral spines in the wall). Small surface thorns are commonly found on the earliest forms. but are rarely found later in the stratigraphic range. Abdomen subcylindrical and generally wavy in the earliest forms to subspherical in the Late Oligocene specimens Termination when observed intact is a simple peristome with irregular pores, or in Late Oligocene to Early Miocene forms it may be closed.

Etymology: To refer to the general shape of this species the name is derived from the Greek nouns gnoma (neutral) token, and abax (masculine) gaming board.

Distinguishing characters: A.gnomabax is distinguished from both A. rossi and A. catalexis by an apical spine that is not entirely included in the cephalis wall, by the mitral arches being located some distance down from the upper part of the cephalic wall, and by three ribs in the thoracic wall originating from the dorsal and primary lateral spines. It differs from Lophocyrtis (Apoplanius) aspera in having small holes or dimples at the base of the apical horn, well differentiated mitral arches and no secondary cephalic horns.

Distribution: Rare in Middle Eocene sediments at Site 94; first occurrence unknown; last occurrence at the Middle Late Eocene boundary. Rare to common in Middle Eocene to Late Oligocene sediments in the investigated Antarctic sites; first occurrence unknown; last occurrence in the Late Oligocene Lychnocanoma conica Zone (25.5 to 25.8 Ma).

Phylogeny: A. gnomabax, the first member of this lineage, evolves into A. rossi through gradual inclusion of the apical spine in the cephalic wall, thickening of the mitral arches and shortening of the apical horn until it is lost, or only a small thorn. The ancestor is unknown, but just below the Eocene Oligocene boundary this species evolves into A. rossi.
Sanfilippo and Caulet 1998


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