Artophormis gracilis Riedel

Artophormis gracilis Riedel, 1959, p.300, pl.2, figs.12-13; Riedel and Sanfilippo, 1970, p.532, pl.13, fig.6; 1971, pl.3B, figs.5-7, pl.3B, figs.5-7, pl.6, fig.7

DESCRIPTION

Shell conical above, subcylindrical below, consisting of four segments separated by moderately distinct strictures. Cephalis globular, hyaline or with a few small pores, bearing an apical spine, which may be simple conical, multiple, or latticed. Thorax truncate-conical and abdomen inflated annular, both of these segments rather thick-walled, with rough surface and pores subcircular or circular, irregular in size and arrangement. Fourth segment consisting essentially of four to twenty longitudinal ribs joined together by a rather delicate lamellar meshwork with irregular pores; the longitudinal ribs terminate in free spines, and in some specimens are not clearly distinguishable from the meshwork (Riedel, 1959).

DIMENSIONS

Based on 30 specimens. Length of apical horn 3-63 m; of cephalis 20-30 m; of thorax 38-58 m; of abdomen 43-78 m; of fourth segment (including terminal spines) 25-145 m. Maximum breadth (at third or fourth segment) 95-145 m (Riedel, 1959).

DISTINGUISHING CHARACTERS

Artophormis gracilis is distinguished from its ancestor, A. barbadensis, by the third segment being generally shorter, and inflated-annular rather than truncate-conical.

Poorly preserved specimens, lacking most of the fourth segment, must frequently be identified by the general form of the first three segments, and the remaining stubs of the longitudinal ribs, which are more widely spaced than the bars between the pores of the distal part of the third segment. Unless care is taken to ensure that the top of the third segment does not have a single row of large pores, incomplete specimens of Eucyrtidium diaphanes Sanfilippo and Riedel (in Sanfilippo et al., 1973, p.221, pl.5, figs.12-14) can be mistakenly identified as A. gracilis. Another character useful in their distinction is that the thorax of E. diaphanes is more spherical than that of Artophormis gracilis (Sanfilippo et al., 1985).

VARIABILITY

This species is broadly defined, to include forms in which the wall of the second and third segments is moderately thick and regularly latticed, or (in some early specimens) irregularly thickened by a network of ridges, and in which the apical horn is simple, multiple, composed of a spongy meshwork, or cylindrical with thorns distally. The total length of the first three segments (excluding the horn) is usually 110-135 m, and the maximum breadth of the third segment is usually 85-125 m.

The fourth segment is more delicately constructed than the first three, and consists of 15-20 longitudinal ribs about 25 m long in early specimens, changing gradually to 6-10 much longer ones in late specimens, these ribs being followed by a porous lamellar subcylinder in the early specimens, and connected by an irregular meshwork in later specimens. Toward the end of its range, many specimens completely lack the meshwork of the fourth segment, which is reduced to 4-8 terminal feet up to 100 m in length. Thus, the fourth segment in early specimens is subcylindrical and latticed, with a row of larger pores (between the short longitudinal ribs) at its junction with the third, in the mid-part of its range consists of irregular lattice between longer ribs, and at the end of its range consists only of a small number of terminal feet (Sanfilippo et al., 1985).

DISTRIBUTION

This species is widespread in assemblages of early Oligocene to early early Miocene age in low and middle latitudes of all oceans, including the Mediterranean region. It evolved from Artophormis barbadensis within the Theocyrtis tuberosa Zone and became extinct at approximately the lower limit of the Cyrtocapsella tetrapera Zone.

PHYLOGENY

Evolved from Artophormis barbadensis just above the Eocene-Oligocene boundary.

REMARKS

Additional illustrations can be found in Moore, 1971, pl.5, figs.10-11; Sanfilippo et al., 1973, pl.2, figs.12-13.

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