Buryella tetradica Foreman

Buryella tetradica Foreman, 1973, p.433, pl.8, figs.4-5, pl.9, figs.13- 14

DESCRIPTION

Shell of four segments subovate, smooth, with none of the strictures expressed externally. Cephalis subspherical, with a few circular pores, bearing a short, sharp horn or thorn. Collar stricture with four collar pores internally, and vertical pore expressed as a short horizontally directed tube. Thorax conical, with circular pores arranged in approximately three vague transverse rows or more irregular. Third segment largest, inflated, with greatest dimension medianly or in the distal half. Uniform circular pores are arranged in longitudinal and four to five transverse rows. Area between the longitudinal rows raised. Last segment inverted truncate-conical, with a smooth thinner wall and two to seven transverse rows of circular to elliptical pores. Distal margin scalloped or even, without a differentiated rim (Foreman, 1973).

DIMENSIONS

Based on 20 specimens. Length overall, exclusive of horn, 120-185 m, length of cephalis and thorax 35-45 m, of third segment 40-65 m, of last segment 70-95 m; greatest width 70-95 m (Foreman, 1973).

DISTINGUISHING CHARACTERS

The third segment is the largest, and its pores are aligned longitudinally and transversely (Riedel and Sanfilippo, 1978a).

B. tetradica is distinguished from B. pentadica Foreman (1973, p.433), its ancestor, by having only one post-cephalic segment above the widest segment, instead of two. The difference from B. clinata is indicated under that species (Sanfilippo et al., 1985).

VARIABILITY

The four-segmented, subovate shell usually shows no external expression of the strictures, though rare early specimens have a lumbar stricture. Also in early specimens, the fourth, more delicate segment is transversely subdivided. The pores are uniform, circular, and rectangularly arranged in longitudinal and transverse rows. The area between the longitudinal rows of pores is usually raised (Sanfilippo et al., 1985).

DISTRIBUTION

This species is found in Paleocene to early early Eocene assemblages from tropical localities in all three major oceans. It has been found in Subantarctic sediments from the Pacific (ODP Site 700) and the Atlantic (DSDP Sites 327 and 329). It is found in the oldest Paleocene material presently available and becomes extinct within the Buryella clinata Zone.

PHYLOGENY

This species evidently developed from B. pentadica, and no descendants have been recognized.

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