Buryella clinata Foreman

Buryella clinata Foreman, 1973, p.433, pl.8, figs.1-3, pl.9, fig.19; Foreman, 1975, p.620, pl.9, figs.35-36

DESCRIPTION

Shell of four segments, with none of the strictures expressed externally. Cephalis subspherical with a few very small circular pores, bearing a broad-based, sharp, bladed, apical horn, its length up to twice that of the cephalis. Ridges from the horn diverge and extend to the collar stricture except ventrally where two ridges rejoin to enclose a vertical pore and form an upwardly directed tube. Collar stricture internally with four collar pores. Thorax truncate-conical with circular pores randomly or quincuncially arranged. Third segment largest, inflated, its greatest dimension medianly or in the distal half, with circular to subcircular pores, quincuncially arranged in transverse and diagonal rows. Fourth segment inversely truncate-conical proximally, cylindrical distally with thinner wall, and pores subcircular to elliptical in transverse rows, termination ragged (Foreman, 1973).

DIMENSIONS

Based on 15 specimens. Length overall 140-195 m, length of cephalis and thorax 40-45 m, of third segment 50-75 m; greatest width 65-85 m (Foreman, 1973).

DISTINGUISHING CHARACTERS

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

The short, well developed apical horn has pronounced blades sufficiently wide to enclose the cephalis, and extending to the collar stricture. The third segment is the largest, with pores characteristically aligned transversely and diagonally (Sanfilippo et al., 1985).

Distinguished from its ancestor, Pterocodon ? anteclinata, by having a bladed rather than a conical apical horn.

VARIABILITY

Although the pores of the third segment typically are strictly aligned transversely and diagonally, very rare specimens have the pores of that segment irregularly arranged. The development of the fourth segment, and the degree of concavity of its outline, vary greatly (Sanfilippo et al., 1985)

DISTRIBUTION

This species is found in late early to earliest middle Eocene assemblages from a few tropical localities, and to about 30S (DSDP Site 248) and about 35N (DSDP Site 10). Its evolutionary transition from Pterocodon ? anteclinata marks the base of the Buryella clinata Zone and it becomes extinct at approximately the lower limit of the Theocotyle cryptocephala Zone.

PHYLOGENY

The limit between B. clinata and its ancestor Pterocodon(?) anteclinata Foreman (1975, p.621) is drawn where the base of the horn becomes as wide as the cephalis. Co-occurring forms with which Buryella clinata might be confused are B. tetradica, in which the abdominal pores are aligned both longitudinally and transversely, commonly with longitudinal ridges separating the rows; and an undescribed form with less prominent horn, conical thorax with rough surface, distinct lumbar stricture, and thick-walled, inflated abdomen with regularly arranged pores (Sanfilippo et al., 1985).

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