Bekoma bidartensis Riedel and Sanfilippo

Bekoma bidarfensis Riedel and Sanfilippo, 1971, p.1592, pl.7, figs.1-7; Foreman, 1973, p.432, pl.3, figs.20- 21, pl.10, fig.6

DESCRIPTION

Cephalis and thorax together campanulate, with three robust feet. Cephalis approximately hemispherical, with very thick wall, not marked off from the thorax externally. Cephalis seems generally poreless, but with numerous depressions resembling infilled pores. In some specimens the apical and vertical spines extend beyond the cephalic surface as two weak, oblique spines. Thorax campanulate, with somewhat rough surface, proximally narrow and forming a type of "neck" merging with the cephalis with no change in contour. Thoracic pores subcircular, smaller and sparser proximally. From (and slightly above) the distinct distal rim of the thorax arise three long, thick, subparallel cylindrical feet with an outwardly-directed thorn in the distal half. In many specimens, the feet are irregularly hollow distally. Fragments of latticed abdomen are commonly present (Riedel and Sanfilippo, 1971).

DIMENSIONS

Based on 9 specimens. Length of cephalis plus thorax 145-165 m; maximum breadth of thorax 130-170 m (Riedel and Sanfilippo, 1971).

DISTINGUISHING CHARACTERS

Feet straight, not divergent, with an internal projection but not joined by a ring (Riedel and Sanfilippo, 1978a).

B. bidartensis differs from B. campechensis Foreman (1973) in not having the three feet joined by a triangular ring supported by inwardly directed thorns, and from B. divaricata Foreman (1973) in having less widely divergent, more robust feet (Sanfilippo et al., 1985).

VARIABILITY

The relatively large, thick-walled cephalis is usually provided with one or two small horns representing the apical and vertical spines. The outwardly directed thorn from each foot is approximately median or in the distal half of the foot, depending on the total length of the feet. Later forms have shorter feet. Weak abdominal meshwork is variably developed (Sanfilippo et al., 1985).

DISTRIBUTION

Few assemblages of late Paleocene to early early Eocene age are available, but this species has been found in the northern tropical Pacific, the northern Atlantic (~35N), the Caribbean region, and in France. Its morphotypic first appearance marks the base of the Bekoma bidartensis Zone and it becomes extinct within the Buryella clinata Zone.

PHYLOGENY

This species appears to have evolved from B. campechensis by degeneration of the ring joining the feet, but this is uncertain because of the unavailability of complete successions. It is the last representative of its genus.

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