Bekoma campechensis Foreman

Bekoma campechensis Foreman, 1973, p.432, pl.3, fig.24, pl.10, figs.1-2

DESCRIPTION

Shell of two segments with three feet joined by branches that form a triangular ring. Cephalis cone-shaped to rectangular in outline, bearing a conical or cylindrical apical horn and vertical horns. Apex of cephalis flattened, causing the angular appearance when viewed dorsally or ventrally. Cephalis and proximal thorax poreless with a thick hyaline wall, surface roughened with minute irregular papillae, occasionally with tiny spines or nodes apically. Collar stricture not defined externally as contour of cephalis merges with that of thorax. Internally six collar pores, the vertical spine rising very sharply so that the cervical pores tend to be almost in a vertical plane. Thorax proximally narrow, necklike, distally expanded, tending to have straight sides. Expanded portion with pores generally subcircular and subregular in size and arrangement, and surface roughened with small, generally pointed nodes. Fragments of thin lamellar lattice extend below the wide, open aperture. Three sturdy, proximally ridged, straight, divergent feet depend from the shell immediately above the smooth apertural rim. They are joined distally by a triangular ring, which is formed by a single inward directed branch of each foot bifurcating and curving slightly downward to join the similar branch of each adjacent foot (Foreman, 1973).

DIMENSIONS

Based on 20 specimens. Length of cephalis and thorax together 100-140 m (majority 120-135 m); width of thorax 105-130 m; length of feet 65-300 m (generally 120-200 m) (Foreman, 1973).

DISTINGUISHING CHARACTERS

This species is distinguished from other members of the genus by the cephalis and proximal, neck-like part of the thorax being thick-walled and hyaline, and by its three, long, circular in cross section, divergent feet that are joined distally by a triangular ring formed by a single inwardly directed branch of each foot (Sanfilippo, unpubl. data).

VARIABILITY

Bekoma campechensis is one of the most commonly found Paleocene radiolarians. It includes forms varying not only in shell size, but also in length, width and angle of divergence of the three feet. The cylindrical apical horn is quite variable in size, from long and robust to short (Sanfilippo, unpubl. data).

DISTRIBUTION

The morphotypic first appearance of this species defines the base of the Bekoma campechensis Zone. It evolved into Bekoma bidartensis in the late Paleocene and its morphotypic last appearance lies within the Bekoma bidartensis Zone.

PHYLOGENY

The origin of this species is not known, but it is the ancestor of Bekoma bidartensis.

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