Anthocyrtidium angulare Nigrini

Anthocyrtidium angulare Nigrini, 1971, p.445, pl.34.1, figs.3a-3b

DESCRIPTION

Cephalis trilobate, elongate, with subcircular pores, bearing a stout three-bladed apical spine of approximately the same length as the cephalis. Thorax shaped like a biretta. Three stout thoracic ribs, which may become external to form short, thorn-like wings, control the shape of the upper thorax. There is a sharp break in shell contour where the ribs terminate, and the lower thorax is approximately cylindrical. Pores circular to subcircular, usually arranged longitudinally. Eight to eleven, three-bladed subterminal teeth, directed outwards, are usually present, but may be absent or much reduced. Distally from the subterminal teeth, the thoracic wall curves sharply inwards and terminates at a narrow, poreless peristome that often bears numerous short, delicate, lamellar, terminal teeth, directed downwards and inwards (Nigrini, 1971).

DIMENSIONS

Based on 20 specimens. Length of apical horn 18-27 m; of cephalis 18-36 m; of thorax 36-81 m; of subterminal teeth 9-18 m. Maximum breadth of cephalis 18-27 m; of thorax 72-100 m (Nigrini, 1971).

DISTINGUISHING CHARACTERS

Three stout thoracic ribs, which may become external to form thorn-like wings and which control the biretta shape of the thorax (Nigrini and Caulet, 1988).

A. angulare may be distinguished from A. ophirense (Ehrenberg, 1872a, p.301) by its smaller size (thoracic length in A. ophirense 81-119 m, and in A. angulare 36-81 m) and by the distinctive shape of its thorax (Sanfilippo et al., 1985).

VARIABILITY

This two-segmented pterocorythid is characterized by angular "shoulders". Some forms show thorn-like projections at the shoulders. When such projections are not present, the shoulder may be rounded, but the shell always has a distinct break in contour between the upper, biretta-shaped part and the lower cylindrical part (Sanfilippo et al., 1985).

DISTRIBUTION

Rare in the tropical Indian Ocean; few to common in the tropical Pacific Ocean.

Rare to common from the upper part of the Pterocanium prismatium Zone to the top of the Anthocyrtidium angulare Zone. Morphotypic last appearance marks the upper limit of the early Quaternary Anthocyrtidium angulare Zone. Both morphotypic first and last appearances are reliable and synchronous datum levels. More common in the Pacific Ocean than in the Indian Ocean.

PHYLOGENY

Possibly derived from A. ophirense (Nigrini and Caulet, 1988).

Subsequent unpublished studies by Nigrini and Caulet suggest that Anthocyrtidium jenghisi (Streeter, 1988, p.63) might be a more likely ancestor for this species.

REMARKS

Additional illustrations can be found in Johnson and Knoll, 1975, pl.1, figs.3a-3b.

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