Thyrsocyrtis (Pentalacorys) triacantha (Ehrenberg)

Podocyrtis triacantha Ehrenberg, 1873, p.254; 1875, pl.13, fig.4; Sanfilippo and Riedel, 1982, p.176, pl.1, figs.8-10, pl.3, figs.3-4 (with synonymy)

DESCRIPTION

General form conical. Cephalis subspherical, with few small pores, bearing cylindrical horn several times its length. Collar stricture distinct. Thorax considerably shorter than abdomen, broadly conical or slightly inflated, with small subcircular pores. Lumbar stricture usually slight. Abdomen broadest segment, robust, with large subcircular pores, distinct peristome, and occasionally thorny surface. Three feet cylindrical, of variable length (often longer than abdomen), arising from peristome, curved with convexity outward, with terminations simple or ragged, rarely forked, and rarely bladed proximally (Sanfilippo and Riedel, 1982).

DIMENSIONS

Based on 25 specimens. Length excluding horn 240-370 m, of cephalothorax 65-85 m, of abdomen 85-125 m; maximum breadth 130-195 m (Sanfilippo and Riedel, 1982).

DISTINGUISHING CHARACTERS

T. triacantha differs from its ancestor T. tensa in the feet being convex outward, from T. tetracantha in having a differentiated peristome and constantly three feet, and from T. lochites in the feet originating from the peristome rather than slightly above it (Sanfilippo et al., 1985).

VARIABILITY

Cephalis with a variable cylindrical or elongate-conical horn. Thorax broadly conical to slightly inflated, decreasing in size in late forms (from 102 m wide in early specimens to 83 in late forms, and in length from 80 m to 62 m). The thick-walled abdomen is broader than the thorax, occasionally thorny, with large subcircular pores (10-13 on a half-circumference in early forms and 5-7 on a half circumference in late forms). From a distinct peristome [originate] three cylindrical feet of variable length, (often longer than the abdomen), curved with the convexity outward, with terminations simple or ragged, rarely forked (Sanfilippo et al., 1985).

DISTRIBUTION

T. triacantha occurs commonly in tropical assemblages from middle middle through late Eocene. Its morphotypic last appearance lies within the Calocyclas bandyca Zone. Its evolutionary transition from Thyrsocyrtis tensa is approximately synchronous with the lower limit of the Thyrsocyrtis triacantha Zone.

PHYLOGENY

This species evolved from T. tensa and into T. tetracantha. T. lochites is an evolutionary offshoot from this lineage.

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