N45

Pterocanium trilobum (Haeckel)

Dictyopodium trilobum Haeckel, 1860, p. 839

Pterocanium trilobum (Haeckel), Nigrini, 1967, p. 71, pl. 7, figs. 3a, b

DESCRIPTION

"Cephalis small, spherical with closely spaced pores, or pits (probably representing infilled pores), and bearing a stout conical apical horn approximately twice its length). Apical and vertical spines free within cephalis; both project to form external horns, the vertical horn being shorter and very much more delicate than the apical. Sometimes additional by-spines form on both cephalis and thorax.

"Thorax is an inflated tetrahedron with circular to subcircular pores, arranged in longitudinal rows. Thoracic ribs strong, becoming stout 3-bladed feet, latticed proximally and tapering to a point distally. Feet are divergent, convex outwards, as long to half again as long as thorax.

"Only traces of an abdomen are preserved. In most specimens a few lattice bars are present, and these seem to form a single row of large pores adjacent to the thorax, and sometimes one large pore bordering on the proximal end of the feet. The rest of the abdomen has smaller pores and apparently hangs free of the feet." (from Nigrini, 1967).

"The Antarctic representatives of this species do not show the great range of variation described by Popofsky [1913]. Most Antarctic individuals are thicker than those previously described. By-spines were rarely detected on the thorax, though th vestiges of by-spines were commonly observed on the cephalis. The basal feet are invariably present and sometimes strongly developed. The mouth is often slightly constricted but never closed." (from Hays, 1965).

"There is great variability in the overall shell size. Some specimens were very similar to P. praetextum, while others were three times as large." (from Renz, 1976).

DIMENSIONS

"Length of apical horn 27-54 microns; of cephalis 18-27 microns; of thorax 63-100 microns; of feet 90-173 microns. Maximum breadth of cephalis 23-27 microns; of thorax 90-136 microns." (from Nigrini, 1967)

"Length of apical horn 23-68; of cephalis 17-30, of thorax 74-144, of feet 57-171." (from Hays, 1965).

Plate 23, figures 4a-c

N46

REMARKS

1. For a more complete synonymy, see Nigrini, 1967.

2. Benson's (1966) description and dimensions of this species (Pterocanium prosperinae Ehrenberg in Benson, p. 405) are consistent with the above, but it is likely that of the three specimens illustrated by him only the one shown in Plate 27, fig. 4 is P. trilobum.

RECENT DISTRIBUTION

1. Hays, 1965; Pterocanium trilobum like Saturnulus planetes, with which it is usually associated in Antarctic sediments, is not found in the Recent surface sediments south of the Polar Front and was only rarely observed from surface sediments north of the Polar Front in this study. It was, however, reported to be cosmopolitan in the Challenger material from the Atlantic and the Pacific. It occurs at depth in some cores ranging back to zone phi."

2. Nigrini, 1967, fig. 37; "Indian Ocean occurrences - P. trilobum is fairly abundant (1% to 13% of the described population) in low latitudes, but south of 35 degrees S. It never forms more than 1 percent of the described population. It appears to be a reliable and potentially useful member of the low latitude assemblage."

3. Nigrini, 1970, fig. 26; belongs to a tropical assemblage derived by recurrent group analysis of North Pacific samples, but the species does range as far north as 40 degrees N right across the Pacific.

4. Molina-Cruz, 1975, Code N28; used in factor analysis of southeast Pacific assemblages; cf. Appendix 10 for percent N28 at each station.