Allonautilus: A new genus of living nautiloid cephalopod and its bearing on phylogeny of the Nautilida

Journal of Paleontology, Nov 1997 by Ward, Peter D, Saunders, W Bruce

Distribution.-Live-caught specimens of A. scrobiculatus have been described from Manus Province, Bismarck Archipelago, Papua New Guinea, where the species occurs with Nautilus pompilius in a 1:7 ratio, and appears to be cohabiting the same deep forereef niche as a scavenger and opportunistic predator, at -150-400 m depth (Saunders et al., 1987; Saunders, 1990). One beach-stranded specimen from Milne Bay, Papua New Guinea, was illustrated by Willey (1902). Drift shells are known from the southern Papua New Guinea region, as far south as the Solomon Islands.

Material.- None of the original type material has been located. Annotated copies of the Portland Catalogue indicate that Dillon purchased specimen 3906 and that George Humphrey(s) bought specimen 3963. Unfortunately, the whereabouts of the two Portland specimens (which would be syntypes) and those of Lister, is presently unknown. Live-caught specimens and drift shells from Manus, Papua New Guinea, are reposited at the U.S. Museum of Natural History, Washington D.C., (USNM 816504, 816505, 885683-885688).

ALLONAUTILUS PERFORATUS (CONRAD, 1847) Figures 7.4-7.5, 8

Nautilus perforatus CONRAD, 1847, p. 214. Nautilus pompilius Var. perforatus, MILLER, 1947: p.13. Nautilus scrobiculatus Lightfoot. SAUNDERS, 1987: p. 36.

Diagnosis.-A species of Allonautilus in which the shell bears reticulate ornament prior to nepionic constriction; postnauta shell smooth, ornamented only by growth lines, with thick rugae, or ribs, in adult shell.

Description.-Allonautilus perforatus is similar in size, umbilication, and coloration to A. scrobiculatus. Mature shells are ca. 180 mm diameter, and sexual dimorphism is apparent, with females being slightly smaller, and with more compressed apertures. The surface of the shell is typically glossy, or porcellaneous, and is smooth across the flanks, but may be covered by very fine, almost imperceptible lirae across the venter. There are a series of coarse undulating plicae, or ribs, on the flanks of the body chamber near the aperture in mature specimens (Figures 7.4, 8). These plicae are ca. 30 mm long, with crests ca. 10 mm apart, and they are strongly expressed on both the interior and exterior of the shell. No soft-parts have yet been observed for this species.

The nauta is large (25-27 mm diameter) with seven septa at hatching and distinct crosshatched ornamentation.

Remarks.-Until live-caught material is available, it is unknown whether this species bears the unique, thick periostracum of A. scrobiculatus, as well as the soft-part features (above) characteristic of the latter species.

Distribution.-The locality for the holotype is unknown, as is also true of most shells of this species in museum collections. However, commercial dealers in Bali, Indonesia, have been selling locally derived shells of both Nautilus pompilius and A. perforatus, with the latter being considerably less common (Donald Dan and Alex Roth, personal commun., 1988). Few details of their origin are available, but the fact that most of the shells exhibit Octopus borings in the body chambers indicates that they are drifted shells, rather than live-caught. This is supported by anecdotal information by the dealers, who were unaware of trapping activity. This information, plus the lack of documented specimens of A. perforatus from the Papua New Guinea region, where A. scrobiculatus is trapped, indicates that the two species are geographically isolated.