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

The hood of Allonautilus scrobiculatus is covered with irregularly shaped papillae, extending upward from the surface of the hood and terminating in multipronged, white-tipped digitations (Figure 1). In extant species of Nautilus, the papillae always form a hood pattern of two straight white lines in the medial region, whereas the hood papillae of A. scrobiculatus are scattered randomly across the surface and decrease in size toward the edges (Figure 1). They are also larger, more complex, and exhibit more relief than in extant Nautilus species.

The gills of Allonautilus scrobiculatus are structurally similar to those of Nautilus, but are much smaller than in similarly sized individuals of Nautilus. They are shorter, narrower, and thinner and have fewer folioles (individual gill elements) per unit length than do Nautilus gills. (Figure 3.4, 3.5). This finding may clarify field observations that newly caught A. scrobiculatus rapidly die if not kept in well-oxygenated seawater. By contrast, Nautilus is highly tolerant to hypoxic conditions (Wells et al., 1992).

The male reproductive system of Allonautilus scrobiculatus differs from that of Nautilus in details of spadix and penis anatomy. Although both Allonautilus scrobiculatus and Nautilus possess five cirri in the spadix, in A. scrobiculatus the tip of the first cirrus is conical and tapered, with no change in overall shape over its length (Figure. 3.1, 3.2). By contrast, the first cirrus in all Nautilus examined to date (N. pompilius, N. belauensis, N. macromphalus) is parallel for most of its length, terminating in a blunt tip, with a distinct nipple-like protuberance (Figure 3.3).

The slime gland of Allonautilus scrobiculatus is elliptical in shape; its edges are differentiated from the first cirrus by a deep canal (Figure 3.2). The slime gland is large compared to the overall length of the first cirrus, making up nearly half its length. In Nautilus, the slime gland is circular, not elliptical; its edges are not differentiated from the first cirrus by a canal; and it is relatively small compared to the length of the cirrus (Figure 3.3; Table 2).

The penis of A. scrobiculatus has a short, blind sac relative to the length of the central structure. In all Nautilus examined, the blind sac is well developed and relatively large compared to the length of the penis central structure.

Remarks.-The taxonomic origin of A. scrobiculatus traces to the 1786 auction catalogue of the collections of the Duchess of Portland. Although this famous document was anonymously prepared, its authorship has been attributed to J. Lightfoot. Discussion of this and justifications for the validity of the various new taxa named in the Portland Catalogue have been discussed by Dance (1962), Kay (1965), and Rehder (1967).

Most recent workers (e.g., Saunders, 1981, 1987; Ward, 1987) have assumed that there is but a single species of living umbilicate nautiloid, A. scrobiculatus Lightfoot, 1786, and that three other described umbilicate taxa (Nautilus umbilicatus Lister, 1685, N. texturatus Gould, 1857, and N. perforatus Conrad, 1847) were nomen dubia. However, our examination of collections at the Academy of Natural Sciences, Philadelphia, plus specimens recently obtained from Bali, Indonesia, suggest that in addition to A. scrobiculatus there is a second taxon, which is both morphologically distinct and geographically isolated from A. scrobiculatus. Based on examination of Conrad's type material, and phylogenetic analysis, we have concluded that the second species, A. perforatus (Conrad), is valid (see below).