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

ABSTRACT-Living ectocochliate cephalopods have long been thought to be restricted to a single genus, Nautilus Linnaeus, 1758, comprising five or six extant species. The shells of two species, N. scrobiculatus Lightfoot, 1786, and N. perforatus Conrad, 1847, are quite distinct, but no soft-parts were known until 1984, when N. scrobiculatus was seen alive for the first time. Dissections show that significant anatomical differences exist between N. scrobiculatus and other Nautilus species, including differences in gill morphology and details of the male reproductive system. These differences, along with phylogenetic analysis of extant and selected fossil nautiloid species, indicate that N. scrobiculatus, and N. perforatus should be distinguished from Nautilus as a newly defined genus, Allonautilus. This analysis contradicts previous phylogenies proposed for the Nautilida, which placed Nautilus as the last-evolved member of the order. We surmise that Allonautilus is a descendent of Nautilus, that the latter is paraphyletic, and first evolved in the Mesozoic, rather than in the late Cenozoic, as has been previously suggested.

INTRODUCTION

NAUTILUS HAS been known to natural historians since the Renaissance (Landman, 1982; Davis, 1987) and since the work of Owen (1832) has been recognized as the last surviving taxon of the once-numerous externally shelled cephalopods. The genus Nautilus was defined by Linnaeus (1758), with the widespread species N. pompilius serving as the type. Because of its uniqueness, Nautilus is often cited as an archetypal living fossil: it appears to have survived for a great length of time at low species diversity (two prime characters used in the definition of living fossils by Schopf, 1984; documented for Nautilus by Ward, 1984). Across its range, however, Nautilus is not rare; it may occur in great numbers in isolated Indo-Pacific archipelagos.

Although eleven species have been named within the genus (Saunders, 1987), most workers have accepted either four or five valid species (Miller,1947; Stenzel, 1964; Saunders,1981, 1987; Ward, 1987). These include N. pompilius, N. macromphalus Sowerby, 1849, N. stenomphalus Sowerby, 1849, N. scrobiculatus Lightfoot, 1786, and N. belauensis Saunders, 1981. All but one of these (N. belauensis) were originally defined on shell morphology differences alone, and, until the 1980s, the softparts of only two (N. pompilius and N. macromphalus) had been described. Even though these two species showed differences in shell morphology (N. macromphalus has an openly coiled shell and lacks an umbilical callus), detailed dissections of the softparts failed to detect a single anatomical difference (Griffin, 1900; Willey, 1902). In the last decade, live-caught specimens of a third species, N. stenomphalus, have been shown to exhibit only slight differences in hood and tentacle ornamentation compared to N. macromphalus and N. pompilius (Saunders and Ward, 1987), whereas a fourth, N. belauensis Saunders, to date has shown no soft-part differences from the others. Thus, four of the five accepted Nautilus species differ only in details of shell coiling or ornament and appear virtually identical in softpart anatomy. In shell form, they are also almost indistinguishable from newly recovered Tertiary representatives of the genus: Eocene N. cookanum Whitfield (Squires, 1988) and Oligocene N. praepompilius Shimansky (recently refigured by Saunders et al., 1996). All of the extant species have been grouped into a single taxon based on their shared shell characters, and all show little intraspecific variation (Saunders, 1987; Ward, 1987).

Such is not the case for the species Nautilus scrobiculatus Lightfoot, 1786, and a morphologically similar, but extremely rare and little-discussed form, N. perforatus Conrad, 1847 (thought until now to be a synonym of N. scrobiculatus). For a century and a half, it has been known that the shell shape, ornament, and shell coloration of these two species differ significantly from other known Nautilus species (Sowerby, 1849). These differences were even deemed sufficient to suggest to Stenzel (1964) that N. scrobiculatus should be set off as a distinct subgenus, although this was never formalized. The hesitation may have been due to the fact that no unambiguous softpart observations were available for either, for until recently, such knowledge was limited to observations based on a decomposing body of N. scrobiculatus found stranded in Milne Bay, Papua New Guinea (Willey, 1902), which indicated only that the hood of N. scrobiculatus differs from that of other Nautilus species. In summary, Nautilus scrobiculatus and N. perforatus show markedly different shell morphologies from other Nautilus species, with unconfirmed indications that anatomy of N. scrobiculatus might be different as well.

In 1984, live specimens of Nautilus scrobiculatus (Figures 1, 3, 7) were obtained for the first time, captured off Manus Island, Papua New Guinea (Saunders and Davis, 1985; Saunders et al., 1987). Tissue samples from this population were subsequently analyzed and compared to Nautilus pompilius, N. belauensis, N. macromphalus and N. stenomphalus, using both gel electrophoretic techniques (Woodruff et al., 1987) and mitochondrial DNA sequencing (Wray et al., 1995). Both studies showed that N. scrobiculatus exhibits the greatest degree of genetic divergence from other Nautilus species, and that the latter species show little genetic difference between one another. By contrast, N. scrobiculatus is so far removed that Wray et al. (1995) considered it to be a functional outgroup, while D. Woodruff (personal commun.) regarded the gel-electrophoresis differences to be so great as to suggest familial differences were they to be found in other groups of mollusks. The genetic results led Wray et al. (1995) to thus suggest that only two taxa could be discriminated genetically: N. scrobiculatus, and a single taxon composed of all other extant Nautilus species.