A MORPHOLOGICAL EVALUATION OF NAUTILOID INTERRELATIONSHIPS

The nautilids Nautilus and Allonautilus are the only extant ectocochleate cephalopods with a phragmocone, distinguishing them from all other known modern cephalopod taxa. However, in the Paleozoic and Mesozoic, the diversity of ectocochleate cephalopods was much greater. This prominent ancestral characteristic explains why extant nautilids have an outsized impact on our understanding of cephalopod evolution. Despite their status as “living fossils,” it is incorrect to assume that extant nautilids always present the ancestral condition. For example, their extensive chemosensory organs are likely a novel adaptation to demersal life in an aphotic environment, and their many arms are a modification of the ancestral ten. Paleozoic nautiloids were highly disparate and likely occupied a variety of ecospaces distinct from the fore-reef slopes that modern nautilids exclusively inhabit. As extant nautilids do not necessarily reflect the plesiomorphic state, it is crucial to consider fossil lineages when reconstructing phylogeny. Molecular analysis has recovered a Cambrian-Carboniferous nautiloid-coleoid divergence, which is commonly interpreted as evidence for a Silurian-Devonian origination and orthocerid ancestry. However, this narrative contradicts longstanding fossil evidence suggesting nautiloids derived from Ordovician tarphycerids and/or oncocerids. Among the extant nautilids, phylogenetic interrelationships are similarly disputed, with population genetics suggesting that several established species are invalid and that multiple reproductively isolated populations constitute undescribed cryptic species. We present preliminary results from the first attempt at a comprehensive morphologically-based nautiloid phylogeny that spans the order’s entire evolutionary history. Through a literature review and the examination of museum specimens, we collected discrete and continuous morphological data from over 64 extant and 20 fossil nautiloid specimens (representing at least 16 species from 9 genera). The resultant data matrix contains 109 discrete and 13 continuous morphological characters. Maximum parsimony tree analysis was performed in TNT. These results should help clarify the complex taxonomy of recent forms and shed light on the group’s controversial origins.