Finding a home for the ram’s horn squid: phylogenomic analyses support Spirula spirula (Cephalopoda: Decapodiformes) as a close relative of Oegopsida

The molluscan clade Decapodiformes (Cephalopoda) comprises a diverse and enigmatic assemblage including inshore and offshore squids, bobtails, cuttlefishes, and the ram’s horn squid (Spirulida: Spirula spirula ). The latter species is of particular interest to paleontologists because it is the only living cephalopod with an internal chambered, spiral-shaped, calcareous shell resembling those seen in some fossil cephalopod taxa. Spirulida has been difficult to place phylogenetically, in part because it shares different features with sepiolids, sepiids, and oegopsids, creating conflict in morphological analyses. Unlike morphological assumptions of a close relationship with sepiids, previous molecular studies have found support for Spirulida as a close relative of Bathyteuthida and Oegopsida. Identifying the correct phylogenetic placement of Spirulida could allow alternative hypotheses of phragmocone evolution, e.g., retention of an ancestral phragmocone in Spirulida and Sepiida vs. independent reacquisition of the phragmocone in these taxa, to be evaluated. In the present study, we combined new, high-quality transcriptome data for a specimen of Spirula spirula with additional new and previously published transcriptome data for decapodiform cephalopods. Phylogenetic analyses of several matrices yielded trees in which Spirula spirula was recovered as the sister group of the oceanic open-eyed squids (Oegopsida). This close relationship of Spirula to oceanic squids, rather than to nearshore, demersal/benthic decapodiform lineages, provides further support for an “onshore to offshore” model of decapodiform evolution.