Syllipsimopodi bideni Whalen & Landman 2022, gen. et sp. nov.

Syllipsimopodi bideni gen. et sp. nov. Etymology. The genus name is derived from the Greek συλλήψιμος (syllípsimos) for prehensile and πόδι (pódi) for foot. The name prehensile-foot is chosen because this is the oldest known cephalopod to develop suckers, allowing the arms, which are modifications of the molluscan foot, to better grasp prey and other objects. The species name is to celebrate the recently inaugurated (at the time of submission) 46th President of the United States, Joseph R. Biden. Holotype. ROMIP 64897 (Royal Ontario Museum). Material. The type and only specimen was donated to the Royal Ontario Museum by B. Hawes in 1988; accession number 88-72717. There is no counterpart. Locality. Bear Gulch Limestone, Heath Formation, Big Snowy Group, Fergus County, Montana, USA 18. The Bear Gulch Limestone is a plattenkalk, or lithographic limestone, similar to the more famous Jurassic Solnhofen Limestone of Germany 19. Deposition occurred in a low-latitude shallow marine bay subject to oscillating semi-arid and tropical conditions18. Exceptional preservation is likely a result of microturbidites deposited by seasonal monsoons18. Monsoonal rainfall would have rapidly introduced terrestrial sediments and biomatter into the bay, feeding algal blooms that created short-lived anoxic zones simultaneous with the saline instability caused by the rapid injection of voluminous freshwater18. Bear Gulch is perhaps best known for the pelagic fauna of the central basin and bay mouth – a diverse array of vertebrates20–22 (especially chondrichthyans22–25 and coelacanthiforms22,26), malacostracans19,27, polychaetes22, and cephalopods13,28–30, which are preserved in such exquisite detail that vascularization can sometimes be distinguished31. Benthic fossils are very rare in the central basin18,22, but marginal facies preserve gastropods, worms, asterozoans, and abundant sponges, which acted as a substrate for various brachiopods, bivalves, and conulariids18,19,22. Crinoids, blastoids, bryozoans, and corals are almost absent; algae (especially dasyclads) are common throughout18,22. Horizon. Bear Gulch Limestone, Arnsbergian E2b (~328.3–324.5 Ma), Serpukhovian (Namurian), Mississippian, Carboniferous 8,18,32. Diagnosis. (Figs. 3, 4) Coleoid with simple, nearly triangular gladius, bearing funnel-like conus and median field with median rib, but no hyperbolar zones, cone flags, or lateral reinforcements; lateral fields unlikely. Lacking chambered phragmocone, primordial rostrum, or rostrum. Ten arms bearing biserial rows of suckers but no hooks or cirri; two arms may be elongated (though this could be taphonomic). Ink sac present. Terminal median fin support and one fin pair present. Description. The gladius median field is simple; it is widest at the extreme anterior with straight lateral edges and a flat (not rounded/pointed) anterior edge (Figs. 3a, 4). The median asymptote angle is ~13.8°. The gladius length to width ratio is 3.17; the median field is ~6.5 cm long, which is ~55% of the total body length. The median field bears a prominent median rib, which is diagenetically distorted and broken in various places (Supplementary Fig. 2). The rib appears to be posteriorly bipartite and anteriorly unipartite (Figs. 2, 3). Gladius growth lines are poorly preserved and only clearly visible in one place on the lateral edge (Supplementary Fig. 3). The funnel-like conus is ~6.8 mm long (Figs. 2, 3). A cigar-shaped central fin support measuring ~13.1 mm long is preserved posterior to the gladius (Fig. 3; Supplementary Fig. 4); an originally cartilaginous composition seems most likely. It is possible that this fin support is a vestige of the phragmocone, but we consider this alternative unlikely because there is no evidence of a siphuncle or septa. These are unlikely to have been dissolved without leaving a trace since septa can clearly be observed in the co-occurring coleoid Gordoniconus 13. Septa are dissolved in Bear Gulch ammonoids30; but since Gordoniconus is a coleoid, we consider it a better taphonomic comparator. Also, the fin support is posterior to and thus external of the conus; the phragmocone should be internal to the conus (Fig. 5). We consider a primordial rostrum identity unlikely because it seems doubtful that a primordial rostrum (or rostrum) would be present in the absence of a phragmocone. The fin support is associated with patches of a shiny fibrous mineral (Fig. 3; Supplementary Fig. 4), presumed to be connective tissue remnants. The holotype appears to preserve a faint outline of a single pair of short terminal lobate fins measuring ~2.3 cm long anteroposteriorly and ~1.7 cm laterally across at the widest position (Figs. 3, 4). (2022) 13:1107 | https://doi.org/10.1038/s41467-022-28333-5 | www.nature.com/naturecommunications Ten sucker-bearing arms are preserved, measuring ~2.1–2.4 mm wide at the midlength (Fig. 4b, c). Developmental evidence and phylogenetic inference have long suggested that the ten-arm condition is ancestral for cephalopods and vampyropods33–35, but no ten-armed fossils have been documented outside of the decabrachian-belemnoid clade prior to the discovery of Syllipsimopodi. Syllipsimopodi is the first and only known vampyropod to possess ten robust, functional appendages; all other known vampyropods have either reduced arm pair II to filaments (i.e., Prototeuthidina7, Loligosepiina7,36, Teudopseina 7, Vampyromorphida), or lost the arm pair entirely (i.e., Octopoda). Two arms, measuring ~4.0 and ~4.1 cm long (~27% of the total body length), might have been elongated relative to the other eight arms (Fig. 4b, c). These elongated arms do not have any obvious manus, as in decabrachian tentacles, and are not significantly thinner than the unmodified arms, as in vampyromorph filaments (Fig. 4b, c). Of the shorter arms, the three best preserved measure ~1.7, ~1.9, and ~1.9 cm long, respectively (~13% of the total body length); the remaining five arms are either incomplete or preserved in a contorted orientation (Fig. 4b, c). All arms appear to have suckers along the base and midlength (Fig. 4d). The better-preserved shorter arms appear to show distal suckers; it is unclear if the elongated arms bear suckers distally (Fig. 4d). Suckers are commonly between ~0.31 and ~0.62 mm in diameter. Suckers appear to have been biserial wherever present, but both rows are not always preserved along the entire arm length. Sucker rows are laterally separated by ~0.5 mm (Fig. 4e–g). Within a row, suckers are immediately adjacent proximodistally or separated by up to ~0.4 mm (Fig. 4e–g). There is no evidence that suckers were stalked. (2022) 13:1107 | https://doi.org/10.1038/s41467-022-28333-5 | www.nature.com/naturecommunications Most arms are incompletely preserved, so it is possible that the apparent elongation of two of the arms is a taphonomic artifact. We consider this unlikely because exactly two arms are elongated and the elongated arms are of approximately similar lengths, suggesting they are from the same arm pair. Furthermore, the better-preserved shorter arms are each of approximately similar lengths. Probability suggests a preservational artifact would result in a non-two number of unequally elongated arms associated with shortened arms of dissimilar lengths. Additional fossil specimens will be necessary to test this hypothesis though. Based on the phylogenetic affinity of Syllipsimopodi, we interpret these as arm pair II. However, precise arm identities cannot be determined; this is an inference not an observation. Syllipsimopodi bideni is conservatively coded unknown in the phylogenetic analysis for all characters related to the modification/elimination of arm pair II (Supplementary Data 1). A possible funnel measuring ~2.4 mm long is preserved at the lateral edge of the head (Fig. 3). Afunnel identification is advocated because suckers appear to be absent on the structure (Fig. 4d), and we cannot easily connect it to one of the ten identified arms (Fig. 4b, c). The buccal apparatus is preserved as a dark rectangular patch within a light circular patch that is distinct from the surrounding arm/head tissues (Fig. 3, Supplementary Fig. 5). The dark structure (~3.5 × ~1.3 mm) appears to be texturally distinct from the other preserved tissues, suggesting it could be a remnant of the beak. An intriguing S-shaped band measuring ~0.33 mm in width is preserved within the buccal apparatus. It is possible that this band is a remnant of the radula, but we suspect it is more likely to be a superimposed sucker-bearing arm because of the relatively long length (in comparison to the dark rectangular patch) and the apparent circular shapes lining parts of the band. A dark, contiguous, anteroposteriorly elongate, saclike structure is preserved laterally offset from the central median ridge of the gladius. We interpret this as the ink sac. The ink sac measures ~2.6 cm long anteroposteriorly and ~0.3 cm wide laterally at the widest point (Figs. 3, 4). Ecological interpretations. The preserved soft tissues and gladius suggest a torpedo-shaped body reminiscent of extant squids. The fins appear to have been large enough to potentially function as a viable supplement to jet swimming, but their apparent circular shape and terminal position would seem to suggest a stabilizing role may have been more important. If the arms are preserved to approximately their total lengths, then one arm pair was considerably longer than the other four pairs, which were shorter (arm-to-body length ratio) than in most extant octobrachians37. It seems likely that the elongated arms captured prey to be confined and manipulated by the shorter arms, analogous to extant decabrachians. The type does not preserve identifiable gut contents, so diet is unknown. While Vampyroteuthis remains the best living analog for understan

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