Plesioteuthis prisca ( Rüppell, 1829 )

Plesioteuthis prisca ( Rüppell, 1829) .

( Figs. 10 View Fig , 11 View Fig , 12 View Fig )

Specimens HT 73/152, HT 77/23 (col. H. Tischlinger).

Stratigraphy Moernsheim Formation, Moernsheimensis subzone, Hybonotum Zone, Lower Tithonian, Jurassic.

Locality Daiting near Monheim, Bavaria, Germany.

Description of arm crown Two specimens of P. prisca are discussed here because they both display a peculiarly preserved arm crown. HT 77/23 ( Figs. 10 View Fig , 11c, d View Fig ) is a complete specimen measuring 317 mm from arm tip to the tip of the gladius. Te mantle length is approximately 250 mm and partially covers the gladius. Te conus is heavily phosphatized like the muscular mantle. Te gladius is hardly phosphatized at its anterior edge and hence not well preserved there. Te ink sac and duct are clearly visible. Te head ( Fig. 11c, d View Fig ) is preserved on a slight angle, possibly due to necrolytical processes. It displays an oval structure, 13 mm long and 11 mm wide, which we interpret to be an imprint of the cephalic cartilage. Te arm crown is preserved as seven phosphatized elongate structures of about 1 mm width each. All but one arm are curled inward, as seen in an exceptionally preserved specimen from the Kimmeridgian of Painten (BMMS 617a, Klug et al., 2015: fig. 2). Te Painten-specimen, however, has much thicker arms, which display their cirri. In most other specimens, including those with landing marks (Klug et al., 2015: fig. 7) that sometimes accurately reflect arm proportions, the arms are much thicker proportionally (Additional File 1).

Specimen HT 73/152 is quite similarly preserved

( Figs. 11a, b View Fig 12 View Fig ). It measures 320 mm from arm tip to gladius tip and also displays a strongly phosphatized mantle, the remains of an ink sac and duct, as well as the gladius. Te head displays a limonitic stain on the phosphatic mass, which is here interpreted as jaw remains. Like in HT 77/23, this specimen preserves seven to eight fine, lightly phosphatized structures, which are about 1 mm wide. Correspondingly, these proportions support the interpretation that it is not the arms but rather the axial nerve cords, which are preserved here.

Ancestral state reconstructions

Te analyses show a strong phylogenetic signal in the habitat distributions of crown group coleoids ( Fig. 13 View Fig ). Te oldest nodes (crown Cephalopoda and crown Coleoidea) contain the highest uncertainty, both slightly favouring a pelagic habitat with a probability of about

2/3. Tis high uncertainty likely stems from the necessarily poor sampling of the Nautilus lineage, but also from the reconstruction of the basal coleoid dichotomy, where the crown octobrachian node is reconstructed with high probability (79%) as pelagic, while the most recent common ancestor of crown decabrachians most likely (88%) had a coastal habitat. Within both superorders, our analysis recovered a single habitat transition. Within Octobrachia , this switch occurred at the base of the Octopodidae , which is reconstructed as coastal with high probability (78%). Conversely, the transition between coastal and pelagic decabrachians was estimated at the node containing Oegopsida + Spirulida (74%). Te transition rate for the change in habitat was estimated to a mean of 0.0019 per million years, with a median of 0.0017 and a 95% highest posterior density interval between 0.0002 and 0.0043. Tus, on average, a single lineage would be expected to transition between habitats only once in 500 million years, indicating a very slow transition rate.