LONG TERM HABITAT CHANGE IN POST JURASSIC NAUTILIDS - FROM WARM SHALLOWS TO DEEP, COOL, FORE REEF SLOPES

The extant species of Nautilus and Allonautilus, the two surviving members of the Nautiloidea, live on tropical fore-reef slopes at depths averaging 200-300m, and therefore not at warm, >20C, but at temperatures from 15-18C or less. New oxygen isotope analyses coupled to high-resolution, ultrasonic transmitter data coming from living species of Nautilus and Allonautilus in their field areas confirm the depths and temperatures where most of their shell material is constructed. We have used oxygen isotope analyses on a large number of species of Cretaceous through Miocene nautilid genera (Nautilus, Eutrephoceras, Cimomia, Cymatoceras, Aturia, Aturoidea) to search for comparisons with the modern day taxa, looking not only for mean temperatures of shell forming habitat, but from temperature, deducing relative depths as well. Only the youngest species of Aturia (late Miocene) show shell formation temperatures approaching the cool temperatures of the ten populations of Nautilus and one of Allonautilus we analyzed for this study. We also present the first ever data on the comparative habitat depths of Nautilus pompilius and Allonautilus scrobiculatus from Ndrova Island, obtained in 2015, with the latter seen for the first time in 30 years. These two taxa appear to live at distinctly different mean depths, with Allonautilus living on near vertical walls, and Nautilus just on deeper, flatter, and sediment rich bottoms bathymetrically below the walls. The long term change in depth may be anti predatory in nature, and the so-called “primitive” eyes and many tentacles may be traits that are more recently evolved adaptations for deeper, darker habitats, where food and mates and are found through chemoreception, not vision.