Aturia from the Miocene Paratethys : An exceptional window on nautilid habitat and lifestyle
Résumé
Many examples of drifted Aturia shells in shallow littoral deposits have been reported worldwide, suggesting that the paleobiogeographic distribution of this Cenozoic nautilid could be a mere post-mortem artifact. An exceptional Lower Miocene deposit from the Central Paratethys yields abundant (about 500 specimens) and very well-preserved newly hatched as well as adult shells, associated with upper and lower jaws, representing the first unequivocal case of autochthonous Aturia and one of the most exceptional nautilid deposits reported so far. Oxygen isotope ratios show that Aturia lived like Nautilus, being nektobenthic at all stages of its development. But unlike Nautilus, both newly hatched and adult Aturia lived at the same water depth and temperature (about 240-330 m and 13-17.6 degrees C) in which the eggs were laid. The dysoxic paleoenvironmental setting in which Aturia occurs in abundance may be interpreted in light of both the capacity of Nautilus to exploit/tolerate oxygen-depleted waters, and the molecular phylogenetic tree of cephalopods, suggesting plesiomorphic physiological traits associated with hypoxia tolerance. Since the last common ancestor of Aturia and Nautilus may be traced back at least into the Jurassic, this sheds new light onto the relative scarcity of Mesozoic and Cenozoic nautilids in well-oxygenated, epicontinental shelf deposits. (C) 2011 Elsevier B.V. All rights reserved