Large heteromorph ammonites from the Upper Cretaceous of Seymour Island, Antarctica

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Abstract
Two species of the heteromorph ammonite genus Diplomoceras Hyatt are described from the Upper Cretaceous of the James Ross–Seymour Islands area, Antarctica. The late Campanian–early Maastrichtian D. lambi Spath has a relatively high rib density, whereas D. maximum n. sp. has a lower rib density and is known only from the uppermost Maastrichtian of the Lopez de Bertodano Formation on Seymour Island. Both species attain an exceptionally large size with the body chamber of D. maximum n. sp. attaining a length in excess of one meter. The structure of the shell wall in Diplomoceras is characterized by the thickening of the nacreous layer below the ribs. The shell thickening results in an inner flat surface and a smooth phragmocone. Reconstruction of the shell suggests at least four parallel shafts and three U-connectives. Estimates of the total density, center of buoyancy, and center of mass in this reconstructed shell indicate a slightly positive buoyant shell with a relatively unstable floating position.

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