![]() We further suggest that rib density covaries with whorl shape in Mariella, following Buckman’s first rule of covariation. ![]() In particular, we show that a significant part of the variation in the number of ribs per whorl within the heteromorphic genera Mariella and Turrilitoides is directly related to shell size and that the strength of tubercles covaries negatively with the number of ribs in Mariella, therefore challenging the diagnostic value of these characters. This led to determine the diagnostic value of various shell characters commonly used to characterize both well-documented and poorly-defined species. The sample size and quality of this new material allowed some extensive morphological and biometric analyses whenever possible (i.e., for subdatasets in which a significant number of measurements is available) and to thoroughly assess intraspecific and ontogenetic variations for a few species. 50% of heteromorphs) constituting a high diversity. In this work, we document an original cephalopod assemblage from Salazac represented by abundant, well-preserved specimens (564 studied ammonites, including ca. Furthermore, the rare assessments of ammonite taxonomic diversity are presumably inaccurate, as they either result from a drastic typological approach or could not benefit from the most recent advances in modern concepts of ammonoid intraspecific and ontogenetic variations. However, most ammonite species have paradoxically been scarcely illustrated up to now. The Salazac locality (Gard, southeastern France) is renowned for the richness of its cephalopod fauna (especially ammonites) from the Mortoniceras fallax Zone (uppermost Albian, Lower Cretaceous). Biostratigraphic range data of the preserved macro- and microfossil assemblages imply an age of latest Albian, contemporaneous with the rifting of Batavia Knoll from Greater India during the broader India–Australia–Antarctica breakup in the mid-Cretaceous. Taphonomic analyses of these fossils and their host sedimentary facies revealed the Batavia Knoll sandstone was deposited in a shallow marine environment during a mass-flow event. Detritivorous and herbivorous taxa comprised 22% and 9%, respectively, with nektic and epifaunal carnivores amounting to 30%. The fossil assemblage was dominated by shallow marine suspension-feeding taxa (39% of the suite). Further recovered were one species of Serpulidae (Polychaeta), two of Trachyleberididae (Ostracoda), and a probable echinoid fragment. Scaphopods (Dentaliidae) and ammonites (two taxa, of Desmoceratidae and Hamitidae) are also present. Bivalves are represented by members of the Nuculanidae, Inoceramidae, Pinnidae, Buchiidae, Lucinidae, Veneridae, and Hiatellidae. In addition, pleurotomariid, ringiculid, and architectonicid gastropod taxa were recovered. and aporrhaids Drepanocheilus bataviensis n. a procerithiid Procerithium arenacollicola n. The fauna consists of 22 species of Mollusca, including new gastropods, a calliotropid Planolateralus acanthanodus n. 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.The first sedimentary rocks from Batavia Knoll, on the western edge of the Perth Abyssal Plain, eastern Indian Ocean, have been recovered, yielding an assemblage of invertebrate fossils hitherto undocumented from this part of the world. Reconstruction of the shell suggests at least four parallel shafts and three U-connectives. The shell thickening results in an inner flat surface and a smooth phragmocone. The structure of the shell wall in Diplomoceras is characterized by the thickening of the nacreous layer below the ribs. attaining a length in excess of one meter. Both species attain an exceptionally large size with the body chamber of D. has a lower rib density and is known only from the uppermost Maastrichtian of the Lopez de Bertodano Formation on Seymour Island. lambi Spath has a relatively high rib density, whereas D. The late Campanian–early Maastrichtian D. Two species of the heteromorph ammonite genus Diplomoceras Hyatt are described from the Upper Cretaceous of the James Ross–Seymour Islands area, Antarctica.
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