TAXONOMIC SIGNIFICANCE OF THE NEANOCONCH OF THE AMMONOID GENUS GAUDRYCERAS FROM THE UPPER CRETACEOUS OF ANTARCTICA

RAFFI MARÍA EUGENIA; OLIVERO EDUARDO B.

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

CONICET-MENDOZA; International Palaeontological Association

Abstract With its long stratigraphic record spanning the Albian-Maastrichtian, Gaudryceras de Grossouvre is considered a conservative ammonoid genus of low biostratigraphic importance. Three species group have been differentiated on the basis of the ribbing style of the adult shell. As this taxonomic character can only be applied to the adult shell, the classification of the juvenile shell is problematic. Juvenile and adult specimens of Gaudryceras from the Santonian-mid Campanian Santa Marta and Rabot formations, James Ross Basin, Antarctica, including ca. 60 specimens recovered in a thickness of 1,350 m, display a regular change in the ornamentation of the neanoconch. In wellpreserved specimens, with the ammonitella and neanic phragmocone filled with sparry calcite, two contrasting ornamentation patterns of the neanoconch are distinguished: a) dense, seven or more, strong, simple, rectirradiate ribs, only found in Santonian-early Campanian specimens, and b) few, three or less, distant ribs, only found in late early and mid Campanian specimens. In both cases, the neanic ornamentation is abruptly replaced by fine, dense lirae and fine ribs typical for Gaudryceras. In combination with the ribbing style, degree of evolution and whorl section, five species of Gaudryceras are distinguished. G. cf. denmanense Whiteaves, with strongly ornamented neanoconch followed by prorsirradiate and straight ribs (Santonian); Gaudryceras sp. nov. B with strongly ornamented neanoconch followed by dense flexuous ribs; Gaudryceras sp. nov. A, with strongly ornamented neanoconch followed by distant flexuous ribs increasing in size at the body chamber; Gaudryceras sp. nov. C with less ornamented neanoconch followed by bifurcated flares; and Gaudryceras cf. varagurense with dense flexuous lirae followed by flexuous ribs. Although the genus Gaudryceras has been mentioned in the Coniacian-upper Campanian of Antarctica, its biostratigraphic importance was not fully appreciated. In ascending stratigraphic order we recognize four stratigraphic successive intervals characterized by different species of Gaudryceras: 1) Gaudryceras cf. denmanense, Santonian, lower part of the Santa Marta Formation; 2) Gaudryceras sp. nov. B, early Campanian, lower mid part of the Santa Marta Formation; 3) Gaudryceras sp. nov. A, mid Campanian, upper part of the Santa Marta Formation; and 4) Gaudryceras cf. varagurense and Gaudryceras sp. nov. C, mid Campanian-late Campanian, upper middle part of the Rabot Formation. Even though the significance of the variable ornamentation of the neanoconch for the juvenile mode of life is still under study, it clearly indicates a morphological feature of taxonomic importance.