SANTONIAN-CAMPANIAN GAUDRYCERATID AMMONOIDS FROM ANTARCTICA IN SPACE AND TIME: PALEOBIOGEOGRAPHICAL IMPLICATIONS

RAFFI MARÍA EUGENIA; OLIVERO EDUARDO B.

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

CONICET-MENDOZA; International Palaeontological Association

Abstract The widespread geographic distribution of the relatively light-ornamented gaudryceratid ammonoids have been considered as typical for an oceanic planktonic drifter mode of life (leiostracan ammonoids) and the long ranging stratigraphic distribution of gaudryceratid species as an indication of its relatively low evolutionary rate. As a consequence, gaudryceratid ammonoids have been rarely used in paleobiogeography and biostratigraphy. However, the stratigraphic and paleobiogeographic distribution of the Santonian-Campanian Antarctic gaudryceratids partly belies this concept. In the James Ross Basin, five successive species of the genus Gaudryceras characterize particular stratigraphic intervals during the Santonian-Campanian. Gaudryceras cf. denmanense (Whiteaves), Santonian; Gaudryceras sp. nov. B, early Campanian; Gaudryceras sp. nov. A, Gaudryceras cf. varagurense (Kossmat) and Vertebrites cf. kayei (Forbes), lower middle Campanian; and Gaudryceras sp. nov. C, upper middle Campanian. In addition, two new species of Anagaudryceras appear near the early-middle Campanian boundary in the Rabot Formation. This interval also bears Metaplacenticeras cf. subtilistriatum (Jimbo), Hoplitoplacenticeras cf. plasticum Paulcke and Baculites subanceps Haughton; which are restricted in the Indopacific region to the base of the mid Campanian. A striking biogeographical contrast is that while the Santonian-early Campanian G. cf. denmanense, G. cf .varagurense and Vertebrites cf. kayei show marked affinities with similar species from the Indopacific or North Pacific regions, the mid-late Campanian gaudryceratid species (Gaudryceras sp. nov. B, A and C and Angaudryceras sp. nov. A and Anagaudryceras sp. nov. B) are apparently restricted to Antarctica. Outside Antarctica the genus Gaudryceras is well-recorded up to the late Maastrichtian. This is in strong contrast with the Antarctic record, where most of the species of Gaudryceras s.s. disappear by the middle Campanian, with only few specimens recorded near the base of the late Campanian. Apparently, gaudryceratid ammonoids follow the same trend detected in the rest of the ammonoid fauna from Antarctica, which is characterized by a cosmopolitan or Indopacific fauna during the Santonian-early Campanian followed by a strongly endemic fauna during the middle Campanian-Maastrichtian. This major faunal change has been interpreted as reflecting a long cooling trend in sea water temperatures. While the local extinction of Gaudryceras is concomitant with the gradual cooling of waters, the possibility of bathymetric controls, i.e. the generalized shallowing of the basin during the middle Campanian-early Maastrichtian, cannot be ruled out.