A NEW, LARGE ACTEONID GASTROPOD (MOLLUSCA) FROM THE LATEST CRETACEOUS OF ANTARCTICA

Abstract

Opisthobranch gastropods are rare in Mesozoic deposits of Antarctica. The first documented occurrence of opisthobranchs from this continent is from Cretaceous shallow-marine deposits of the James Ross Basin, Antarctic Peninsula, where Wilckens (1910, p. 95–96, pl. 4, fig. 19) described the minute ringiculid Cinulia sp., based on five specimens from two localities on Snow Hill Island. No further work has been done on the Snow Hill Island species. The next mention of Antarctic Cretaceous opisthobranchs was by Zinsmeister et al. (1989, p. 733, fig. 2, p. 734, fig. 3), who cited the occurrence of the ringiculid “ Oligoptycha cf. O. concinna Meek and Hayden, 1858,” in the lowermost part of Unit 8 of the Lopez de Bertodano Formation (Maastrichtian) on the southern half of Seymour Island (Fig. 1). We report the first record of Acteonidae from the Mesozoic of Antarctica, Acteon (Tenuiactaeon) antarctihadrum n. sp., discovered in shallow-marine Maastrichtian deposits. Figure 1 —Index map of Seymour Island, Antarctic Peninsula, showing the distribution of Maastrichtian and Danian strata on the southern two-thirds of the island. Diagonal hatched, Lopez de Bertodano Formation. The numbers correspond to Macellari's lithologic units (1–5, Upper Campanian; 6–9, Maastrichtian, and 10, Danian). Cross hatched, Sobral Formation; Stippled region, Cross Valley Formation, Late Paleocene; Unshaded northern region of Seymour Island, Eocene La Meseta Formation. Diagonal black lines crossing the Sobral and units 8 through 10 of the Lopez de Bertodano Formations represent a Neogene dike complex. Thick lines between units 9 and 10 represent the K-T boundary. Locality 1620 is 692 m above the base of the sequence (dip of strata 8 degrees) Seymour Island provides an exceptionally rich and well-preserved fossil record across the Cretaceous-Tertiary (K-T) boundary. The diverse suites of molluscs from this small Antarctic isle offer important insight on the composition …