THE AGE OF A THEROPOD DINOSAUR, BASED UPON ASSOCIATED PALYNOASSEMBLAGES, FROM THE SNOW HILL ISLAND FORMATION (MAASTRICHTIAN) AT THE NAZE, JAMES ROSS ISLAND, ANTARCTICA

From the marine deposits of the Cape Lamb Member of the Snow Hill Island Formation in Antarctica, a theropod dinosaur was recovered during an expedition sponsored by the Office of Polar Programs, National Science Foundation. The dinosaur had floated from shore and sank to the bottom of a Late Cretaceous shallow seaway. Now, these marine deposits are exposed on a peninsula, known as The Naze, on the northern margin of James Ross Island, east of the Antarctic Peninsula. A 90-m-thick section begins at sea level, ends below a basalt sill, and is composed of interbedded green-grey massive and laminated fine-grained sandstones and mudstones. The palynoassemblages associated with the theropod include fourteen samples, yielding moderately diverse assemblages with a total of 100 relatively well-preserved species. The principal continental groups are represented by lycophytes (6 species), pteridophytes (12 species), gymnosperms (13 species), and angiosperms (10 species). Aquatic palynomorphs belong to chlorococcaleans (8 species), dinoflagellates (49 species), and the acritarch Michrystridium piliferum. Some trilete spores (Perotrilites majus), pollen grains (Podocarpidites, Nothofagidites), dinoflagellates (Isabelidinium cretaceum- gravidum, Operculodinium radiculatum, Spiniferites ramosus complex, Xenascus plotei), and chlorococcaleans (Nummus monoculatus) always occur. M. piliferum and other dinoflagellates (e.g., Batiacasphaera grandis, Canninginopsis ordospinosa, Manumiella spp., Trichodinium castaneum-T. chilensis complex) show quantitative variations possibly related to sea-level fluctuations and/or nutrient availability within an inner platform with contributions from the littoral/inland environments. Although assemblages show no major shifts up section, the appearance of Paleocystodinium pilosum and the disappearance of Odontochitina porifera at ca. 20m from the base and the appearance of Manumiella seelandica, Alterbidinium acutulum and T. castaneum-chilensis complex at ca. 25m and Spinidium essoi at ca. 35m indicate the early/late Maastrichtian boundary within this interval; the dinosaur was found between 41 and 47m, suggesting the specimen was deposited during the early-late Maastrichtian Age.