Upper Jurassic Fossils from Ellsworth Land, West Antarctica, and notes on upper Jurassic Biogeography of the South Pacific Region

Abstract

Abstract Fossils from Lyon Nunataks (74° 52′ S, 74° 02′ W) are described: Conodicoelites spp., Rotularia sp. indet., indet. pectinacean: cf. Entolium, and Variamussium lyonensis sp. nov. Upper Jurassic (?Lower Kimmeridgian) age. The Conodicoelites spp. have strong affinities with those of the New Zealand Lower Kimmeridgian. Ammonites, belemnites, and Inoceramus, all with strong Indo-Pacific affinities, are present in the Kimmeridgian and Lower Tithonian of Central and South America, and the Lyon Nunataks fossils appear to be a southerly extension of this fauna. These fossil occurrences strongly suggest that a trans-Pacific migration route, probably skirting Antarctica, was available in Kimmeridgian and Lower Tithonian times. Faunal movements along this southern route are discussed in terms of: (1) Migration across existing oceans; (2) Migration between existing land masses, aided by connecting shelf areas; (3) Migration skirting formerly contiguous land masses. Continental drift is seen as a means of providing shallow-water migration routes between New Zealand, West Antarctica, and South America. Various continental reconstructions are considered, and one first published by King (1958) is preferred. Oxygen-isotope paleotemperature determinations indicate that in the Jurassic New Zealand had a temperate climate and was probably in middle latitudes, either close to its present position or in an equivalent position as part of Gondwanaland. Two major faunal realms, Boreal and Tethyan, developed in the later half of the Jurassic. The boundary between the two realms is thought to have been essentially climatically controlled, the Boreal faunas probably consisting of temperate or cooltemperate stenothermal animals, and the Tethyan faunas and derivatives warm or warm-temperate stenothermal. In the late Jurassic, differentiation within the Tethyan realm, perhaps as a result of short-term orogenic barriers, allowed the evolution of distinct Indo-Pacific faunas. The relationship of the West Antarctic fossils to these faunal groupings is discussed. The geographic distributions of the Jurassic faunas are considered in relation to both present-day latitudes and to paleolatitudes determined from paleomagnetic and continental drift studies. The apparent absence of a Southern Hemisphere equivalent of the Boreal realm raises the question whether the Southern Hemisphere continents may. have been grouped away from the Jurassic South Pole. Northern Hemisphere contments on the other hand may have been grouped close to the Jurassic North Pole. This factor, as well as perhaps improved oceanic circulation in the Southern Hemisphere, may have allowed Tethyan and related faunas to populate latitudes in the Southern Hemisphere that in the Northern Hemisphere were populated by Boreal faunas.