Judd A. Case, 2006. "The late Middle Eocene terrestrial vertebrate fauna from Seymour Island: the tails of the Eocene Patagonian size distribution", Cretaceous–Tertiary High-Latitude Palaeoenvironments: James Ross Basin, Antarctica, J. E. Francis, D. Pirrie, J. A. Crame
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The Middle Eocene Antarctic terrestrial vertebrate palaeofauna from the La Meseta Formation on Seymour Island (= Isla Marambio), Antarctic Peninsula has a U-shaped, bimodal distribution of body sizes. This palaeofauna includes a wide range of body sizes from small insectivorous, omnivorous and granivorous marsupials, a rodent-like non-therian gondwanathere, large-sized ungulates, a sloth and cursorial birds (a ratite and a phororachoid). Medium-sized, homeothermic animals in the size range represented by rabbit to small ungulate-sized animals have not been found.
For comparison, the Early Eocene Casamayoran (Vacan ‘subage’) mammalian palaeofauna from Patagonia has a reasonably normal distribution of body sizes, with the modal class represented by medium-sized mammals, a distribution that is the direct opposite of the Antarctic palaeofauna. A comparison of the Middle Eocene Antarctic palaeofauna from the La Meseta Formation to the early Eocene Vacan-aged mammal palaeofauna is appropriate, due to the taxonomic affinities of the Antarctic palaeofauna to Riochican (latest Palaeocene) and Vacan-aged palaeofaunas of Patagonia. If these Patagonian mammalian palaeof aunas (PMP) were the source for the La Meseta palaeofauna (LMP), then a similar normal distribution with less taxonomic diversity would be expected. However, the LMP does not meet this expectation or even a distribution where all size classes are equally represented. Thus, the pattern of size distribution is quite different from the PMPs.
Floral data for the Early Eocene of Patagonia indicate subtropical conditions with mean annual temperatures (MAT) of 15.6 °C and equable winter temperatures (>10 °C) generating high taxonomic diversity at the species level. Floral data from the La Meseta Formation of equivalent age to the mammalian fauna indicate a cooler MAT of 11–13 °C with a highly seasonal climate, where the mean winter temperature could have ranged from –3 to 2 °C. There is also a significant drop in floral taxonomic diversity, which is dominated by Nothofagus.
A bimodal body size distribution pattern is not an unusual pattern for higher latitude mammalian faunas. Modern boreal mammalian faunas of North America have a low frequency of species in the medium body size range in response to cold winter temperatures in these higher latitudes. The smaller-sized mammals have adapted their physiology to the cold winter temperatures. The larger animals have adapted to the cold winter conditions by conserving heat through small surface-area-to-volume ratios as a result of their greater bulk. The low frequency of medium-sized animals is due to the fact that neither of these thermal strategies is available to them and thus they are at a selective disadvantage.
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High-latitude settings are sensitive to climatically driven palaeoenvironmental change and the resultant biotic response. Climate change through the peak interval of Cretaceous warmth, Late Cretaceous cooling, onset and expansion of the Antarctic ice sheet, and subsequently the variability of Neogene glaciation, are all recorded within the sedimentary and volcanic successions exposed within the James Ross Basin, Antarctica. This site provides the longest onshore record of Cretaceous–Tertiary sedimentary and volcanic rocks in Antarctica and is a key reference section for Cretaceous–Tertiary global change. The sedimentary succession is richly fossiliferous, yielding diverse invertebrate, vertebrate and plant fossil assemblages, allowing the reconstruction of both terrestrial and marine systems. The papers within this volume provide an overview of recent advances in the understanding of palaeoenvironmental change spanning the mid-Cretaceous to the Neogene of the James Ross Basin and related biotic change, and will be of interest to many working on Cretaceous and Tertiary palaeoenvironmental change.