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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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