Considerable interchange of mammals between South America and Australasia occurred during the first half of the Tertiary, including the presence of placental mammals in Australia. This challenges the old assumption that the marsupial radiation in Australia was made possible by the absence of placental competition, and suggests that two properties of marsupial organization may have favoured their survival in the increasingly arid climates that developed after the separation of Australasia from Antarctica. The basal metabolic rates of marsupials are about 70% of equivalent placentals, so their maintenance requirements for energy, nitrogen and water are lower, whereas their field metabolic rates are about the same, which means that they have a greater metabolic scope to call on when active. This may have given marsupials an advantage in semi-arid environments. The lengthy and complex lactation of marsupials enables the female to exploit limited resources over an extended period without compromising the survival of the young. Both these properties of marsupials enabled them to survive the double constraints of low fertility soils and the uncertain climate of Australia throughout the Tertiary. The arrival of people was followed first by the extinction of the large marsupials and, much later, by the wholesale decline or extinction of the small-to-medium sized species. The common factor in both extinctions may have been the constraints of marsupial reproduction.
Seeking to resolve conflicting literature on cytoskeletal structure in mammalian "primitive" generation erythrocytes, we have utilized the circulating blood of developing marsupials. In young of the Tammar Wallaby (Macropus eugenii) and the Gray Short-tailed Opossum (Monodelphis domestica), relatively large, nucleated primitive erythrocytes constituted nearly 100% of the circulating population at birth (= day 0) and in fetuses (Tammar) several days before birth. These cells were discoidal or elliptical, and flattened except for a nuclear bulge. Their cytoskeletal system, consisting of a marginal band of microtubules enclosed within a cell surface-associated network (membrane skeleton), closely resembled that of non-mammalian vertebrate erythrocytes. By day 2 or 3, much smaller anucleate erythrocytes of "definitive" morphology, lacking marginal bands, appeared in abundance. These accounted for greater than 90% of the circulating population of both species by day 6-8. Non-nucleated erythrocytes of a different type, constituting 1-6% of the cells in most blood samples up to day 7, were identified as anucleate primitives on the basis of size, shape, and presence of a marginal band. Thus, loss of erythrocyte nuclei in mammals appears to begin earlier than generally recognized, i.e., in the primitive generation. Counts of these anucleate primitives in young of various ages implicated nucleated primitives as their probable source. Pointed erythrocytes, occasionally found in younger neonates of both species, occurred in greatest number in fetuses (Tammar) prior to birth. This is in accord with previous work on non-mammalian vertebrates suggesting that such cells are morphogenetic intermediates. The results confirm the long-suspected similarity between mammalian primitive erythrocytes and the nucleated erythrocytes of all non-mammalian vertebrates.
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