The origin of eutherian mammals

Abstract

Palaeontologically recognizable eutherians originated no later than the Early Cretaceous in warm, probably moderately seasonal climates. Immediate ancestors were small, sharing many anatomical, physiological and reproductive features with small modern marsupials. Development of characteristically eutherian features involved interactions of body size, rates of metabolism, energetic costs of reproduction, anatomical/physiological processes of development and effects of each upon rates of population growth. In contrast to eutherians, marsupials have a narrow range of basal metabolic rates (lacking high rates), and show no direct links between rate of energy expenditure and gestation period, postnatal growth rate, fecundity or reproductive potential. Biological implications of this contrast are most pronounced at small body sizes. When resources are abundant, the relatively higher growth rates and earlier maturation of small eutherians (particularly those with high rates of metabolism) can lead to rapid population growth; among most marsupials, however, both pre-and postnatal constraints apparently preclude attainment of such high rates of reproduction. Also, only eutherians among the amniotes combine intimacy of placentation with prolonged active intra-uterine morphogenesis. Once established, that 28 1 002&4066/87/110281+56 $03.00/0 0 1987 The Linnean Society of Lond 282 J. A. LILLEGRAlJEN E l AL.(,ombination permitted (and even favoured) increases in diversity of adaptation in such disparate asperts as elevated metabolic rate, increased pre-and postnatal growth rates, increased cnccphalization, greater longevity, increased gregariousness, greater karyotypic flexibility, and augmented variability in adult morphology. However, all such boosts in diversity were probably secondary and dependent upon prior innovation of trophoblastic/uterine wall immunological protection of foetal tissues during prolonged intra-uterine development. Increased metabolic rates followrd thrreafter, with synergisms that may have speeded evolution among early eutherians. Eutherian-style trophoblast probably originated in the Mesozoic. Dependent adaptations, variably expressed, evolved later in sundry descendant lineages. Reproductivc differences between marsupials and eutherians are not biologically trivial; to the contrary, breakthroughs among eutherians assured their dominance: (1) in high intensity food habits; (2) at small body masses: and (3) in very cold climates.

Phylogenetic diversity as a window into the evolutionary and biogeographic histories of present-day richness gradients for mammals

“…Africa and Southeast Asia contain greater PD than predicted from species richness, while South America and western North America capture less PD than predicted from species richness. Using plots of the richness of ancestral lineages through time, we show that Africa has a greater diversity of old lineages, probably reflecting the early African origins of many major clades of extant mammals [102]. By contrast, South America shows low richness of old lineages, with tropical lineage diversity only approaching that for Africa within the last 20 Mya.…”

Environmental correlates and energetics of winter flight by bats in southern Alberta, Canada

“…Hibernating animals save considerable energy and can survive on limited resources (i.e., body fat or cached food) for extended periods, up to months at a time (Humphries et al 2003). Benefits of hibernation are obvious (Geiser 2004;Geiser and Brigham 2012) but there are associated costs, such as suppressed molecular synthesis (Lillegraven et al 1987), ceased or delayed reproduction (Racey 1969; Barnes et al 1986), and immunosuppression (Bouma et al 2010). For these reasons and likely others, nearly all hibernating mammals arouse periodically (Willis 1982; French 1985), ostensibly to excrete metabolic wastes (Baumber et al 1971), mount immune responses (Burton and Reichman 1999), mate (Thomas et al 1979), eat (Humphries et al 2001), and possibly drink (Thomas and Cloutier 1992;Thomas and Geiser 1997; Ben-Hamo et al 2013).…”

Craniofacial development in marsupial mammals: Developmental origins of evolutionary change

“…The evolutionary origins of these differences, as well as the potential evolutionary constraints imposed by one or the other strategy have long been topics of broad interest and controversy (e.g., Tyndale-Biscoe, 1973;Lillegraven, 1975;Kirsch, 1977a,b;Lee and Cockburn, 1985;Lillegraven et al, 1987;Tyndale-Biscoe and Renfree, 1987;Hughes and Hall, 1988;Cockburn, 1989;Maier, 1993Maier, , 1999Sears, 2004;and references therein).…”

“…Several evolutionary scenarios have been presented to account for the differences between marsupials and eutherians. Some scenarios emphasize constraint, claiming that marsupials have been unable to develop suffi-ciently effective placentation methods or immunological protection of the fetus (e.g., Lillegraven, 1975;Lillegraven et al, 1987), while others argue that the marsupial mode of reproduction has evolved in response to distinct selective pressures and has significant advantages in some circumstances over the mode of reproduction seen in eutherians (e.g., Kirsch, 1977a,b;Parker, 1977;Hayssen et al, 1985). Others have claimed that marsupials and placentals represent ends of a continuum and that the reproductive strategies should not be seen as distinct alternatives, but merely differences in emphasis, perhaps the consequences of initial minor differences that have become magnified over time (e.g., Renfree, 1983Renfree, , 1995Tyndale-Biscoe and Renfree, 1987).…”