Introduction: relictual lineages, marsupials andDromiciops
The discovery of living representatives of groups that were thought long extinct opens a window in time to improve our understanding of their biology, as they represent invaluable material to test evolutionary hypotheses on adaptation. These relict species (sensu Habel, Assman, Schmidtt, & Avise, 2010) generate an enormous amount of valuable knowledge regarding ecological, morphological, and physiological traits of past lineages, as they could serve as a ‘window to the past’ that allow us to understand the conditions that allowed them to survive for so long (Habel et al., 2010; Tan, Kelly, & Jiang, 2013; Yoder et al., 2010). Here we address one of these cases, the relict monito del monte (“little mountain monkeys”, with two recognised species,Dromiciops gliroides Thomas 1894 and D. bozinovici D’Elia et al. 2016), an outstanding mammal from southern South America. From fossil evidence (Goin & Abello, 2013) and ancestral habitat reconstruction of present-day marsupials (Mitchell et al., 2014), this marsupial (Dromiciops , hereafter) seems to have retained the ecological niche of its Gondwanan marsupial ancestors.
The ancestors of Marsupialia (crown-clade Metatheria) diverged from placental mammals (crown-clade Eutheria) at least 125 Mya in Laurasia (Bi et al., 2018; Luo, Yuan, Meng, & Ji, 2011), originating in today’s China and spreading to North America, where the earliest evidence of true marsupials is known (O’Leary et al., 2013). Those early mammals remained confined to Laurasia until the late Cretaceous, when they dispersed to Gondwana, following a North America – South America path. About the same time, they suffered particularly dire consequences of the KT extinction in Laurasia, which ultimately drove them to extinction on the supercontinent (Case, Goin, & Woodburne, 2005; Sanchez-Villagra, 2013). In South America, marsupials thrived and diverged, eventually spreading further south and reaching Antarctica about 65–70 Mya (Mitchell et al., 2014), presumably via dispersal of Microbiotherians (Nilsson et al., 2010; Prevosti, Forasiepi, & Zimicz, 2013). This order reached Australia through Antarctica and gave origin to Australasian marsupials, which dominated the continent and adjacent islands, occupying much the same ecological niches that placental mammals did in every other continent (Long, Long, Archer, Flannery, & Hand, 2002; Mitchell et al., 2014).
Today, marsupials are taxonomically less diverse than placental mammals, but their long and often isolated evolutionary history has resulted in a comparable morphological and ecological diversity (Sanchez-Villagra, 2013). Extant marsupials are grouped into three American (Didelphimorphia, Microbiotheria, and Paucituberculata) and four Australasian orders (Dasyuromorphia, Diprotodontia, Notoryctemorphia, and Peramelemorphia). The evolutionary relationships among marsupial orders have long been assessed using a wide range of methods that have often yielded contradictory and intensively debated results. Particularly puzzling is the position and origin of Microbiotheria. In this regard, Szalay (1982) proposed that Microbiotheria is nested within the modern Australasian clade: Australidelphia (Meredith, Westerman, Case, & Springer, 2008; Nilsson et al., 2010). The revolution of genetic and genomic methods during the last couple of decades has helped to disentangle the topology of the marsupial phylogeny (Eldridge, Beck, Croft, Travouillon, & Fox, 2019), clearly positioning Dromiciopswithin Australidelphia, the sister group of all living Australasian marsupials (Euaustralidelphia) and confirming the monophyly of the rest of American marsupials (Duchêne et al., 2018; Mitchell et al., 2014).