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).