The Common Ancestor for the Australian Marsupial Dispersal?
There are two great things about doing research on the evolution of Australia’s marsupials:
1] When you are wondering where to look for the successor population of a given population, if you are already in Australia, you don’t have to go anywhere else! If you can’t find the fossilized link in Australia, you know you aren’t going to find them somewhere else.
and 2] if you can differentiate the fossil of an extinct line of marsupials well enough, you don’t have far to go to study the genomes of the living successor population. If the fossil comes from a “dead end” population, the current living specie or species may not descend from the “dead end”, but they will at least be “cousin” lineages.
This thread is a review of the basic milestones of research in the radiation and dispersal of the Australian marsupials. In my view, the evidence arrayed in Australia is far more interesting and compelling than what we see in the Galapagos islands. Yes… the Finches there are very nice… but there’s only so much diversity on a set of islands, even isolated ones. While the Finches have diversified to eat different things and live in different ways, the usual YEC complaint is that they are still “just finches”.
However, if one branch of the finches (genetically related) looked and acted like a Bald Eagle… that would be a pretty good example of speciation - - even in the minds of some YECs!
Well, in Australia, we have that kind of radiation and dispersal in the “mammalian” realm of the marsupials. And we apparently owe the existence of all these types of marsupials to a single ancestral population!
6 Panther like
But we’ll have to chase the rabbit a little bit to get to that “reveal”, because the “mother population” formed outside of Australia! But the branches that we follow within Australia apparently all lead back to that external mother population.
The link below is for an article that’s just 3 years old:
Mammals from ‘down under’: a multi-gene species-level phylogeny of marsupial mammals
by Laura J. May-Collado, C. William Kilpatrick, Ingi Agnarsson
Published February 26, 2015 PubMed 25755933
Figure 1: Summary cladogram of all the analyses showing
support for relationships among major clades within Metatheria.
So, let’s look at the the “mother” of all Marsupials (later postings will “drill down” into the details):
Australidelphia is the superorder that contains roughly three-quarters of all marsupials, including all those native to Australasia and a single species [stranded] in South America. (All other … marsupials [in the Americas] are members of the Ameridelphia.)
Analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials has shown that the South American monito del monte’s lineage is the most basal of the superorder.
[Source: Tracking Marsupial Evolution Using Archaic Genomic Retroposon Insertions
"Tracking Marsupial Evolution Using Archaic Genomic Retroposon Insertions" by Maria A. Nilsson, Gennady Churakov, Mirjam Sommer, Ngoc Van Tran, Anja Zemann, Jürgen Brosius, and Jürgen Schmitz
PLoS Biol. 2010 Jul; 8(7): e1000436. Published online 2010 Jul 27. PMCID: PMC2910653 PMID: 20668664 ]
The Tracking Marsupial Evolution article tells us that kangaroos and opossums are the marsupials most closely related to placental mammals, “having shared a common ancestor around 130 millions years ago”.
The Australian branches of marsupials have been lumped together under the name Australidelphians as a continent-wide clade, for which the name Euaustralidelphia (“true Australidelphia”) has been proposed (the branching order within this group is yet to be determined).
‘Tracking Marsupial Evolution (2010)’ is quoted: “Australidelphia is significantly supported by both molecular and morphological data and comprises the four Australasian marsupial orders and the South American order Microbiotheria, indicating a complex, ancient, biogeographic history of marsupials.”
Further: “The four Australasian orders share a single origin with Microbiotheria as their closest sister group, supporting a clear divergence between South American and Australasian marsupials. In addition, the new data place the South American opossums (Didelphimorphia) as the first branch of the marsupial tree. The exhaustive computational and experimental evidence provides important insight into the evolution of retroposable elements in the marsupial genome. Placing the retroposon insertion pattern in a paleobiogeographic context indicates a single marsupial migration from South America to Australia. The now firmly established phylogeny can be used to determine the direction of genomic changes and morphological transitions within marsupials.”
In the first study mentioned above [Mammals from ‘down under’ (2015)] re-affirms the the conclusions from the older research:
“The study also showed that the most basal of all marsupial orders are the other two South American groups (Didelphimorphia and Paucituberculata, with the former probably branching first). This indicates that Australidelphia arose in South America along with the other major divisions of extant marsupials, and likely reached Australia [80 mya while dinosaurs still dominated] via Antarctica in a single dispersal event after Microbiotheria split off.”