Did marsupials originate in South America?
Above you see the phylogenetic trees for Metatheria in the papers for Tsagandelta (Rougier 2015) and Lotheridium (S. Bi 2015).
Both ultimately focus on deltatheroideans – both taxa are part of this group, after all-, but they reach a similar conclusion in regards to metatherian phylogeny, which I took care to highlight above: the closest relative of the clade leading to modern marsupials – just outside of the last common ancestor between them and sparassodonts – (highlighted in red) is the specimen known as the “Gurlin Tsav Skull” (highlighted in orange).
For those that don’t know, the Gurlin Tsav Skull is a specimen from the Late Cretaceous… of Mongolia.
Traditionally, marsupials are thought to have descended from north american species that reached South America in the Paleocene or Late Cretaceous. This makes sense: metatherian diversity was very high in North America during the Late Cretaceous, Australia was completely isolated from the northern continents during this epoch, and ultimately metatherian diversity is higher in South America than in Australia, the latter bearing exclusively one clade, Australidelphia.
Yet, as the cladograms above show, nearly all north american metatherians group within an entirely seperate clade from the south american ones, the only ones not being within this clade being a few deltatheroideans, which are even less closely related to marsupials. The graph clearly seems to indicate that south american and north american metatherian diversities were distinct radiations, with the closest relative being an asian species.
Granted, groups classically considered “stem-marsupials”, the herpetotheriids and “peradectids”, are absent. The latest study I can find on the matter seems to deem the former as part of the “northern” assemblage, while the latter are arranged as closer to the southern line, but given that most of their next closest relatives are “northern species” in these cladograms it seems likely that “peradectids” may be “northern” species as well.
Therefore, for now, we have to default to the Gurlin Tsav Skull.
This leaves us with two possibilities. The first is perhaps the most straightforward and admitely the most well supported: in the midst of the “nothern metatherians”, there was still a north american species more closely related to the Gurlin Tsav skull than to these, and it eventually reached South America and produced the “southern metatherians”. Many taxa evade the fossil reccord, after all, so there is credence to this, especially if this lineage was indeed rare; if “peradectids” – already thought to be paraphyletic – turn out to be close to marsupials after all, they immediately justify this assertion.
The other option, more radical, is that “southern metatherians” actually evolved from asian forms, that arrived to Australia during the Cretaceous or earliest Paleocene, and only later on dispersed across Antarctica to South America during the Paleocene and Eocene.
This hypothesis is not entirely uneresonable. We do know asian groups colonised eastern Gondwana in the Cretaceous: India and Madagascar in the Late Cretaceous bears a distinctively asian squamate and eutherian mammal groups (Sahni 1987 & 1994, DW et al 2006), mekosuchine crocodiles group among Paleogene eurasian clades (Alistair Glen 2014), and there is evidence of possible Cretaceous choristodere teeth in Timor (Umgrove 1949).*
Therefore, we have evidence that some tetrapod groups, even warm-blooded mammals, made the crossing from Asia to Gondwanna during this epoch, perhaps by island hopping from island systems precursor to modern Indonesia.
There is some evidence that metatherians dispersed multiple times across Antarctica, between Australia and South America. Microbiotheria, a group currently represented only by a south american species, were not only represented in the Palaeocene of Bolivia, but also the Eocene of Seymour Island and the contemporary Tingamarra fauna in Australia (Shiewe 2010, Nisson 2010). Though the possible australian Chulpasia species has since been found to be a possibly unrelated metatherian, the newly discovered Archaeonothos shows similarities with non-marsupial south american metatherians (Beck 2015).
Additionally the generally poor fossil Paleogene fossil reccord of Australia – only one pre-Oligocene assemblage, the Murgon Fossil Site -, and virtually none in the Cretaceous, it is a lot easier to make an argument for missing key species than in the very well documented Late Cretaceous and Paleocene north american faunas. Simply put, our window to Australia’s fauna during this epoch is very limited, and the fact that the Tingamarra Fauna contains both groups present in South America as well as a wealthy amount of incertae sedis forms makes it very likely that Australia had a non-australidelphian assemblage as diverse as South America’s.
Instead of a single metatherian lineage colonising Australia, it seems instead likely that there was a diversity comparable to South America’s, and that the latter simply saw a better fossil reccord and a higher number of post-Oligocene survivors.
Additionally, even if “peradectids” are close to “southern metatherians”, its just as equally likely that they were then recent invaders from South America to North America. There is precedent in this: the most basal of pantodonts is the south american Alcidedorbignya, implying a south american origin for this eutherian clade (Muizon 2015), and the fact that the closest known relative to dinoceratans is the south american Carodnia may similarly imply a southern origin for these “ungulates”, among the meridiungulate radiation (Burger 2015). More damning is the presence of a gondwanatherian in the Late Cretaceous of Mexico, a member of a clade that, as the name implies, is primarily gondwanan in range (SVP 2015).
In conclusion, the idea that marsupials and other southern metatherians colonised Australia first rather than South America has some merit. It should definitely be something that needs further exploration, as there is plenty of evidence of laurasian faunal groups having colonised eastern Gondwanna from Asia, and the mysterious Gurlin Tsav skull and Tingamarra Fauna both offer tantalising clues in that direction.
Multiple links in the text
G. W. Rougier, B. M. Davis, and M. J. Novacek. 2015. A deltatheroidan mammal from the Upper Cretaceous Baynshiree Formation, eastern Mongolia. Cretaceous Research 52:167-177
S. Bi, X. Jin, S. Li and T. Du. 2015. A new Cretaceous metatherian mammal from Henan, China. PeerJ 3:e896
Ashok Sahni, New evidence for palaeogeographic intercontinental Gondwana relationships based on Late Cretaceous-Earliest Palaeocene coastal faunas from peninsular India, Washington DC American Geophysical Union Geophysical Monograph Series 01/1987; 41:207-218. DOI: 10.1029/GM041p0207
Ashok Sahni, Guntupalli V R Prasad, Jaeger jean-jacques, C. K. Khajuria, Eutherian mammals from the Upper Cretaceous (Maastrichtian) Intertrappean Beds of Naskal, Andhra Pradesh, India, · June 1994
Krause, D.W., O’Connor, P.M., Rogers, K.C., Sampson, S.D., Buckley, G.A. and Rogers, R.R. 2006. Late Cretaceous terrestrial vertebrates from Madagascar: Implications for Latin American biogeography (subscription required). Annals of the Missouri Botanical Garden 93(2):178–208.
Alistair Glen, Christopher Dickman, Carnivores of Australia: Past, Present and Future, Csiro Publishing, 05/11/2014
J. H. F. Umbgrove, Structural History of the East Indies, 1949
Robin M.D. Beck (2015). “A peculiar faunivorous metatherian from the early Eocene of Australia”. Acta Palaeontologica Polonica 60 (1): 123–129. doi:10.4202/app.2013.0011.
Christian de Muizon, Guillaume Billet, Christine Argot, Sandrine Ladevèze & Florent Goussard (2015) Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny and palaeobiology. Geodiversitas 37 (4): 397-634.
BURGER, Benjamin J., THE SYSTEMATIC POSITION OF THE SABER-TOOTHED AND HORNED GIANTS OF THE EOCENE: THE UINTATHERES (ORDER DINOCERATA), Utah State University Uintah Basin Campus, Vernal, UT, United States of America, 84078, 2015