Argyrolagus: a South American kangaroo!

Earlier we looked at giant South American wombats, like Toxodon and Pyrotherium. And, of course, the basal stem marsupial, Didelphis, still lives in North America. So don’t be too surprised to find kangaroos (Figs. 1-4)  and other unrecognized derived marsupials in South America. They were there!

First you find them in the ground,
then you find them in a large gamut cladogram where taxa nest themselves with a miniimum of traditional bias.

Simpson 1970 reported, “Argyrolagids are marsupials, but show no clear affinity with any others known. They probably arose from didelphids independently of other known families and are distinct at the superfamily level, at least.” Well, all marsupials and placentals arose from didelphids in the large reptile tree, but let’s save that cladogram (Fig. 4) for later.

Sanchez-Villagra and Kay 1997 reported, “The Argyrolagidae are one of the most enigmatic extinct groups of South American mammals.” Several workers have even questioned the marsupial affinities of this clade. What they needed was the LRT.

Argyrolagids look like little kangaroos.
They probably hopped like little kangaroos. So what set them apart from kangaroos? Turns out, not much…

Argyrolagus palmeri and Proargyrolagus bolivianus (Fig. 1; Ameghino 1904; Simpson 1970, Sanchez-Villagra and Kay 1997; Late Oligocene, Plio-Pleistocene)) was just added to the large reptile tree (LRT, 1031 taxa), and it nested with a big kangaroo, Macropus (Fig. 2).

So why was Argyrolagus
EVER considered an enigma? Is it because kangaroos don’t live in South America? Or is this yet another case of taxon exclusion?

Figure 1. The skull of Proargyrolagus and an illustration of Argyrolagus. The traits shown here align very closely with Macropus, the kangaroo, to no one's surprise... so why was this considered an enigma taxon?

Figure 1. The skull of Proargyrolagus and an illustration of Argyrolagus. The differences are subtle yet notable. In either case, the traits shown here align very closely with Macropus, the kangaroo (see figure 2). So why were these considered enigma taxa?

Perhaps so.
Online I don’t see many cladograms that include both taxa. In the large gamut  reptile tree Argyrolagus shares nearly all of its tested traits with Macropus, the kangaroo (Fig. 2) and it’s easy to see why. The small list of differences incude: smaller size, four premaxillary teeth, naris open ventrally, caudals 3x longer than tall, lack of a maxillary diastema.

Figure 2. The extant kangaroo, Macropus, nests as a sister to Proargyrolagus in the LRT.

Figure 2. The extant kangaroo, Macropus, nests as a sister to Proargyrolagus in the LRT. Even the pelvis in lateral view looks like the one n Argyrolagus.

Based on their similar small size
Argyrolagus has been compared to extant ricocheting rodents, like kangaroo rats. Surprisingly and apparently argyrolagids haven’t yet been compared to real kangaroos in a phylogenetic analyses.

Argyrolagids and toxodontids are found in South America.
Kangaroos and wombats come from Australia. 180–140 million years ago, during the Jurassic, Gondwana split these related taxa apart. So their genesis and radiation must have been earlier, perhaps in the early Jurassic.

If you know of a paper
that includes both Argyrolagus and Macropus as taxa, and they nest far from each other, let me know. I’d like to learn the details. This could be a grand case of homoplasy. But at present, we’re looking at overlooked homology and both giant wombats and tiny kangaroos in Bolivia.

On a similar note
Smithsonian online mentioned the roots of kangaroos and wombats in South America. The article referenced Nilsson et al. (Fig. 3) who studied the genomics of Australian and South American marsupials. They report, “The evolutionary relationships among the seven marsupial orders have, however, so far eluded resolution.  In particular, the relationships between the four Australasian and three South American marsupial orders have been intensively debated since the South American order Microbiotheria was taxonomically moved into the group Australidelphia.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. Placing the retroposon insertion pattern in a paleobiogeographic context indicates a single marsupial migration from South America to Australia.” (Marsupials are resolved in the LRT).

Figure 3. Phylogenetic tree of marsupials derived from retroposon data from Nilsson et al. No extinct taxa are shown here.

Figure 3. Phylogenetic tree of marsupials derived from retroposon data from Nilsson et al. No extinct taxa are shown here.

Nilsson et al. 2010 further report,
Dromiciops is clearly only distantly related to Australian marsupials, supporting a single Gondwanan migration of marsupials from South America to Australia.” In the LRT Dromiciops, a South American opossum, is closely related to tested marsupials (Fig. 4) from Australia and South America.

Figure 4. Geographic distribution of basal Theria, focusing on marsupials.

Figure 4. Geographic distribution of basal Theria, focusing on marsupials. Patterns are just beginning to emerge with this number of taxa all radiating before the mid-Jurassic splitting of Gondwana.

The geographical radiation of basal therians
(Fig, 4) indicates a world-wide distribution with both marsupials and placentals (metatherians and eutherians) arising out of Asia before being restricted to North America (Didelphis), South America, Madagascar and Australia. At least that’s how it looks with this admittedly small sample set.

References
Ameghnino F 1904. Nuevas especies de mamíferos, cretáceos y terciarios de la República Argentina [New species of mammals, Cretaceous and Tertiary, from the Argentine Republic]. Anales de la Sociedad Cientifica Argentina 56–58:1-142.
Nilsson MA et al. (6 co-authors) 2010. Tracking marsupial evolution using archaic genomic retroposon insertions. https://doi.org/10.1371/journal.pbio.1000436
Sanchez-Villagra MR and Kay RF 1997. A skull of Proargyrolagus, the oldest argyrolagid (Late Oligocene Salla Beds, Bolivia), with brief comments concerning its paleobiology. Journal of Vertebrate Paleontology 17(4):717-724.
Simpson GG 1970. The Argyrolagidae, extinct South American marsupials. Bulletin of the Museum of Comparative Zoology 139, 1–86.

wiki/Argyrolagus

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