A surprising ancestor for kangaroos: Interatherium

Traditionally the short-faced kangaroo,
(genus: Procoptodon; Owen 1870; Pleistocene; Figs. 1, 3) was considered an aberrant taxon with a weirdly shortened face, so unlike that of traditional kangaroos, like Macropus (Fig. 1). However, by adding taxa, like Procoptodon and Dendrolagus (Fig. 1), to the large reptile tree (LRT, 1248 taxa, subset Fig. 4) Interatherium (Figs. 1,2) shifts over to become an ancestral kangaroo, despite lacking hopping legs and diprotodont teeth. The skulls of Interatherium and Procoptodon are incredibly similar, even if the post-crania and dental formula of Procoptodon has evolved.

Figure 1. The skulls of Toxodon, Procoptodon and Interatherium resemble one another more than their post-crania might suggest. Now they nest together in the LRT (subset in figure 2).

Figure 1. The skulls of Toxodon, Procoptodon and Interatherium resemble one another more than their post-crania might suggest. Now they nest together in the LRT (subset in figure 2).

Interatherium (Mid-Miocene)
has not been linked to Procoptodon (Pleistocene) before.

And why should it?

  1. Balbaroo (Flannery, Archer and Plane, 1983; Black et al. 2014, Middle Miocene) was hailed as a kangaroo ancestor, but in the LRT it nests with the phalanger, Phalanger
  2. Cookeroo bulwidarri (Butler et al. 2016; Late Oligocene, Early Miocene, 23-18mya)was hailed as a non-hopping kangaroo ancestor. The LRT has not tested it yet, but it looks like Macropus (Fig. 1).
  3. Palaeopotorous priscus (den Boer and Kear 2018; middle Miocene) was hailed as a non-hopping kangaroo ancestor, based on teeth.
  4. Tradition considers Interatheriidae “an extinct family of notoungulate (placental) mammals from South America, known from the Eocene through the Miocene. These animals were principally small-sized, occupying a habitat like hares and marmots.The majority were very small, like rodents.”
  5. Interatherium has four fingers (Fig. 2), lacking a thumb (convergent, it turns out, with Protypotherium, a placental herbivore traditionally considered related). Kangaroos retain five fingers (but I’d like to see a good X-ray or something similar).

Figure 2. Interatherium is the surprising ancestor of kangaroos, with a special affinity to the short-face kangaroo.

Figure 2. Interatherium is the surprising ancestor of kangaroos, with a special affinity to the short-face kangaroo.

Current DNA studies
place a small wallaby, Lagostrophus, at the base of their kangaroo cladogram, but Lagostrophus already has diprodontid teeth. That’s too easy. We’re looking for an earlier, more primitive taxon, without obvious kangaroo traits.

Figure 4. Procoptodon is a basal kangaroo, close to Interatherium (Fig. 3).

Figure 3. Procoptodon is a basal kangaroo, close to Interatherium (Fig. 3). Here longer legs and longer feet differentiate this taxon from Interatherium.

Interatherium (Miocene) represents a late-surviving member
of a much earlier (Late Jurassic) kangaroo radiation, in which the interathere clade lost its thumb. Alternate scenario: perhaps the thumb was never collected in the matrix. The epipubes were likewise somehow overlooked, though I think I see them is an online image of an in situ fossil. More data needed here.

This Late Jurassic kangaroo genesis
is based on the Early Cretaceous appearance of Anebodon, a kangaroo cousin more closely related to the extant marsupial mole, Notoryctes. These burrowers, in turn, have more kangaroo-like sister taxa, today represented by the bandicoot Perameles and the biliby, Macrotis, which combine long hind limbs and digging front limbs.

Note, the front dentary teeth of Interatherium
(Fig. 1). The change to diprotodonty (two anterior fangs) has not happened yet in Interatherium, but the canines are on the way out and the squamosals are very tall.

Figure 4. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

Figure 4. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

Interatherium rodens (Ameghino 1887, 1894; Middle Miocene; 50cm in length) the Interatheridae and Interatherium were long considered members of the Notoungulata, a clade that has broken up in the LRT. Here (Fig. 4) Interatherium nests at the base of the kangaroos, derived from the more basal marsupials like Eomaia. Interatherium retains several small incisors, but apparently has lost its thumb, unlike kangaroos.

Note
that Interatherium, nesting at the base of the kangaroo clade (Fig. 4), is also the sister to the Toxodon + the wombat (genus: Vombatus) clade. There the diprotodont dental pattern appears by convergence because, like Interatherium, basal taxa (genus: Eurygenium, late Oligocene, and Toxodon) lack a diprotodont dental pattern.

Goodbye, Diprotodontia.
The clade Diprotodontia is no longer monophyletic (Fig. 4) and can no longer be exclusively defined by the diprotodont dental pattern, which now appears twice within the Metatheria. Please test this heresy and let me know what you get. Taxon exclusion is once again the problem here.

References
Ameghino F 1887. Observaciones generales sobre el orden de mamíferos estinguidos sud-americanos llamados toxodontes (Toxodontia) y sinopsis de los géneros y especies hasta ahora conocidos. Anales del Museo de La Plata 1:1-66.
Ameghino F 1894. Enumeration synoptique des especes de mammifères fossiles des formations éocènes de Patagonie. Boletin de la Academia Nacional de Ciencias en Cordoba (Republica Argentina) 13:259-452.
Black KH et al. 2014. A New Species of the Basal “Kangaroo” Balbaroo and a Re-Evaluation of Stem Macropodiform Interrelationships. PloseOne https://doi.org/10.1371/journal.pone.0112705
den Boer W and Kear BP 2018. Is the fossil rat-kangaroo Palaeopotorous priscus the most basally branching stem macropodiform? Journal of Vertebrate Paleontology; e1428196 DOI: 10.1080/02724634.2017.1428196
Butler K, Travouillon KJ,Price GJ, Archer M and Hand SJ 2016. Cookeroo, a new genus of fossil kangaroo (Marsupialia, Macropodidae) from the Oligo-Miocene of Riversleigh, northwestern Queensland, Australia. Journal of Vertebrate Paleontology. doi:10.1080/02724634.2016.1083029.
Cooke BN 2000. Cranial remains of a new species of balbarine kangaroo (Marsupalia: Macropodoidea) from the Oligo-Miocene freshwater limestone deposits of Riversleigh World Heritage Area, Northern Australia. Journal of Paleontology 74(2) 317-26.
Flannery TF, Archer M and Plane MD 1983. Middle Miocene kangaroos (Macropodoidea: Marsupialia) from three localities in northern Australia, with a description of two new subfamilies. Bureau of Mineral Resources, Journal of Australian Geology and Geophysics 7: 287–302.
Owen R 1873. Procoptodon goliah, Owen. Proceedings of the Royal Society of London 21, 387.

wiki/Interatherium
wiki/Lagostrophus
wiki/Procoptodon
http://www.abc.net.au/news/2016-02-19/extinct-non-hopping-species-may-be-ancestors-of-kangaroo/7185650
Palaeopotorous PR: https://www.sciencedaily.com/releases/2018/04/180411111019.htm

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.

Figure 1. Subset of the LRT focusing on Metatheria after the addition of Diprotodon and Palorchestes. Some new clades are proposed here.

Figure s. Added June 9, 2019 with more metatheres added. Subset of the LRT focusing on Metatheria after the addition of Diprotodon and Palorchestes. Some new clades are proposed here. Proargyrolagus is still related to Macropus.

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

Anebodon: another symmetrodont or marsupial mole sister?

Updated March 09, 2018 with the nesting of this taxon with the golden mole, Notoryctes.

Bi et al. 2016
reported on “Anebodon luoi (STM 38-4, Tianyu Museum of Nature, Shandong Province, China, Fig. 1) a new genus and species of zhangheotheriid symmetrodont mammal from the Lujiatun site of the Lower Cretaceous Yixian Formation, China. The fossil is represented by an associated partial skull and dentaries with a nearly complete dentition.”

Figure 1. Anebodon luoi subjected to DGS tracing and phylogenetic analysis nests with Macropus, the extant kangaroo, not with Zhangheotherium. The kangaroo kink is just starting here with a concave/convex maxilla. The canines are present, but tiny. The anterior dentary teeth extend anteriorly, the first step toward the kangaroo's 'tusks'.

Figure 1. Anebodon luoi subjected to DGS tracing and phylogenetic analysis nests with Macropus, the extant kangaroo, not with Zhangheotherium. The kangaroo kink is just starting here with a concave/convex maxilla. The canines are present, but tiny. The anterior dentary teeth extend anteriorly, the first step toward the kangaroo’s ‘tusks’.

Bi et al. noted
Anebodon lacked the high molar count typical of derived symmetrodonts. Their diagnosis focused on dental differences compared to Maotherium and Kiyatherium.

By contrast
the large reptile tree (LRT) nested Early Cretaceous Anebodon with the the golden mole Notoryctes (Fig. 1). A septomaxilla was identified in Anebodon.

The clade
Symmetrodonta is considered paraphyletic at Wikipedia. The clade is based on teeth characterized by the triangular aspect of the molars when viewed from above and the absence of a well-developed talonid. Perhaps such teeth were common to basal placentals.

References
Bi S-D, heng X-T, Meng J, Wang X-L, Robinson N and Davis B 2016. A new symmetrodont mammal (Trechnotheria: Zhangheotheriidae) from the Early Cretaceous of China and trechnotherian character evolution. Nature Scientific Reports 6:26668 DOI: 10.1038/srep26668