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