Toxodon and Pyrotherium are wombats

Earlier the LRT nested two taxa, Vintana and Zalambdalestes, with the wombat, Vombatus. I did not know then that this would start a trend among enigmatic and unusual taxa.

Today
the enigmatic South American ‘ungulates’ Toxodon (Fig. 1) and Pyrotherium (Fig. 2) move over from the decimated Notoungulata and Ungulata to nest with marsupial wombats. They lack epipubes and have only three molars (per side x4), but other related marsupials also share those traits. It’s no longer a rule that marsupials have to have four molars.

Figure 1. Toxodon was a notoungulate placental. Now it's a wombat marsupial.

Figure 1. Toxodon was a notoungulate placental. Now it’s a wombat marsupial with only three toes on each foot. Note the septomaxilla and posterior placement of the jaw glenoid.

Toxodon platensis
(Owen 1837; Pliocene-Pleistocene 2.6-.016 mya; 2.7 m in length) lost the medial and lateral toes on all four hoofed extremities. That makes it look like a perissodactyl ungulate. The high neural spines and low position of the skull remind one of North American bison. Prehistoric humans hunted them, based on the arrow tips found with skeletons.

Figure 2. Pyrotherium was another notoungulate. Now it is another wombat.

Figure 2. Pyrotherium was another notoungulate. Now it is another wombat. From what I’ve seen, I think the dorsal view is fairly damaged. Note the septomaxilla (orange) and posterior placement of the jaw joint. I show the fossil and drawings because not all the parts match up.

Pyrotherium sorondoi
(Ameghino 1889, 1894, 1895; Early Oligocene, 29-21 mya; 3m long); data here comes only from its skull, but post-cranial pieces are known indicating a graviportal stance. Wikipedia called Pyrotherium an ungulate. Other workers (i.e. Shockley et al. 2004) had trouble nesting it, too. It converges with Arsinoitherium and elephants. Provided with a wide gamut of mammals to nest with, Pyrotherium nests with Toxodon close to Vombatus.

Figure 3. Two large wombats, Vombatus and Phascolonus for comparison.

Figure 3. Two large wombats, Vombatus and Phascolonus for comparison. These wombats have epipubes.

The wombats
are becoming much more diverse than earlier imagined. Credit ‘taxon inclusion’ for this insight and others.

References
Ameghino F 1889Contribución al conocimiento de los mamíferos fósiles de la República Argentina, obra escrita bajo los auspicios de la Academia Nacional de Ciencias de la República Argentina para presentarla a la Exposición Universal de Paris de 1889. Actas Academia de Ciencias. de Córdoba 6:11027.
Ameghino F 1894. Sur les oiseaux fossiles de Patagonie; et la faune mammalogique des couches à Pyrotherium. Boletin del Instituto Geographico Argentino 15:501-660.
Ameghino F 1895. Premiére contribution à connaissance de la fauna mammalogique de couches à Pyrotherium. Boletín Instituto Geográfico Argentino 15:603660.
Owen R 1837. Description of the cranium of the Toxodon platensis. Proceedings of the Geological Society of London 2:541-542.
Shockey BJ & Anaya F 2004. Pyrotherium macfaddeni, sp. nov. (late Oligocene, Bolivia) and the pedal morphology of pyrotheres. Journal of Vertebrate Paleontology. 24 (2): 481–488. doi:10.1671/2521.

wiki/Pyrotherium
wiki/Toxodon

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