Hey! Some of those Miocene ‘ungulates’ are marsupials!

More heresy today
courtesy of taxon inclusion.

Cassini 2013
looked at several traditional Miocene South American ‘ungulates’ (Fig. 1) unaware that these taxa do not nest in monophyletic clade any more specific than Theria in the large reptile tree (LRT, 1401 taxa). Cassini was reporting the results of “an ecomorphological study based on geometric morphometrics of the masticatory apparatus.” So he was working from prior cladograms and focusing on the mechanics of eating.

Figure 1. Image from Cassini 2013. Pink taxa are marsupials. Others are placentals.

Figure 1. Image from Cassini 2013. Pink taxa are marsupials. Others are placentals.

Earlier
here and here the traditional clade Notoungulata was splintered by the LRT into several clades, some among the marsupials, other among the placentals.

Traditional ‘Litopterna’
Diadiaphorus (Fig. 1) nests at the base of this clade. Theosodon (Fig. 1) nests as a derived taxon. Also included, but not listed: Chalicotherium and other chalicotheres.

Considering its member taxa,
the clade Litopterna (Ameghino 1889) is a junior synonym for Chalicotheridae (Gill 1872) in the LRT.

Considering its member taxa,
the clade Astrapotheria (Lydekker 1894) is a junior synonym for Meniscotheriinae (Cope 1882) and both nest within Phenacodontidae (Cope 1881).

Interatheriidae (Ameghino 1887) traditionally includes Interatherium, Protypotherium, Miocochilius and other taxa listed here. In the LRT Interatherium nests close to the ancestry of the Toxodon clade + the kangaroo clade within Metatheria. By contrast, Protypotherium and Miocochillus nest with Homalodotherium deep within the Eutheria. Homalodotherium traditionally nests with the the metatherian Toxodon. According to the LRT, all the above taxa developed similar enough traits by convergence that all were mistakenly lumped together in the invalid placental clade ‘Notoungulata’.

This is not the first time
metatherians were split from convergent eutherians. Most creodonts are marsupial predators, phylogenetically distinct from their traditional sisters in the clade Carnivora, within the clade Eutheria (Placentalia).

New taxa added to the LRT:

  1. Hegetotherium (Fig. 1) nests with Mesotherium between Interatherium and the Toxodon clade in the Metatheria. 
  2. Diadiaphorus (Fig. 1), the horse-mimic, nests at the base of the Litopterna/Chalicotheriidae, just basal to Litolophus.

I did not know these two, so I added these two to better understand them.

References
Cassini G 2013. Skull Geometric Morphometrics and Paleoecology of Santacrucian (Late Early Miocene; Patagonia) Native Ungulates (Astrapotheria, Litopterna, and Notoungulata). Ameghiniana 50 (2):193–216. DOI: 10.5710/AMGH.7.04.2013.606

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