Toxodon: now closer to kangaroos than to wombats

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).

Another short one today
just to dash off a progress report as I wrestle with metatherian data. Like everyone else, I’m learning as I go. Toxodon and Eurygenium (Fig. 1) were always close to Interatherium, which was recently nested at the base of all kangaroos with the addition of the short-faced kangaroo, Procoptodon to the large reptile tree (LRT, 1258 taxa). Bulky, quadrupedal Toxodon and Eurgenium previously nested with quadrupedal wombats. 

The loss of pedal digit 1
found in kangaroos, interatheres and
Toxodon + Eurygenium turns out to be not a convergence, but a homoplasy as the Toxodon + Eurygenium node shifts over to the Interatherium node. All three are quadrupeds.

Now basal wombats with five pedal digits,
like the koala, no longer have four-toed taxa, like Eurygenium and Toxodon, separating them from their five-toed ancestors. 

Interatheres are getting to be more interesting. 

Figure 2. Subset of the large reptile tree focusing on the Metatheria. The tree is fully resolved, but many bootstrap scores under 50 indicate that only one or two characters separate those nodes with low scores. Scores higher than 50 are separated by three or more traits.

Figure 2. Subset of the large reptile tree focusing on the Metatheria. The tree is fully resolved, but many bootstrap scores under 50 indicate that only one or two characters separate those nodes with low scores. Scores higher than 50 are separated by three or more traits.

BTW
loss of pedal digit 1 also occurs alone in the wolf-like marsupial carnivore, the Tasmanian wolf, Thylacinus, by convergence.  By convergence, pedal digit 1 also shrinks in the bandicoot (Perameles) clade.

Geologically
western Gondwana (Africa + South America) separated from eastern Gondwana (Madagascar, India, Australia, Antartica) about 180 mya (Jurassic). That’s when the ancestors of South American Interatherium and Toxodon separated from the Australian ancestors of kangaroos. This is one way to estimate the antiquity of mammal clades.

Final thought for paleontologists and soon-to-be-paleontologists:
Reexamining data, like this, is good science. Making mistakes. like this, goes with the territory. Naiveté and enthusiasm go hand-in-hand. Apologies often follow. Gaining experience is a slog, but worthwhile when the puzzle pieces fit better in the end. More work often brings new insight. 

 

 

4 thoughts on “Toxodon: now closer to kangaroos than to wombats

    • According to the LRT Toxodon nests in a clade with Interatherium at its base. And, yes, the interathere clade has a last common ancestor with the kangaroo clade apart from the wombats. At that point, all were quadrupeds. Note the weak support (<50). More taxa may change things. This is a hypothesis of interrelationships.

  1. hold your hypothetical horses, do I see amphicyonids and hyenodonts, some very widely agreed on placentals being mixed in with sparassodonts? does that mean that they were metatherians? and thylacinus there, does that mean that thylacinus wasn’t a true marsupial?, what does that mean for the rest of dasyuromorphia? i have so many questions

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