This used to be big news,
when a taxon no longer nested in a traditional clade and moved to another. But with the breakup of the Notoungulata, this has become commonplace for former members of this clade and I expect the trend to continue.
in the large reptile tree (LRT) Notostylops murinis (Ameghino 1897, Riggs and Patterson 1935; 75cm in esitmated length; Eocene; FMNH-P13319) nests with the Ectocion specimen that nests with elephants, not the one that nests with carnivores (Fig. 2).
In Notostylops, the anterior incisors were enlarged as in other elephant and sea cow ancestors, but the anterior jaws narrowed, so the incisors became close together, like those of rodents. The mandible itself was more robust. No canines were present. The premolars were molarized.
Of course all this is
based in the literature, not firsthand observation of the pertinent specimens. I encourage others to test these taxa to confirm these hypothetical interrelationships.
I became aware of this taxon
after reading Billet (2010), who wrote: “Intriguing similarities are also detected in the anterior dentition of Pyrotherium and the Casamayoran notoungulate Notostylops. These resemblances suggest a unique relationship between Pyrotheria and Notoungulata, specifically between Pyrotheria and Notostylops.” The large reptile tree does not confirm that relationship. Rather the former (Fig. 3) is a derived marsupial while the latter is a derived placental.
Ameghino F 1897. Mamiferos Cretaceos de la Argentina – Boletin Instituto Geografico Argentino 18:406-521.
Billet G. 2010. New Observations on the Skull of Pyrotherium (Pyrotheria, Mammalia) and New Phylogenetic Hypotheses on South American Ungulates. Journal of Mammal Evolution. 17:21-59.
Riggs ES and Patterson B 1935. Description of some notoungulates from the Casamayor (“Notostylops) beds of Patagonia. Proceedings of the American Philosophical Socity 75(2):163-215.