What are brontotheres?

Mihlbacher 2008 turned his 2004 PhD dissertation
into a comprehensive AMNH look at the Brontotheriidae (Fig. 1), those rhino-like perissodactyls that ranged throughout the Eocene of North America, Asia and a bit of Europe, and then disappeared. Mihlbacher 2008 reported, “The present study reevaluates the species taxonomy of brontotheres based on craniodental materials. All known taxa were considered for revision except North American representatives of Eotitanops, Palaeosyops, and Megacerops.”

Presently only two brontotheres are included
in the large reptile tree (LRT, 2032 taxa; Fig. 2): small Early Eocene Eotitanops and giant Late Eocene Embolotherium. Those are the alpha and omega taxa in every brontothere study. Add another or another dozen brontotheres and they all nest between these two. Today our interest returns to what was around a day, a week, and a million years before brontotheres had their genesis.

The LRT recovered one proximal outgroup taxon,
from 2030 competing candidates (Fig. 2): the one-ton Miocene hornless rhinoceros, Aceratherium (Fig. 3), a taxon not mentioned in Mihlbacher 2008.

Distinct from Milbacher’s outgroup traditions
In the LRT Hyracotherium nests at the base of the wide, short-legged rhinos, including Amynodon, far from brontotheres.

In the LRT Pachynolophus nests with Juxia and Paraceratherium, far from brontotheres.

In the LRT Danjiangia and Lambdotherium nest together close to Sus, the pig, far from the Perissodactyla and far from brontotheres.

So…
Why would anyone choose any outgroup taxa? That’s guesswork.
Why would anyone borrow outgroup taxa from others? That’s trust.
Why would anyone want a professor to ‘provide’ (= demand) a short list of approved outgroup taxa? That’s tradition and/or obedience.
Instead, let software choose your outgroup taxa from such a wide gamut of taxa that exclusion is minimized. That way you’ll have accuracy and falsifiable validity, without guesswork, trust, tradition and/or obedience.

References
Mihlbachler MC 2004. Phylogenetic Systematics of the Brontotheriidae (Mammalia, Perissodactyla). PhD dissertation. Columbia University. p. 757.
Mihlbachler MC 2008. Species taxonomy, phylogeny and biogeography of teh Brontotheriidae (Mammalia: Perissodactyla). Bulletin of the American Museum of Natural History 311:475pp.

wiki/Embolotherium
wiki/Eotitanops
wiki/Titanotherium
wiki/Aceratherium
wiki/Brontotheriidae