Here another putative stem brontothere,
Lambdotherium (Cope 1880, Mader 1998; Eocene, 50mya; Fig. 1) likewise moves away from the basal brontothere, Eotitanops. In the LRT Lambdotherium nests with Ancodus (Fig. 2), another basal artiodactyl close to extant pigs.
I don’t know of any post-crania
for Lambdotherium. Note that Ancodus (Fig. 2), like Eotitanops, has a pentadatyl manus. Lambdotherium was traditionally considered a brontothere based on its teeth. The LRT employs relatively few dental traits. And maybe some specimens need to be reexamined. The very high arch of the Lambdotherium squamosal, among many other traits, is more similar to pig-like taxa, than to basal brontotheres, which here nest closer to rhinos, than to horses, contra the Wikipedia report on brontotheres.
Distinct from both rhinos and horses,
brontotheres have four toes on the forefeet. All are derived from a sister to Hyrachyus, which likewise has four toes.
Cope ED 1880. The bad lands of the Wind River and their fauna. The American Naturalist 14(10):745-748.
Mader BJ 1998. Brontotheriidae. In Janis CM, Scott KM, and Jacobs LL (eds.), Evolution of Tertiary Mammals of North America 1:525-536.
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.