Toxodonts versus Rhinos
So, in a recent conversation, it has been suggested to me that the absence of american rhinos from the Pliocene onwards might be a result of not only climatic changes, but also competition with toxodontids (and ground sloths, though north american sloths and rhinos did co-exist in the late Miocene).
Meridungulates, while largely affected by northern invaders, have not been uniformly displaced by laurasithere ungulates, some groups even prospering and diversifying well until mankind reached South America in the “Holocene”. Toxodonts are one such example, being not only relatively diverse, but also perhaps among the most common components of Pleistocene south american megafauna.
Toxodon itself is a primarily Pleistocene critter, and one of the more omnipresent megafaunal herbivores of the continent from the epoch. In Brasil, what was once thought to be a fauna dominated by a single Toxodon species, Toxodon platensis, is now thought to represent a myriad of different taxa, such as Trigodonops/Toxodon lopesi and Mixotoxodon larensis, a diversity comparable to the contemporary Pleistocene proboscideans. Within Toxodontidae, Toxodontinae proper appears to have been the most successful Pleistocene clade, with 3-5 taxa dating to the Pleistocene/”Holocene”, although Haplodontheriinae is also well established, in the form of Mixotoxodon larensis and Trigodonops lopesi (though the later might be a species of Toxodon instead).
Thus, toxodonts were doing rather fine after the Interchange, being about as speciose as modern rhinos, and without the benefit of much larger geographical seperations. In Brasil at least, we observe a clearly range partitioning between tropical/subtropical haplodontheriines and Toxodon platensis, exemplifying enough diversity to warrant competitive speciations, and similar to historical eurasian rhino ranges. With such unimpeeded diversity, rhinos would have had little chance of settling down in South America like tapirs and proboscideans did, their niches as browsers and robust grazers already taken by a successful toxodont fauna.
Much like ground sloths, toxodonts also crossed Central America, invading the rhino homeland. Most of the north american remains belong to Mixotoxodon, a Pleistocene genus that would have conquered the area well after the last north american rhinos became extinct, and thus probably doesn’t reflect the ecological replacement of rhinos by toxodonts. Earlier remains do occur in the Pliocene of Central America, though, so as it seems toxodonts formed a solid barrier to rhinos, preventing them from colonising the south, and thus perhaps factoring in their demise when the glaciations occured.
* Ricardo Mendonça 2008
* Lundelius et al. 2013