Leptobos and the origin of Bos and Bison

In the most recent phylogenies based on mitochondrial DNA, Bos turns out to be paraphyletic. The yak is grouped as a sister taxon to Bison, and banteng and gaur form a clade that is sister to yak and bison [1,2]. The wisent is a sister taxon to cattle in this phylogeny, was has been interpreted as the result of hybridization. The most recent study, however, considers it more likely that this is the result of incomplete lineage sorting [1].  

Regarding the origin of Bos, there are two conflicting hypothesis. One that suggests an African origin, and one that suggests an Asiatic origin. Advocates of the "out of Africa" hypothesis suggest an origin from Pelorovis oldowayensis [3]. Pelorovis, however, is considered to be too different by other authors. They suggest an origin from Leptobos instead, which is described as more similar to Bos [4]. 

Leptobos comprises a group of fossil bovine species with very variable horns. Leptobos is also suggested to be the direct ancestor of Bison. And indeed the cranium of L. vallisarni bears striking resemblance to bison skulls, so that some authors list it as Bison vallisarni instead. Leptobos stenometopon, on the other hand, has a horn curvature that is reminiscent of that of aurochs, kouprey and yak. This horn curvature could either be basal to the Bos-Bison clade or evolved several times. The fact some species of Leptobos resemble Bos species while others resemble bison in cranial anatomy might imply that different species of Leptobos gave rise to the modern Bos clades (aurochs/cattle, the banteng-gaur and possible also kouprey clade) and bison. In this case, Leptobos would be a genus that includes the basalmost members of the Bos-Bison clade. This is only my personal speculation. More complete skeletal material would be needed in order to better resolve the paleontological evidence for the origin of the Bos and bison species. 

Leptobos etruscus - is Leptobos the ancestor of the Bos-Bison clade?

[1] Wang et al.: Incomplete lineage sorting rather than hybridization explains the inconsistent phylogeny of the wisent. 2018. 
[2] Verkaar et al.: Maternal and paternal lineages in cross-breeding bovine species. Has the wisent a hybrid origin? 2004. 

[3] Martinez-Navarro et al.: The Olduvai buffalo Pelorovis and the origin of Bos. 2007. 

[4] Tong et al.: New fossils of Bos primigenius (Artiodactyla, Mammalia) from Nihewan and Longhua of Hebei, China. 2014.