There is a world somewhere between reality and fiction. Although ignored by many, it is very real and so are those living in it. This forum is about the natural world. Here, wild animals will be heard and respected. The forum offers a glimpse into an unknown world as well as a room with a view on the present and the future. Anyone able to speak on behalf of those living in the emerald forest and the deep blue sea is invited to join.
"Leo" in quotes means the species is not Panthera leo but as it has not been discovered yet (and given a proper taxonomic name), the authors will call this (currently missing) link "Panthera "leo"" to have some ID to refer to. "ssp." means subspecies. So that point on the tree is indicating a subspecies of the ancestral Panthera "leo" was isolated in Eurasia and became the cave lion, while the original African Panthera "leo" became the modern lion.
The modern lion split is not shown in the tree. The P. fossilis evolution into P. atrox in the Americas (not shown correctly as a dichotomy due to spatial constraints) is shown.
Correct on the relationship between P. fossilis and P. atrox, Sotnikova and Foronova (2014) very well describe such. The paper is in the library post on the first page.
Postcranial-wise I do not remember seeing any robusticity difference between the American and Cave lion forms. Cranially, P. spelaea was quite more robust.