Which smaller relative within the same order is mentioned as highlighting diversity within Litopterna?

Answer

*Thoatherium*, which had hooves akin to a modern gazelle

*Macrauchenia*'s closest relatives within South America included smaller, more swift-running forms like *Thoatherium*, which specialized in lighter grazing.

Which smaller relative within the same order is mentioned as highlighting diversity within Litopterna?
animalextinctmammalungulatemacrauchenia