There's a new fossil ungulate-related article available online if anyone is interested:
Nicolás R. Chimento; Federico L. Agnolin (2020). Phylogenetic tree of Litopterna and Perissodactyla indicates a complex early history of hoofed mammals. Scientific Reports 10 (1): Article number 13280. doi:10.1038/s41598-020-70287-5.
The perissodactyl affinities of litopternans have been previously suggested by DNA-based analyses, but the paper by Chimento and Agnolin is the first morphology-based cladistic analysis to support a relationship between Perissodactylia and Litopterna. Moreover, some families assigned to the polyphyletic group "Condylarthra" are recovered as related to Perissodactyla, while the condylarth family Phenacodontidae is recovered along with Pleuraspidotheriidae as sister to Afrotheria (which includes elephants, aardvarks, hyraxes, elephant shrews, and sirenians).