There is uncertainty regarding the placement of Perissodactyla; a grouping with Ferae has been proposed as the clade Zooamata while a grouping with Cetartiodactyla has been proposed as the clade Euungulata ("true ungulates").
His research involves the evolution of Eurasian land mammals and terrestrial environments during the Neogene, ecomorphology of ungulates, developmental biology, the function and evolution of mammalian teeth, and scaling problems (changes in size with growth or as species evolve).
Souss-Massa also holds captive-breeding programmes for four threatened North African ungulates: Scimitar Oryx, Addax, Dama gazelle and Dorcas gazelle, that are kept in separate enclosures within the park.
While a close relationship between these and ungulates was already proposed by Frank Evers Beddard in 1900, and is generally accepted today, cetaceans have diverged so radically from their ancestors' lifestyle as to confound analyses of fossils and perhaps even DNA.