Convergence of Afrotherian and Laurasiatherian Ungulate-Like Mammals: First Morphological Evidence from the Paleocene of Morocco

Abstract

Molecular-based analyses showed that extant “ungulate” mammals are polyphyletic and belong to the two main clades Afrotheria (Paenungulata) and Laurasiatheria (Euungulata: Cetartiodactyla-Perissodactyla). However, paleontological and neontological studies hitherto failed to demonstrate the morphological convergence of African and Laurasian “ungulate” orders. They support an “Altungulata” group including the Laurasian order Perissodactyla and the African superorder Paenungulata and characterized especially by quadritubercular and bilophodont molars adapted for a folivorous diet. We report new critical fossils of one of the few known African condylarth-like mammal, the enigmatic Abdounodus from the middle Paleocene of Morocco. They show that Abdounodus and Ocepeia display key intermediate morphologies refuting the homology of the fourth main cusp of upper molars in Paenungulata and Perissodactyla: Paenungulates unexpectedly have a metaconule-derived pseudohypocone, instead of a cingular hypocone. Comparative and functional dental anatomy of Abdounodus demonstrates indeed the convergence of the quadritubercular and bilophodont pattern in “ungulates”. Consistently with our reconstruction of the structural evolution of paenungulate bilophodonty, the phylogenetic analysis relates Abdounodus and Ocepeia to Paenungulata as stem taxa of the more inclusive new clade Paenungulatomorpha which is distinct from the Perissodactyla and Anthracobunidae. Abdounodus and Ocepeia help to identify the first convincing synapomorphy within the Afrotheria–i.e., the pseudohypocone–that demonstrates the morphological convergence of African and Laurasian ungulate-like placentals, in agreement with molecular phylogeny. Abdounodus and Ocepeia are the only known representatives of the early African ungulate radiation predating the divergence of extant paenungulate orders. Paenungulatomorpha evolved in Africa since the early Tertiary independently from laurasiatherian euungulates and “condylarths” such as apheliscids. The rapid early Tertiary radiation of the Afrotheria and Paenungulatomorpha, as illustrated by the Paleocene Moroccan mammals, is concurrent with that of the Laurasiatheria in a general, explosive mammal evolution in both the South and North Tethyan continents following the K/Pg event.

Early African Fossils Elucidate the Origin of Embrithopod Mammals

“…It is supported by analyses constraining the Afrotheria and Afroinsectiphilia (analyses 7-10), and it is also found in unconstrained analysis (analysis 1) and in analysis with TNT's ''implied weighting'' option (analysis 2; Figure 4A). Of note, it is most usually associated with the clade Paenungulatomorpha [7]. The alternative, previously consensual sister-group relationship of the Embrithopoda and the Proboscidea is found marginally in three analyses (analyses 1, 3, and 7), especially with unordered features (analysis 3).…”

“…This is especially true for the Arabo-African Island, which was the center of radiation of one of the main placental clades, the Afrotheria. The Ouled Abdoun phosphate series, in Morocco, yielded the only known Paleocene and early Eocene fossil remains of stem and early crown paenungulate afrotherians [3][4][5][6][7][8]. The paenungulates, today represented by orders of hyraxes (Hyracoidea), elephants (Proboscidea), and sea cows (Sirenia), were major components of the African endemic herbivorous megafauna during the Paleogene.…”

“…Comparisons emphasize the primitive morphology of the two Moroccan species within the Embrithopoda. They differ from all other embrithopods, including Palaeoamasiidae, by their small size and plesiomorphic traits, such as a primitive hyperdilambdodont state with less lingual paracone and metacone and correlatively shorter pseudolophs, and by retention of prominent postparacrista, postmetacrista, and pseudohypocone (metaconule in Paenungulata [7]). Other primitive features of Stylolophus with respect to palaeoamasiids are the lower crown of cheek teeth, M 1-2 with only one lingual root and with separated paracone and protocone roots, P 2 single rooted and simple, and P 2 double rooted and simple.…”

“…The paenungulate radiation also includes the extinct order Embrithopoda of uncertain interordinal relationships, which is best known for the giant and strangely specialized Oligocene genus Arsinoitherium. The Ouled Abdoun basin, Morocco, yielded exceptional Paleocene-Eocene fossils documenting the early paenungulate evolution [3][4][5][6][7][8]. Here we report two new small Ypresian species, Stylolophus minor n.g., n.sp.…”

Improvements in the fossil record may largely resolve the conflict between morphological and molecular estimates of mammal phylogeny

“…If genuine fossil taxa exhibit these character combinations, then their discovery and inclusion in phylogenetic analyses might be sufficient to largely resolve the current conflict between molecular and morphological analyses, even using morphological matrices that currently show extensive conflict with molecular data, such as that of O’Leary et al [3]. There is increasing evidence that this may indeed be the case; for example, well-preserved remains of Ocepeia from the Palaeocene of Morocco reveal that it combines features of “insectivoran” and “ungulate” afrotherians [40], and other African fossils show that dental similarities between “ungulate” afrotherians and laurasiatherians are homoplastic [41].…”