Characters from the deciduous dentition and its interest for phylogenetic reconstruction in Hippopotamoidea (Cetartiodactyla: Mammalia)

Abstract

Teeth are frequently used in phylogeny in order to better characterize the evolution of extinct mammal species. While most studies have focused on the adult dentition, the consideration of characters from the deciduous dentition could also contribute to reinforce phylogenetic assumptions or disentangle phylogenetic issues. We chose to investigate the characters of the deciduous dentition in cetartiodactyl taxa in relation to the disputed relationships within hippopotamoids, especially the position of Hippopotamidae. We describe the deciduous dentition of 51 species, among a dataset of 70 cetartiodactyls. We noticed that second and third deciduous premolars have a much lower degree of molarization, and are more suitable for coding than fourth deciduous premolars. Thirty-nine resulting characters were thus added to a previously published matrix, and parsimony and maximum-likelihood analyses were performed. Both analyses provided a better resolved topology for most taxa than without these characters, and with better support for most nodes in the parsimony analysis. Moreover, this analysis provides additional characters supporting the hypothesis of an emergence of hippopotamids within bothriodontine anthracotheres from the Palaeogene of Africa. The promising use of characters from the deciduous dentition in phylogeny should allow discussion of the underlying ontogenetic mechanisms at the origin of dental homology.

The axial skeleton of Brachyodus onoideus (Mammalia, Anthracotheriidae): taxonomic and functional implications

“…Their analyses therefore did not test the hypothesis proposed by Pickford (2008aPickford ( , 2008bPickford ( , 2015. The missing taxa and missing characters of the taxa included in the data base compiled by Gomes-Rodrigues et al (2021) render their results unlikely in a biological context. Interpreting the evidence from the sacrum is less easy because the fossil record of this bone in doliochoeroids is poor, the sole published specimen (Choeromorus petersbuchensis) preserving only the S1 and S2 (Pickford, 2017).…”

“…The new evidence from the atlas, axis and sacrum of Brachyodus underscores the conclusion published by Pickford (2008aPickford ( , 2008bPickford ( , 2015 that it is extremely unlikely that hippos descended from anthracotheres, but more likely that they are related to doliochoeroids and/or siderochoerids (sometimes known as Old World Peccaries, not to be confused with New World Peccaries, which are rather different from them) (Pickford, 2017). If this is so, then the concept of Hippopotamoidea as employed by Gomes-Rodrigues et al (2021) to embrace anthracotheres and hippopotamids within the same superfamily, needs to be tested by adding characters from the postcranial skeleton (especially the axial skeleton) and taxa (especially the Doliochoeridae and Siderochoeridae) to their character matrix which included 221 characters gleaned from 70 taxa. Comparisons of the crania of Brachyodus onoideus and 'Brachyodus' aequatorialis reveal that the former species is hypsicephalic while the latter is platycephalic.…”

“…The new data also prompt a reassessment of the systematic and phylogenetic relationships between anthracotheriids and hippopotamids, which have been an active subject of debate ever since it was hypothesised that hippos are more closely related to whales than to artiodactyls (Montgelard et al, 1997a(Montgelard et al, , 1997bGatesy, 1998Gatesy, , 1999Gatesy et al, 1996Gatesy et al, , 1999Boisserie et al, 2005aBoisserie et al, , 2005bGomes-Rodrigues et al, 2021). It is herein concluded that hippopotamids are unlikely to have descended from anthracotheres, because such a scenario would require evolutionary reversals in several anatomical complexes (dentition, atlas, axis, sacrum, radio-ulna, talus and metapodials among others).…”

First report of the lower dentition of Siamotherium pondaungensis (Cetartiodactyla, Hippopotamoidea) from the late middle Eocene Pondaung Formation, Myanmar

“…A small mesiostylid occurs mesiolingually and a short distostylid occupies the distal rim of the crown that seems to form a short and narrow talonid shelf. The mesiostylid is connected to the protoconid by a very short preprotocristid, unlike in microbunodontines and bothriodontines that display a clearly longer cristid on their d3 (GOMES RODRIGUES et al 2020). There is no lingual or buccal cingulid (Fig.…”

“…Variation in the d4 roots might also occur within each ungulate family (especially the root under the d4 protoconid according to GOMES RODRIGUES et al 2020), and only a much more thorough investigation would provide reliable phylogenetic information. Similarly, the very short preprotocristid that connects the mesiostylid and the protoconid on the d3 of S. pondaungensis might correspond to an anthracotheriine or even a basal anthracothere plesiomorphic feature, because this crest is usually better developed in microbunodontines and bothriodontines (GOMES RODRIGUES et al 2020).…”