New phylogeny of extant and fossil sirenians

[VD231991]

Heritage and Seiffert (2022) have recently published a new phylogenetic and biogeographic analysis of extant and fossil sirenians, putting huge emphasis on genetic, morphological, temporal, and biogeographic data to offer the most up-to-date state-of-the-art knowledge of sirenian phylogeny. There are a few aspects of the cladistic analysis of fossil and extant Sirenia by Heritage and Seiffert that deserve attention:
- Recovery of Miosireninae outside Trichechidae in a sister relationship to Halitherium (=Kaupitherium)
- Placement of Metaxytherium crataegense and M. floridanum as members of Dugonginae
- Recovery of Metaxytherium albifontanum as more derived than Rytiodontinae and other basalmost stem dugongids but more primitive than the clade formed by Metaxytherium and crown Dugongidae
- Placement of Metaxytherium arctodites as the most basal member of Hydrodamalinae (which includes the Steller's sea cow)
 
Although it is no surprise that Metaxytherium is consistently recovered as paraphyletic within Dugongidae in many recent cladistic studies, the recovery of Metaxytherium crataegense and M. floridanum as dugongines begs the question of whether the genus name Hesperosiren should be resurrected for M. crataegense because that taxon is the type species of Hesperosiren. The recovery of M. albifontanum outside the Metaxytherium+Dugonginae+Hydrodamalinae clade (contra Vélez-Juarbe and Domning 2014) is more consistent with the older age of M. albifontanum, considering that the cladistic analysis of Vélez-Juarbe and Domning (2014) recovered M. krahuletzi as more primitive than albifontanum despite the former being a chronologically younger species. Even with the recovery of the Metaxytherium type species M. medium in a clade with Metaxytherium subapenninum, M. serresii, and M. krahuletzi basal to crown Dugongidae, the resurrection of Pontotherium Kaup, 1840 as the oldest available name for M. subapenninum and M. serresii may be necessary if future study confirms subapenninum and serresii as sufficiently distinct from M. medium at the genus level. This would the utilization of Metaxytheriinae Kretzoi, 1941 for the clade formed by M. medium, M. krahuletzi, M. subapenninum, and M. serresii. Likewise, the recovery of Miosireninae as sister to Halitherium (=Kaupitherium) suggests that there could be phylogenetic support for elevating Halitheriinae to familial status and restricting this clade to Halitherium and Miosireninae, given that many dugongids once assigned to Halitheriinae have been recognized as more closely related to the dugong and Steller's sea cow than to Halitherium. Time will tell if the phylogenetic arrangement for dugongids less derived than Hydrodamalinae and Dugonginae obtained by Heritage and Seiffert is supported by future studies, but it's more likely than not that new genera will be erected for Metaxytherium albifontanum, M. krahuletzi, and M. arctodites and that there will be cladistic support for reviving Pontotherium and Hesperosiren for a few species usually assigned to Metaxytherium, in no small part to fill ghost lineages for Hydrodamalinae, Dugonginae, and the Metaxytherium clade and take into account the late Oligocene age of albifontanum. 

Heritage, S., and Seiffert, E.R., 2022. Total evidence time-scaled phylogenetic and biogeographic models for the evolution of sea cows (Sirenia, Afrotheria). PeerJ 10:e13886 https://doi.org/10.7717/peerj.13886

Kaup, J., 1840. Notizen über die fossilen sogenannten Gras-fressenden Wale. Neues Jahrb. Min. Geogn. Geol. Pet. 1840: 673-676.

Kretzoi, M. 1941. Sirenavus hungaricus, ein neuer Prorastomide aus dem Mitteleozän (Lutetium) von Felsögalla in Ungarn. Annales Musei Nationalis Hungarici 34:146–156.

Vélez-Juarbe, J., and Domning, D.P., 2014. Fossil Sirenia of the West Atlantic and Caribbean region. Ix. Metaxytherium albifontanum, sp. nov. Journal of Vertebrate Paleontology 34(2):444-464.