This page covers the stem line of the hippos (family Hippopotamidae, Suborder Whippomorpha; Flacke and Decher, 2019). There are only two extant species: Hippopotamus amphibius (the Common hippopotamus), which is distributed widely in sub-Saharan Africa, and Choeropsis liberiensis (the Pygmy hippopotamus), which is confined to the lowland tropical forests of Liberia, Côte d’Ivoire, Sierra Leone, and Guinea in West Africa (Encyclopaedia Britannica).
The phylogeny of the stem-Hippopotamidae is currently a topic of debate. Recently published trees (e.g. Gomes Rodrigues et al, 2021; Gernelle et al, 2023) are in quite close agreement for the clade closest to the crown group, but there are significant differences for more basal clades. The following time tree, taken from Gomes Rodrigues et al (2021), represents a possible topology:
The phylogeny of the stem-Hippopotamidae is currently a topic of debate. Recently published trees (e.g. Gomes Rodrigues et al, 2021; Gernelle et al, 2023) are in quite close agreement for the clade closest to the crown group, but there are significant differences for more basal clades. The following time tree, taken from Gomes Rodrigues et al (2021), represents a possible topology:
Figure 1. Time tree of the stem-Hippopotamidae
Several species share the distinction of being the oldest known members of the hippo stem group: Anthracokeryx tenuis, Myaingtherium kenyapotamoides and Bothriogenys gorringei. These are all of Middle Eocene (Bartonian) age. Public-domain images are not available for any of these species, but other members of the stem group are illustrated below (for a larger view, click on image):
Figure 2. Images of stem-Hippopotamidae
The above images are placed in left-to-right order from most basal towards the crown group, but the small number of images, mainly of jaws and teeth only, does not allow any trends to be discerned. Indeed, the life restorations suggest that some of these species showed little resemblance to modern hippos. The only exception is Anthracotherium magnum, which does look rather like a hippo but is interpreted to occupy a very basal position in the tree (Figure 1). It would be interesting to know on what evidence the artist based the image.
The oldest known member of the hippo crown group is a fossil of the extant species Hippopotamus amphibius, found in the Pliocene Makapansgat Formation at Makapansgat in South Africa (Butler and Greenwood, 1976).
Taking into account the ghost lineage of at least 6 million years on the hippo stem line (shown as a blue bar in Figure 1), the stem-to-crown transition of the Hippopotamidae appears to have lasted between 42 and 53 million years, from the Early Eocene to the Pliocene (Figure 1).
The oldest known member of the hippo crown group is a fossil of the extant species Hippopotamus amphibius, found in the Pliocene Makapansgat Formation at Makapansgat in South Africa (Butler and Greenwood, 1976).
Taking into account the ghost lineage of at least 6 million years on the hippo stem line (shown as a blue bar in Figure 1), the stem-to-crown transition of the Hippopotamidae appears to have lasted between 42 and 53 million years, from the Early Eocene to the Pliocene (Figure 1).
References
Butler PM, and Greenwood M (1976) Elephant-shrews (Macroscelididae) from Olduvai and Makapansgat. In R Savage and S Coryndon (eds.): Fossil Vertebrates of Africa, Vol. 4. London: Academic Press, pp. 1–56.
Flacke, G. L., & Decher, J. (2019). Choeropsis liberiensis (Artiodactyla: Hippopotamidae). Mammalian species, 51(982), 100-118.
Gernelle, K., Lihoreau, F., Boisserie, J. R., Marivaux, L., Métais, G., & Antoine, P. O. (2023). New material of Parabrachyodus hyopotamoides from Samane Nala, Bugti Hills (Pakistan) and the origin of Merycopotamini (Mammalia: Hippopotamoidea). Zoological Journal of the Linnean Society, zlac111.
Gomes Rodrigues, H., Lihoreau, F., Orliac, M., & Boisserie, J. R. (2021). Characters from the deciduous dentition and its interest for phylogenetic reconstruction in Hippopotamoidea (Cetartiodactyla: Mammalia). Zoological Journal of the Linnean Society, 193(2), 413-431.
Flacke, G. L., & Decher, J. (2019). Choeropsis liberiensis (Artiodactyla: Hippopotamidae). Mammalian species, 51(982), 100-118.
Gernelle, K., Lihoreau, F., Boisserie, J. R., Marivaux, L., Métais, G., & Antoine, P. O. (2023). New material of Parabrachyodus hyopotamoides from Samane Nala, Bugti Hills (Pakistan) and the origin of Merycopotamini (Mammalia: Hippopotamoidea). Zoological Journal of the Linnean Society, zlac111.
Gomes Rodrigues, H., Lihoreau, F., Orliac, M., & Boisserie, J. R. (2021). Characters from the deciduous dentition and its interest for phylogenetic reconstruction in Hippopotamoidea (Cetartiodactyla: Mammalia). Zoological Journal of the Linnean Society, 193(2), 413-431.
Image credits – stem-Hippopotamidae
- Header (Hippopotamus (Hippopotamus amphibius) at Philadelphia Zoo): Art G., CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
- Figure 2(Microbunodon minimum): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2(Heptacodon sp.): Alton Dooley, licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.
- Figure 2(Bothriodon brachyrhynchus): Robert Bruce Horsfall, Public domain, via Wikimedia Commons
- Figure 2(Skull and jaws of Anthracotherium magnum): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2(Anthracotherium magnum): Dmitry Bogdanov, under Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
- Figure 2(Aepinacodon americanus): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2(Elomeryx sp.): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2(Jaws of Elomeryx borbonicus): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2(Elomeryx armatus, life restoration): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2(Merycopotamus dissimilis): Murchison, Public domain, via Wikimedia Commons
- Figure 2(Archaeopotamus pantanellii): Open Access article Martino, R., Rook, L., Mateus, O., & Pandolfi, L. (2023). The Late Miocene hippopotamid, Archaeopotamus pantanellii nov. comb., from the Casino Basin (Tuscany, Italy): paleobiogeographic implications. Historical Biology, 1-14.