This page covers the stem group of the ceratomorphs (suborder Ceratomorpha, order Perissodactyla). This clade comprises the rhinoceroses and the tapirs.

Very few analyses of the phylogeny of the stem-Ceratomorpha have been published. Two recent ones, by Bai et al (2020) and Vautrin et al (2020) are combined in the time tree shown below:
​Figure 1. Time tree of the stem-Ceratomorpha
​The oldest known member of the stem-Ceratomorpha is Lophiaspis maurettei, described from Early Eocene (Early Ypresian) sediments at Palette in Provence, Southern France and at other localities in France, Spain and Portugal (Vautrin et al, 2020). No images are available of this species in the public domain, but other stem-group fossils for which public-domain images are available are shown below (click on image for larger view):
Names in   red indicate   that the fossil is younger  than the oldest known crown-group fossil.
​Figure 2. Images of stem-Ceratomorpha
​The above images, which generally depict only jawbones or teeth, are placed in left-to-right order from most basal towards the crown group, but no obvious trends can be seen. Note that Lophiodon sp. and Lophiodon lautricense, illustrated above, are located near the end of a long side branch of the phylogenetic tree (Figure 1, Clade B) and are thus not closely related to the crown group.
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The above time tree (Figure 1) indicates that the stem-to-crown transformation for the Ceratomorpha occurred entirely within the early Ypresian, a time period of no more than 4 million years.

References

Bai, B., Meng, J., Zhang, C., Gong, Y. X., & Wang, Y. Q. (2020). The origin of Rhinocerotoidea and phylogeny of Ceratomorpha (Mammalia, Perissodactyla). Nature Communications Biology, 3(1), 509.
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Vautrin, Q., Tabuce, R., Lihoreau, F., Bronnert, C., Gheerbrant, E., Godinot, M., ... & Billet, G. (2020). New remains of Lophiaspis maurettei (Mammalia, Perissodactyla) from the early Eocene of France and the implications for the origin of the Lophiodontidae. Journal of Vertebrate Paleontology, 40(6), e1878200.
Image credits – stem-Ceratomorpha
  • Figure 2 (Homogalax protapirinus): Open Access article Bai, B., Wang, Y., Meng, J., Li, Q., & Jin, X. (2014). New Early Eocene basal tapiromorph from Southern China and Its phylogenetic implications. PLoS One, 9(10), e110806.
  • Figure 2 (Cardiolophus radinskyi): Open Access article Bai, B., Wang, Y., Meng, J., Li, Q., & Jin, X. (2014). New Early Eocene basal tapiromorph from Southern China and Its phylogenetic implications. PLoS One, 9(10), e110806.
  • Figure 2 (Orientolophus hendongensis): Open Access article Bai, B., Wang, Y. Q., & Meng, J. (2018). The divergence and dispersal of early perissodactyls as evidenced by early Eocene equids from Asia. Nature Communications Biology, 1(1), 115.
  • Figure 2 (Chowliia laoshanensis): Open Access article Bai, B., Wang, Y., Meng, J., Li, Q., & Jin, X. (2014). New Early Eocene basal tapiromorph from Southern China and Its phylogenetic implications. PLoS One, 9(10), e110806.
  • Figure 2 (Karagalax mamikhelensis): Open Access article Bai, B., Wang, Y., Meng, J., Li, Q., & Jin, X. (2014). New Early Eocene basal tapiromorph from Southern China and Its phylogenetic implications. PLoS One, 9(10), e110806.
  • Figure 2 (Gandheralophus spp.): Pieter Missiaen and Philip D. Gingerich, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Meridiolophus expansus): Open Access article Bai, B., Wang, Y., Meng, J., Li, Q., & Jin, X. (2014). New Early Eocene basal tapiromorph from Southern China and Its phylogenetic implications. PLoS One, 9(10), e110806.
  • Figure 2 (Isectolophus latidens): Edward L. Troxell, Public domain, via Wikimedia Commons
  • Figure 2 (Lophiodon sp.): Daderot, CC0, via Wikimedia Commons
  • Figure 2 (Lophiodon lautricense): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA)  license