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Tissier and Smith 2026 ran perissodactyls through analysis and traditional taxonomic problems arose

Tuesday, September 1, 2026 6:01

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Tissier and Smith 2026
revealed through phylogenetic analysis “Hyracotherium is Not a Horse: For over a century, Hyracotherium (also called Eohippus) was taught as the earliest direct ancestor of horses. Tissier and Smith showed it is actually a basal (stem) perissodactyl, sitting on the family tree before the evolutionary split between horses, rhinoceroses, and tapirs.”

You heard that here earlier. The large reptile tree (LRT, 2340 taxa) nested Hyracotherium (Fig 1) basal to the clade Perissodactyla for a decade (2016).

Figure 1. Photos and caption from 2016. Hyracotherium is an Eocene horse sister in the LRT. Skull bones are colorized here.

From the abstract
“Here, we investigate the early evolution of perissodactyls by analyzing a new dataset focusing on early species to obtain a new expansive phylogeny. The phylogeny also supports a potential Indo-Pakistani origin of perissodactyls. In addition, this large-scale phylogeny shows that many species that were named as “Hyracotherium” in the past (or sometimes “Eohippus”, the “dawn-horse”) and considered as “horses”, are not actually closely related to horses (Hippomorpha), including Hyracotherium itself, but can be considered as basal perissodactyls.”

In the LRT the direct ancestor of horses + indricotheres is Hyracotherium (Fig 1). It is also the direct ancestor of rhinos and tapirs.

Figure 2. GIF movie (3 frames) showing what is known of the skeletons of Baluchitherium and Indricotherium. Note the more horse-like morphology.

Indricotheres are not mentioned in the text.
Rhinos are mentioned only twice. in the LRT indricotheres are giant horses, not hornless rhinos in the LRT. They put their weight on one central toe, have a long neck, long legs and other traits common to horses, not rhinos.

The authors used 71 terminal taxa.
Some (rhinos, tapirs, horses, Hyracotherium) nested within the traditional clade Perissodactyla. Other traditional perissodactyls used by the authors (Brontotherioidea, Lophiodontidae, Chalicotheriidae, Isectolophidae) nested well outside the Perissodactyla with other taxa in the LRT, which tests 600+ mammals and synapsids in order to minimize taxonomy exclusion and, in this case, inappropriate inclusion. The authors outgroup taxa were cherry-picked = not determined by an over-arching analysis that determined precise outgroup taxa.

The LRT is a hypothesis that needs to be tested in whole and in part.
If authors continue to use university textbooks for their inclusion sets, we’re going to be stuck in the same rut that Tissier and Smith found themselves in – with the approval of editors and referees.

YouTube video on horse evolution

References
Tissier J and Smith T 2026. Earliest perissodactyls reveal large-scale dispersals during the PETM. PNAS Jan 27;123(4):e2519690122. doi: 10.1073/pnas.2519690122.Epub 2026 Jan 12.

PETM = Paleocene-Eocene Thermal Maximum (56 Mya)

wiki/Perissodactyla

https://www.youtube.com/watch?v=fhZJKQaQ7po

Source: https://pterosaurheresies.wordpress.com/2026/09/01/tissier-and-smith-2026-ran-perissodactyls-through-analysis-and-traditional-taxonomic-problems-arose/

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