Metamynodon: hippo-like rhino? or rhino-like hippo?
According to Wikipedia – Metamynodon:
“The various species were large, displaying a suit of semiaquatic adaptations more similar to those of the modern hippopotamus, despite their closer affinities with rhinoceroses.”
“Metamynodon is a member of the extinct family Amynodontidae, sometimes called “swamp rhinos” as they were once all believed to be semi-aquatic.”
“Metamynodon planifrons, the largest species, was about 4 metres (13 ft) in length and 1.8 metric tons (2 short tons) in weight, and, although it was distantly related to modern rhinos, looked more like a hippo (Hippopotamus amphibius). Its front legs had four toes instead of the three found in modern rhinos.”
“The ribcage was broad and Metamynodon had a barrel-like chest similar to the hippo,”
“Like hippos, males may have used their large tusks for fighting or for finding food in river banks.”
That’s a long list of hippo-like traits in this traditional rhino.
Rhino anterior teeth are tiny to absent (Fig 2), distinct from Metamynodon.
Amynodon (Fig 2) remains in the Amynodontidae, a rhino clade.
Figure 1. Metamynodon skeleton, skull and Hexaproton neonate skull.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/metamynodon_588.jpg?w=179″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/metamynodon_588.jpg?w=584″ class=”size-full wp-image-94485″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/metamynodon_588.jpg” alt=”Figure 1. Metamynodon skeleton, skull and Hexaproton neonate skull. ” width=”584″ height=”981″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/metamynodon_588.jpg?w=584&h=981 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/metamynodon_588.jpg?w=89&h=150 89w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/metamynodon_588.jpg?w=179&h=300 179w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/metamynodon_588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Metamynodon skeleton, skull and Hexaproton (pygmy hippo) neonate skull. Given this data, Metamynodon appears to be a result of phylogeenti miniaturization, despite its 4m length. Smaller ancestors of Metamynodon are not yet known. DGS colors added here.
When added to and tested in
the large reptile tree (LRT, 2337 taxa) Metamynodon nests at the base of the hippo clade – Uniquely (among hippos) Metamynodon migrates the orbit to the front half of the skull, as in rhinos (Fig 2). In addition the retroarticular process (rear flange of the mandible) does not flare laterally, again matching rhinos.
Figure 2. Amynodon skull in lateral view. DGS colors added here.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/amynodon_skull_insitu588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/amynodon_skull_insitu588.jpg?w=584″ class=”size-full wp-image-94487″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/amynodon_skull_insitu588.jpg” alt=”Figure 2. Amynodon skull in lateral view. DGS colors added here. ” width=”584″ height=”476″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/amynodon_skull_insitu588.jpg?w=584&h=476 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/amynodon_skull_insitu588.jpg?w=150&h=122 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/amynodon_skull_insitu588.jpg?w=300&h=244 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/09/amynodon_skull_insitu588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Amynodon skull in lateral view. DGS colors added here.
So how can that typically posterior hippo orbit migrate anteriorly?
An anterior orbit = a short rostrum is a neonate trait (Fig 1) lost in adult hppos (Fig 3) during allometric ontogeny. Allometric ontogeny unites crocs, birds and mammals in the revised Archosauromorpha (in the LRT). This change during growth strategy is distinct from Lepidosauromorpha (including pterosaurs) and turtles which develop more isometrically = neonates are much more similar to adults in proportion.
As everyone knows,
a cladogram, like the LRT, nests taxa according to a long list of characters to arrive at a family tree based on maximum parsimony = majority rule. The few traits that converge with other taxa are scored and considered, but the weight of the rest of the scores = the majority, nests the taxon. That’s why Metamynodon now nests in the LRT with hippos, not rhinos, despite the rhino-like anterior orbit and flat retroarticular process.
Figure 3. Hexaprotodon (= Choeropsis) skeleton compared to Hippopotamus. Skull with DGS colors applied. Note the lateral flange of the retroarticular process along with the posterior position of the orbit. This is the adult morph of the neonate skull shown in figure 1.
This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here. Or make the case that hippo-like Metamynodon is closer to rhinos.
References
wiki/Metamynodon
The skull of Metamynodon planifrons, a semiaquatic amynodont rhino that lived in North America and Asia from the late Eocene until the early Oligocene.
byu/Pardusco inPaleontology
Source: https://pterosaurheresies.wordpress.com/2025/09/14/metamynodon-hippo-like-rhino-or-rhino-like-hippo/
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