Phylogenetic miniaturization at the genesis of desmostylians

The addition of a new taxon,
Cornwallius juvenile (Figs 1,1A) now links tiny Didymoconus, a phylogenetically miniaturized hippo relative, to the base of the desmostylians. This clarifies hippo interrelationships in the LRT (Fig 3), which are unsettled at best and beset by traditional myth from the LRT’s point-of-view.

I wondered for years what to do with the distinctively different Cornwallius juvenile skull. Here it resolves a phylogenetic issue: where did desmostylians come from?

If one stretches the short -neotonous – skull of Didymoconus (Fig 1 frame 2) it becomes a close match to the long Cornwallius juvenile skull.

Figure 1. the Cornwallius juvenile taxon forms a transitional taxon linking tiny Didymoconus to the much larger basal desmostylian, Cornwallius. ” data-image-caption=”

Figure 1. the Cornwallius juvenile taxon forms a transitional taxon linking tiny Didymoconus to the much larger basal desmostylian, Cornwallius.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/cornwallius-juvenile588.gif?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/cornwallius-juvenile588.gif?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87029″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/cornwallius-juvenile588.gif” alt=”Figure 1. the Cornwallius juvenile taxon forms a transitional taxon linking tiny Didymoconus to the much larger basal desmostylian, Cornwallius. ” width=”584″ height=”417″ />

Figure 1. Te Cornwallius juvenile taxon forms a transitional taxon linking tiny Didymoconus to the much larger basal desmostylian, Cornwallius. In frame 2 Didymoconus is stretched to fit the juvenile Cornwallius.

Didymoconus berkeyi
(Matthew and Granger 1924, Gingerich 1981, IVPP V11983, Early Oligocene 52mya) is a former enigma taxon here and now nesting basal to the desmostylian, Cornwallius.

According to Wikipedia,
“It is the type genus of the Didymoconidae, a family of eutherian mammals with uncertain taxonomic affinities.”

Data for the holotype skull AMNH 19124 currently is a century-old line diagram with mismatching parts in dorsal and lateral views.

The IVPP specimen (Wang et al 2001, Fig 1) based on photographic data from two views was scored instead.

Figure 1A. The basal desmostylian Cornwallius and its juvenile to scale. Note the allometry of the mture downturned snout. The juvenile retains the straight jaw margin of its ancestor, Didymoconus in figure 1.

At the same time, and just as importantly,
Cambaytherium and Anthracobune (Fig 2) now find closer relatives among two oreodonts, Agriochoerus antiquus and Rostriamynodon (Fig 2), which we looked at a few days earlier here. This change moves these two taxa further from the earlier nesting basal to desmostylians, to a more basal, pre-hippo node.

These transitional taxa in the LRT now evolve more gradually, with fewer changes and better matches.

Figure 2. Cambaytherium and Anthracobune are better nested with these two oreodonts, Agriochoerus antiquus and Rostriamynodon, a traditional rhinoceros. ” data-image-caption=”

Figure 2. Cambaytherium and Anthracobune are better nested with these two oreodonts, Agriochoerus antiquus and Rostriamynodon, a traditional rhinoceros.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/rostriamynodon.grangeri588-1.jpg?w=161″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/rostriamynodon.grangeri588-1.jpg?w=548″ tabindex=”0″ role=”button” class=”size-full wp-image-87031″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/rostriamynodon.grangeri588-1.jpg” alt=”Figure 2. Cambaytherium and Anthracobune are better nested with these two oreodonts, Agriochoerus antiquus and Rostriamynodon, a traditional rhinoceros.” width=”584″ height=”1092″ />

Figure 2. Cambaytherium and Anthracobune are better nested with these two oreodonts, Agriochoerus antiquus and Rostriamynodon, a traditional rhinoceros.

Here’s how the hippo lineage of taxa
now lines up in the LRT (Fig 3). Yellow bars indicate phylogenetically miniaturized taxa retaining neotonous traits at the genesis of new clades.

Figure 2. Subset of the LRT focusing on hippos, desmostylians and mysticetes. Yellow taxa are small, phylogenetically miniaturized. ” data-image-caption=”

Figure 2. Subset of the LRT focusing on hippos, desmostylians and mysticetes. Yellow taxa are small, phylogenetically miniaturized.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/hippos-cladogram588.jpg?w=200″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/hippos-cladogram588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87032″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/06/hippos-cladogram588.jpg” alt=”Figure 2. Subset of the LRT focusing on hippos, desmostylians and mysticetes. Yellow taxa are small, phylogenetically miniaturized.” width=”584″ height=”877″ />

Figure 3. Subset of the LRT focusing on hippos, desmostylians and mysticetes. Yellow taxa are small, phylogenetically miniaturized.

On the subject of hippos, Pickford 2008 wrote:
“The origin of Hippopotamidae has been the subject of enquiry for more than a century and a half, yet at the beginning of the 21st Century it is still a matter for lively debate.”

“The problem of missing taxa and missing data (of included taxa) is universal, and renders all phylogenetic analyses liable to modification.”

Well said, but not followed.

Ironically, for a study on hippo origins,
many of the taxa in this subset of the LRT (Fig 3)  were not mentioned in the text of Pickford 2008. So taxon exclusion is an issue in Pickford’s work, despite his addressing the problem (see above). Instead of a wide gamut phylogenetic analysis, like the LRT, Pickford reviewed historical studies and that focused on a few characters. This led to no new discoveries. By limiting his taxon list Pickford was forced to conclude, “that hippos are not particularly closely related to anthracotheres but are considerably closer to Palaeochoeridae”

Anthracotheres include Anthracotherium.

Palaeochoeridae include old world peccaries.

Both of these clades nest within artiodactyls close to pigs (Sus),
far from the oreodont-hippo clade in the LRT.

Figure 4. Merycopotamus skull and mandible with colors identifying the reappearances of the prefrontal and postfrontal. ” data-image-caption=”

Figure 4. Merycopotamus skull and mandible with colors identifying the reappearances of the prefrontal and postfrontal.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2019/01/merycopotamus_skull588.jpg?w=203″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2019/01/merycopotamus_skull588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-35330″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2019/01/merycopotamus_skull588.jpg” alt=”Figure 4. Merycopotamus skull and mandible with colors identifying the reappearances of the prefrontal and postfrontal.” width=”584″ height=”864″ />

Figure 4. Merycopotamus skull and mandible with colors identifying the reappearances of the prefrontal and postfrontal.

Pickford commented on
“the anthracothere genus Merycopotamus.” In the LRT Merycopotamus (Fig 4) is a hippo, not close to Anthracotherium, pigs and peccaries.

Here’s the problem: Pickford was taught at university by well-meaning professors that hippos are artiodactyls. Wikipedia – Artiodactyl promotes this traditional myth, and mistakenly adds odontocetes.

Here’s the solution: The LRT tests a wider gamut of taxa and nests hippos independent of camels, pigs, sheep, deer and other traditional artiodactyls.

Add taxa to find this out for yourself.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here. Sorry this took so long to recover. These are taxa that I had to think about. They were all odd. Now they nest together.

References
Gingerich PD 1981. Radiation of Early Cenozoic Didymoconidae (Condylarthra, Mesonychia) in Asia, with a New Genus from the Early Eocene of Western North America. Journal of Mammalogy 62(3):526-538.
Matthew WD and Granger W 1924. New Carnivora from the Tertiary of Mongolia. American Museum Novitates (104).
Pickford M 2008. The myth of the hippo-like anthracothere: The eternal problem of homology and convergence. Revista Espanola de Paleontologia 23 (1): 31-90.
Wang X et al 2001. Didymoconus (Mammalia: Didymoconidae) from Lanzhou Basin, China and its stratigraphic and ecological significance. Journal of Vertebrate Paleontology. 21 (3): 555–564.

wiki/Didymoconus

Revisiting the rhino Rostriamynodon: now an oreodont