An overlooked baleen whale ancestor from the oreodont beds of South Dakota

When the juvenile of the pre-desmostylian Cornwallius was first described
(Beatty 2006ab ) the author reported a short list of juvenile differences from the down-turned rostrum of the adult skull (Fig 1). This indicates a measure of isometric ontogeny in the growth strategy.

Isometric growth is a trait of the new Archosauromopha in the Large Reptile Tree, but not scored in the LRT.

Now a wide-skulled Oligocene oreodont, Agriochoerus antiquus,
(Leidy 1869, USNM V 437, Fig 1) is recognized to share a suite of traits with the juvenile Oligocene Cornwallius, demonstrating transitional traits from wider-skulled agriochoerid oreodonts to desmostylians, apparently skipping the hippo clade/grade.

Note the gradual reduction of the postfrontal bar and premolars as these taxa evolve.

In addition,
the flat-skulled “basal tapir or perissodactyl” Lophiaspis (Fig 1) also joins this clade in the LRT based on similarities to the juvenile Cornwallius.

You might remember,
In September 2024 flat-skulled Rostriamynodon was attracted to more narrow-skulled Cambaytherium and Anthracobune, not far from basal notoungulates like Periphragnis.

These bulky, aquatic taxa are all deep ancestors of mysticetes (= baleen whales) in the LRT all prior to the posterodorsal migration of the naris.

Long-time readers will remember odontocetes had a completely separate ancestry, invalidating the clade ‘Cetacea’ and hobbling our concept of what a ‘whale‘ is.

Including juvenile taxa
in the LRT is rare, but in this case warranted as it appears to shed light on the origin of desmostylians from oreodonts.

It’s worth noting
the basal artiodactyl with claws, Agriochoerus latifrons. does not nest with Agriochoerus antiquus (Fig 1) in the LRT. Someone needs to rename one of these genera. They are not congeneric.

It’s also worth noting
the origin of the South American clade, Notoungulata, here represented by Periphragnis, splits off close to these featured taxa (Fig 2).

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here. Sorry it took so long to get to. Some things are obvious only on hindsight.

References
Beatty, BL 2006a. Rediscovered specimens of Cornwallius (Mammalia, Desmostylia) from Vancouver Island, British Columbia, Canada. Vertebrate Palaeontology. 1(1):1–6.
Beatty, BL 2006b. Specimens of Cornwallius sookensis (Desmostylia, Mammalia) from Unalaska Island, Alaska”. Journal of Vertebrate Paleontology. 26(3):785–87.
Cornwall IE 1922. Notes on the Sooke Formation, Vancouver Island, B.C. Canadian Field Naturalist. 36:121–23.
Depéret C 1910. Etudes sur la famille des Lophiodontidés. Bull. Soc. géol. Fr., (4)10: 558-577.
Hay OP 1923. Characteristics of sundry fossil vertebrates. Pan-American Geologist. 39:101–20.
Leidy J 1869. The extinct mammalian fauna of Dakota and Nebraska, including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North America Journal of the Academy of Natural Sciences, Philadelphia. 2: 1–472.
Reinhart RH 1959. A review of the Sirenia and Desmostylia. University of California Publications in Geological Sciences 36(1):1–146.

wiki/Cornwallius
wiki/Lophiaspis
wiki/Whale

Cornwallius: not a desmostylian, an ancestor to desmostylians

Source: https://pterosaurheresies.wordpress.com/2024/11/03/an-overlooked-baleen-whale-ancestor-from-the-oreodont-beds-of-south-dakota/