Notostylops nests with Hegetotherium and Mesotherium

Notostylops has been a challenge to nest. Full stop.
When I finally found a dorsal and ventral view of Mesotherium (Fig 1), the solution became obvious. DGS colors were modified here and there on the Notostylops CT scan that lacked certain sutures, but revealed a curious bent = torsioned skull in dorsal view, replicated in related and unrelated taxa, like tenrecs.

The takeaway here is
tthis clade of South American herbivores, Interatheridae, (former and traditional Notoungulata), converge with placental oreodonts, astrapotheres and hippos found elsewhere on the planet from the Eocene to the near present.

The kicker is they were derived from meat-eating cat-like marsupials like Patriofelis, and hippo-like herbivores like Periphragnis, with premaxillary AND maxillary fangs, an unexpected and overlooked evolutionary transition we looked at earlier here.

Figure 1. Mesotherium, Hegetotherium and Notostylops now nest together in the LRT. This was a struggle to recover. ” data-image-caption=”

Figure 1. Mesotherium, Hegetotherium and Notostylops now nest together in the LRT. This was a struggle to recover.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/mesotherium-cristatum.skull588.jpg?w=184″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/mesotherium-cristatum.skull588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87465″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/mesotherium-cristatum.skull588.jpg” alt=”Figure 1. Mesotherium, Hegetotherium and Notostylops now nest together in the LRT. This was a struggle to recover. ” width=”584″ height=”950″ />

Figure 1. Mesotherium, Hegetotherium and Notostylops now nest together in the LRT. This was a struggle to recover. Arrows indicate torsion in the skull, otherwise seen in tenrecs and odontocetes, both of which locate food by echolocation. This offsets one ear against another AND the auditory canal is raised in these oreodont-like to hippo-like taxa. Perhaps they were singing, like hippos do.

Mesotherium cristatum
(originally ‘Typotherium‘; Serres 1867; Middle Pleistocene; 1.2m long) was considered a typotherid, mesotheriid notoungulate, with rodent-like incisors. Here it nests with Hegetotherium in the LRT. Note the deep jugals without a descending process. Fernandez-Monescillo 2023 reported, “It is the last member of the Typotheria lineage.” 

Hegetotherium mirabile
(Ameghino 1887; Mid Miocene, 17mya; skull length 10cm) was originally considered a notoungulate. Tiny third incisors, canines and first premolars complete the dental arcade without a diastema. Note the slightly elevated orbits.

Notostylops murinis
(Ameghino 1897, Riggs and Patterson 1935; 75cm in estimated length; Eocene; FMNH-P13319) is widely considered a ‘notoungulate’ a clade that has been split into several parts in the large reptile tree. The jaws narrowed anteriorly below broad cheek bones.

Figure 3. Subset of the LRT focusing on marsupial creodonts, including derived herbivorous taxa. ” data-image-caption=”

Figure 3. Subset of the LRT focusing on marsupial creodonts, including derived herbivorous taxa.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/creodonta2024lrt588.jpg?w=156″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/creodonta2024lrt588.jpg?w=531″ tabindex=”0″ role=”button” class=”size-full wp-image-87474″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/creodonta2024lrt588.jpg” alt=”Figure 3. Subset of the LRT focusing on marsupial creodonts, including derived herbivorous taxa.” width=”584″ height=”1126″ />

Figure 2. Updated subset of the LRT focusing on marsupial creodonts, including derived herbivorous taxa.

Interesting historical note from Darren Croft’s webisite:
“Mesotherium was named “middle beast” in reference to Serres’ belief that it represented an evolutionary intermediate between rodents – due to the animal’s large upper incisors – and “pachyderms” (i.e., ungulates) – due to its size and proportions.”

Taxon exclusion was a much bigger problem back when fossils were first being discovered and so few were known and described.

Torsioned skulls in Notostylops and Mesotherium
(Fig 1) would make a good study for a young student. This skull bend in dorsal view is otherwise seen in tenrecs and odontocetes, both of which locate food by echolocation. This angle offsets one ear against another perhaps to enhance binaural listening accuracy. The auditory canal is raised in these oreodont-like to hippo-like taxa. Perhaps they were singing and listening in shoulder-deep water, like hippos do.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here. This is yet another case where pattern recognition failed until persistence beat it out.

Frustrated in your own studies? Don’t give up. Keep pushing.

References
Ameghino F 1887. Observaciones generales sobre el orden de mamíferos estinguidos sud-americanos llamados toxodontes (Toxodontia) y sinopsis de los géneros y especies hasta ahora conocidos. Anales del Museo de La Plata 1:1-66.
Ameghino F 1897. Les mamiferes crétacés de l´Argentine. Boletín Instituto Geográfico Argentino 18: 405–521.
Riggs ES and Patterson B 1935. Description of some notoungulates from the Casamayor (“Notostylops) beds of Patagonia. Proceedings of the American Philosophical Society 75(2):163-215.
Serres, AERA 1867. De l’ostéographie du Mesotherium et de ses affinitiés zoologiques. Comptes Rendus de la Académie des Sciences, Paris 65:6-17, 140-148, 273-278, 429-437, 593-599, 740-748, 841-848.
Fernandez-Monescillo M, Croft DA, Pujos F and Antoine P-O 2023. Taxonomic history and intraspecific analysis of Mesotherium cristatum (Mammalia, Notoungulata, Mesotheriidae) from the Early-Middle Pleistocene of Buenos Aires Province, Argentina. Historical Biology 35(6):1028–1040.

wiki/Interatheriidae
wiki/Notoungulata

Another fang transition from maxilla to premaxilla

Mesotheriidae