Revisiting Early Paleocene Triisodon and two tiny mesonychids

Housekeeping continues
in the mammal subset of the LRT.

As of today
an Early Paleocene mesonychid, Triisodon, nests with two phylogeneticaly mniaturized mesonychids, early Oligocene Didymoconus from Asia and Paleoene Ocepeia (Fig 1) from Morocco. These latter two tiny taxa (Fig 1) apparently led to no other taxa. This clade of three taxa is basal to the one that produced the large extant mammal, Hippopotamus. All are derived from Mesonyx (Fig 2), a plant-eater with molars flattened by a lifetime of grinding and crushing tough vegetation, such as grasses and reeds, as in hippos.

Figure 1. Triisodon and the tiny hippos, Wyolestes (= Didymoconus) and Ocepeia. ” data-image-caption=”

Figure 1. Triisodon and the tiny hippos, Wyolestes (= Didymoconus) and Ocepeia.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/triisodon.skull_.compared588.jpg?w=124″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/triisodon.skull_.compared588.jpg?w=425″ class=”size-full wp-image-85918″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/triisodon.skull_.compared588.jpg?w=584&h=1407″ alt=”Figure 1. Triisodon and the tiny hippos, Wyolestes (= Didymoconus) and Ocepeia.” width=”584″ height=”1407″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/triisodon.skull_.compared588.jpg?w=584&h=1407 584w, https://pterosaurheresies.files.wordpress.com/2024/05/triisodon.skull_.compared588.jpg?w=62&h=150 62w, https://pterosaurheresies.files.wordpress.com/2024/05/triisodon.skull_.compared588.jpg?w=124&h=300 124w, https://pterosaurheresies.files.wordpress.com/2024/05/triisodon.skull_.compared588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Triisodon and the tiny hippos, Wyolestes (= Didymoconus) and Ocepeia.

According to Wikipedia – Mesonyx,
“They were probably active hunters.” This is widely believed, but incorrect based on phylogeny and tooth wear. Hippos and their relatives are and were herbivores, but the defensive tusks of fossil taxa have been mistaken for offensive fangs in mesonychid-hippos like Early Miocene Simbakubwa of Kenya (hippo territory).

Figure 2. Harpagolestes and kin inluding Mesonyx and Triisodon. ” data-image-caption=”

Figure 2. Harpagolestes and kin inluding Mesonyx and Triisodon.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/harpagolestes588.jpg?w=230″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/harpagolestes588.jpg?w=584″ class=”size-full wp-image-85921″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/harpagolestes588.jpg?w=584&h=763″ alt=”Figure 2. Harpagolestes and kin inluding Mesonyx and Triisodon.” width=”584″ height=”763″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/harpagolestes588.jpg?w=584&h=763 584w, https://pterosaurheresies.files.wordpress.com/2024/05/harpagolestes588.jpg?w=115&h=150 115w, https://pterosaurheresies.files.wordpress.com/2024/05/harpagolestes588.jpg?w=230&h=300 230w, https://pterosaurheresies.files.wordpress.com/2024/05/harpagolestes588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 2. Harpagolestes and kin inluding Mesonyx and Triisodon.

Triisodon quivirensis
(Cope 1881; Early Paleocene) is a basal mesonychid known from most of a 3D skull. Note the large canine. The premaxillary teeth have three cusps. Cope 1881 considered this ‘the oldest known mammal’, suggesting an earlier, perhaps Cretaceous, origin for mesonychids and their ancestors.

Didymoconus berkeyi
(Gingerich 1981, IVPP V11983, Eocene 52mya) is a former enigma taxon here nesting with the small mesonychid, Ocepeia. The zygomatic arches are wider posteriorly and the orbits are dorsally open, distinct from Ocepeia, which also has dorsally open orbits.

Ocepeia daouiensis
(Gheerbrant et al 2001, 2014; Paleocene, 60 mya; 9 cm skull length) is a phylogentically miniaturized mesonychid from Morocco and a sister to Didymoconus. The pneumatized skull contains many air spaces. Slightly larger skulls have larger canines and so are considered male. This specimen (Fig 1) has a tiny canine. The molar teeth are very large and the jugal deepens below the orbit, hiding the posterior molars in lateral view. The premaxilla is transverse. The upper canine rubs against the lower one creating a facet, as in hippos and mesonychids. Ocepeia was found with aquatic taxa and was probably amphibious. The name Ocepeia derives from the initials of Office Chérifien des Phosphates (O.C.P.), the national Moroccan phosphate mining company.

According to Wikipedia – Mesonyx,
“In the generally accepted cladogram by Spaulding et al. (2009), Mesonyx is classified, together with other mesonychians, outside of ungulates.”

Some of this is confirmed in the LRT, which tests more taxa.
A closer interrelationship between mesonychids, hippos and mysticetes (= not all ‘cetaceans’) is recovered. Also arising from Mesonyx: artiodactyls, beginning with the Miocene Homalodotherium + Cainotherium clades. Perissodactyls in turn arise from the artiodactyls, specifically from the Early Eocene Litolophus + Lophiodon clades. So parts of the Spaulding et al 2009 cladogram are upside down when more taxa are included.

Add taxa to confirm, refute or modify this hypothesis of interrelationships.

By the late Miocene
the Mesonyx – hippo – desmostylian line had produced Miocaperea, a basal mysticete (baleen whale), about the time the first pig relatives are recovered, but much later than the earliest horse relatives. So the Paleocene remains an under-explored radiation event.

According to Wikipedia – Didymoconidae,
citing Morio and Nagel 2002, “Its classification has been the subject of controversy, and the family has been placed in various orders since it was erected in 1943, sometimes being placed in its own order Didymoconida.

According to Morlo and Nagel, Three prior authors ‘regarded’ didymoconids as mesonychids, matching the LRT. Other prior authors ‘regarded’ didymoconids as members of the Lipotyphla, Creodonta, Caniformia, Arctocyonoidea, Palaeoryctoidea and Leptictida. Most of these are not related to one another in the LRT.

According to Wikipedia – Ocepeia,
the presented cladogram nested Ocepeia with the giant otter shrew, Potamogale, close to Orycteropus, the aardvark, and Ptolemaia, a basal condylarth. None of these are related to one another in the LRT.

References
Cope ED 1872. Descriptions of some new Vertebrata from the Bridger Group of the Eocene. Proceedings of the American Philosophical Society 12:460-465.
Cope ED 1881. On Some Mammalia of the Lowest Eocene Beds of New Mexico. Proceedings of the American Philosophical Society. 19 (109): 484–495.
Gheerbrant E, Amaghzaz M, Bouya B and Goussard F and Letenneur C 2014. Ocepeia (Middle Paleocene of Morocco): The Oldest Skull of an Afrotherian Mammal. PLoS ONE. 9 (2): e89739. doi:10.1371/journal.pone.0089739.
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.
Hatcher JB 1901. Report of the Princeton University Expeditions to Patagonia 1869-1899. Mammalia of the Santa Cruz Beds. IV. Astrapotheria. Scott WB ed. Vol. 6, Paleontology 3. Princeton, NJ Stuttgart 1909-1928.
Morlo M and Nagel D 2002. New Didymoconidae (Mammalia) from the Oligocene of Central Mongolia and first information on the tooth eruption sequence of the family. Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen. 223 (1): 123–144.
Szalay FS and Gould SJ 1966. Asiatic mesonychidae (Mammalia, Condylarthra). Bulletin of the American Museum of Natural History 132(2):127–174
Wortman JL 1901. Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum. The American Journal of Science, series 4 12(70):281-296.

wiki/Mesonyx
wiki/Harpagolestes
wiki/Ocepeia
wiki/Triisodon
wiki/Didymoconus
wiki/Simbakubwa