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The first radiation of Placentalia 1 taxa in the LRT to scale

Wednesday, July 8, 2026 12:49

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Several difficult-to-nest = oddball taxa
now nest together in the LRT (Fig 1) at the origin of the Placentalia 1 clade that includes Carnivora, Primates and Artiodactyla, as well as the solitary taxon, Kopidodon. Perhaps this is why they were individually difficult to nest elsewhere.

BTW, I am aware that something is off about this layout, image shifted right and lacking a caption. This is due to WordPress’s presets in dealing with such a tall image, which I am unable to overcome at present.

Figure 1. Taxa at the start of the Placentalia 1 clade in the LRT. These include extant Eupleres, and several extinct taxa including Alcidedorbignya, Dissacus, and Trigonostylops. The outgroup taxon is the extant mouse-sized marsupial from South America, Monodelphis domestica. ” data-image-caption=”

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This clade evolved from the outgroup taxon, `
the South American mouse-like marsupial without a pouch, Monodelphis domestica (Fig 1 top).

Figure 2. Eupleres, the near extinct Western falanouc of Madagascar.

Eupleres
Restricted to Madagascar, the extant falanouc (Fig 2) is the long, slinky ancestor to similar to North American Vulpavus basal to the Carnivora, North American Notharctus basal to the Primates and Kopidodon from the Eocene Messe Pit in Europe. Tradition, university textbooks and Wikipedia all agree that falanoucs ‘rafted’ from Africa 18-20 mya. In a competing hypothesis Madagascar was a refugium for falanoucs, lemurs, aye-ayes, tenrecs and vivverids since the big island split from Africa 170 million years ago during the Early Jurassic. Only two slender molars are present in this taxon, fewer than its closest relatives, but it had a long time to lose those other molars.

Alcidedorbignya
This Early Paleocene Bolivian squirrel-sized omnivore (Figs 1, 3) is best compared to Bolivian-to-Brazilian Monodelphis domestica, rather than the more distant falanouc, Eupleres (Fig 3). Originally considered the earliest pantodont, basal to Pantolambda. In the LRT that hypothesis does not work out quite so well. Note the upturned mandible tip, a trait that continues in related taxa. Five teeth are molariform. Two are premolariform. Canines are long.

Dissacus
This Early Paleocene to Early Eocene omnivore (Fig 1) is known from 20 less complete species spread across the northern continents. It is a traditional mesonychid, perhaps because this blogpost is the first time a competing hypothesis of interrelationships has been proposed that introduces a new clade. Dissacus compares well with Alcidedorbignya, especially so in dorsal view.

Figure 4. Drawing and museum mount of Meniscotherium.

Meniscotherium
This Paleocene–Eocene dog-sized hooved herbivore (Fig 4) from the western USA lived in herds. Meniscotherium has been allied with several traditional clades prior to today’s blogpost.

Trigonostylops
This Late Paleocene-Late Eocene herbivore from South America was often compared to South American elephant ancestors, the Astrapotheria and to Menisoctherium. The post-crania remains unknown. Here the missing snout region is replaced with ghosted data from Alcidedorbignya.

Figure 5. Sminthopsis compared to the song, Macroscelides, and the unrelated sengi, Rhynchocyon.

Currently there are two other origins for the placenta in the LRT
1. From the extant tiny mouse-like marsupial Sminthopsis: the extant ‘sengi’ Macroscelides (Fig 5) alone, all by itself. The LRT labels this Placentalia 3.
2. From the marsupial sugar glider Petaurus (Fig 6) including tree shrews (Fig 6), shrews, rodents (including manatees), multituberculates, moonrats, sengis, cingulate, aardvarks tenrecs and cingulates (sloths, armadillos, anteaters, pangolins) and giant tenrecs leading to odontocetes.

Figure 6. The marsupial, Petaurus, compared to the placentals, Ptiloceercus and Tupaia.

This appears to be a novel hypothesis of interrelationships.
If not please provide a citation so I can promote it here. Of course, Wortman 1902 was the first to propose three origins for placental reproduction, but not with these extinct and extant taxa.

Figure 7. Five extant basal Placentalia 1 taxa all display ringed tails. Eupleres in figure 2 does not share this pattern.

If there are competing hypotheses
for the origin of these three placental clades in the literature, please also make those citations known.

References
Ameghino F 1897. Les mamiferes crétacés de l´Argentine. Boletín Instituto Geográfico Argentino:18: 405–521.
Cope ED 1874. Report of the Chief of Engineers for. 1874 Vol. II, Pt. II, Appendix FFa of Appendix FF, pp. 115-130.
MacPhee RDW, et al. (5 co-authors) 2021. Cranial Morphology and Phylogenetic Relationships of Trigonostylops wortmani, an Eocene South American Native Ungulate. Bulletin of the American Museum of Natural History 449(1), 1-183. https://doi.org/10.1206/0003-0090.449.1.1
Simpson GG 1933. Structure and affinities of Trigonostylops. American Museum Novitates 608: 1–28.
Williamson TE and Lucas SG 1992. Meniscotherium (Mammalia, “Condylarthra”) from the Palaeocene-Eocene of Westeran North America. New Mexico Museum of Natural Hstory and Science Bulletin 1.
Wortman JL 1901-1903. Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum. Am. Jour. Sci. 11:333–348.

YouTube Video on the viral origin of the placenta
Another YouTube Video on the viral origin of the placenta

wiki/Eupleres
wiki/Alcidedorbignya
wiki/Dissacus
wiki/Meniscotherium
wiki/Trigonostylops

Wortman 1902 was first: Three origins for placental mammals hypothesis

Source: https://pterosaurheresies.wordpress.com/2026/07/08/the-first-radiation-of-placentalia-1-taxa-in-the-lrt-to-scale/

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