Four former enigmas now clade together in the LRT

Today’s taxa are phylogenetically ‘different’ from other mammals,
but all are similar to one another. Where manual and pedal elements are known, hoof-ish claws are described. To that point: these four taxa now nest outside of the traditional Ungulate clade in the LRT, so this convergence is likely the cause of previous puzzlement.

In other words, clades are defined and determined by a last common ancestor, not their traits. A long list of traits describe a clade, but these may and too often do converge. That means the presence of hoof-like unguals does not mean the taxon is a member of the Ungulata or close to it. That would be “Pulling a Larry Martin.”  Don’t do that. Plug in all the traits and a long list of taxa to let the software tell you where a taxon or a clade nests.

One astrapothere-like taxon, Trigonostylops (see below), has been readmitted to the LRT despite lacking a large portion of its rostrum. That loss of data kept it out of the LRT to increase resolution elsewhere. Now that taphonomic loss has been minimized by more completely known closely related taxa surrounding it.

Tetraclaenodon puercensis
(Cope 1881, Scott 1892; Kondroshev and Lucas 2012, Middle Paleocene, New Mexico) was originally considered a member of the Phenacodontidae, close to Phenacodus. Kondroshev and Lucas wrote, “The ungual phalanx is intermediate in morphology between a claw-like and a hoof-like phalanx”.

Here in the LRT Tetraclaenodon is even more primitive: transitional between the mouse-sized extant marsupial Monodelphis domestica and the rest of the members in the clade Pantodonta, which, in turn, is basal to several herbiovorous Placental 1 taxa with fangs and a diastema, including Condylarthra and Artiodactylia.

Deltatherium fundaminis
(Cope 1881, middle Paleocene 60 mya, North America, 14 cm skull length, cat-sized, AMNH 16610) this former creodont and former tillodont now nests with Prodinoceras, among the basal pantodonts. The large cranial crest, common to all four clade members, anchored large chewing jaw muscles.

Trigonostylops wortmani
(Ameghino 1897; Simpson 1933, Late Paleocene—Early Eocene; est. 1.5m length, South America) now nests apart from the astrapotheres, now close to Prodinoceras. Deltatherium and Tetraclaenodon. The premolars were molarized. The snout remains unknown. The zygomatic arch narrowed posteriorly in dorsal view, an odd morphology.

Simpson 1933 wrote about Trigonostylops,
“The general conclusions regarding the affinities of Trigonostylops are:
1. That it is an extraordinarily isolated placental of the general ungulate cohort, showing no evidence of close affinity with any other known group.
2. That it is a very aberrant branch from some ancient and primitive stock, retaining an archaic character despite its specialization in many features.
3. That it is not at all close to the Notoungulata and cannot be referred to that Order.
4. That it shows some evidence of collateral relationship to the astrapotheres on one hand and litopterns on the other, and is perhaps slightly closer to the former.
5. That it may hence be very tentatively concluded that from a primitive and remote ungulate stock, probably in or very near the Condylarthra, arose the litopterns, retaining rather more of these ancestral characters, and astrapotheres, more strongly aberrant, and that Trigonostylops also came from this remote ancestry, possibly nearer to or even in the most ancient astrapothere line, but diverging strongly in a third direction.”

In the LRT Trigonostylops is so primitive that it is convergent in that ‘third direction”, derived from the mouse-like marsupials Asioryctes and Monodelphis, basal to the rest of the Pantodonta along with the other three clade members featured in this post.

Since 1933
several taxa have been added to the astrapotheres, including Trigonstylops. One of these new taxa is…

Eoastrapostylops riolorense
is known from a largely complete, Late Paleocene, Argentina, 9cm long skull (the size of a baseball) with short rostrum, large orbit and other juvenile-ish traits. Tentatively adding Late Paleocene Eostrapostylops to the LRT nests it basal to Dissacus + Kopidodon and reroutes several other taxa and clades. In other words: nowhere near astrapotheres or Trigonostylops. Wikipedia reports, “Those researches would indicate that Eoastrapostylops was one of the most basal member of the South American ungulates, and differentiated before the separation between astrapotheres, pyrothere and notoungulates.” In the LRT none of these odd clades are related to one another.

I’m looking for better data = several views of the real skull of Eostrapostylops.
Until then, it does not enter the LRT.

Prodinoceras martyr
(Matthew, Granger and Simpson 1929, Late Paleocene, Mongolia, 2.9 m long) is traditionally considered the most basal uintathere. In the LRT it now nests as the largest taxon in the primitive pantodont Trigonostylops clade.

These four taxa now nest together
at the base of the Pantodonta in the LRT, with fangs, diastema, high cranial crest and hoof-lke claws where known.

Click on the above links to view the data.

In the LRT,
taxa following the mouse-like marsupial from South America, Monodelphis domestica, now split into 1. borhyaenids (including Early Cretaceous Vincelestes and extant Sarcophilus,the Tasmanian Devil) on one branch and placental 1 taxa on the other branch.

Thereafter the placental 1 taxon splits into Carnivoramorpha (long-torso arboreal Sinopa at the base and including Carnivora + Primates) and Pantodonta (Tetraclaenodon at the base and including large herbivorous placental mammals) on the other branch.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here. This hypothesis could also change, because it is a hypothesis and based on fragile interrelationships. Even so, the corralling of these four former enigma taxa together and at a primitive node with a gradual and impressive increase in size through time, apart from all other more popular taxa, I hope bodes well for the validity of this hypothesis. It’s been a long slog with these four.

References
Ameghino F 1897. Les mamiferes crétacés de l´Argentine. Boletín Instituto Geográfico Argentino:18: 405–521.
Cifelli RL 1993. The phylogeny of the native South American ungulates; pp. 195–216 in F. S. Szalay, M. J. Novacek, and M. C. McKenna (eds.), Mammal Phylogeny: Placentals. Springer-Verlag, New York.
Cope ED 1881. On some Mammalia of the lowest Eocene beds of New Mexico. Palaeontological Bulletin 33:484-495.
Flerov KK 1952. Novye Dinocerata iz Mongolii (New Dinocerata from Mongolia). Doklady Akademii Nauk SSSR 86, 1029-32.
Kondrashov PE and Lucas SG 2012. Nearly complete skeleton of Tetraclaenodon (Mammalia, Phenacodontidae) from the Early Paleocene of New Mexico: Morpho-Functional Analysis. Journal of Paleontology 86(1):25–43.
Lucas SG and Kondrashov PE 2004. A new species of Deltatherium (Mammalia, Tillodntia) from the Paleocene of New Mexico. In Lucas SG, Zeigler KE. and Kondrashov PE, eds., 2004, Paleogene Mammals, New Mexico Museum of Natural History and Science Bulletin No. 26.
Matthew WD, Granger W and Simpson GG 1929. Additions to the fauna of the Gashato Formation of Mongolia. American Museum Noviates 376: 1–12.
Simpson GG 1933. Structure and affinities of Trigonostylops. American Museum Novitates 608:1–28.
Soria MF and Powell JE 1981. Un primitivo Astrapotheria (Mammalia) y la edad de la Formación Río Loro, provincia de Tucumán, República Argentina. Ameghiniana 18(3-4):155-168.

wiki/Prodinoceras
wiki/Trigonostylops
wiki/Deltatherium – not yet posted
wiki/Tetraclaenodon
wiki/Eoastrapostylops

 

Source: https://pterosaurheresies.wordpress.com/2026/05/07/four-former-enigmas-now-clade-together-in-the-lrt/