Early Cretaceous Acristatherium compared to Myrmecobius, the numbat, the most primitive living therian in the LRT

Today we’re taking a fresh look at Early Cretaceous Acristatherium (Fig 1) from China and compare it to Myrmecobius (Fig 1), the termite-eating numbat from Australia as revisions to the mammal subset of the LRT continue with occasional novel insights, like this, arising.

The molar and premolar count are also under review today.

The first thing to notice on these two taxa are the teeth. Five premolars (blue) and three molars (green) erupt from the maxilla (Fig 1) in both taxa. Note that it is difficult to determine where they divide. In Acristatherium the molars are spaced apart and rather shapeless. That trend continues in the posterior premolars. Similarly after 125 million years, In Myrmecobius the molars are simple, tiny, spaced out cheese box-like shapes at best, absent of any cusps and the posterior premolars are just as simple. The canines (orange) in both taxa are large and sharp. Both have four premaxillary teeth.

The second thing to notice is the skull shape. Both are similar to one another despite the 2.5x size difference. They have similar scores in the matrix, more similar to each other than to other taxa and so they now nest together in the LRT. The premaxillae are noticeably different. So are the postorbital ‘eyebrows’ = more extensive in the extant numbat.

The numbat is considered to be a member of the Dasyuromorphia. That might be due to the exclusion of fossil taxa. In the large reptile tree (LRT, 2340 taxa) several purported members do not nest with Dasyurus and its close kin. The numbat is one of those unrelated taxa in the LRT.

The molar count issue. Most placentals have three molars. Many marsupials have four. That standard count clade attribution has been applied to Acristatherium, Eomaia and related marsupials with the pattern of three molars and five premolars. In the literature these two taxa are considered eutherians = placentals.

This method is called ‘Pulling a Larry Martin‘ = determining clade membership by one to a dozen diagnosing traits rather than the correct method: the last common ancestor after phylogenetic analysis method, which remains the best way to weed out convergence from homology in paleontology.

Acristatherium and Myrmecobius nest so closeto the origin of marsupials that only six tested taxa in the LRT separate these two from monotremes. This clade had Early Jurassic ancestors among the multituberculates, which indicates these are both late survivors of a Late Triassic origin and radiation – probably world-wide considering their origins in and outside of Australia still part of Pangaea back then.

References

Tate GHH 1951. The banded anteater, Myrmecobius Waterhouse (Marsupialia). American Museum Novitates 1521, 8 pp.
Waterhouse GR 1836. Myrmecobius fasciatus. Proc. Zool. Soc. London 4: 69–131.
Waterhouse GR 1841. Description of a new genus of mammiferous animals from Australia, belonging probably to the order Marsupialia. Trans. Zool. Soc., London2, aricle. 11, p 149.

The numbat Myrmecobius has a new nesting partner: Early Cretaceous Eomaia

wiki/Numbat

wiki/Myrmecobius

wiki/Dasyuromorphia

wiki/Acristatherium

Source: https://pterosaurheresies.wordpress.com/2026/10/06/early-cretaceous-acristatherium-compared-to-myrmecobius-the-numbat-the-most-primitive-living-therian-in-the-lrt/