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Placental vs marsupial tooth formulas compared

Tuesday, July 21, 2026 14:22

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Today we’re going to look at
the number of cheek teeth in Early Cretaceous Eomaia from China, and two descendants: Early Paleocene Pucadelphys from Bolivia and Early Eocene Vulpavus from North America (Fig 1).

The similarity in skull shapes here (Fig 1) is not accidental. These three taxa nest without intervening taxa in the current round of the mammal subset of the LRT. Like the LRT I had not noticed the similarity and proximity until today. Always glad to see this happen.

Notably
the teeth are different. But not too different = evolution.

Figure 1. On frame 1, Eomaia, Pucadelphys and Vulpavus to scale. On frame 2, Pucadelphys to size with Vulpavus. Note the tooth counts: the same for Pucadelphys and Vulpavus, but the number of premolars vs molars is switched. Note the size and shape of the teeth. The largest premolar is the posterior one.

According to Wikipedia – Marsupial
“Most Australian marsupials outside the order Diprotodontia have a varying number of incisors between their upper and lower jaws. Early marsupials had a dental formula of 5.1.3.4 (uppers) 4.1.3.4 (lowers)t, consisting of five (maxillary) or four (mandibular) incisors, one canine, three premolars, and four molars, totaling 50 teeth. While some taxa, like the opossum, retain this original tooth count, others have reduced numbers.”

Figure 2. Asioryctes has 4 premolars and 4 molars, as in other basal marsupials. 50 teeth total. Note the reduced canines on their way to replacement by premolar 1.

According to Wikipedia – Vulpavus
“Vulpavus (“ancestor of foxes”) is an extinct paraphyletic genus of placental mammals from clade Carnivoraformes, that lived in North America from the early to middle Eocene.”

Figure 3. Skull of Monodelphis domestica in 4 views. Upper tooth formula is 5-1-3-4 as in didelphid marsupials.

Figure 4. At left: The extant termite-eating numbat, Myremeciobius is the closest living relative to Early Cretaceous Eomaia (at right), which was also a likely insectivore.

According to Wikipedia Numbat
“The numbat genus Myrmecobius is the sole member of the family Myrmecobiidae, one of four families that make up the order Dasyuromorphia, the Australian marsupial carnivores.”

No longer is Myremecobius (Fig 4) the sole member, according to the LRT.

“The numbat genus Myrmecobius is the sole member of the family Myrmecobiidae, one of four families that make up the order Dasyuromorphia, the Australian marsupial carnivores.”

“The species is not closely related to other extant marsupials; the current arrangement in the order Dasyuromorphia places its monotypic family with the diverse and carnivorous species of Dasyuridae. Genetic studies have shown that the ancestors of the numbat diverged from other marsupials between 32 and 42 million years ago, during the late Eocene.”

Add Eomaia to your taxon list to test the LRT hypothesis.

This still appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here. Add taxa to your own LRT to test this.

References
Ji et al 2002. The earliest known eutherian mammal, Nature 416:816-822.
Waterhouse 1841.
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.

References

wiki/Eomaia
wiki/Pucadelphys
wiki/Vulpavus
wiki/Marsupial

Source: https://pterosaurheresies.wordpress.com/2026/07/22/placental-vs-marsupial-tooth-formulas-compared/

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