The differences between Eutheria and Metatheria are a little fuzzy

Eutheria are placental mammals. Metatheria are marsupial mammals. Traditionally the latter have a pouch and epipubic bones. Placentals do not. According to Wikipedia eutherians have:

  1. Enlarged malleolus process on the distal tibia.
  2. First metatarsal offset medially compared to second metatarsal
  3. Various features of the jaws and teeth.

Not really much to go on, is it? But all is not lost:
According to Wikipedia, metatherians have:

  1. “Four pairs of molar teeth in each jaw, whereas eutherian mammals (including true placentals) never have more than three pairs.”

UnfortunateIy
I found several eutherians with four pairs of molar teeth, including Asioryctes (Fig. 1), Leptictis and Maiacetus, the stem whale with legs.

Figure 1. Skulls of mammals at or near the Metatheria/Eutheria split. Here Asioryctes has four pairs of molars, like Didelphis and Monodelphis. Leptictis also has four pairs of molars.

Figure 1. Skulls of mammals at or near the Metatheria/Eutheria split. Here Asioryctes has four pairs of molars, like Didelphis and Monodelphis. Leptictis also has four pairs of molars. Note the similarities and differences here as each of these taxa are basal to a long list of other mammals. They should look alike because phylogenetically they are not that far from each other.

On the same subject, Carroll 1988 reports on marsupials:

  1. Tooth replacement is limited to the 3rd premolar, not the molars.
  2. Most have 3 premolars and 4 molars, while most placentals have 4-5 premolars and 3 molars.
  3. The jugal extends to the jaw joint
  4. The reflected angular process is present in marsupials… and some early eutherians.
  5. Tooth details

I noted earlier
that the purported marsupial Monodelphis (Fig. 1) does not have a pouch, but does have epipubic bones. Also note that in Asioryctes the jugal also extends to the jaw joint.

All is not lost.
As we learned earlier with basal reptiles (=amniotes) traditional traits should not be used to identify basal members of any clade. Those traits might not get it right. Rather provisional basal taxa are identified during phylogenetic analysis as the last common ancestors of clade members. >After< nesting their traits can be listed.

In the large reptile tree (776 taxa) the basal eutherians Eomaia and Monodelphis are distinguished from tested outgroup metatherians by the following traits (subject to change/evolve in derived eutherians, and I’m including pre- mid- and post-node changes for these two taxa, since the line of division has not been officially set, or is due to be upset after this):

  1. Post-orbital cranial length greater than pre-orbital rostrum length
  2. Naris opening is anterior, with the premaxilla filling in any anterolateral exposure
  3. The frontal nasal angle is anteriorly oriented, not zig-zag
  4. The squamosal descends only to mid skull, not the tooth row.
  5. An ectotympanic develops to ventrally cover and isolate the tiny ear bones
  6. The opisthotic no longer descends as a separate bone, but becomes fused to surrounding occipital bones or extends laterally.
  7. The second sacral rib (transverse proceess) is fused to the first.
  8. Prepubis (epipubis) bone is absent (but I expect this to change with future discoveries as some derived taxa either redevelop or retain it)
  9. Longest metatarsal(s): 3 and 4

In the end
the clade Mammalia is identified (after phylogenetic analysis) by the presence of mammary glands, implying toothless hatchlings/new borns and single tooth replacement, along with a firm dentary squamosal joint and the detachment of the posterior mandible elements that continue to shrink to become middle ear bones). The clade Eutheria is a little more difficult to describe, but I hope the above list leads to further refinement of this issue and the removal of false paradigms that might confuse students reading texts based on Carroll 1988 or other widely read academic works.

Figure 2. Vincelestes skull. Note the many differences in tooth size and count here along with the shorter rostrum on this basal Cretaceous carnivore.

Figure 2. Vincelestes skull. Note the many differences in tooth size and count here along with the shorter rostrum on this basal Cretaceous carnivore.

It’s worth noting
that the basal carnivore, Vinceletes (Fig. 2) appears to be quite derived (big canine, fewer teeth, shorter rostrum, etc.) compared to proximal sisters, like Nandinia, in the LRT. This implies a much older split from Nandinia (Fig. 1) than the present Early Cretaceous (130 mya) first appearance of Vincelestes in the fossil record.

And for those who care the update on Liaoconodon was posted yesterday.

References
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.

 

 

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