A little eutherian/metatherian convergence

between marsupials (metatherians) and placentals (eutherians) is rather commonplace. Today we’ll take a quick look at two mice-like taxa with arboreal affinities and omnivorous diets, Dromiciops (metatherian) and Ptilocercus (eutherian) (Fig. 1).

Figure 1. Dromiciops (marsupial) is similar to Ptilocercus (placental), until you look at the teeth and other details.

Figure 1. Dromiciops (marsupial, left) is similar to Ptilocercus (placental, right), until you look at the teeth and other details. Here, the marsupial has a larger braincase. The processes angularis is the hook-like shape at the bottom rear of the mandible.

Despite their similarities
these two nest separately in the large reptile tree. The NHC website notes the following differences between marsupial and palcental skulls:

  1. large face/small braincase – not true in figure 1.
  2. rear part of jaw turned inward (inflected jaw angle) in marsupials. Judging by the shadows this appears to be true.
  3. marsupials have more teeth including 3 premolars + 4 molars – true in figure 1.
  4. placental mammals typically have two sets of teeth, marsupials replace only some of their teeth

According to Wikipedia
distinct from placentals, marsupials have:

  1. foramen lacrimale in the front of the orbit- in both taxa in figure 1
  2. the cheekbone is enlarged and extends further to the rear – not true in figure 1
  3. the angular extension (processus angularis) of the lower jaw is bent toward the center. Judging by the shadows this appears to be true.
  4. hard palate has more openings.
  5. different number of incisors in the upper and lower jaws (except wombats)
  6. more premaxillary incisors than placentals – true in figure 1.
  7. more molars than premolars – true in figure 1.

Hope this helps.
This is all fresh data for me. And BTW, none of these traits divide metatherians from eutherians in the large reptile tree. There’s a whole other list — but, it works only with the few taxa I’ve worked with so far. What would happen with additional taxa is currently beyond the realm of this study.


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