Traditional marsupial vs placental skull traits

In paleontology sometimes all we have to go on
is a fossil skull we can attribute to the Mammalia based on single replacement teeth and a list of plesiomorphic traits held by pre-mammals. Breast milk is THE key mammal trait. Single replacement teeth implies toothless hatchlings/newborns who depend on their mother’s breast milk for their first few weeks/months/years of life. That’s the logic stream for mammal vs non-mammal identification.

If the mammal hatched from an egg,
it is a monotreme.

If the mammal was born as an embryo that can crawl into its mother’s pouch,
it is a marsupial.

If the mammal was born at a more advanced stage
due to the presence of a placenta and the mother lacks a pouch it is a placental.

Some transitional marsupials (e.g. Monodelphis, Fig 1) lack a pouch AND lack a placenta. So, it’s never so simple as we were taught in grade school.

Note that none of these developmental differences are present in the skull, and sometimes all we have to go on is a fossil skull.

Figure 3. Monodelphis mother with her growing brood of young clinging to her fur and nipples. ” data-image-caption=”

Figure 3. Monodelphis mother with her growing brood of young clinging to her fur and nipples.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2021/06/monodelphys_mom.jpg?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2021/06/monodelphys_mom.jpg?w=584″ class=”size-full wp-image-56538″ src=”https://pterosaurheresies.files.wordpress.com/2021/06/monodelphys_mom.jpg?w=584&h=385″ alt=”Figure 3. Monodelphis mother with her growing brood of young clinging to her fur and nipples.” width=”584″ height=”385″ srcset=”https://pterosaurheresies.files.wordpress.com/2021/06/monodelphys_mom.jpg?w=584&h=385 584w, https://pterosaurheresies.files.wordpress.com/2021/06/monodelphys_mom.jpg?w=150&h=99 150w, https://pterosaurheresies.files.wordpress.com/2021/06/monodelphys_mom.jpg?w=300&h=198 300w, https://pterosaurheresies.files.wordpress.com/2021/06/monodelphys_mom.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Monodelphis mother with her growing brood of young clinging to her fur and nipples.

According to
an online listing sponsored by UC Berkeley:

Marsupial skulls can be distinguished from those of placentals by:

1. Participation of the jugal in the jaw joint at the glenoid (mandibular) fossa
2. Presence of posterior palatal vacuities
3. Auditory bullae derived at least in part from the alisphenoid
4. Angular process of the dentary usually inflected medially
5. Primitive dental formula of 5134 uppers, 4134 lowers
6. Small braincase

Due to fossil crushing, often n1 and n5 are the only traits clearly shown. N5 does not pertain to derived marsupials like wombats and kangaroos. That leaves only n1 as a trait commonly encountered in fossils. Is it accurate? Can it be trusted? See figure 2.

Placental skulls can be distinguished by:

1. Absence of the jugal in the mandibular fossa
2. Absence of posterior palatal vacuities (there are exceptions)
3. Complex auditory bullae without an alisphenoid contribution
4. Angular process of the dentary usually not inflected medially
5. Primitive dental formula of 3143/3143
6. Disproportionately large braincase

These traits were discussed above by inference.

The other method for lumping and splitting clades
is phylogenetic analysis. In analysis every taxon affects every other taxon, so less complete taxa connect to more complete taxa for which confirmation can be ascertained, especially when the soft tissue is known.

Given the present limitations, what does the LRT recover?
The basalmost marsupial in the LRT, Ukhaatherium has the marsupial tooth formula 5134, but the jugal does not reach the jaw joint. That occurs in a more derived taxon, Eomaia.

Figure 1. Panthera, the placental lion, has a marsupial jugal that extends to the jaw joint – by convergence. ” data-image-caption=”

Figure 1. Panthera, the placental lion, has a marsupial jugal that extends to the jaw joint – by convergence.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/hesperocyon-panthera588.jpg?w=197″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/hesperocyon-panthera588.jpg?w=584″ class=”size-full wp-image-85797″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/hesperocyon-panthera588.jpg?w=584&h=888″ alt=”Figure 1. Panthera, the placental lion, has a marsupial jugal that extends to the jaw joint – by convergence.” width=”584″ height=”888″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/hesperocyon-panthera588.jpg?w=584&h=888 584w, https://pterosaurheresies.files.wordpress.com/2024/05/hesperocyon-panthera588.jpg?w=99&h=150 99w, https://pterosaurheresies.files.wordpress.com/2024/05/hesperocyon-panthera588.jpg?w=197&h=300 197w, https://pterosaurheresies.files.wordpress.com/2024/05/hesperocyon-panthera588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 2. Panthera, the placental lion, has a marsupial jugal that extends to the jaw joint – by convergence.

Speaking of participation of the jugal in the jaw joint
(see marsupial list above), that also occurs in the extant genus Panthera (Fig 2), a confirmed placental. So exceptions and convergence is a real problem.

Trying to identify vertebrate clades
by having or lacking certain traits (see the UC Berkeley liists above) is not the best idea. Exceptions abound. Convergence is out there. That’s why the LRT attempts to recover clades that are defined by a last common ancestor and all of its descendants while minimizing taxon exclusion based on prior assumptions and textbooks.

The reason for this post
was to show the difficulties due to convergence one tends to find in wide gamut phylogenetic analyses and why so many published analyses lack complete resolution.

This is why creodonts (carnivorous marsupials) have been traditionally considered members of the placental Carnivora.

References
berkeley.pressbooks.pub/morphology/chapter/mammal-skulls/