Tree shrews are basal to all marsupials and placentals
We’re going to expand a definition today
Tree shrew-like taxa are all related to one another (Fig. 1). The traits they share are inherited, not convergent. They form traditionally overlooked links between more familiar clades. That’s because ultimately some of these tree shrew-like taxa evolve to other shapes and sizes, like bats, giraffes, kangaroos, killer whales and humans. It’s about time we understood just exactly where we humans and we placentals came from.
Unfortunately, O’Leary et al. 2013 is the current standard
and doggone it, that crew had to invent a hypothetical ancestor for placentals because they chose to exclude pertinent taxa… and they relied on genomic testing. Two major sins.
Small arboreal trees shrew-like and and terrestrial mouse-like marsupials
like Caluromys and Monodelphis (Figs. 2, 3), precede small arboreal placental tree shrews, like Tupaia and Ptilocercus (Fig. 5) in the large reptile tree (LRT, 1870+ taxa; subset Fig. 1).
Caluromys also precedes
a clade of largely carnivorous marsupials (traditional creodonts) in the LRT (Fig. 6).
The large tree shrew-like Virginia opossum
(Didelphis) precedes a clade of largely herbivorous marsupials in the LRT (Fig. 6). This opossum can be thought of as a large tree shrew if you have the smallest inkling of imagination.
As you might imagine,
taking care of growing, hungry Monodelphis youngsters can be quite a chore for the mom (Fig. 3) especially in Late Jurassic and throughout the Cretaceous when this scene was common if you knew where to look (with night vision goggles). One of life’s mysteries is how this sort of maternal care was selected for. It looks extremely debilitating, but somehow it worked! This is why we celebrate Mother’s Day.
The last common ancestor of all placentals
in the LRT, Early Eocene Vulpavus palustris (Fig. 2), is basically a large tree shew. We’ll all be looking for its little sister in Jurassic formations.
The two extant taxa the dictionary considers tree shrews,
Ptilocercus (Fig. 3) and Tupaia (Fig. 3) are basal to two different clades of placentals, Volitantia and Glires. This breaks tradition, which nests these two in a traditional clade, Scandentia. In the LRT that clade name is a junior synonym for Placentalia.
Taxa with a tree shrew morphology are common
along the backbone of the mammal cladogram (Fig. 1). All basal marsupial and placental clades have either a tree shrew-, squirrel-, mouse- or phalanger-like morphology, with sizes up to the Virginia opossum. Thereafter some become bats (Fig. 7). Others become primates. Still others become rodents, mulittuberculates and tenrecs.
Monotremes appear to have rarely climbed trees.
Most of these mouse-like invertebrate-eaters don’t have any obvious arboreal traits. Long-limbed, Late Jurassic Juramaia might be the exception. Extant monotremes do not attempt climbing. Late surviving, Late Cretaceous Ukhaatherium, the basalmost therian in the LRT, has a supple backbone and long limbs, ideal for a tiny tree climber. In the LRT, Ukhaatherium is the basalmost member of Theria (Parker and Haswell 1897), aka Marsupialia (Illiger 1811).
Genomic analyses
do not recover a similar tree topology for mammals. None provide transitional taxa between monotremes and marsupials and placentals. Some indicate that hyraxes, elephants and armadillos are basal placental taxa. Avoid deep time genomic analyses. Through no fault of their own, genes become corrupted by epigenetic interactions with continental viruses. The majority of paleontologists have not yet embraced that crack in the glass.
“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”
– MARK TWAIN
References
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of placentals. Science 339:662-667. abstract
Source: https://pterosaurheresies.wordpress.com/2021/06/11/tree-shrews-are-basal-to-all-marsupials-and-placentals/
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