Basal placentals illustrated in phylogenetic order

Eutherian (= placental) mammals
are divided into clades like Primates, Ungulata, Carnivora, etc. Known basal taxa for each of these clades are related to one another in a ladder-like fashion, each one nesting at the base of a bushy clade in the large reptile tree (LRT, 1366 taxa).

Today,
an illustration of skeletons (Fig. 1) and skulls (Fig. 2) in phylogenetic order documents the minor changes (microevolution) between basal taxa that nest at the bases of several increasingly derived placental clades.

FIgure 1. Skeletons of taxa that nest at the bases of several major placental clades, divided between Cretaceous and Paleocene taxa.

FIgure 1. Skeletons of taxa that nest at the bases of several major placental clades, divided between Cretaceous and Paleocene taxa, divided by four different scales. Basal taxa are several degrees of magnitude smaller.

The following placental skulls are not to scale
yet continue to demonstrate the minor changes (microevolution) that occur at the bases of several major placental clades. For instance, Chriacus is basal to bats, while Anagale is basal to odontocete whales. It is difficult, if not impossible, to determine such future developments in these basal taxa without the benefit of a wide gamut analysis, like the LRT.

Figure 2. A selection of placental skulls in phylogenetic order and divided into Cretaceous and Paleocene taxa.

Figure 2. A selection of placental skulls in phylogenetic order and divided into Cretaceous and Paleocene taxa.

The lesson for today:
Sometimes quantity, without firsthand observation, is needed to put together the ‘Big Picture’ before one is able to pick apart the details that each specific specimen reveals during firsthand study. Traditionally paleontologists have been putting the latter ahead of the former by (too often) excluding pertinent taxa revealed and documented by the more generalized and wide gamut phylogenetic analysis provided by the LRT. Like Yin and Yang, both must be considered. ‘Avoid taxon omission‘ is the single most important rule when constructing a cladogram of interrelationships.

References
See ReptileEvolution.com and links therein.

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