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).
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.
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.
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.
See ReptileEvolution.com and links therein.