SVP 2018: Placentals in the Cretaceous

Halliday et al. 2018
wonder about “the traditional lack of Cretaceous placental fossils when results from diverse dating analyses favor a Cretaceous origin of Placentalia.”

Unfortunately
they use an outdated cladogram that includes the following invalid clades (superorders) that Halliday et al. surmise should be present in Cretaceous sediments:

  1. Atlantogenata = Afrotheria + Xenarthra (elephants and anteaters in one clade?)
  2. Laurasiatheria = shrews, pangolins, bats, whales, carnivorans and ungulates (whales and bats in the same clade?)
  3. Euarchontoglires = rodents, lagomorphs, tree shrews, colugos and primates (lacking only carnivores, these are basal eutherians

Together these three clades
comprise the entirety of extant Eutheria (placental mammals). All of the above clades are extant. Where are the extinct clades, like Multituberculata?

By contrast,
the large reptile tree  (LRT, 1313 taxa) recovers Middle and Late Jurassic placentals (multituberculate rodents) along with several Early Cretaceous taxa, like the pangolin ancestor, Zhangheotherium (Fig. 1). So “the traditional lack of Cretaceous placental fossils” has been updated in the LRT.

Figure 1. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

Figure 1. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

Halliday et al. conclude: “The lack of definitive Cretaceous placental mammals may therefore be explained by high predicted morphological similarity among stem and basal crown eutherians, providing an avenue for partially reconciling the fossil record and molecular divergence estimates in Placentalia.”

No.
Taxon exclusion has given Halliday et al. an outdated tree topology. There is plenty of evidence for Mesozoic placentals in the LRT. Adding taxa provides every included taxon new opportunities to nest more parsimoniously. A good starter list can be found here (LRT subset Fig. 2). Many taxa from this list are candidates for discovery in the Mesozoic based on the discovery of multituberculates in the Mesozoic.

Figure 3. Subset of the LRT, focusing on basal Eutheria and Heterocephalus. Aqua taxa are arboreal. Tan taxa are terrestrial. Blue taxa are aquatic.

Figure 3. Subset of the LRT, focusing on basal Eutheria and Heterocephalus. Aqua taxa are arboreal. Tan taxa are terrestrial. Blue taxa are aquatic.

References
Halliday TJ et al. (5 co-authors) 2018. Delayed increase in morphological rates of evolution after the origin of the placental mammal crown group. SVP abstracts.