During the reign of the dinosaurs
tree shrews, like Ptilocercus (Fig. 1) and Tupaia (Fig. 1), stayed in the trees, evolving into tree-dwelling members of the Carnivora (Genetta, Fig. 1), Volitantia (bats, pangolins and dermopterans), Glires (including multituberculates led by Tupaia) and Primates (Microcebus, Fig. 1) in the large reptile tree (LRT, 1818+ taxa) distinct from all gene studies and all other prior trait studies (due to taxon exclusion). The LRT is the first study that found tree-dwelling Caluromys (Fig. 1), an extant tree shrew-like marsupial, as the proximal outgroup to the Placentalia. Based on chronological bracketing, Caluromys relatives lived in the Early Jurassic.
After the Cretaceous some tree shrews became terrestrial.
Leptictids, elephant shrews (Rhynchocyon, Fig. 2 and tenrecs (Tenrec) were phylogenetically among the first of the former tree shrews to become fully terrestrial. They were all small. After the Cretaceous some terrestrial tree shrew descendants began to increase in size. Some became elephants, others horses, still others baleen whales, all following Cope’s Rule.
Once established on the ground
and spreading beyond the jungles, the following Early Paleocene terrestrial placentals became cat to tiger size: Onychodectes (Fig. 3), Alcidedorbignya (Fig. 3) and Pantolambda (Fig. 3).
By the late Paleocene
taxa like massive Barylambda showed further increases in size. This taxon was basal to giant glyptodonts and ground sloths, some of which ultimately became smaller and returned to the trees as tree sloths.
PS… saving the best for last.
Writing this blogpost inevitably brought my gaze back to Fruitafossor (Luo and Wible 2005), a small, Late Jurassic digging mammal with four robust fingers, xenarthran lumbars and single cusp, tubular teeth. When first encountered and based on these traits the LRT mistakenly nested Fruitafossor with edentates for the last four years. That Late Jurassic temporal discontinuity in an otherwise Tertiary clade of edentates required a review and revision of taxon scores for Fuitafossor. That review ultimately re-nested Fruitafossor more plausibly and parimoniously basal to echidnas in the LRT. Fruitafossor is a basal echidna from Colorado. That story comes to you tomorrow.
Luo Z-X and Wible JR 2005. A late Jurassic digging mammal and early mammal diversification. Science 308:103–107.