Updated January 3, 2019, seven years and about 1000+ taxa later.
We don’t talk about mammals much,
but as reptiles they (we) do qualify as subjects to be covered by ReptileEvolution.com.
A new online study by Wu et al. (2012) finds evidence for a post-Cretaceous origin for rodents. Rodents (everything from porcupines and guinea pigs to squirrels, mice and multituberculates) are related to rabbits (lagomorphs).
How are they all related?
Near (but not at) the base of the primates is an interesting set of taxa known as tree shrews, like Tupaia (Raffles 1821, Fig. 1). Essentially they are basal rodents/rabbits/multituberculates.
The most common tree shrew, Tupaia was found to be basal to the equally arboreal and highly derived Plesiadapis (Fig. 3) and terrestrial rabbits, and rodents, like the porcupine. It’s worthwhile to see the porcupine skull and how close it resembles that of Plesiadapis.
The other arboreal tree shrew,
Ptilocercus, is basal to Tupaia, derived from basal primates and carnivores.
Based on the nesting of multituberculates
all these tree shrews, rodents and rabbits had their origin in the Jurassic, not the Paleocene (contra Wu et al. 2012.
Wu S, Wu W, Zhang Z, Ye J, Ni X, Sun J, Edwards SV, Meng J and Organ CL 2012. Molecular and Paleontological Evidence for a Post-Cretaceous Origin of Rodents. PLoS ONE 7(10): e46445. doi:10.1371/journal.pone.0046445