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 and mice) are related to rabbits (lagomorphs) which are related to primates (including readers of this blog and lemurs) which all were derived from arboreal carnivores like Vulpavus.
The most common one, Tupaia (Raffles 1821) was found to be basal to the equally arboreal Plesiadapis (Fig. 3) and by extension to the terrestrial rabbits, and by further extension to rodents (keeping on topic), like the porcupine. It’s worthwhile to see the porcupine skull and how close it resembles that of Plesiadapis.
I don’t know much about rodents, but given what I do know about the initial appearance of their outgroups, the Wu et al. (2012) study makes perfect sense in the present context!
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