Rodentia is the largest and most diversified mammalian clade, comprising 40% of all mammalian species, and inhabiting all continents with the exception of Antarctica. Within Rodentia, there are 3 groups, with 2 of them widely distributed, namely Myodonta including genera such as Rattus and Mus, and Sciuromorpha with genera such as Sciurus, Tamias, and Marmota. The 3rd group, Hystrocomorpha, is the least studied, with a few species living in Africa (e.g, Heterocephalus glaber), but most of the species living in South-America, including genera such as Hydrochoerus (capybara, the largest living rodent), Chinchilla and Octodon. According to phylogenetic studies, the first rodents appeared after the K/Pg boundary, around 65 MY ago, and the first record of the group Hystrocomorpha is 58 MY ago, and for Sciuromorpha is 56 MY ago, while the group Myodonta including Muruidea and Dipodoidea originated around 45 MY ago (see Marivaux et al. 2004, Wu et al. 2012, Fabre et al. 2012).
