Evolution of bipedalism in Dipodoidea (Rodentia: Mammalia): A molecular and paleobiological investigation

Bipedalism evolved in jerboas (Dipodoidea). To date however, we know little about how this specialized mode of locomotion evolved. To understand the pattern of bipedal evolution, dipodoid phylogeny was constructed based on a broad taxon sampling and eleven nuclear genes under Bayesian and maximum likelihood criteria. According to the phylogeny, the traditional classification, which divides the superfamily Dipodoidea into two Families, Zapodidae and Dipodidae, is incorrect. I present a new classification that groups dipodoids into three families, the Family Sicistidae (including Sicistinae), the Family Zapodidae (including Zapodidae) and the Family Dipodidae (including Euchoreutinae, Allactaginae, Dipodinae and Cardiocraniinae). Additionally, a molecular clock analysis was conducted to date the origin and diversification of rodents, an issue that has remained controversial. While paleontological evidence supports a rodent radiation in the early Cenozoic, molecular studies place rodents as one of the earliest placental orders, diversifying in the Cretaceous. To reconcile the molecular clock with fossil data, I estimated the divergence times across major lineages of rodents based on eight fossil calibration points and two molecular dating methods. The present results support the origin of rodents in the early Paleocene, in accordance with the paleontological record, and therefore, challenge the hypothesis of previous molecular investigations, which suggest a Cretaceous origin and diversification of rodents, and thus other crown groups of placental mammals.;Rodent bipedalism is widely assumed to be an adaption to dry, open habitats. The phylogeny indicates that bipedalism most likely evolved in jerboas in the late part of the Early Miocene -- a period of global warming and high humidity, indicating that bipedalism probably evolved in wet, vegetated environments. Moreover, early jerboas were found to be associated with mammalian browsers, suggesting that they lived in a humid C3 environment. Furthermore, the evolution of specialization for bipedal locomotion in jerboas is independent from the evolution of dental specialization. The cheek teeth of bipedal jerboas underwent directional evolution: crown height increased with time since the Late Miocene, while the jerboas' leg/head-body length ratio remained static. These results suggest that bipedalism evolved in jerboas as an exaptation to humid woodlands or forests for vertical jumping in complex environments. Fast running is probably then a by-product of selection for jumping, which became advantageous in open plains later on.