Molecular and Paleontological Evidence for a Post-Cretaceous Origin of Rodents

Abstract

The timing of the origin and diversification of rodents remains controversial, due to conflicting results from molecular clocks and paleontological data. The fossil record tends to support an early Cenozoic origin of crown-group rodents. In contrast, most molecular studies place the origin and initial diversification of crown-Rodentia deep in the Cretaceous, although some molecular analyses have recovered estimated divergence times that are more compatible with the fossil record. Here we attempt to resolve this conflict by carrying out a molecular clock investigation based on a nine-gene sequence dataset and a novel set of seven fossil constraints, including two new rodent records (the earliest known representatives of Cardiocraniinae and Dipodinae). Our results indicate that rodents originated around 61.7–62.4 Ma, shortly after the Cretaceous/Paleogene (K/Pg) boundary, and diversified at the intraordinal level around 57.7–58.9 Ma. These estimates are broadly consistent with the paleontological record, but challenge previous molecular studies that place the origin and early diversification of rodents in the Cretaceous. This study demonstrates that, with reliable fossil constraints, the incompatibility between paleontological and molecular estimates of rodent divergence times can be eliminated using currently available tools and genetic markers. Similar conflicts between molecular and paleontological evidence bedevil attempts to establish the origination times of other placental groups. The example of the present study suggests that more reliable fossil calibration points may represent the key to resolving these controversies.

The divergence of small mammals in Xinjiang, China, as revealed by phylogentic analyses of COI and Cytb

“…and Dipus sp. diverged separately in the Pliocene, which was consistent with the estimates of Zhang et al (2012). Furthermore, L. yarkandensis is an endemic species in Xinjiang and also emerged in the Pliocene (~3.3 Ma).…”

“…Many phylogenetic reconstmctions regarding these small mammals were generated and consisted of the Leporidae (Matthee et al, 2004;Liu et al, 2011), Ochotona (Niu et al, 2004), Dipodidae (Zhang et al, 2012), Alticola (Lebedev et al, 2007), Phodopus (Meshchersky & Feoktistova, 2009), Microtus (Jaarola et al, 2004), Sciuridae (Oshida et al, 2000;Harrison et al, 2003), Castoridae (Hom et al, 2011) and Gerbillinae (Chevret & Dobigny, 2005;Ito et al, 2010) with cosmopolitan distributions. Importantly, changes in the palaeoenvironment and paleoclimate expedite the process of evolutionary divergence in mammals.…”

“…There are eightyeight extant small mammal species from eight families of Rodentia and Lagopmorpha in Xinjiang arid region, including several species that are endemic to the region (e.g., Lepus yarkandensis, Meriones chengi: Gao, 2005;Smith et al, 2010). In addition, the region is the origin center of some jerbora species (e.g., genera Allactodipus, Allactaga and Alactagulus) and the divergence between the two subfamilies Allactaginae and Dipodinae occurred there (based on fossil evidence and divergence dating; Wu et al, 2012;Zhang et al, 2012). Consequently, this study aimed to describe the species differentiation of small mammals under the influence of palaeoenvironmental changes in Xinjiang.…”

“Septal compass” and “septal formula”: a new method for phylogenetic investigations of the middle ear region in the squirrel-related clade (Rodentia: Mammalia)

“…The squirrel-related clade is positioned at the base of the extant rodents (Huchon et al 2002;Mercer and Roth 2003;Blanga-Kanfi et al 2009;Wu et al 2012) and normally is used to reconstruct the basal rodent morphotype in anatomical studies (Wöhrmann-Repenning 1982;Ade 1999;Frahnert 1999;Mess 1999). The squirrel-related clade comprises three families: the Sciuridae (squirrels), Gliridae (dormice), and Aplodontidae (mountain beavers) (Adkins et al 2003;Blanga-Kanfi et al 2009;Fabre et al 2012).…”

Rate variation during molecular evolution: creationism and the cytochrome c molecular clock

“…Recent analysis based upon both fossil evidence and molecular clocks place the origin of rodents at approximately 60 million years ago during the period of very high cytochrome c substitution rate. See Wu et al (2012). introns. By 2003 Zhaolei Zhang and Mark Gerstein had identified 49 cytochrome c pseudogenes distributed over 18 different human chromosomes .…”