Abstract

BackgroundTamquammys has been considered one of the basal ctenodactyloid rodents, which has been documented in the earliest to middle Eocene (~ 56.0–48.5 Ma) in China. It was the most abundant and widespread rodent genus in the Erlian Basin (Nei Mongol, China) and dominated Arshantan small-mammal faunas of that region. Here for the first time we describe the morphology of the astragalocalcaneal complex in Tamquammys robustus (larger) and T. wilsoni, and interpret it against the background of locomotor adaptations of basal Euarchontoglires (rodents, lagomorphs, tree shrews, and primates).ResultsThe comparative morphology of the tarsal elements in Tamquammys robustus and T. wilsoni shows overall slenderness of the bones and their similarity to the tarsal elements of Rattus, a generalist species, and those of small rock squirrels (e.g. Sciurotamias). The two species differ slightly in their cursorial ability; smaller T. wilsoni shows some adaptations to climbing. The results of principal component analysis of the calcaneus and astragalus support this observation and place T. robustus in-between Rattus and ground/rock squirrel morphospace, and T. wilsoni closer to euarchontans, Tupaia and Purgatorius.ConclusionsThe morphology of the tarsal elements in Tamquammys indicates a generalist rodent morphotype with no particular adaptations to arboreality. We suggest that Tamquammys as a basal ctenodactyloid is closer to the ancestral astragalocalcaneal morphology of rodents than that of more derived North American paramyines of similar age. Overall similarity in Tamquammys tarsal elements structure to Purgatorius, a basal primate, may point to the antiquity of the tarsal structure in Tamquammys and a generally unspecialized foot structure in early Euarchontoglires.

Highlights

  • Tamquammys has been considered one of the basal ctenodactyloid rodents, which has been documented in the earliest to middle Eocene (~ 56.0–48.5 Ma) in China

  • Four species of Tamquammys have been recognized in the Nei Mongol deposits far; Tamquammys wilsoni (Fig. 1b, c) was the most abundant and persistent in the fossil record, ranging from the earliest Eocene (Bumbanian) to the early middle Eocene (Irdinmanhan) [3]

  • This paper focuses on the tarsal bones morphology of the two most abundant species of Tamquammys in Nei Mongol: T. robustus and T. wilsoni (Fig. 1)

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Summary

Introduction

Tamquammys has been considered one of the basal ctenodactyloid rodents, which has been documented in the earliest to middle Eocene (~ 56.0–48.5 Ma) in China It was the most abundant and widespread rodent genus in the Erlian Basin (Nei Mongol, China) and dominated Arshantan small-mammal faunas of that region. The earliest species of Tamquammys were coeval with the early Eocene rodents Advenimus, Chenomys, Cocomys, Tamquammys was most abundant and diversified in the Erlian Basin, Nei Mongol, where its earliest, scarce, remains were found, from the upper part of the Nomogen Formation (NS-3), representing the Bumbanian Asian Land-Mammal Age [3, 8, 9]. In the early middle Eocene (Irdinmanhan) another species, T. fractus, coexisted with T. wilsoni, both being relatively small

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