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On Tsaganomyidae (Rodentia, Mammalia) of Asia

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American Museum of Natural History
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Abstract

Abstract The taxa within the family Tsaganomyidae are systematically redefined. Cyclomylus lohensis is a valid taxon. It is generically different from Tsaganomys in both tooth and skull morphology, including having lower crowned cheek teeth with closed roots, more prominent lophs and cusps on occlusal surfaces, a reduced dentine part, different microstructure of the incisor enamel, as well as other features. Two new species, C. intermedius and C. biforatus, are included in the genus Cyclomylus, mainly based on differences in crown height and root number. The strongly hypsodont tsaganomyids can be separated into two genera: Tsaganomys and a new genus, Coelodontomys, based on differences in cheek tooth morphology and relations of cheek teeth with incisors. As a result, Tsaganomyidae includes three genera and five species: Tsaganomys altaicus, Cyclomylus lohensis, C. intermedius, C. biforatus and Coelodontomys asiaticus. Tsaganomys is the sister group to Cyclomylus plus Coelodontomys. Cyclomylus lohensis, the most primitive species, made its first appearance in early early Oligocene; C. intermedius also then but slightly later. Cyclomylus biforatus and Coelodontomys asiaticus are restricted to late early Oligocene. Tsaganomys altaicus made its first occurrence in late early Oligocene and survived to early late Oligocene (or possibly earliest Miocene), by which time Cyclomylus and Coelodontomys had already become extinct. The Tsaganomyidae differ from Hystricognathi and Ctenodactyloidea in lacking the derived features of the two latter groups, such as hystricomorphous skull, large and deep pterygoid fossa, well-developed lower masseteric crest, fused malleus and incus, large and anteriorly expanded malleus head, proximal fusion of tibia and fibula, and large hypocone on upper cheek teeth. The autapomorphic features of the Tsaganomyidae are extremely enlarged temporal fossa, prominent sagittal and occipital crests, robust and strongly laterally convex zygomatic arch with well-developed jugal meeting premaxilla, high ascending ramus with high coronoid process, and triangularly convex surface of condyloid process. The angular process in the tsaganomyids is large and strongly flared, which is so different from the typical hystricognathous lower jaw that it is called tsaganomys-type hystricognathous mandible in this paper. The Tsaganomyidae presumably have unelongated masseter and pterygoid muscles but well-developed temporal muscle and steplike occlusal surface of the cheek tooth row. These features may be associated with crushing and transverse chewing. Obviously, the Tsaganomyidae evolved in a direction fairly distinct from both the Ctenodactyloidea and Hystricognathi.

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