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An assemblage of the early Miocene Sayimys giganteus (Ctenodactylinae, Rodentia) from its type locality Keseköy, and smaller assemblages of Yapıntı and Horlak (Anatolia, Turkey) are described. Almost all Sayimys species are known from small collections of isolated cheek teeth. The very large assemblage from Keseköy is of special interest, because it allows the study of the intra-specific variation of dental morphology: size, the morphological changes through wear and the criteria for recognition of the tooth position of molars. The results have been used to review the formally named Sayimys species and to evaluate the dental features used to define the species. The review of the early and middle Miocene succession of Sayimys in the Siwaliks (Pakistan) resulted in the resurrection of S. minor; S. baskini is considered to be a junior synonym of S. minor. S. sivalensis and S. obliquidens are nomina dubia, and restricted to their holotypes, S. hintoni n. sp. is established for mediumsized Sayimys from the Kamlial, lower Chinji and Manchar Formations and S. chinjiensis is resurrected for Sayimys from the upper Chinji and Nagri Formations. It is suggested that the middle Miocene species Sayimys intermedius and S. assarrarensis from Saudi Arabia are synonymous.

Major crown-groups of rodents were well established in the early Tertiary, and fossils provide an invaluable window into their evolutionary history. The main focus of this project was to perform a cladistic assessment of the dental evidence for early Tertiary rodent cladogenesis – the masticatory apparatus and teeth are the most frequently preserved anatomical features in the fossil record. We focused on groups that existed in a period corresponding to their early history, combining fossils belonging to extinct lineages and to stem-groups leading to modern lineages. While the monophyly of some groups is not systematically explored, our results have important implications for high-level rodent relationships and systematics. These results are consistent with those of recent molecular phylogenies and reliably congruent with the stratigraphic record, thus enhancing the pertinence of dental characters for phylogenetic inference. Our approach provides evidence of a fundamental dichotomy in early rodent history. Two major clades have been identified: (1) the earliest ‘ctenodactyloid’ (Ctenodactylidae, Chapattimyidae, Yuomyidae, Diatomyidae) and hystricognathous (Tsaganomyidae, Baluchimyinae, ‘phiomorphs’, ‘caviomorphs’) rodents, and (2) the earliest ‘ischyromyoid’ rodents with their closest relatives (Muroidea + Dipodoidea + Geomyoidea + Anomaluroidea + Castoroidea + Sciuravidae + Gliroidea, and Sciuroidea + Aplodontoidea + Theridomorpha). This topology has led us to endorse Ctenohystrica as the first clade and propose a new taxon, Ischyromyiformes, for the second. Although minimized in our working hypothesis, the homoplasy in dental characters remains significant. However, a number of homoplasic characters reveal structuring in their internal distribution, allowing us to discern evolutionary morphological patterns, notably the pentalophodonty of molars, zygomasseteric complex and incisor enamel microstructure. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 142, 105–134.

The ctenodactylids from the Lower and Middle Miocene of Saudi Arabia are represented by two species: Sayimys intermedius (Sen and Thomas, 1979) and Sayimys assarrarensis sp. nov. This new taxon differs from the other species of Sayimys by its larger size together with an association of characters such as the presence of a metalophulid II in d4 and the strength of the anteroloph and posteroloph in P4. A cladistic analysis involving Prosayimys flynni, Sayimys obliquidens, S. assarrarensis sp. nov., S. intermedius, S. sivalensis, S. baskini, and Metasayimys curvidens is provided. Sayimys assarrarensis sp. nov. nests between Sayimys obliquidens and Metasayimys curvidens as a relatively primitive taxon.

A new species of Sayimys (Ctenodactylidae, Mammalia), Sayimys giganteus sp. nov., is described from the Early Miocene localities of Keseko¨ y 1, Keseko¨ y 2, and Hisarcik (Turkey). Sayimys giganteus sp. nov. differs from other known species of Sayimys by its larger size and several plesiomorphies (e.g., metalophulid II on dp4, strong anterolophid and posterolophid on P4, and a long paraflexus and metaflexus on upper molars), but also in the presence of some derived characters. Two species of Sayimys are currently recognized in Turkey: Sayimys cf. Sayimys intermedius (at Pas¸alar, Middle Miocene) and Sayimys giganteus sp. nov. Based on a cladistic analysis involving all known species of Sayimys, Metasayimys, and Prosayimys, Sayimys giganteus sp. nov. emerges as the sister taxon of Sayimys obliquidens.