Morphology and Evolution of the Glires Rostral Cranium

Glires is recently considered as the best supported hypothesis at the supraordinal level within Eutheria based on morphology. Large, evergrowing gnawing incisors and the associated differentiation of the rostral cranium are the most conspicuous corresponding feature of rodents and lagomorphs. A detailed morphological analysis of these characters within therians with special emphasis on rodents and lagomorphs evaluates their implications for the still not generally monophyly of Glires. Morphotype characters of this clade are reconstructed. The Glires morphotype is characterized by at least two incisor in each jaw (d12, 13). In the upper jaw, the 13 did not lie distinctly medial and approximated to the gliriform incisor. In a short diastema behind the incisors the Inflexa pellita were located. The nasopalatine duct opened closely behind the incisors into the oral cavity. Jacobson's organ opened into the floor of the nasal cavity anterior to the nasopalatine duct. With no special rostral extension of the masseter muscle the facial skull was still broad, the nasolacrimal duct ran lateral to the posterior part of the incisor, and the lacrimal foramen was positioned laterally inside the orbit at the border between lacrimal and maxillary. The premaxillary had a posterodorsal process which connected to the frontal. Based on this reconstruction, character transformations in the evolution towards lagomorphs and rodents are traced. Within the character set three synapomorphies for the Glires are confirmed. The transformations demonstrated appear plausible in the organismic context. No evidence that contradicts the Glires hypothesis was found.