Cranial Morphology And Phylogenetic Analysis Of Tsaganomyids Based On A New Skull From The Early Oligocene Of Saint-Jacques,Nei Mongol

Tsaganomyidae are characterized by the protrogomorphous skull and mainly found in the early Oligocene of China,Mongolia and Kazakhstan of Asia.Until now the family Tsaganomyidae consist of three genera and five species.They are useful in biochronological correlation as they are widely distributed in the Oligocene of Central Asia and Norther China.However,the fossils are often represented by fragmentary skulls or isolated teeth,this has led to taxonomic misidentification.Also only a few studies on Tsaganomyid phylogenetic relationship have been conducted at the species level.This paper describes a new fossil skull of the early Oligocene from Saint-Jacques,Nei Mongol Autonomous Region.The skull is relatively complete and lacks the zygomatic arch.This study employs the high-resolution microscopic computed tomography(Micro CT)to reconstruct the morphology of cranial bones.Furthermore,the morphological characters were incorporated into the data matrix to analyze the phylogenetic relationship.1.After description and comparison,the skull was referred to the Tsaganomys altaicus based on the following features are: left and right anterpremolar crest curved;P4 is located medial to the upper incisors;the dentin was higher than Coelodontomys and the medullary cavity was shorter than Cyclomylus;the root is open.2.The morphological characteristics of the skull and cheek teeth of the new material were observed and described using micro-CT,and the virtual skull data were reconstructed.Internal morphological data were obtained:(1)the root is open;(2)P4 is located medial to the upper incisors;the P4 root is located on the lingual side of the upper incisor,and the crown extends to the buccal side through the lingual and dorsal side of the upper incisor;(3)The bending and extension directions of the crown and root of P4 are different from those of M1-3.3.Phylogenetic analysis was carried out using TNT.Following the character list of Bryant and Mc Kenna(1995),we selected 7 taxa and 61 characters to build a morphological data matrix for phylogenetic analysis.The tree supports that Cyclomylus is a monophyletic group.Within Cyclomylus,Cyclomylus lohensis and Cyclomylus intermedius form a sister group.Tsaganomys altaicus and Coelodontomys asiaticus are closely related and form a sister group.Together with Cyclomylus,they form the family Tsaganomyidae.