New Study on Dental and Skeletal Features of the Cretaceous “Symmetrodontan” Mammal Zhangheotherium

• Published in: Journal of mammalian evolution Vol. 12; no. 3-4; pp. 337 - 357
• Main Authors: Luo, Zhe-Xi, Ji, Qiang
• Format: Journal Article
• Language: English
• Published: New York Springer Nature B.V 01.12.2005
• Subjects: Ankle; Calcaneus; Dentition; Fossil mammals; Fossils; Growth stage; Hominids; Pelvis; Premolars; Skeleton; Vertebrae; Zhangheotherium quinquecuspedens
• ISSN: 1064-7554; 1573-7055
• DOI: 10.1007/s10914-005-6958-x

A new partial skeleton of the Cretaceous “symmetrodontan” mammal Zhangheotherium quinquecuspedens from the Yixian Formation of Liaoning, China has shed light on the dental and skeletal features of this taxon. The new fossil is a juvenile individual of late growth stage, preserved with interesting features of the premolar replacement. This fossil also provides new information on the vertebral column, the pelvis, the hindlimb and pes. Zhangheotherium has a typical diphyodont replacement of its premolars that is characterized by an alternating pattern (p1 → p3 → p2). This alternating replacement of premolars is a derived condition shared by Dryolestes, Slaughteria, and some basal eutherians, and differs from the plesiomorphic sequential replacement of anterior postcanines in eutricondontans, in most multituberculates and in stem mammaliaforms. The calcaneus and astragalus in the ankle joint of Zhangheotherium lack superposition. This shows that the trechnotherian clade, of which Zhangheotherium is a basal taxon, has retained the primitive condition of mammaliaforms in which the astragalus is in juxtaposition with the calcaneus. Coupled with recent evidence from the earliest metatherians and eutherians, this suggests that the superposition of astragalus and calcaneus evolved in parallel in metatherians and eutherians.