Eggysodontines are a group of medium-sized rhinocerotoids, including the European Eggysodon and the Asian Allacerops, previously found only in Oligocene deposits from Europe and Asia. Scientists from Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences reported a new primitive late Eocene eggysodontine, Proeggysodon qiui gen. et sp. nov., from Erden Obo, Siziwangqi, Nei Mongol, China. The new material provides important information about the origin and evolution of Eggysodontinae, as well as their phylogenetic relationship with other rhinocerotoids, according to a paper published in the latest issue of Vertebrata PalAsiatica 2012(3).

The new material is a lower jaw of an old individual, with incisor, canine, premolar and molar teeth preserved, recovered from a bed equivalent to the “Middle White” or “Gray” layer described by Walter Granger at Erden Obo during the Central Asiatic Expedition. The specific name honors Prof. Dr. Zhanxiang Qiu, who described the first Chinese eggysodontines, for his great contributions to the study of Chinese perissodactyls.

The mandible of Proeggysodon has two paired of well-developed, spatulate incisors, large, vertical canines, very short diastemae between the canines and cheek teeth, relatively prominent ectoflexids on the cheek teeth, and lower molars that are not elongated. The large, vertical canines and very short diastemae between the canines and cheek teeth are characteristic of eggysodontines.

“Proeggysodon differs from other eggysodontines in being smaller; in having two pairs of well-developed, spatulate lower incisors, non-molarized lower premolars, a more lingually aligned cristid obliqua of p4, and an angled junction between the cristid obliqua and the hypolophid of each lower molar; and in lacking buccal cingulids on the lower cheek teeth”, said Dr. BAI Bin, first author of the study, “we propose that eggysodontines and forstercooperes are closely related, mainly because they share the following features: large and erect canine, relatively short diastema between canine and cheek teeth, and dorsoventrally deep and anteroposteriorly abbreviated symphysis associated with nearly vertically implanted incisors”.

The Eocene/Oligocene transition reflects an important global climatic change from a warm and humid Eocene to a cool and arid Oligocene. The Eocene/Oligocene faunal turnover in Europe is named the “Grande Coupure”, and it is thought that European Eocene faunas were displaced by Oligocene immigrants either from Asia or from North America via Asia.

“The new specimen narrows the time gap between the Early to Middle Eocene forstercooperes and the Oligocene eggysodontines. The discovery of Proeggysodon in the upper Eocene deposits of Nei Mongol is a further indication that the European Oligocene form Eggysodon most likely had an Asian ancestry and accordingly represents a case that fits the hypothesis that the European ‘Grande Coupure’ across the Eocene/Oligocene boundary resulted from immigration from Asia”, said Dr. WANG Yuanqing, research designer and corresponding author of the study.

This work was supported by National Basic Research Program of China (973 Program), the National Natural Science Foundation of China, the Basic Work Program of MST of China, the Key Laboratory of Evolutionary Systematics of Vertebrates, and the Special Fund for Fossil Excavation and Preparation, Chinese Academy of Sciences. 

Fig.1 The lower jaw of Proeggysodon qiui (IVPP V 18099); A. line drawing of right lower dentition in occlusal view; B. dorsal view of lower jaw; C. lateral view of lower jaw. (Image by BAI Bin)