Abstract
Deperetellidae is a clade of peculiar, Asian endemic tapiroids from the early and middle Eocene. The previously published material mainly comprises maxillae, mandibles, and some postcranial elements. However, the absence of cranial materials and primitive representatives of the deperetellids obscures their phylogenetic relationships within Tapiroidea. Furthermore, derived deperetellids have completely molarized premolars, but the pattern of their evolution remains unclear. Here, we report a nearly complete skull and some carpals of a new basal deperetellid tapiroid, Irenolophus qii gen. et sp. nov., from the late early Eocene of the Erlian Basin, Inner Mongolia, China. We suggest that deperetellids (along with Tapiridae) probably also arose from some basal ‘helaletids’, based on the reduced, flat, lingually depressed metacones on the upper molars, the trend towards the bilophodonty on the lower molars, and a shallow narial notch with the premaxilla in contact with the nasal. The molarization of the premolars in Deperetellidae from Irenolophus through Teleolophus to Deperetella was initiated and gradually enhanced by the separation between the paraconule and the protocone. That pattern differs from the protocone-hypocone separation in helaletids, tapirids, and most rhinoceroses, and the metaconule-derived pseudohypocone in amynodontids. However, the specific relationship of deperetellids within Tapiroidea and the roles of different patterns of premolar molarization in perissodactyl evolution need further and comprehensive study.
Highlights
Deperetellidae is a group of endemic Asian tapiroids known from Eocene deposits in China, Mongolia, Kyrgyzstan, and Myanmar [1,2,3]
On the basis of the evolutionary lineage from the new primitive deperetellid through Teleolophus to Deperetella, we propose a special premolar molarization process through the separation of the paraconule and protocone in Deperetellidae, and discuss the distribution of this pattern in other perissodactyls
Most optical photographs were taken with magnifications under 20X to identify the Hunter-Schreger Bands (HSB) and Schmelzmuster
Summary
Deperetellidae is a group of endemic Asian tapiroids known from Eocene deposits in China, Mongolia, Kyrgyzstan, and Myanmar [1,2,3]. Deperetellid fossils are relatively abundant and common in the Eocene Asian deposits, their phylogenetic relationships with other tapiroids have long been remained controversial. We report a nearly complete skull and some carpals of a new primitive deperetellid from the late early Eocene of the basal Arshanto Formation of the Erlian Basin, Inner Mongolia, China, which sheds light on the origin of deperetellids and their phylogenetic position among tapiroids. On the basis of the evolutionary lineage from the new primitive deperetellid through Teleolophus to Deperetella, we propose a special premolar molarization process through the separation of the paraconule and protocone in Deperetellidae, and discuss the distribution of this pattern in other perissodactyls. Two other patterns of premolar molarization in perissodactyls are investigated and compared with that of deperetellids
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