Tuesday 20 December 2022

Borissiakia huangheensis: A Schizotheriine Chalicothere from the Early Miocene Xianshuihe Formation of Gansu Province, China.

The Chalicotheres are an unusual group of extinct Perissodactyls known from Oligocene and Early Miocene deposits in Europe, Asia, North America, and Africa. Unlike other Perissodactyls they lacked hooves, instead having claws on their limbs similar to those of carnivorous Mammals, although these were combined with a dentition typical of herbivorous Perissodactyls. The Chalicotheres can be divided into two groups, the Chalicotheriinae and Schizotheriinae, based upon their dental and postcranial characters.

In a paper published in the Journal of Mammalian Evolution on 1 September 2022, Zhaoyu Li of the State Key Laboratory of Continental Dynamics at Northwest University, and the Department of Palaeobiology at the Swedish Museum of Natural HistoryThomas Mörs, also of the Swedish Museum of Natural History, and of the Bolin Centre for Climate Research at Stockholm University, and Yunxiang Zhang, Kun Xie, and Yongxiang Li, also of the State Key Laboratory of Continental Dynamics at Northwest University, describe the mandible of a Schizotheriine Chalicothere from the Early Miocene Xianshuihe Formation of the Lanzhou Basin in Gansu Province, China.

The specimen described, NWU V 1542 is a left mandible belonging to an adult individual, preserving diastema, the roots of  premolar 2 to molar 1 and the crowns of molars 3 and three; the symphysis and rostrum are broken. 

Left mandible of NWU V 1542 from Huangyangtou village, Lanzhou Basin: (a) labial view; (b) lingual view; (c) occlusal view. Scale bar equals 5 cm. Li et al. (2022).

NWU V 1542 closely resembles the mandible of Borissiakia betpakdalensis, a Schizotheriine Chalicothere known from sandstone outcrops on the Golodnaya Steppe of Southern Kazakhstan, which have been dated to the Late Oligocene or Early Miocene. However, the new specimen is significantly smaller. This is not necessarily a problem in a Schizotheriine Chalicothere, a group known to have shown significant sexual dimorphism. However, two size morphs, thought to represent different sexes, are known for Borissiakia betpakdalensis, with a specimens, including some interpreted as subadults,  being significantly larger than NWU V 1542. 

NWU V 1542 also resembles another Schizotheriine Chalicothere, IVPP V 9959, known from the Xianshuihe Formation. This specimen, described as Phyllotillon huangheensis, was first described from an outcrop close to the one which produced NWU V 1542 in 1968, with the differences between then within the bounds of predicted intraspecific variation.

Since IVPP V 9959 also resembles closely, Li et al. conclude that the name Phyllotillon huangheensis is in fact invalid, and that IVPP V 9959 actually represents a new species of Borissiakia, which they identify as Borissiakia huangheensis. NWU V 1542 is also referred to this new combination.

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