Wednesday, March 9, 2022

[PaleoIchthyology • 2022] Paleoschizothorax diluculum • A New Cyprinid (Cypriniformes: Cyprinidae) from the Oligocene of Qaidam Basin, north-eastern Tibetan Plateau, and Its Implications

Paleoschizothorax diluculum
 Yang, Liang, Cai, Gu, Han, Chen, Wang, Bao & Defei, 2022

The Qaidam Basin, lying in the north-eastern Tibetan Plateau, China, is key to understanding the Cenozoic climatic and biological changes that have occurred on the plateau; however, information from a palaeontological perspective on this Palaeogene basin is scant. Recently, fossil cyprinids were found in the middle portion of the Shangganchaigou Formation (= Upper Ganchaigou Formation; Oligocene) in the north-western Qaidam Basin. These share many osteological characters with barbines and ‘morphologically primitive clade schizothoracines’ (= primitive schizothoracines; i.e. Schizothorax and Aspiorhynchus), and they closely resemble the latter in the size and shape of the scales: body scales are small and oval; the number of lateral line scales exceeds 100; and the long-oval scales from the pre-anal region are very similar to the ‘anal scales’, which are unique to schizothoracines. Compared with another fossil fish, Paleoschizothorax qaidamensis, from the same formation, the major differences are: (1) the posterior part of the entopterygoid in the new material is normal and not expanded while that of P. qaidamensis is obviously expanded; and (2) the premaxilla process of maxilla is well developed in P. qaidamensis but weak in the new specimens. Therefore, they are described as a new species of the fossil genus Paleoschizothorax (subfamily Schizothoracinae): Paleoschizothorax diluculum sp. nov. A phylogenetic analysis, which included 13 extant genera and three fossil forms of barbines and schizothoracines and 70 morphological characters, also supports the close relationship between P. diluculum and primitive schizothoracines. A preliminary correlation analysis suggests that the degree of reduction of body scale size is negatively correlated with habitat mean temperatures among most Chinese cyprinids. Moreover, we speculate that the ‘primitive schizothoracines’ likely originated in the north-eastern–central Tibetan Plateau based on the fossil records and molecular phylogeny of the extant taxa.
Keywords: Tibetan Plateau, Qaidam Basin, fossil cyprinids, palaeoenvironment, Oligocene

Paleoschizothorax diluculum sp. nov.
 A, holotype HTG18021a, head to right; B, counterpart HTG18021b, head to left;
C, opercle, HTG19001; D, cleithrum, HTG19007;
E, F, two entopterygoids, left, in lateral view, HTG19010 and HTG21023.
 Scale bars: A, B = 10 mm; C-F = 5 mm.

Systematic palaeontology 
Superorder Ostariophysi Sagemehl, 1885 
Order Cypriniformes Bleeker, 1859/60 

Family Cyprinidae Bonaparte, 1840 
Subfamily Schizothoracinae Berg, 1912 
(= Lineage Schizothoracini Howes, 1991) 

Genus Paleoschizothorax Yang et al., 2018 
Paleoschizothorax diluculum sp. nov. 
Etymology. The species name ‘diluculum’, from the Latin, means first light of the day, referring to the expectation of more discoveries about the evolutionary implications of primitive schizothoracines.

Locality and horizon. Fossil locality HTG F27, about 15 km north-east of the town of Huatugou, north-western Qaidam Basin, China. Middle part of the Shangganchaigou Formation, Oligocene (Fig. 1).

Tao Yang, Weiyu Liang, Jiahao Cai, Haoran Gu, Lei Han, Hongyu Chen, Haojian Wang, Lin Bao and Defei. 2022. A New Cyprinid from the Oligocene of Qaidam Basin, north-eastern Tibetan Plateau, and Its Implications. Journal of Systematic Palaeontology. DOI: 10.1080/14772019.2021.2015470  
Tao Yang, Li Zhang, Wenjia Li, et al. 2018. New schizothoracine from Oligocene of Qaidam Basin, northern Tibetan Plateau, China, and its significance. Journal of Vertebrate Paleontology. 38, e1442840. DOI: 10.1080/ 02724634.2018.1442840