New Jurassic spinicaudatans from Xinjiang Uygur Autonomous Region of northwestern China and their evolutionary implications

Abstract

New Jurassic clam shrimps have been discovered respectively from the Hettangian lower part of the Badaowan Formation in the Junggar Basin, and the Oxfordian upper part of the Qiketai Formation in the Turpan Basin of Xinjiang, northwestern China. Through the examination under a scanning electron microscope (SEM) of well-preserved specimens we have described a new genus Punctatestheria n. gen., which includes two new species Punctatestheria lianmuqinensis n. gen. n. sp. and Punctatestheria karamayensis n. gen. n. sp. Given their similar punctate carapace ornamentation pattern, the British Bajocian–Bathonian species Euestheria trotternishensis is herein attributed to the new genus. Punctatestheria n. gen. first occurred in the Hettangian and ranged into the Oxfordian. This finding supports the previous hypothesis that punctatestheriids have first derived from the Triassic lineage Aquilonoglypta, and then gave rise to triglyptids in the Bathonian, because in Triglypta three kinds of ornaments have ontogenetically developed on the carapace, the juvenile-stage dorsal part of which is ornamented by evenly distributed puncta.

Introduction

Clam shrimps are large bivalved crustaceans with a chitinous carapace, which is often mineralized as calcium phosphate in the fossil records (Stigall et al., 2008). Extant clam shrimps usually live in ephemeral fresh water environments, like rain pools, rice fields, playas, and shallow lakes (Chen and Shen, 1985, Rohn et al., 2005, Guériau et al., 2016, Schneider and Scholze, 2016). This is consistent with their relatively short lifespan of about several weeks, such as in the recent species Eulimnadia stoningtoensis Berry, 1926, its death occurring about 23 days after an individual hatches from the egg (Chen and Shen, 1985). Although nowadays only four families remain (Brtek, 1997), clam shrimps were very prosperous in the Mesozoic and evolved rapidly (Chen et al., 2007, Li et al., 2009a, Li et al., 2009d, Li et al., 2010, Scholze et al., 2015, Scholze et al., 2016). This made them a very useful fossil group for the biostratigraphic subdivision of Mesozoic non-marine strata (Li et al., 2006, Li et al., 2007a, Li et al., 2014b, Kozur and Weems, 2010, Gallego et al., 2011, Gallego et al., 2013, Teng et al., 2016), and the reconstruction of a freshwater ecosystem (Boukhalfa et al., 2015, Li et al., 2017).

Xinjiang Uygur Autonomous Region is one of the most important fossil localities for Jurassic clam shrimps in China (Chen, 2003, Li and Matsuoka, 2012). In northern Xinjiang, three abundant and diverse Jurassic clam shrimp faunas have been recognized: (1) an Early Jurassic Palaeolimnadia baitianbaensis fauna from the Badaowan and Sangonghe formations in the Junggar Basin (Fu, 1998, Deng et al., 2010); (2) a Middle Jurassic Triglypta Fauna from the lower Toutunhe Formation in the Junggar Basin (Wang, 1985); (3) a Middle–Late Jurassic Sinokontikia fauna from the Qiketai Formation in the Turpan Basin (Novojilov, 1958, Wang, 1985, Li and Matsuoka, 2012). Triglypta Wang, 1984 was an important component of the well-known Middle–Late Jurassic Yanliao Biota (Wang and Wang, 2010, Zhou et al., 2010, Lü and Bo, 2011, Lü et al., 2011, Liu et al., 2012, Xu et al., 2011, Xu et al., 2016, Peng et al., 2012); it was later recovered in the Junggar Basin, and further in the Middle Jurassic rocks of Mongolia (Li et al., 2014a). Wang (2014) proposed that Triglypta manasica Wang, 1985 phylogenetically derived from the British species Euestheria trotternishensis Chen and Hudson, 1991 in the Bathonian. In order to search for the ancestral form of the triglyptids in the Lower and Middle Jurassic rocks we carried out a field work in northern Xinjiang in 2015. We have recovered well-preserved clam shrimp specimens respectively from the Junggar and Turpan basins (Fig. 1). Through the examination of specimens under an SEM we carried out a taxonomic study on the newly collected clam shrimps. We have described new clam shrimp taxa, and further discussed the origin of triglyptids as reported below.