Upper Devonian (Famennian) and Lower Carboniferous (Tournaisian) ammonoids from western Junggar, Xinjiang, northwestern China-stratigraphy, taxonomy and palaeobiogeography

Seven ammonoid levels/assemblages are recognised in the Devonian (Famennian) and Carboniferous (Tournaisian) of the northwestern Junggar Basin, Xinjiang, northwestern China. Eleven genera and 20 species, including six new species (Becanites sp. nov., Protactoclymenia junggarensis sp. nov., Prot. magna sp. nov., Cyrtoclymenia? parallela sp. nov., Sporadoceras impressum sp. nov. and Mimimitoceras transiens sp. nov.) are described from the Hongguleleng and Heishantou formations in the Bulongguoer, Hebukehe, Emuha and Hashatu sections. The first assemblage is characterised by Platyclymenia subnautilina (Sandberger), Prionoceras frechi (Wedekind) and Sp. impressum sp. nov. It is assigned to the basal upper Famennian Pl. annulata Zone (UD IV-A) and indicates that the global, transgressive Annulata Event enabled the first spread of ammonoids to the region. The second assemblage is characterised by large-sized Cyrtoclymeniaceae (Protactoclymenia and Cyrtoclymenia?) and Sporadoceras aff. muensteri (Munster) and correlates with the orbiculare Zone (UD IV-C) of North Africa, which also contains very large clymeniids and goniatites. The third regional assemblage consists of rare Gonioclymenia sp. and a related, more evolute form. The fourth assemblage is a peculiar Cymaclymenia Mimimitoceras fauna, including C. striata (Munster) s.l., C. cf. involvens Lange, Sporadoceras sp., Mimimitoceras cf. geminum Korn, M. transiens sp. nov. and M. cf. rotersi Korn, most of which are discovered for the first time in the vast Junggar and adjacent areas. Based on conodont (associated Protognathodus fauna) and palynomorph data (LN Zone), this assemblage corresponds to the lower Hangenberg Event Interval (UD VI-E). Rare records from a fifth level suggest that cymaclymeniids survived regionally into the conodont Pr. kockeli Zone, as in other regions. The sixth level consists of a single Lower Tournaisian Acutimitoceras (Stockumites)?. The seventh level also has a low diversity and includes Weyerella angularia (Liang and Wang) and Becanites sp. nov. in the Emuha section. It falls in the Middle Tournaisian, as do Weyerella faunas that were previously reported from the region. The endemic species and the distinctive lack of otherwise widespread genera provide a strong regional signature of the overall ammonoid fauna.