Churkites, a trans-Panthalassic Early Triassic ammonoid genus from South Primorye, Russian Far East

The taxonomy, biostratigraphy and paleobiogeographic origin of Churkites, an Early Triassic acute-ventered arctoceratid ammonoid genus, are reported based on carefully controlled bed-by-bed sampling of several sections in South Primorye, Russian Far East. Churkites syaskoi is herein regarded as a synonym of the type species, C. egregius, which was described from the upper Smithian (loewr Olenekian) Anasibirites Zone of Russia. C. egregius is relatively abundant in sandstone laminae in the laminated mudstone beds of distal turbidites, but rare in the sandstone beds of relatively proximal turbidites. In contrast, Anasibirites is abundant in the sandstone beds, but very rare in the mudstone beds. This disparity in occurrence makes a precise biostratigraphic correlation difficult for the upper Smithian Anasibirites Zone in the mudstone facies, but C. egregius makes it possible to correlate the zone in the mudstone facies. The different modes of occurrence among ammonoid taxa strongly suggest that they either were preserved by different taphonomic processes or had different habitats. Churkites most likely evolved from Arctoceras tuberculatum on the eastern side of the Panthalassa during the middle Smithian and then crossed the Panthalassa and gave rise to C. egregius on the western side of the Panthalassa during the late Smithian. This westward dispersal across the Panthalassa was likely aided by westward equatorial currents as well as by stepping stones provided by the shallow waters around reefs and island terranes.