Lycopsid Lepacyclotes Emmons from the Middle Triassic of the Ordos Basin, North China and reviews of the genus

Abstract

Lepacyclotes Emmons is one of the most important Early-Middle Triassic lycopsids, a genus that became extinct during the Late Triassic-Jurassic. Due to its relatively short geological ranges, fairly wide geographic distributions, and strict palaeoecological requirements, this genus is important in stratigraphical correlation and palaeoenvironmental interpretation. Although this lycopsid has been studied for more than a century, there are less than 20 species of Lepacyclotes are known. In this paper, we report abundant new materials from the upper Middle Triassic of the Ordos Basin, North China for the first time. Two species are identified: Lepacyclotes zeilleri (Fliche) Retallack and L. ordosensis Deng sp. nov., with the former species yielding numerous in situ megaspores. Precise absolute dating of tuff layers intercalated in the strata bearing Lepacyclotes point to the early Ladinian (Middle Triassic). A comprehensive analysis of the previous data shows that the genus Lepacyclotes originated during the Early Triassic in multiple areas, flourished in late Early Triassic to early Middle, and strongly declined in the Middle Triassic. A few relicts survived in the Late Triassic and Early Jurassic. Lepacyclotes was adapted to salty, seawater-associated, or evaporative environments. Additionally, the generic diagnosis of Lepacyclotes is emended, and the relationships between the genus and its related genera have been discussed.