Systematics and evolution of cyathophylloidid and stauriid rugose corals (Late Ordovician–mid-Silurian)

Rugose corals attributed here to the families Cyathophylloididae and Stauriidae are common and internationally widespread in the Upper Ordovician–Wenlock strata. They were formerly considered as closely related constituents of the Stauriidae. Here, based on the described, mostly type material mainly housed at palaeontological institutions of China and Estonia, as well as a few new specimens, a great many taxa of the two families are systematically revised, and a comparative study of all known records is performed. The study highlights the evolutionary and taxonomic significance of modes of corallite increase. The Cyathophylloididae, consisting of Palaeophyllum, Cyathophylloides, Favistina, Crenulites and Palaeolithostrotion, is defined here as a group of non-dissepimented Stauriida exhibiting exclusively lateral increase. In contrast, the Stauriidae consists of Stauriida characterised by a septal parricidal increase (new term), in which new separating walls are formed from pre-existing septa of the parent. This is in contrast to aseptal parricidal increase (new term), in which pre-existing septa are not involved in the formation of neowalls. Within the Stauriidae, three subfamilies (i.e. the Stauriinae, Paraceriasterinae n. subfam., and Heininae n. subfam.) are recognised based on their essentially different septal increase modes. These are termed here KLAC, KAC, and KA, in which neowalls develop from respectively the two counter-lateral (KL), two alar (A) and cardinal (C) septa, the counter (K), two alar and cardinal septa, and the counter and two alar septa. Two new genera, Heina and Yuina, are proposed and included in the Heininae n. subfam. An updated summary of the stratigraphic distribution of both families is also given, offering insights into their origin and evolution. Cyathophylloidids might have originated from a Favistina-like ancestor during the late Sandbian and diversified rapidly during the Katian. However, only Palaeophyllum, Cyathophylloides and Palaeolithostrotion survived the Late Ordovician Mass Extinction around the Katian–Hirnantian boundary. Few representatives of the former two persisted into the early Llandovery, while those of Palaeolithostrotion continued into the Wenlock. Stauriids might have emerged by the earliest Silurian from ancestral forms whose septa tended to meet at the axis. This was followed by a flourishing of all three subfamilies through much of the Llandovery, which was subsequently succeeded by the extinction of Paraceriasterinae n. subfam. and Heininae n. subfam., along with a substantial decline in Stauriinae during the early Telychian. The surviving stauriinids extended into the late Sheinwoodian (early Wenlock) but did not endure beyond that interval.