VI - The British Silurian Rugose Corals with Acanthine Septa

Abstract

This paper seeks to elucidate the phylogeny of those British Silurian Rugose Corals which have acanthine septa, i.e ., whose septa can be seen by the naked eye each to consist of a row of spines. Laboratory studies have been supported by field work, with the object of discovering the range of the species in time and space, and the conditions of life under which they lived ; and of determining whether the evolution of any character received impetus from a change in such conditions. It would appear that ancestors of that Rugose coral fauna which became dominant in the Wenlock, entered the British area in Llandovery times. The Streptelasmoid fauna characteristic of the Ordovician first shows intermixture with Wenlock forms at the top of the Lower Llandovery beds of Llandovery, where Calostylis occasionally occurs. In the Upper Llandovery the Calostylidae increase, and the Acanthocyclidae and primitive members of the Pycnactis-Phaulactis lineage appear. From Pycnaclis a large proportion of the Wenlock and Ludlow Rugose corals descended. Streptelasma seems to have died out at the top of the Wenlock. In the shaly facies of the British Wenlock-Ludlow, a characteristic fauna of Pycnactis mitrata, Syringaxon siluriensis, Acanthocyclus spp. and Spongophylloides spp. slowly evolved. When more calcareous conditions appeared, this was enriched by new forms. With the reef facies of the Wenlock limestone, a large number of new species appeared. The British area is a most interesting, though not the most useful, area for the study of the evolution of the Silurian Rugose corals, for here reef conditions were confined to the top of the Wenlock. It has to be decided which new forms evolved from species already present in the area, and which came in from other areas with the reef conditions. Such migrations can only be proved by reference to Scandinavia and N. America, where reef conditions occurred throughout the Silurian. In the present study such migrations cannot be taken into account, because field work has so far been confined to the British area.