The glacially induced Middle Ordovician rise of the Plectambonitoidea (Brachiopoda) and the phylogenetic placement of the Baltic genus Ukoa

Abstract

The rare Middle Ordovician brachiopod genus Ukoa has remained unassigned to any existing family. The original description by Öpik in 1932 was based primarily on the external morphology; all later revisions of the genus also lack information regarding the interior of the dorsal valve, as well as information on the shell ultrastructure. As these are critical morphological features for designating a placement within the superfamily Plectambonitoidea – or indeed the order Strophomenida – not even a higher-level taxonomical determination has hitherto been attempted for Ukoa. The current study has examined new material, as well as museum specimens, that allowed for an analysis of the shell ultrastructure confirming the genus placement within the plectambonitoids. Moreover, as dorsal cardinalia are preserved in one of the new specimens, we further place Ukoa within the family Taffiidae Schuchert and Cooper, 1931, assigned to a new subfamily, the Ukoainae n. subfam. This corresponds well with the stratigraphic range of the Taffiidae, and as such, Ukoa, therefore represents yet another short-ranged plectambonitoid genus that originated during the Middle Ordovician Darriwilian Stage. As this coincides with both the onset of the main phase of the Great Ordovician Biodiversification Event (GOBE) and suddenly cooling climate, the current study also reviews the early diversification of the Plectambonitoidea within that context. Based on a specific focus on Baltica, we suggest that the unusual morphology that characterizes the plectambonitoids could be a particularly successful functional morphological adaptation to the unfolding Darriwilian ice age. As sea level dropped – possibly in the same order of magnitude as that experienced during the Quaternary glaciations – increased siliciclastic input washed onto the epicontinental seas globally providing muddy substrates, and thus new ecospace for the plectambonitoids to exploit and utilize during the early Darriwilian sea-level lowstands. The millennial glacioeustatic oscillations further accelerated plectambonitoid diversity through multiple allopatric speciation events, contributing to the overall accumulation of biodiversity that characterises the GOBE.