Lunulite bryozoan biogeography - a convergent global success with a distinct Western Australian twist

Abstract

Lunulites are a polyphyletic group of marine bryozoans that have been a conspicuous element of marine shelf faunas since the Late Cretaceous into the present day. They are easily recognizable by their domed colony form and free-living mode of life on sea floor sediments. Here we explore the waxing and waning of the major lunulitiform groups and their unique morphology and mode of life from the Cretaceous to the present day. Because relatively few and simple modifications are needed to transition from an encrusting form into this highly specialized lifestyle, shared colonial features are rampant and we find examples of both convergent and iterative evolution across several unrelated clades, although detailed phylogenetic relationship remain largely unresolved. The early chapter of the lunulite story is focused on the Late Cretaceous European Chalk Sea, which appears to have been a crucible for the evolution of lunulites. At least six, and likely more, cheilostome groups independently evolved the free-living mode of life in this tropical shelf area. To what extent any of these free-living clades gave rise to post-Cretaceous groups remains unclear. The Cenozoic chapter is more complex, comprising at least three independently evolved major clades, two of which are extant; (1) the Lunulitidae s. str., a North American/European cluster, comprising the classic Lunulites, which became extinct in the late Neogene, (2) the Cupuladriidae, which reached circum-global tropical and sub-tropical distribution in the Miocene, and (3) the Austral lunulite cluster, which is almost certainly polyphyletic and through most of its history confined to Australia and New Zealand, bar a comparatively brief colonization of the southern part of South America, with the earliest representatives from NW Western Australia.