Ostracods had colonized estuaries by the late Silurian.

Harrison Bush,Christopher P Stocker,Julien Legrand,C Giles Miller,David J Siveter,Toshihiro Yamada,Anna Mcgairy,Toshifumi Komatsu,Mark Williams,Phong Duc Nguyen,Thomas H P Harvey

doi:10.1098/rsbl.2021.0403

Abstract

The fossil record of terrestrialization documents notable shifts in the environmental and physiological tolerances of many animal and plant groups. However, for certain significant components of modern freshwater and terrestrial environments, the transition out of marine settings remains largely unconstrained. Ostracod crustaceans occupy an exceptional range of modern aquatic environments and are invaluable palaeoenvironmental indicators in the fossil record. However, pre-Carboniferous records of supposed non-marine and marginal marine ostracods are sparse, and the timing of their marine to non-marine transition has proven elusive. Here, we reassess the early environmental history of ostracods in light of new assemblages from the late Silurian of Vietnam. Two, low diversity but distinct ostracod assemblages are associated with estuarine deposits. This occurrence is consistent with previous incidental reports of ostracods occupying marginal and brackish settings through the late Silurian and Devonian. Therefore, ostracods were pioneering the occupation of marginal marine and estuarine settings 60 Myr before the Carboniferous and they were a component of the early phase of transition from marine to non-marine environments.

Highlights

Identifying the physiological adaptations and environmental contexts that allowed marine organisms to colonize brackish and freshwater niches is necessary to understand the development of complex terrestrial ecologies
Ostracod fossils and casts are stored at the Geological Museum, General Department of Geology and Minerals of Vietnam (DGMV), Hanoi: figured specimens are on rock slabs numbered BT1 to BT22
Palynological data indicate the Si Ka Formation is of Silurian, late Ludlow to early Pridoli age [17]

Summary

Introduction

Identifying the physiological adaptations and environmental contexts that allowed marine organisms to colonize brackish and freshwater niches is necessary to understand the development of complex terrestrial ecologies. Ostracods are the most abundant arthropods in the fossil record and occupy basal positions in aquatic trophic networks [1]. Despite their abundance and ubiquity as primary consumers and detritivores in aquatic settings today, and their widespread use as palaeoenvironmental indicators in the fossil record, the timing of their transition from marine to non-marine aquatic niches remains unclear. With fossil evidence of complex terrestrial ecosystems by the late Silurian [2,3,4], including both early vascular plants and predatory arthropods, the apparent absence of contemporaneous ostracods in non-marine aquatic settings presents a conundrum.

Methods

Results

Conclusion