Renaudie, J. | Search bioRxiv

bioRxiv2024

Diversity and evolution of Radiolaria: Beyond the stars of the ocean

Since Ernst Haeckel and the Challenger expedition (1872-1876), Radiolaria have been known as ubiquitous and abundant star-shaped oceanic plankton. Their exquisite biomineralized skeletons have left an extensive fossil record which is extremely valuable for biostratigraphic and paleo-environmental research. In contemporary oceans, there is growing evidence that Radiolaria are significant contributors to marine food webs and global biogeochemical cycles. Here we provide a comprehensive morpho-molecular framework to assess the extant diversity, biogeography and evolutionary history of Radiolaria. Our analyses reveal that half of radiolarian diversity is morphologically undescribed, with a large part forming three hyper-diverse environmental clades, named Rad-A, Rad-B and Rad-C. We suggest that most of this undescribed diversity likely comprises skeleton-less life forms or endosymbionts, explaining their elusive nature. Phylogenetic analyses highlight the need for major revision of high-level Radiolaria taxonomy, including placement of the Collodaria within the order Nassellaria. Fossil calibration of a molecular clock revealed the first appearance of Radiolaria [~]760 million years ago (Ma), the development of the skeleton in the early Paleozoic ([~]500 Ma) and the onset of photosymbiotic relationships during the mid to late Mesozoic ([~]140 Ma), related to geological periods of oligotrophy and anoxia. The results presented here provide an extensive and robust framework for developing new perspectives on early eukaryotic diversification, paleo-environmental impacts on plankton evolution, and marine microbial ecology in rapidly evolving ecosystems.

evolutionary biology↗

bioRxiv2023

Response of siliceous marine organisms to the Permian-Triassic climate crisis based on new findings from central Spitsbergen, Svalbard

Siliceous marine ecosystems play a critical role on the Earths climate system through its influence on organic carbon burial and rates of marine authigenic clay formation (i.e. reverse weathering). The ecological demise of silicifying organisms associated with the Permian-Triassic mass extinction is postulated to have elevated rates of marine authigenic clay formation, resulting in a prolonged greenhouse climate during the Early Triassic. Yet, our understanding of the response of siliceous marine organisms during this critical interval is poor. Whilst radiolarians experienced the strongest diversity loss in their evolutionary history and perhaps also the greatest population decline of silica-secreting organisms during this event, only a small number of Griesbachian (post-extinction) localities that record siliceous organisms are known. Here, we report newly discovered latest Changhsingian to early Griesbachian (Clarkina meishanensis - Hindeodus parvus Zone) radiolarians and siliceous sponge spicules from Svalbard. This fauna documents the survival of a low-diversity radiolarian assemblage alongside stem-group hexactinellid sponges making this the first described account of post-extinction silica-secreting organisms from the Permian/Triassic boundary in a shallow marine shelf environment and a mid-northern palaeolatitudinal setting. Our findings indicate that latitudinal diversity gradients for silica-secreting organisms following the mass extinction were significantly altered, and that silica productivity was restricted to high latitude and deep water thermal refugia. This result has potential to further shape our understanding of changes to marine porewater and seawater dissolved silica levels and in turn rates of reverse weathering, with implications for our understanding of carbon cycle dynamics during this interval. This also suggests that the export of organic carbon to the deep ocean was not as severely impacted at non-equatorial latitudes. Key PointsO_LIWe document the first occurrence of siliceous sponge spicules and radiolarians (biogenic silica) from a mid-northern paleolatitude following the mass extinction event C_LIO_LIHoldover radiolarian species show poleward range shifts C_LIO_LIThe ecological composition and the restriction to shallow water oxygenated facies suggests a shallow mid-latitude refuge for siliceous marine organisms C_LIO_LIThis result has potential to further shape our understanding of changes to marine dissolved silica levels and in turn rates of reverse weathering, with implications for our understanding of Permian-Triassic carbon cycle dynamics. C_LI

evolutionary biology↗