Highly dynamic marine redox state through the Cambrian explosion highlighted by authigenic δ²³⁸U records

The history of oceanic oxygenation from the late Neoproterozoic to the early Cambrian is currently debated, making it difficult to gauge whether, and to what extent environmental triggers played a role in shaping the trajectory of metazoan diversification. Uranium isotope (U) records from carbonates have recently been used to argue for significant swings in the global marine redox state from the late Neoproterozoic to the early Cambrian. However, geochemical signatures in carbonates—the U isotope archive most commonly employed to argue for redox shifts—are susceptible to diagenetic alteration and may have variable offsets from seawater values. Therefore, there is an impetus to reconstruct seawater U isotopic evolution using another sedimentary archive, in order to verify that these excursions can indeed be linked to global shifts in marine redox landscape. Here we report new U isotope data from two fine-grained siliciclastic upper Ediacaran to lower Cambrian (ca. 551–515 Ma) successions in South China. We find large δ²³⁸U swings between -0.63‰ and +0.39‰ for calculated values of authigenic U in the siliciclastic rocks, consistent with correlative records from the carbonates. The replication of these patterns in both carbonate and siliciclastic units provides confirmatory evidence that the early Cambrian seawater was characterized by highly variable U isotope compositions. These new δ²³⁸U data also provide higher-resolution records of global oceanic redox conditions during Cambrian Age 3, coeval with a critical interval of the Cambrian explosion. These δ²³⁸U data bolster the case that the Ediacaran-Cambrian transition experienced massive swings in marine redox state, providing a dynamic environmental backdrop for and potentially even a key driver of the emergence and radiation of metazoans

Early Palaeozoic ocean anoxia and global warming driven by the evolution of shallow burrowing

The evolution of burrowing animals forms a defining event in the history of the Earth. It has been hypothesised that the expansion of seafloor burrowing during the Palaeozoic altered the biogeochemistry of the oceans and atmosphere. However, whilst potential impacts of bioturbation on the individual phosphorus, oxygen and sulphur cycles have been considered, combined effects have not been investigated, leading to major uncertainty over the timing and magnitude of the Earth system response to the evolution of bioturbation. Here we integrate the evolution of bioturbation into the COPSE model of global biogeochemical cycling, and compare quantitative model predictions to multiple geochemical proxies. Our results suggest that the advent of shallow burrowing in the early Cambrian contributed to a global low-oxygen state, which prevailed for ~100 million years. This impact of bioturbation on global biogeochemistry likely affected animal evolution through expanded ocean anoxia, high atmospheric CO2 levels and global warming

Bioturbation feedbacks on the phosphorus cycle

Bioturbation—sediment mixing and ventilation by burrowing animals—provides one of the most prominent examples of how animals shape their surroundings. A critical but longstanding question is when and how in Earth's history bioturbators began to similarly influence marine biogeochemistry. Recent work has proposed that even though the development of well-mixed sediments was a protracted process, the initial rise of bioturbators led to a decrease in marine phosphate levels, global productivity crises and ultimately deoxygenation events in the early Cambrian oceans. Herein, using a diagenetic model, we provide a new view of how bioturbators impact the global phosphorus cycle. Our work focuses on incorporating more realistic representation of bioturbation with a more complete and mechanistic parameterization of benthic phosphorus cycling than has previously been used to explore the early Paleozoic. We find, in contrast to previous modeling studies, that bioturbation does not uniformly or unidirectionally mediate increased phosphorus burial. Enhanced biodiffusion of sedimentary particles can mediate enhanced phosphorus burial. In contrast, bioirrigation—nonlocal transport of solutes via burrows—enhances phosphorus recycling and may therefore stimulate rather than stymy productivity. Given evidence from the geologic record that bioirrigation rather than biodiffusion was predominant during the ramping up of bioturbation (through the early Paleozoic), the emergence of bioturbation is unlikely to have driven deoxygenation events

The ‘Tully monster’ is a vertebrate

Problematic fossils, extinct taxa of enigmatic morphology that cannot be assigned to a known major group, were once a major issue in palaeontology. A long-favoured solution to the ‘problem of the problematica’1, particularly the ‘weird wonders’2 of the Cambrian Burgess Shale, was to consider them representatives of extinct phyla. A combination of new evidence and modern approaches to phylogenetic analysis has now resolved the affinities of most of these forms. Perhaps the most notable exception is Tullimonstrum gregarium3, popularly known as the Tully monster, a large soft-bodied organism from the late Carboniferous Mazon Creek biota (approximately 309–307 million years ago) of Illinois, USA, which was designated the official state fossil of Illinois in 1989. Its phylogenetic position has remained uncertain and it has been compared with nemerteans4,5, polychaetes4, gastropods4, conodonts6, and the stem arthropod Opabinia4. Here we review the morphology of Tullimonstrum based on an analysis of more than 1,200 specimens. We find that the anterior proboscis ends in a buccal apparatus containing teeth, the eyes project laterally on a long rigid bar, and the elongate segmented body bears a caudal fin with dorsal and ventral lobes3,4,5,6. We describe new evidence for a notochord, cartilaginous arcualia, gill pouches, articulations within the proboscis, and multiple tooth rows adjacent to the mouth. This combination of characters, supported by phylogenetic analysis, identifies Tullimonstrum as a vertebrate, and places it on the stem lineage to lampreys (Petromyzontida). In addition to increasing the known morphological disparity of extinct lampreys7,8,9, a chordate affinity for T. gregarium resolves the nature of a soft-bodied fossil which has been debated for more than 50 years.</p

The ‘Tully monster’ is a vertebrate

Problematic fossils, extinct taxa of enigmatic morphology that cannot be assigned to a known major group, were once a major issue in palaeontology. A long-favoured solution to the ‘problem of the problematica’1, particularly the ‘weird wonders’2 of the Cambrian Burgess Shale, was to consider them representatives of extinct phyla. A combination of new evidence and modern approaches to phylogenetic analysis has now resolved the affinities of most of these forms. Perhaps the most notable exception is Tullimonstrum gregarium3, popularly known as the Tully monster, a large soft-bodied organism from the late Carboniferous Mazon Creek biota (approximately 309–307 million years ago) of Illinois, USA, which was designated the official state fossil of Illinois in 1989. Its phylogenetic position has remained uncertain and it has been compared with nemerteans4,5, polychaetes4, gastropods4, conodonts6, and the stem arthropod Opabinia4. Here we review the morphology of Tullimonstrum based on an analysis of more than 1,200 specimens. We find that the anterior proboscis ends in a buccal apparatus containing teeth, the eyes project laterally on a long rigid bar, and the elongate segmented body bears a caudal fin with dorsal and ventral lobes3,4,5,6. We describe new evidence for a notochord, cartilaginous arcualia, gill pouches, articulations within the proboscis, and multiple tooth rows adjacent to the mouth. This combination of characters, supported by phylogenetic analysis, identifies Tullimonstrum as a vertebrate, and places it on the stem lineage to lampreys (Petromyzontida). In addition to increasing the known morphological disparity of extinct lampreys7,8,9, a chordate affinity for T. gregarium resolves the nature of a soft-bodied fossil which has been debated for more than 50 years.</p