Early Cambrian Animals Research Articles

The Emu Bay Shale: A unique early Cambrian Lagerstätte from a tectonically active basin.

The Emu Bay Shale (EBS) of South Australia is anomalous among Cambrian Lagerstätten because it captures anatomical information that is rare in Burgess Shale-type fossils, and because of its inferred nearshore setting, the nature of which has remained controversial. Intensive study, combining outcrop and borehole data with a compilation of >25,000 fossil specimens, reveals that the EBS biota inhabited a fan delta complex within a tectonically active basin. Preservation of soft-bodied organisms in this setting is unexpected and further underscores differences between the EBS and other Cambrian Lagerstätten. Environmental conditions, including oxygen fluctuations, slope instability, high suspended sediment concentrations, and episodic high-energy events, inhibited colonization of the lower prodelta by all but a few specialist species but favored downslope transportation and preservation of other largely endemic, shallow-water benthos. The EBS provides extraordinary insight into early Cambrian animal diversity from Gondwana. These results demonstrate how environmental factors determined community composition and provide a framework for understanding this unique Konservat-Lagerstätte.

Multiple S-isotopic constraints on paleo-redox and sulfate concentrations across the Ediacaran-Cambrian transition in South China

Changes in oceanic chemical conditions are frequently invoked as a primary driver of biodiversification and mass extinction events in Earth history. During the Ediacaran–Cambrian transition, two major biological events, including the extinction of the Ediacaran biota and the Cambrian explosion, occurred approximately coincident with perturbations of the carbon and sulfur cycle. These dramatic biological changes and coeval geochemical events are commonly thought to be linked, but their causal relationships remain controversial. Here we present multiple S-isotopic compositions of pyrite (δ34Spy and Δ33Spy) from the Xiaotan section spanning the Ediacaran-Cambrian transition interval in South China. Our isotopic results show that negative Δ33Spy values, together with positive δ34Spy values, occurred across the Ediacaran-Cambrian boundary. The negative Δ33Spy data suggest mixing of sulfide from deep waters during the upwelling of sulfidic waters into oxygenated shallow waters. The coincidence between the negative Δ33Spy and the approximate timing of the extinction of the Ediacaran biota implies that paleo-redox perturbations may have contributed to the disappearance of the Ediacaran biota. Negative Δ33Spy values coupled with positive δ34Spy values were also observed in the Cambrian Zhongyicun Member when the phosphorites were deposited, suggesting a strong linkage between the upwelling of nutrient-rich sulfidic deep waters and the deposition of the phosphorites in oxygenated shelf areas. In addition, using box models and stoichiometry calculations, we quantitatively estimated elevated burial rates of organic matter and pyrite based on the parallel positive δ13Ccarb, δ34Spy and Δ33Spy excursions observed at Xiaotan within Cambrian Stage 2. Our mass balance modelling results provide an independent constraint on sulfate concentrations which were 4 mM or lower in the early Cambrian oceans. The probable increase in oxygen levels resulting from the enhanced burial of pyrite and organic matter could have provided a more habitable environment for early animals and thus paved the way for the radiation of early Cambrian animals.

Early Cambrian animal diapause embryos revealed by X-ray tomography

Early Cambrian animal diapause embryos revealed by X-ray tomography

Heterogenous oceanic redox conditions through the Ediacaran-Cambrian boundary limited the metazoan zonation

Recent studies have enhanced our understanding of the linkage of oxygenation and metazoan evolution in Early Cambrian time. However, little of this work has addressed the apparent lag of animal diversification and atmospheric oxygenation during this critical period of Earth history. This study utilizes the geochemical proxy and N isotope record of the Ediacaran–Cambrian boundary preserved in intra-shelf basin, slope, and slope basin deposits of the Yangtze Sea to assess the ocean redox state during the Early Cambrian metazoan radiation. Though ferruginous conditions appear to have prevailed through the water column during this time, episodes of local bottom-water anoxia extending into the photic-zone impacted the slope belt of the basin. Heterogenous oceanic redox conditions are expressed by trace element concentrations and Fe speciation, and spatial variation of N isotopes. We propose that the coupling of ocean chemistry and Early Cambrian animal diversification was not a simple cause-and-effect relationship, but rather a complex interaction. Specifically, it is likely that animal diversification expanded not only temporally but also spatially from the shallow shelf to deep-water environments in tandem with progressive oxygenation of the extensive continental margin.

A highly redox-heterogeneous ocean in South China during the early Cambrian (∼529–514 Ma): Implications for biota-environment co-evolution

Abstract The “Cambrian Explosion” is known for rapid increases in the morphological disparity and taxonomic diversity of metazoans. It has been widely proposed that this biological event was a consequence of oxygenation of the global ocean, but this hypothesis is still under debate. Here, we present high-resolution Fe–S–C–Al-trace element geochemical records from the Jinsha (outer shelf) and Weng'an (outer shelf) sections of the early Cambrian Yangtze Platform, integrating these results with previously published data from six correlative sections representing a range of water depths (Xiaotan, Shatan, Dingtai, Yangjiaping, Songtao, and Longbizui). The integrated iron chemistry and redox-sensitive trace element data suggest that euxinic mid-depth waters dynamically coexisted with oxic surface waters and ferruginous deep waters during the earliest Cambrian, but that stepwise expansion of oxic waters commenced during Cambrian Stage 3 ( ∼ 521 – 514 Ma ). Combined with data from lower Cambrian sections elsewhere, including Oman, Iran and Canada, we infer that the global ocean exhibited a high degree of redox heterogeneity during the early Cambrian, consistent with low atmospheric oxygen levels ( ∼ 10 – 40 % of present atmospheric level, or PAL). A large spatial gradient in pyrite sulfur isotopic compositions (δ34Spy), which vary from a mean of − 12.0 ‰ in nearshore areas to + 22.5 ‰ in distal deepwater sections in lower Cambrian marine units of South China imply low concentrations and spatial heterogeneity of seawater sulfate, which is consistent with a limited oceanic sulfate reservoir globally. By comparing our reconstructed redox chemistry with fossil records from the lower Cambrian of South China, we infer that a stepwise oxygenation of shelf and slope environments occurred concurrently with a gradual increase in ecosystem complexity. However, deep waters remained anoxic and ferruginous even as macrozooplankton and suspension-feeding mesozooplankton appeared during Cambrian Stage 3. These findings suggest that the “Cambrian Explosion” in South China may have been primarily a consequence of locally improved oxygenation of the ocean-surface layer rather than of the full global ocean. Our observations are inconsistent with predicted changes in ocean chemistry driven by early Cambrian animals, suggesting that the influence of early Cambrian animals on contemporaneous ocean chemistry, as proposed in previous studies, may be overly exaggerated.

The Role of Red Algae in Abrupt Appearance of Animals in the Early Cambrian on Siberian Platform

Calcified rock-forming marine filamented benthic micro- and macrophytes of Dzhelindia Kolosov, 1970 (Figure 1(а)) and Chaptchaica Kolosov, 1975 (Figure 1(b)) genera, thrived in Neoproterozoic (1000-541 million years ago) Siberia together with cyanobacteria. Their belonging to Rhodophyta is well-grounded. These and other Neoproterozoic rodofits, as well as Epiphyton Bornemann 1886 (Figure 1(c)) and other Early Cambrian (541-513 million years ago) benthic algae, oxygenating floor of epicontinental sea basin, formed conditions, favorable for the animals. In combination with other biotic, and such abiotic factors as: equatorial position and tectonic feature of Siberian platform; volcanism; transgression of the sea; diversity of ecologic niches, these conditions were prerequisites of skeleton and shell acquisition by Early Cambrian animals, and as a consequence, accelerated morphologic evolution, increased diversity of taxons of different groups of animals.

Gut contents as direct indicators for trophic relationships in the Cambrian marine ecosystem.

Present-day ecosystems host a huge variety of organisms that interact and transfer mass and energy via a cascade of trophic levels. When and how this complex machinery was established remains largely unknown. Although exceptionally preserved biotas clearly show that Early Cambrian animals had already acquired functionalities that enabled them to exploit a wide range of food resources, there is scant direct evidence concerning their diet and exact trophic relationships. Here I describe the gut contents of Ottoia prolifica, an abundant priapulid worm from the middle Cambrian (Stage 5) Burgess Shale biota. I identify the undigested exoskeletal remains of a wide range of small invertebrates that lived at or near the water sediment interface such as hyolithids, brachiopods, different types of arthropods, polychaetes and wiwaxiids. This set of direct fossil evidence allows the first detailed reconstruction of the diet of a 505-million-year-old animal. Ottoia was a dietary generalist and had no strict feeding regime. It fed on both living individuals and decaying organic matter present in its habitat. The feeding behavior of Ottoia was remarkably simple, reduced to the transit of food through an eversible pharynx and a tubular gut with limited physical breakdown and no storage. The recognition of generalist feeding strategies, exemplified by Ottoia, reveals key-aspects of modern-style trophic complexity in the immediate aftermath of the Cambrian explosion. It also shows that the middle Cambrian ecosystem was already too complex to be understood in terms of simple linear dynamics and unique pathways.

Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary

Animal-like multicellular fossils appeared towards the end of the Precambrian, followed by a rapid increase in the abundance and diversity of fossils during the Early Cambrian period, an event also known as the 'Cambrian explosion'. Changes in the environmental conditions at the Precambrian/Cambrian transition (about 542 Myr ago) have been suggested as a possible explanation for this event, but are still a matter of debate. Here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 Myr. We find a transient molybdenum isotope signal immediately after the Precambrian/Cambrian transition. Using a box model of the oceanic molybdenum cycle, we find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed Early Cambrian molybdenum isotope signal. Our findings suggest that the Early Cambrian animal radiation may have been triggered by a major change in ocean circulation, terminating a long period during which the Proterozoic ocean was stratified, with sulphidic deep water.

Preservation of Early Cambrian animals of the Chengjiang biota

The Chengjiang biota of Yunnan, China, documents the earliest extensive radiation of the Metazoa recorded in the fossil record. Gauging preservational bias is crucial in providing an assessment of the completeness of this biota and thereby elucidating whether it represents a comprehensive depiction of Early Cambrian life. We here present a model to explain the nature of the exceptional preservation of the Chengjiang biota and details of the decay process. This study indicates that Chengjiang fossils were preserved through two taphonomic pathways that may have captured tissues of distinct compositions, and this finding should provide a foundation for the interpretation of Chengjiang fossils. Many Chengjiang fossils are preserved by pyrite (later pseudomorphed by iron oxides); the clay-rich host sediment was deficient in organic carbon but replete in available Fe, and this composition ensured that a decaying carcass acted as a local substrate for Fe- and S-reducing bacteria. Pyrite morphology probably reflects contrasts in the decay rate, and hence the H 2 S production rate, of different tissues in a carcass. Reactive, rapidly decaying tissues would have quickly supplied H 2 S, producing many pyrite nuclei, resulting in framboidal habits. More recalcitrant tissues would have produced H 2 S more slowly, so that crystal growth operated on fewer nuclei, resulting in larger euhedral pyrite crystals. Reflective films, especially common on Chengjiang arthropods, represent the remains of degraded carbon.

New early Cambrian animal and onychophoran affinities of enigmatic metazoans

THERE is much interest in the early evolution of metazoans with the restudy of the Middle Cambrian 'soft-bodied' fauna of the Burgess Shale1. Several other, newly discovered Cambrian 'soft-bodied' faunas2 provide a wealth of new data. One of the oldest and best-preserved faunas was discovered in 1984 in Chengjiang in southern China3. This fauna is of early Cambrian age, about late Atdabanian4 (∼520–530 Myr BP)2. We now describe a new 'armoured lobopod' from the Chengjiang fauna. This animal shows close affinity with the enigmatic Microdictyon5. The conundrum Halludgenia6 is reinterpreted as another 'armoured lobopod', as are Xenusion7 and Luolishania8. The large plates set in pairs along the trunk are a synapomorphy of this group, which flourished soon after the 'Cambrian explosion'. Soft-part anatomy suggests that the group has affinities with the Burgess Shale 'lobopod' Aysheaia9. All these marine, Cambrian forms are here grouped with the extant, terrestrial velvet worms in the phylum Onychophora.