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The rise of land plants during the early Palaeozoic had profound effects upon subsequent Earth history and evolution. The sequestration of standing biomass and carbon burial caused a primary shift in the distribution of active carbon within the biosphere and surficial Earth systems. This manifested itself in a dynamic decline in pCO2 during Silurian–Devonian time, affecting both terrestrial and marine ecosystems. We examined first-order correlations between terrestrialization and pCO2 by comparing the GEOCARB III data with time-constrained fossil events in the early evolution of land plants. We compared the same GEOCARB III data with the species/genus richness of lower Palaeozoic acritarchs. The correlation between the rise of woody plants and pCO2 is built into the GEOCARB model for the Late Devonian and later, but pCO2 begins to decline in the Cambrian long before the origin of woody trees (lignophytes). The influence of early phases in plant evolution may be seen in a two-stage pCO2 decline corresponding to fossil evidence for the origin of thalloid bryophytes in the Middle Cambrian and the origin of tracheophytes near the Ordovician–Silurian boundary. The decline of the acritarchs shows a highly correlated lag of about 10 Ma with respect to the pCO2 decline. The relation between pCO2 and acritarch species richness suggests a tight coupling between the evolution of the marine phytoplankton and atmospheric CO2, supporting previous suggestions that pCO2 was a significant causal factor in the near extinction of acritarchs by the end of the Devonian.

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