Was there a Cambrian explosion on land?

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The colonisation of land was a key transition in the history of life. Molecular clock analyses imply a rapid radiation ofarthropods and plants on land in the early Palaeozoic, overlapping with the Cambrian explosion in the sea, but the fossilrecord is frustratingly limited. The project will combine palaeontological and computational methods to bridge this gap inthe fossil record and establish a timescale for the evolution of terrestrial ecosystems. Early Palaeozoic terrestrial depositswill be investigated using a low-manipulation acid extraction procedure to isolate small carbonaceous fossils (SCFs).SCFs are a unique preservation window for small organisms that rarely fossilise. Arthropod SCFs from numerousterrestrial deposits have been reported in the 1980 and 1990s but have been overlooked since. Preliminary analyses ofcoal balls reveal exceptional SCFs suitable for comparison with extant and Palaeozoic taxa. The phylogenetic positionand non-marine origin of problematic SCFs will be tested using geochemical techniques, providing reliable criteria forrecognising the oldest terrestrial ecosystems. The SCF record will be complemented by revising arthropods from theRhynie chert and Mazon Creek biotas in museum collections using tomography. This fossil evidence will provide a set ofrobust calibrations for a new timescale of terrestrialisation, which will in turn serve as a backbone for testing hypothesesabout the drivers of the diversification of life on land.