Earth’s magnetic field remained weak for 40 million years after the Cambrian radiation

The Ediacaran-Cambrian interval marks major environmental change and the emergence of complex life, coinciding with some of the weakest magnetic field strengths in Earth history. It has been proposed that inner core nucleation near this boundary triggered a rapid strengthening of the geodynamo during the early Cambrian, but direct constraints on magnetic field behavior across this interval remain limited. Here, we present new paleointensity results from two Cambrian igneous sequences in Brazil (~525 million years ago) and Nova Scotia (~505 million years ago). Using multiple complementary methods, both records indicate a reversing geomagnetic field that remained weak. Combined with a reanalysis of published data, our results show that weak magnetic field conditions persisted for tens of millions of years after the Cambrian radiation. These findings limit the magnitude and timing of any Cambrian geodynamo strengthening and imply that Earth's magnetospheric shielding remained strongly suppressed during a key phase of early animal evolution.