The Cambrian-Ordovician (541 million to 443 million y ago) climate record is poorly constrained due to the sparsity of carbonate fossils typically used for temperature reconstructions. We demonstrate that the abundant carbonate sediments from this time can preserve isotopic compositions and trends similar to those of fossil material and combine sediment records into a high-resolution proxy record of Late Cambrian and Ordovician climate. Examination of this record in the context of evolutionary and tectonic changes demonstrates that Earth's climate was tightly coupled to tectonic and volcanic activity and played a strong role in biodiversity trends during the time of early animal evolution.