The authors regret that an acknowledgement section was missing when this article was initially published. The article has been updated to include the below acknowledgement:

“We are indebted to Dr Tim Mohun, who prepared all the episcopic datasets used to prepare the figures showing the features of cardiac development whilst he was working at the Crick Institute in London. We also thank the staff of the Human Developmental Biology Resource at Newcastle University, who made the histological sections of the human embryos available for our use.”