The ventricle of the mammalian heart is functionally divided into right and left ventricles by septal structures. This form of full ventricular septation is also observed in archosaurs (crocodilian reptiles and birds) and appears to have evolved multiple times. But how and when this happened is unclear. Now, Bjarke Jensen and colleagues examine the hearts of monitor lizards, which are non-crocodylian reptiles that exhibit an intermediate degree of ventricular septation. They provide a detailed description of how the monitor heart is partially divided by three septa – the vertical septum, the muscular ridge and the bulbuslamelle – that show functional and anatomical similarities to mammalian ventricular structures. They further demonstrate that orthologues of transcription factors expressed in the mammalian ventricular septum (such as Tbx5, Irx1 and Irx2) are expressed in the septal structures of developing monitor hearts. The authors also report that the myocardium adjacent to the vertical septum of monitors is enriched in transcripts associated with the atrioventricular bundle of mammals; accordingly, the mapping of cardiac action potentials reveals that the earliest signs of electrical activation are always observed in this region. Together, these findings lead the authors to propose that ventricular septation relies on evolutionarily conserved building blocks that allow variations on a common design.

• © 2019. Published by The Company of Biologists Ltd