A 32-year-old previously well man presents with sharp central chest pain that is worse on inspiration and when lying flat, and improved on sitting forward. He had a viral upper respiratory infection 1 week ago. ECG shows widespread concave ST elevation in leads I, II, aVL, aVF and V2-V6, with PR depression in several limb leads and no reciprocal ST depression (except in aVR). Troponin is normal. Which ECG feature most strongly supports a diagnosis of acute pericarditis rather than ST-elevation myocardial infarction?