The rhythm is chaotically irregular. The QRS morphology allows one to determine that the origin of the rhythm is supraventricular. There are no P waves.
Irregularity is the electrocardiographic hallmark of atrial fibrillation.
Fibrillation means disorganized electrical activity. However, P waves are evidence of organized electrical activity within the atria. Excepting the few cases of paroxysmal atrial fibrillation, the presence of P waves is therefore incompatible with an electrocardiographic diagnosis of atrial fibrillation. Experimentally, a critical mass of myocardium is required to sustain the arrhythmia of fibrillation. That's why large animals with otherwise normal hearts such as horses, and the occasional giant breed dog, can develop atrial fibrillation seemingly in the absence of structural heart disease.