Fin ray patterns at the fin-to-limb transition

Significance To explain how limbs evolved from fins, paleontologists have traditionally studied the endoskeleton. Here, we provide a comparative analysis of the other skeletal system of fins, the dermal skeleton. We describe dermal ray anatomy for 3 species of tetrapodomorph fishes. These data show that, prior to the origin of digits, dermal rays were simplified, the fin web became reduced in size, and the top and bottom of the fin became more asymmetric. These changes reveal how fins became adapted for interacting with the substrate prior to the fin-to-limb transition and that dorsoventral asymmetry is an important, understudied axis of diversification in paired fins.