Discovery of fossil lamprey larva from the Lower Cretaceous reveals its three-phased life cycle

Significance Lampreys are one of the two surviving jawless vertebrate groups that hold the key to our understanding of early vertebrate evolution. Although the fossil records have shown the emergence of many general features of extant lamprey adults as early as the Late Devonian, the origin of the three-phased life cycle in lampreys still eludes us because we know little about fossilized lamprey larvae or transformers. This paper reports the first to our knowledge discovery of exceptionally preserved premetamorphic and metamorphosing larvae of the fossil lampreyMesomyzon mengae from the Lower Cretaceous of Inner Mongolia, China. These fossil ammocoetes look surprisingly modern and show the three-phased life cycle emerged essentially in their present mode no later than the Early Cretaceous.