Journal Article

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

Zhang, Jiangyong, Wu, Feixiang, Miao, Desui, Chang, Mee-mann

2014

Overview

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 lamprey Mesomyzon 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. Lampreys are one of the two surviving jawless vertebrate groups and one of a few vertebrate groups with the best exemplified metamorphosis during their life cycle, which consists of a long-lasting larval stage, a peculiar metamorphosis, and a relatively short adulthood with a markedly different anatomy. Although the fossil records have revealed that many general features of extant lamprey adults were already formed by the Late Devonian (ca. 360 Ma), little is known about the life cycle of the fossil lampreys because of the lack of fossilized lamprey larvae or transformers. Here we report the first to our knowledge discovery of exceptionally preserved premetamorphic and metamorphosing larvae of the fossil lamprey Mesomyzon mengae from the Lower Cretaceous of Inner Mongolia, China. These fossil ammocoetes look surprisingly modern in having an eel-like body with tiny eyes, oral hood and lower lip, anteriorly positioned branchial region, and a continuous dorsal skin fin fold and in sharing a similar feeding habit, as judged from the detritus left in the gut. In contrast, the larger metamorphosing individuals have slightly enlarged eyes relative to large otic capsules, thickened oral hood or pointed snout, and discernable radials but still anteriorly extended branchial area and lack a suctorial oral disk, which characterize the early stages of the metamorphosis of extant lampreys. Our discovery not only documents the larval conditions of fossil lampreys but also indicates the three-phased life cycle in lampreys emerged essentially in their present mode no later than the Early Cretaceous.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: adults / Animals / Biological Sciences / Blood vessels / Capsules / China / Embryonic structures / evolution / Fossils / Fresh water / Lampreys - anatomy & histology / Lampreys - growth & development / Larva - anatomy & histology / Larva - growth & development / Larvae / Larval development / Life cycle / Life Cycle Stages / Metamorphosis / Petromyzontiformes / Vertebrates