Mesomyzon mengae is an extinct lamprey from freshwater strata of the Early Cretaceous-aged Yixian Formation, in China.

[1] The animal's exquisitely preserved fossils show a creature very similar to modern-day lampreys, having a well-developed sucking oral disk, a branchial basket, at least seven pairs of gill pouches and corresponding gill arches, impressions of gill filaments, and at least 80 myomeres of its musculature.

[1] It had the same three phase life cycle found in modern lampreys.

[2] A phylogenetic analysis conducted in 2018 shows that Mesomyzon is the fossil lamprey most closely related to modern taxa, though it is not closely related to any modern group.

[3] On the other hand, Brownstein & Near (2022) found it to be a member of the lamprey crown group, most closely related to Petromyzontidae.