''Mesomyzon mengae'' is an extinct

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

from freshwater strata of the Early Cretaceous-aged

Yixian Formation The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its exq ...

, in China. So far, ''M. mengae'' is the only lamprey known from post-Paleozoic fossils. 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

myomere Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by connective fascia (myosepta) and most easily seen in larval fishes or in the olm. Myomere c ...

s of its musculature. 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. On the other hand, Brownstein & Near (2022) found it to be a member of the lamprey crown group, most closely related to northern lampreys.

References

Prehistoric Hyperoartia genera Lampreys Mesozoic jawless fish Cretaceous fish Barremian genus first appearances Early Cretaceous fish of Asia Aptian genus extinctions {{Paleo-jawless-fish-stub