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, 111 (43), 15486-90

Discovery of Fossil Lamprey Larva From the Lower Cretaceous Reveals Its Three-Phased Life Cycle

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Discovery of Fossil Lamprey Larva From the Lower Cretaceous Reveals Its Three-Phased Life Cycle

Mee-mann Chang et al. Proc Natl Acad Sci U S A.

Abstract

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.

Keywords: Lower Cretaceous; fossil lamprey larva; three-phased life cycle.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Larvae of M. mengae from the Lower Cretaceous of China. These 125-My-old ammocoetes are the earliest known examples of lamprey larvae in the fossil records, showing almost identical anatomy and life cycle with their living counterparts. (A) Photograph and (B) drawing of IVPP V15114.6A in left view. (C) Photograph and (D) drawing of IVPP V15165.2A in right view. (E) Photograph of in IVPP V15114.5B in left view and (F) that of the head and anterior part of the body of E. (G) Photograph of IVPP V15681A in right view. (H) Photograph and (I) drawing of head and anterior part of body of G. (bv, blood vessels; cf, caudal fin; df, dorsal fin; dt, detrital remains in digestive tract; bo, external branchial opening; bp1, bp7, first and seventh branchial pouch; le, left eye; lo, left otic capsule; lp, lower lip; lv?, possible remains of liver; ms, myoseptum; nc, notochord; oh, oral hood; re, right eye; ro, right otic capsule; sc, spinal cord).
Fig. 2.
Fig. 2.
Early transformers of M. mengae. (A) Photograph of IVPP V15030 in left view. (B) Photograph of IVPP V15032 in left view. (C) Box area in B in higher magnification, showing the radials. (D) Photograph and (E) drawing of the head and anterior part of the body of A. (F) Photograph and (G) drawing of the head and anterior part of the body of B. (For abbreviations, see Fig. 1).
Fig. 3.
Fig. 3.
Specimens and restorations of larva, early transformer, and adult of M. mengae, showing its three-phased life cycle, which emerged at least 125 Ma. (A) Larva (ammocoete) of M. mengae. a1, restoration of larval M. mengae; a2, restoration of burrowing larvae of M. mengae (cf. fig. 4.1 in ref. , not to scale); a3, photograph of IVPP V15034B in right view. (B) Early transformer of M. mengae. b1, restoration; b2, photograph of IVPP V15506B in right view (original photo horizontally flipped). (C) Adult or late transformer of M. mengae. c1, photograph of holotype (IVPP V14719) of M. mengae in right view; and c2, its restoration. (the circle in white dotted line in a1, b1, and c1 representing the position of otic capsule).

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