As one of only two groups of living jawless Vertebrates, Lampreys (Petromyzontiformes) have an important place in our understanding of the history of the group. They have a unique feeding style, with a sucker mouth which they use to attach to their prey, before either detaching a chunk of tissue to be consumed or remaining attached and draining their host's blood. Fossil Lampreys are known from the Carboniferous, showing that they have been around for at least 360 million years, but unfortunately the post-Carboniferous fossil record of the group to-date comprises only two species from the Cretaceous Jehol Biota of China. These Jehol Lampreys are apparently little different from their modern relatives, implying that some significant changes had taken place between the Carboniferous and the Cretaceous, including a re-arranging of the arrangement of the keratinous teeth, the appearance of a worm-like ammocoete larval stage, the invasion of fresh-water environments, and the adoption of an anti-tropical distribution (i.e. being found outside the tropics in both hemispheres, but being absent from tropical areas).
In a paper published in the journal Nature Communications on 31 October 2023, Feixiang Wu of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Philippe Janvier of the Muséum national d’Histoire naturelle, and Chi Zhang, also of the Key Laboratory of Vertebrate Evolution and Human Origins at the , Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, describe two new species of Lamprey from the Middle Jurassic Yanliao Biota Lagerstätte of northeastern China, and discuss the implications of these for the evolution of Lampreys as a group.
Both of these Jurassic Lampreys have a feeding apparatus which includes well-developed movable biting plates on the tongue-like piston, something known in the living Pouched Lamprey, Geotria australis, which is found in New Zealand, Chile, Argentina, the Falkland Islands, South Georgia and the southwest and southeast corners of Australia, but which has not previously been seen in any fossil Lamprey. The discovery of this trait in Jurassic Lampreys has important implications for the history of the group, suggesting that this may be a lost ancestral trait in the group, not an advanced development in the Pouched Lamprey, as had previously been assumed.
Both new species are placed in a new genus, Yanliaomyzon, meaning 'Yanliao sucker' in reference to the Yanliao Biota and the feeding apparatus of Lampreys. Both members of the genus have oral discs with well-toothed anterior and lateral fields, the teeth on these fields are closely arranged, dorsally truncated, and spatulate in shape with the slightly concaved under-surface of the free edge protruding as a shallow blade.
The first new species is named Yanliaomyzon occisor, where 'occisor' means 'killer' in reference to the presumed hunting habit of the species. The species is described from two specimens, the first being complete and coming from the Daxishan locality in Jianchang County, Liaoning Province, and the second comprising a head and the forepart of the body, and coming from the Nanshimen Village locality in Hebei Province. Both are from the Tiaojishan Formation, which is thought to be between 160 and 158.58 million years old. Yanliaomyzon occisor has a supraoral lamina which completely spans the lateral rims of the oral aperture, the central cusps of which are flanked immediately by two smaller projections. It has 16 circumoral teeth, and a tail which takes up 28% of its bodylength.
The second new species is named Yanliaomyzon ingensdentes, where 'ingensdentes' means 'large teeth', in reference to the large cuspid laminae on the gouging piston of this species. This species is described from a complete specimen and a separate preserved oral disk, both from the Daohugou Beds at Wubaiding Village in Reshuitang County, Liaoning Province, a locality dated to about 163 million years ago. Yanliaomyzon ingensdentes has a supraoral lamina occupying roughly one-third of the rim of the oral aperture; as well as a transverse lingual lamina which almost equals the supraoral lamina in width. It has about 23 circumoral teeth, and a tail which makes up about 40% of its bodylength.
Both species of Yanliaomyzon are large, with the complete specimen of Yanliaomyzon occisor measuring 642 mm; among extant Lampreys this is exceeded only by the Anadromous Sea Lamprey, Petromyzon marinus (maximum adult length 1200 mm), Pacific Lamprey, Entosphenus tridentatus (850 mm), Pouched Lamprey, Geotria australis (788 mm), and Arctic Lamprey, Lethenteron camtschaticum (790 mm).
The most distinctive feature of these Lampreys is the extensively toothed oral disc and tongue-like piston, which is similar in morphology to that of the extant Poached Lamprey, a species with a Southern Hemisphere distribution, which is capable of delivering a powerful bite and removing large chunks of flesh from its prey.
Both species of Yanliaomyzon have gular pouches, a feature seen in 12 species of living Lamprey, as well as the Cretaceous Mesomyzon mengae, but unknown in other fossil Lampreys. This feature has been suggested to be connected to courtship displays in male Lampreys, or serving as an energy reserve during anadromous (sea-to-freshwater) migrations; seven of the extant species in which this feature is found undertake such migrations.
Also seen in both species of Yanliaomyzon is a long dorsal fin extending anteriorly until the level of the fourth gill pouch, and a a long precloacal skin fold, which extends anteriorly to the anterior branchial region.
A phylogenetic analysis recovered Yanliaomyzon as stem group Lampreys (i.e. more closely related to living Lampreys than to any other living group, but not decended from the last common ancestor of all living Lampreys). Notably, including Yanliaomyzon in the analysis led to Mesomyzon mengae also being recovered as a stem group Lamprey, where previous studies had recovered it as a member of the crown group. In this analysis all fossil Lampreys lie outside the crown group (the crown group comprises everything descended from the last common ancestor of all living members of a group), which now comprises only living species.
In this new analysis, Geotria australis, the only member of the family Geotriidae, is recovered as the outgroup to all other Lampreys, with the genus Mordacia, with two species forming the family Mordaciidae, forming the sister group to the family Petromyzontidae (Northern Hemisphere Lampreys), which includes all other living Lampreys. This is another new interpretation, with previous analyses having suggested either that the Geotriidae and Mordaciidae are sister groups, on a separate branch to the Petromyzontidae, or that the Petromyzontidae and Geotriidae are sister groups, with the Mordaciidae being sister to the pair.
Yanliaomyzon occisor is the largest fossil Lamprey known to science, and would be large for a modern Lamprey. Among living Lampreys large size is associated with longer migrations, a wider range, larger clutches of eggs, and a greater tolerance for salt water. Many small Lamprey species do not feed at all after metamorphosing from their ammocoete larval stage. Based upon this, and the prevailance of anadromous migrations among Lampreys recovered as basal within the crown group by Wu et al.'s phylogenetic analysis, Yanliaomyzon occisor appears likely to have been an anadromous migratory species with a triphasic life cycle (this is known to have been the case in Mesomyzon mengae, a Cretaceous species found to be less closely related to the crown group than Yanliaomyzon occisor in the phylogenetic reconstruction).
The long dorsal fin and ribbon-like precloacal skin fold seen in both species of Yanliaomyzon also suggest that these Lampreys were powerful swimmers. Similar arrangements are seen in the European Eel, Anguilla vulgaris, and African Knifefish, Gymnarchus niloticus, both of with are capable of swimming against powerful currents, something likely to be useful in a Lamprey migrating upstream to reproduce.
Lampreys appear to have switched from a simple non-migratory life cycle lacking a separate larval stage to the modern three stage, anadromous migratory life cycle some time after the Carboniferous, and the discovery of Yanliaomyzon spp. strongly suggests that this had occurred by the Middle Jurassic. This change in lifestyle appears to have also been connected to a sharp increase in the body size of Lampreys, probably as a result of the interactions between Lampreys and a changing prey-community.
Lampreys first appeared in the Devonian, and have generally been assumed to have been either carnivorous or predatory from the outset. However, Wu et al. point out there is little evidence for such behaviour in Palaeozoic Lampreys, which are very small, lack an ammocoete larval stage, and have simply structured and tiny dentition and a small buccal cavity (the space where the glands which secrete anticoagulants are found in modern Lampreys). The oral disks of these Palaeozoic Lampreys were capable of attaching, but had little biting capacity. Furthermore, the majority of Palaeozoic Fish were covered with thick scales or amoured plates, which it is unlikely even a modern Lamprey could penetrate, and which it is highly unlikely that the much smaller and less well armed Palaeozoic Lampreys could have overcome. As an alternative, Wu et al. suggest that early Lampreys may have specialised in scraping Algae from the bodies of larger Animals, using their oral disks to stay attached when their hosts moved about. Adopting a specialist niche such as this would have enabled Lampreys to flourish in an environment where they faced competition from a large number of other Jawless Fish species, notably the Conodonts from which they are thought to have derived, and which were armed with similar feeding apparatus. This evolutionary jump could help to explain the rapid range expansion of Palaeozoic Lampreys, which were restricted to the southern polar region in the Devonian, but which had reached equatorial regions by the Late Carboniferous.
Wu et al.'s phylogenetic reconstruction suggests that the ancestors of Mesomyzon mengae diverged from the ancestors of Yanliaomyzon spp. and modern Lampreys in the Early Jurassic, suggesting that more powerful oral disks associated with predation and parasitic behaviour had evolved by this point. This may have been linked to the rise of Teleost and Acipenseriform fish in the Early Jurassic, which typically have much thinner scales than the Gannoid Fish they supplanted, as well as the disappearance of potential competitors such as the Conodonts in the Permian and Triassic extinctions, which would have created new opportunities for Lampreys, leading to the development of more the specialized feeding apparatus and the increase in size seen in later members of the group. This increase in size would have facilitated the invasion of freshwater environments and the development of a migratory reproductive cycle.
The feeding apparatus and gut of Yanliaomyzon spp. appear similar to that of the modern carnivorous Pouched Lamprey, Geotria australis, indicating that this lifestyle had appeared by the Middle Jurassic. Wu et al. hypothesise that carnivory is the ancestral state for crown group Lampreys, and that parasitism arose as specialization derived from this, the reverse of the previously assumed scenario. The adaptation to a carnivorous lifestyle would have provided Lampreys with a high energy diet, enabling the evolution of larger body sizes, and longer migrations.
Modern Lampreys have an anti-tropical distribution, found in temperate and sub-arctic waters in both hemispheres, north and south of the 30° parallel and the 20°C isotherm. This preference for cool waters was also seen in the earliest Lampreys, and while the group have at times moved into more equatorial waters, this appears to have coincided with cooler intervals in the geological record.
Lampreys were present in palaeoequatorial regions of Euramerica during the Late Carboniferous Ice Age, and all known Mesozoic fossils are restricted to temperate regions of the Northern Hemisphere. The size and morphology of these Mesozoic Lampreys suggests that they were probably stronger swimmers than even the most widespread current species, such as the Pouched Lamprey, Geotria australis, and Pacific Lamprey, Entosphenus tridentatus. Both of these predatory Lampreys are capable of following shoals of Fish for long distances, and reaching considerable depths; Pacific Lampreys are typically found at depths of 0-500 m, but the maximum depth at which the species has been recorded is 1485 m. A Lamprey capable of sustained swimming at such depths would be capable of migrating across the equator without ever having to enter warm water.
Crown group Lampreys were assumed to have arisen in the Southern Hemisphere between 280 and 220 million years ago, before the breakup of Pangea, and then Gondwana, leading to the anti-tropical distribution of the group. Wu et al.'s study suggests that the crown group is younger than previously thought, and probably arose in the Southern Hemisphere, with a subsequent migration of some groups back into the Northern Hemisphere. Wu et al. suggest that the crown group may have appeared around the end of the Cenomanian-Turonian Thermal Maximum, an event likely to have wiped out the stem group Lampreys, with the subsequent migration of Lampreys back into the Northern Hemisphere having occurred before the Palaeocene-Eocene-Thermal-Maximum, an event which would have excluded from lower latitudes.
Crown group Northern Hemisphere Lampreys, Petromyzontidae, are thought to have arisen in the late Oligocene in western North America, and subsequently spread around the hemisphere following the development of ice caps in Greenland and the Arctic Sea, enabling them to spread to eastern North America and then Europe. The absence of such cold water close to the major landmasses of the Southern Hemisphere has led to the uneven species richness in the two hemispheres.
Wu et al. suggest that the morphology and lifestyle of Lampreys is not as conservative as previously thought, and that the group underwent a major evolutionary leap in the Jurassic, including a significant increase in size and swimming ability and the modification of the feeding apparatus into a more modern configuration. Crown group Lampreys are suggested to be much younger than previously hypothesized, and to have arisen in the Southern Hemisphere rather than the Northern Hemisphere, as previously thought.
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