2023-11-01 18:35:20
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Over approximately 360 million years, lampreys gradually evolved from herbivores to carnivorous parasites. However, our understanding of this transition has many gaps due to the scarcity of the fossil record. Recently, scientists shed light on the subject following uncovering two exceptionally well-preserved fossils, dating from 160 million years ago, whose dentition would indicate the emergence of predation in this lineage.
Lampreys are one of two groups of living jawless vertebrates and include 49 different species. They are characterized by their suction cup mouth with numerous teeth, allowing them to cling securely to their hosts in order to consume their blood. Their oral cavities also have a pair of glands secreting an anticoagulant substance, allowing them to suck up as much blood as possible.
This mode of feeding gives them a key ecosystem role by contributing to the control of the population of certain species. However, if introduced, they can easily become invasive and cause heavy losses for local fisheries. Understanding their evolutionary biology might further support the efforts of ecologists to adopt the best measures to preserve or control them.
A transition from herbivore to carnivore
While the fossil record of these fish dates back 360 million years, they are extremely unevenly distributed during the post-Carboniferous period, with only two taxa known from the Cretaceous (145.5 to 65.5 years ago). million years). Analysis of fossils dating from the Paleozoic (541 to 252.2 million years ago) indicates that their mouths have changed considerably, gradually resembling those of current species. These transformations concern in particular the size, shape and arrangement of keratin teeth.
Indeed, while current carnivorous lampreys can measure up to 120 centimeters, their ancestors only measured a few centimeters and had very simple and reduced teeth. According to researchers at the Chinese Academy of Sciences, the mouthparts of these primitive lampreys suggest that they were neither predators nor parasites. They would have fed more on algae and did not have anticoagulant glands. “ Their feeding possibilities were rather limited, as the vast majority of their potential hosts all had scales or thick armor, which they would not have been able to penetrate. », explain the researchers in their studypublished in the journal
Nature Communications. Their diet would not have evolved until much later.
However, gaps remain in the understanding of this evolutionary scenario, in particular due to a lack of sufficiently reliable fossil records. The newly discovered fossils bridge the gap between existing lineages and fossilized specimens from the Paleozoic. Baptized Yanliaomyzon occisor et Y. ingensdentes, the two new specimens date from the Jurassic (201.3 million to 145 million years ago) and are exceptionally well preserved. “ These fossil lampreys have been exceptionally well preserved, with a complete suite of feeding structures “, wrote the researchers. This reveals valuable clues regarding the evolutionary history of the lineage.
A major change in the biological cycle
The two fossils described in the new study were exhumed at a fossil site located in northeast China. They have mouth cavities with many teeth and bear a striking resemblance to modern-day pocket lampreys (Geotria australis). This suggests that the lineage was already beginning to consume blood at least 160 million years ago.
However, Y. occusor, the larger of the two specimens discovered, measures only 64.2 centimeters long, although it is the largest lamprey fossil discovered so far. For comparison, sea lampreys (Petromyzon Marinus) today measure up to 120 centimeters and Pacific lampreys (Entosphenus tridentatus) 85 centimeters. Chinese experts suggest that the predatory lifestyle probably led to the increase in their size since the Jurassic.
Furthermore, the transition was also accompanied by a major change in their biological cycle, shortly before the Cretaceous. By its large size, Y. occusor has similarities with later taxa that developed a three-stage life cycle, including larval (ammocoete), metamorphic, and adult stages. This suggests that, like current species, this one probably lived at the rhythm of a migratory cycle, in fresh water as a larva, only to return there during the breeding season.
Source : Nature Communications
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