The long accepted theory of origin of vertebrates supported by petrified fish larvae

A new study from the University of Chicago, the Canadian Museum of Nature and the Albany Museum disputes a long hypothesis that the blind, filter-feeding larvae of modern lampreys are a hold from the distant past, resembling the ancestors of all living things. vertebrates, including ourselves. The new fossil discoveries suggest that old lamprey hatchers look more like modern adult lampreys, and that they were completely different from their modern larvae. The results are on March 10 in Nature.

Lampreys – unusual jawless, eel-like creatures – have long provided insight into the evolution of vertebrates, the first author Tetsuto Miyashita, Ph.D., formerly a Chicago Fellow at the University of Chicago and now a paleontologist at the Canadian Museum of Nature, said. “Lampreys has a ridiculous life cycle,” he said. “The larvae first bury themselves in the riverbed and filter feed before finally turning into blood-sucking adults. They are so different from adults that scientists originally thought they were a completely different group of fish.

“Modern lampreyl larvae were used as a model of the ancestral condition that gave rise to the vertebrate lines,” Miyashita continued. “They look primitive enough, comparable to worm-like invertebrates, and their characteristics correspond to the preferred narrative of vertebrate descent. But we have no evidence that such a rudimentary form goes all the way back to the beginning of evolution of vertebrates.”

Newly discovered fossils in Illinois, South Africa and Montana change the story. By connecting the dots between dozens of samples, the research team realized that different stages of the ancient lamprey life cycle had been preserved so that paleontologists could follow their growth from hatching to adulthood. On some of the smallest specimens, about the size of a fingernail, the remains of a yolk sac are shown in the retention of soft tissues, indicating that the fossil recordings captured these lamp journeys shortly after they hatched.

Importantly, these petrified juveniles are very different from their modern counterparts (known as “ammocoetes”), and rather resemble modern adult lamp journeys, with large eyes and teeth of sugar mouths. Most exciting is that this phenotype can be seen during the larval phase in several different species of ancient lamprey.

“Remarkably, we have enough specimens to reconstruct a path from hatching to adulthood in several independent generations of early lampreys,” says Michael Coates, Ph.D., a professor in the Department of Organic Biology and Anatomy at UChicago, ‘and they each have the same pattern: the larval form was like a miniature adult. ‘

The researchers say that these results challenge the 150-year-old evolutionary story that modern lampreyl larvae offer a glimpse into the deep ancestral vertebrate conditions. By showing that ancient lamps never went through the same blind, filter-feeding stage as in modern species, the researchers falsified this cherished ancestral model.

“We basically removed lampreys from the position of the ancestral condition of vertebrates,” Miyashita said. “Now we need an alternative.”

After looking back at the fossil record, the team now believes that extinct armored fish, known as ostracoderms, may rather represent better candidates for the root of the vertebrate pedigree, while modern lampreyl larvae are a more recent innovation. The team believes that the evolution of filter feeding larvae may have caused lamp trips in rivers and lakes. Fossil lampreys reported in the new study all come from marine sediments, but modern lampreys mostly live in freshwater.

According to the researchers, this is the kind of discovery that textbooks can rewrite. “Lampreys are not quite the swimming time capsules we once thought,” Coates said. “They remain important and essential to understanding the profound history of vertebrate diversity, but we must also realize that they too have developed and specialized in their own right.”