Fossilized Fish Larvae Discovery Upends Theory on Vertebrate Origins

An examination of the life cycle of the ancient, eel-like lamprey shows the evolutionary path of hatchlings diverged considerably from the modern adult vertebrates.

Artist’s reconstruction showing the life stages of the fossil lamprey Priscomyzon riniensis. It lived around 360 million years ago in a coastal lagoon in what is now South Africa. Clockwise from right: A tiny, yolk-sac carrying hatchling with its large eyes; a juvenile; and an adult showing its toothed sucker. (Courtesy of Kristen Tietjen)

(CN) — A 150-year-old theory that hatchlings of modern lampreys — a kind of blind, jawless fish — closely resemble their ancestral vertebrates has been upended after new fossil analysis showed both ancient and modern lamprey larvae are dissimilar, according to a study released Wednesday.

The eel-like lampreys have a half billion-year-old history in the world’s oceans and bodies of freshwater.

The lamprey hatchlings buried themselves in riverbeds and coastal lagoons and filter-fed before “metamorphosing into blood-sucking adults,” according to Tetsuto Miyashita, paleontologist at the Canadian Museum of Nature.

Ammocoetes — the filter-feeding hatchlings of modern lampreys — have long been a critical foundation for theories on the evolution of vertebrates, Miyashita said in a statement by researchers released with the study.

“Modern lamprey larvae have been used as a model of the ancestral condition that gave rise to the vertebrate lineages,” said Miyashita, who is first author of the study. “They seemed primitive enough, comparable to wormy invertebrates, and their qualities matched the preferred narrative of vertebrate ancestry. But we didn’t have evidence that such a rudimentary form goes all the way back to the beginning of vertebrate evolution.”

Miyashita and researchers from the University of Chicago, the Canadian Museum of Nature and the Albany Museum in South Africa set out to interrogate the theory by analyzing the fossils of four species of larval lampreys from the Palaeozoic era.

Using fossil samples collected in Illinois, South Africa and Montana, the team sought to uncover whether the sand-burrowing fish existed in an ammocoete-like form in any part of its ancestral lineage.

Researchers examined dozens of samples of tiny, fossilized lampreys ranging from 310 to 360 million years old in order to chart out the evolutionary path of the jawless fish.

The smallest sample was the size of a fingernail and carried a larvae’s “yolk sac,” signaling the fossilization occurred just after hatching. 

The findings showed the fossils of the four lamprey larvae species (scientific names hardistiella, mayomyzon, pipiscius, and priscomyzon) had none of the defining traits of ammocoetes, according to the study published Wednesday in the science journal Nature.

The fossils instead had features closer in resemblance to adult modern lampreys; posteriorly united branchial baskets, large eyes and a ringed, tooth-lined sucker, according to Michael Coates, a University of Chicago professor.

“Lampreys are not quite the swimming time capsules that we once thought they were,” Coates said in the statement. “They remain important and essential for understanding the deep history of vertebrate diversity, but we also need to recognize that they, too, have evolved and specialized in their own right.” 

The results show the ancient lampreys never passed through the evolutionary phase of blind, filter-feeding that modern species went through, the study said, adding that the now extinct armored fishes known as ostracoderms are possible roots of the vertebrate ancestral tree.

“Thus, lampreys and cephalochordates remain important branches for constraining a suite of characters at the crown vertebrate node, but neither is sufficient for the reconstruction of the ancestral state,” researchers wrote in the study. “The presence of such a generalist larval phase prior to the specialized adult form may have contributed to the long-term survival of this lineage, after the apparent extinction of their exclusively non-freshwater relatives.”

The ammocoete phase — along with lampreys evolution from filter feeder fish to predator — evolved within the 200 million-year period in which the lamprey habitat expanded to include freshwater, the study said.

Researchers believe the evolution of filter-feeding larvae opened a path for lampreys to populate rivers and lakes but that the ancient, blood-sucking lampreys would’ve struggled to find prey there.

“Lampreys solved this problem by burrowing in sand and slurping up whatever food particles were available until they could mature enough to start looking for prey with blood,” Miyashita said. “But you don’t have to be too complicated for this kind of lifestyle. So even though lampreys invented this filter feeding larval phase anew, the larvae themselves looked simple and primordial. Until now we were misinterpreting this simplicity for primitiveness.”

Researchers did not immediately respond to an emailed request for further comment by press time.

The study was funded by the National Science Foundation.

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