Vertebrate Origin Theory Now Questionable With Discovery of Fossilized Fish Larvae

According to a recent report from the University of Chicago, the blind, filter-feeding larvae of modern lampreys are a holdover from the far past, resembling the ancestors of all living vertebrates, including ourselves, according to a recent report from the University of Chicago Canadian Museum of Nature, and the Albany Museum.

In the latest fossil discoveries, ancient lamprey hatchlings resembled modern adult lampreys more closely than their modern larvae counterparts. The findings were announced in Nature on March 10th.

Lamprey

Lampreys-bizarre jawless, eel-like creatures-have long offered observations into vertebrate evolution, according to first author Tetsuto Miyashita, Ph.D., a paleontologist at the Canadian Museum of Nature and a recent Chicago Fellow at the University of Chicago. "Lampreys have an absurd life span," he explained. "Once the larvae have hatched, they burrow into the riverbed and filter feed before metamorphosing into blood-sucking adults. They're so dissimilar to adults that scientists mistakenly mistook them for a separate genus of fish.

Miyashita added, "Modern lamprey larvae have been used as a model of the ancestral state that gives birth to the vertebrate lineages." "They seemed primitive enough, resembling wormy invertebrates, and their characteristics suited the preferred vertebrate ancestry tale. However, there was no suggestion that such a primitive type existed at the onset of vertebrate evolution."

Fossil Findings

New fossils found in Illinois, South Africa, and Montana are altering the narrative. The research team discovered that various stages of the ancient lamprey lifecycle had been preserved by connecting the dots between hundreds of fossils, enabling paleontologists to track their development from hatchling to adult. Soft tissue preservation also reveals the remnants of a yolk sac on some of the tiniest specimens, around the size of a fingernail, suggesting that these lampreys were caught shortly after hatching.

Importantly, these fossilized juveniles resemble modern adult lampreys, with wide eyes and toothed sucker mouths, rather than their modern relatives (known as "ammocoetes"). This phenotype can be seen in several ancient lamprey species during the larval period, which is especially exciting.

"Remarkably, we've got enough specimens to recreate a pathway from hatchling to adult in many independent lineages of early lampreys, and they all have the same pattern: the larval shape was like a miniature adult," said Michael Coates, Ph.D., a professor in the Department of Organismal Biology and Anatomy at UChicago.

According to the researchers, the findings dispute the 150-year-old evolutionary narrative that modern lamprey larvae have a snapshot of deep ancestral vertebrate environments. The researchers disproved this commonly believed ancestral paradigm by showing that ancient lampreys never went through the same blind, filter-feeding period as modern species.

Lamprey Exclusion

Miyashita explained, "We've practically excluded lampreys from the role of the ancestral state of vertebrates." "As a result, we now need an alternative."

Since studying the fossil record, the researchers conclude that ancient armored fishes known as ostracoderms are more likely candidates for the vertebrate family tree's base. At the same time, modern lamprey larvae are a more recent invention. According to the researchers, lampreys may have colonized rivers and lakes as a part of the emergence of filter-feeding larvae. The study's fossil lampreys all come from marine sediments, but modern lampreys are mostly freshwater.

Importance of the Breakthrough

According to the researchers, this is the kind of breakthrough that will totally alter textbooks. "Lampreys aren't exactly the swimming time capsules we once thought," Coates explained. "They're still important and essential for understanding the deep past of vertebrate diversity, but we must also acknowledge that they've evolved and specialized in their own right."