Fossil evidence of Earth's earliest vertebrates is sparse, but many scientists agree that a model for the ancestors of the all living vertebrates can still be found in the coastal and fresh waters of most temperate regions.

Evolutionary biologists have long contended that the blind, filter-feeding larvae of modern lampreys represent an evolutionary holdover from millions of years ago -- an anatomical anachronism.

However, new fossil evidence -- described Wednesday in the journal Nature -- threatens to undermine the long-accepted theory.

Scientists have unearthed fossilized fish larvae suggesting ancient lamprey hatchlings looked a lot more like modern adult lampreys than their modern larvae counterparts.

"Nineteenth and early 20th century scientists, not unreasonably, saw lamprey larvae as a good model of general, and likely primitive, conditions for all animals with a backbone," study co-author Mike Coates told UPI in an email.

"The theory persisted, not least because it fits with a general idea of evolution from-simple to-complex, and that development from egg to adult might recapitulate evolutionary history," said Coates, a biologist with the University of Chicago.

Despite the theory's popularity, scientists have never found direct evidence that the simple worm-like body plan of modern lamprey larvae is traceable to the beginning of vertebrate evolution.

For the latest study, scientists analyzed the remains of ancient lamprey larvae and hatchlings recovered in Illinois, Montana and South Africa.

By looking at dozens of ancient specimens from various Palaeozoic-era deposits, researchers were able to reconstruct the lifecycle of the primitive lampreys -- tracing their development from hatchling to adult.

Most importantly, the well preserved juvenile specimens showed ancient lampreys, in their larval and hatchling stages, were quite unlike their modern counterparts, filter-feeding juveniles called ammocoetes.

"The new fossil discoveries -- fossils of larval and juvenile lampreys -- show the features present in today's adults, including well developed eyes, a gill basket for supporting the gills, and an oral disc with toothlets, much like the rasping sucker present in today's adult lampreys," said Coates.

Scientists originally pointed to the blind, filter-feeding larvae of modern lampreys as a model for early vertebrate evolution because they didn't have much fossil evidence to go on. Over the last several decades, however, scientists have begun to unearth well-preserved remnants of early vertebrates.

The authors of the latest paper say it's time to pay closer attention to that fossil evidence.

"The earliest vertebrates might be better understood by taking more account of the fossil evidence, and using this to qualify the evidence from lamprey development," Coates said.

Moving forward, Coates and his colleagues will continue to look for the remains of primitive Palaeozoic vertebrates, as well as use the data they already have to more precisely characterize the function and ecology of early lampreys.

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