A fresh look at forgotten fossils has revealed two new species of giant, filter-feeding fish that swam Earth's oceans for 100 million years, occupying the ecological niche now filled by whales and whale sharks.
Until now, that ancient niche was thought to be empty, and such fish to be a short-lived evolutionary bust.
"We knew these animals existed, but thought they were only around for twenty million years," said Matt Friedman, a University of Oxford paleobiologist. "People assumed they weren't important, that they were an evolutionary failure that was around for a brief time and winked out. Now we realise that they had a long and illustrious evolutionary history."
In a paper from the 18th February in Science, Friedman and five other paleobiologists describe Bonnerichthys gladius and Rhinconichthys taylori. They belong to the pachycormid genus, an extinct group of immense fish that ate by drifting slowly, mouth agape, sucking down plankton and other tiny aquatic life.
Prior to the paper's publication, pachycormids were known from fossils of a single species, Leedsichthys problematicus. (The species name derives from the fragmented remains of its first fossils.) Leedsichthys was an impressive creature, reaching lengths of 10 and perhaps even 15 meters, but its fossils have only been found in western Europe and are between 160 and 145 million years old -- a brief, relatively unexceptional footnote to animal history.
However, during a chance visit by Friedman to the University of Kansas, researchers from their Natural History Museum told him of odd recoveries from a newly-prepared fossil deposit: delicate plates and long rods of bone, jumbled beyond recognition. As Friedman put the pieces together, he realised that the plates were part of a jaw, and the rods were gills. That configuration was known from Leedsichthys, but this clearly belonged to a new species.
Working with other museums, Friedman found more examples of the species, which he dubbed B. gladius. They had been collected in the 19th century and mistakenly classified as Leedsichthys, or dismissed as uninteresting. By the time he was finished, Friedman found B. gladius fossils as old as 172 million years, and as young as 66 million years. In the dusty recesses of London's Natural History Museum, He also found another pachycormid species, R. taylori; it had been mischaracterised and forgotten by Gideon Mantell, the English paleontologist credited with starting the scientific study of dinosaurs.
Altogether, the fossils showed that pachycormids were not a footnote, but an evolutionary chapter that spanned more than 100 million years.
"That's longer than the duration of any living groups of feeders," said Friedman. "That's longer than the Cenozoic, when mammals ascended to ecological dominance."
The disappearance of B. gladius from the fossil record coincides with the Cretaceous-Paleogene mass extinction, which wiped out the dinosaurs and bequeathed terrestrial Earth to birds, mammals and insects. Then, extinction was likely caused by an asteroid strike or period of prolonged volcanic activity that shrouded the planet in dust, or both, causing massive die-offs in bottom-of-the-food-chain plants.
With a diet based on photosynthesising algae, the pachycormids "had the perfect profile of a victim and became extinct," wrote Lionel Cavin, a paleontologist at Geneva's Natural History Museum, in an accompanying commentary.
Ten million years after B. gladius disappeared, sharks and rays rose to prominence. Twenty-five million years after that, modern whales evolved. As described in another Science paper, the whales' evolution coincided with a rebirth of the photosynthetic algae that had once fed B. gladius and the other pachycormids.
Friedman plans to continue studying the pachycormids, and hopes his story will inspire other researchers.
"We've just flagged off a couple examples of these animals," he said. "We know there must be others in the fossil record. Often, when people are collecting fossils in the field, they leave behind the fish, because they're not thought to be important. We hope they keep them."