436-Million-Year-Old Fish Fossil Rewrites the Origin of Vertebrate Bones

The fish was three centimeters long. It lived 436 million years ago in what is now Chongqing, China, when the region was a shallow sea and nothing recognizable as a land animal yet existed. It was found, after more than a decade of fieldwork, by a team from the Chinese Academy of Sciences. And it is, according to two papers published simultaneously on the cover of Nature, the oldest complete bony fish fossil ever discovered - predating all previously known bony fish remains and pushing the origins of the group that includes virtually every fish familiar to us, and all tetrapods including humans, further back in time than anyone had shown before.

The creature is named Eosteus chongqingensis. Despite its age and small size, it is exceptionally well preserved - head to tail, every fin accounted for. That completeness is unusual for fossils this old and made it possible for the researchers to reconstruct its anatomy in detail using high-resolution computed tomography.

A mosaic of old and new features

What Eosteus reveals is a fish caught in an evolutionary transition. It has the streamlined body and single dorsal fin of early ray-finned fishes. Its scales include caudal fulcra - a feature also seen in later ray-finned lineages. But it lacks lepidotrichia, the bony fin rays that are a defining characteristic of bony fishes as a whole. And it has an anal fin spine previously known only in cartilaginous fishes and the armored placoderms, groups that diverged long before bony fishes appeared.

That combination of features is precisely what paleontologists call a mosaic: a mix of primitive and derived traits that shows an organism occupying a position between ancestral and descended forms. Eosteus sits at that junction - it is a bony fish, or rather a stem bony fish, before the split between the two great lineages that would eventually colonize the planet's oceans and, in one case, its land surfaces.

The giant that solved a dental mystery

The second discovery is different in almost every respect. Megamastax amblyodus, from the Late Silurian deposits of Qujing, Yunnan - roughly 423 million years ago - grew to over a meter in length, making it the largest vertebrate of its time. It was already known to science before this study, but its internal anatomy was not. The team spent nearly a decade and made dozens of attempts before successfully using advanced three-dimensional reconstruction from computed tomography to reveal the complete cranial anatomy of this animal.

The payoff was considerable. Megamastax has inner and outer tooth rows, with the inner row consisting of tooth cushions on blunt bases. That structure had been puzzling paleontologists for fifty years. Isolated tooth cushions from Silurian deposits in the Baltic region had been known since the 1970s, but their taxonomic placement was uncertain. Seeing the same structure in a complete specimen finally resolved where those isolated fossils belong and clarified the early evolutionary trajectory of jaws and teeth in bony fishes.

Filling the gap before the great divergence

The broader significance of both finds is what they reveal about the moment just before bony fishes split into their two great surviving lineages. Ray-finned fishes eventually gave rise to over 30,000 species - most of the fish in any ocean or river today. Lobe-finned fishes, far fewer in species, gave rise to tetrapods: amphibians, reptiles, birds, and mammals, including us.

What did the last common ancestor of those two groups look like? That question has resisted a clear answer because nearly all well-studied early bony fish fossils are already specialized ray-finned or lobe-finned forms from the Devonian period, after the split had already happened. Evidence from before the divergence was missing. Eosteus and Megamastax fill that gap. Phylogenetic analyses by the research team place both species in the bony fish stem group - the ancestral lineage before the ray-fin/lobe-fin split - making them the most primitive bony fishes known to date.

The analyses also settle a dispute about which modern lineage the ancestral bony fish most resembled. The evidence from both fossils refutes the hypothesis that the ancestor was more similar to lobe-finned fishes, pointing instead toward a more generalized body plan from which both lineages diverged.

Southern China as an evolutionary center

Both fossils come from southern China, which has now yielded multiple foundational discoveries about early vertebrate evolution. The researchers note that these findings reinforce the region's status as a cradle for the origin of jawed vertebrates as a whole - a claim supported by earlier discoveries from the same research group and others working in the area.

The study was led by Professors Zhu Min, Lu Jing, and Zhu You'an from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences. It was supported by the Key Program of the National Natural Science Foundation of China and the International Research Center of Big Data for Sustainable Development Goals.