After more than a decade of fieldwork and exhaustive laboratory work, a team from the Chinese Academy of Sciences’ Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) have uncovered the oldest known fossils of bony fish, revealing key anatomical features of two primitive species.
These features paint an interesting picture of what fish looked like before they split into two major evolutionary groups: ray-finned fish and lobe-finned fish.
As a group, bony fishes (osteichthyans) account for more than 50% of all living vertebrate species. Today, 99% of fish belong to the ray-finned group, but during the Early Paleozoic (around 400 million years ago) they weren’t quite so dominant, sharing the waters with a variety of lobe-finned fish, as well as long-extinct, armoured fish known as ostracoderms and placoderms.
At least one lineage of lobe-finned fish from this period eventually crawled out of the water, ultimately giving rise to all terrestrial vertebrates, including us!
While this chapter of the ‘fish to human’ story is well-known - or at least supported by several lines of evidence - the origin of bony fishes millennia prior has long been shrouded in mystery. This is largely down to the fact that most early fish fossils are of highly specialised, ray-finned or lobe-finned forms that lived during the Devonian (419-359 million years ago), after the major evolutionary split.
This is what makes the fossils recently uncovered by the Chinese research team so important; falling before the divergence of ray-finned fish and lobe-finned fish, they reveal the anatomy of the last common ancestor of these two groups and shed some much-needed light on this previously enigmatic period of evolutionary history.
In their first of two articles published in the journal Nature last week, the Chinese research team announced the discovery of Eosteus chongqingensis - a brand-new species and, at 436 million years old, the oldest complete bony fish ever found.
At just 3cm in length, E.chongqingensis was no bigger than a neon tetra. It also displays a variety of derived features, such as a single dorsal fin and specialised, hardened scales known as caudal fulcra, which resemble those of early ray-finned fishes. However, it lacks the bony fin rays typical of later bony fishes and it has an anal fin spine that’s only ever been seen in cartilaginous fishes, like sharks, and placoderms.
The fact this variety of traits was present in a fish from the Early Silurian suggests the core characteristics of bony fish evolved a lot earlier than previously thought.
Using high-resolution computed tomography (HRCT), the researchers were able to reconstruct the skull and teeth of another primitive bony fish, Megamastax amblyodus. These findings were presented in their second article and revealed this 1m-long fish - the largest of its time - had two rows of teeth, consisting of ‘tooth cushions’ atop blunt bases.
This strange dental morphology represents the primitive condition for bony fish and helps solve a half-century-old puzzle surrounding the origin of M.amblyodus’ tooth plates, which have previously been found as isolated fossils in Silurian-aged rocks from the Baltic region.
Both E.chongqingensis and M.amblyodus were discovered in South China, but at sites roughly 1,000km apart. The former was found in Xiushan, Chongqing - hence its name - while the latter was found in Qujing, Yunnan.
These discoveries highlight South China as a hotspot for early vertebrate fossils and suggest more groundbreaking discoveries may be made there soon.
“The origins of jawed vertebrates is a major evolutionary event, but it has long been clear that parts of this evolutionary puzzle were missing. New fossil finds from China keep throwing up surprises and help us unlock these deep mysteries. These findings, both big and small, won’t be the last,” said the University of Manchester’s Robert Sansom, an early vertebrate researcher who was not involved in the latest studies.
You can read more about the IVPP’s latest studies here and here.
Top image: Life reconstruction of the oldest osteichthyan: Eosteus chongqingensis. Credit: NICE PaleoVislab, IVPP, Chinese Academy of Sciences
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