Study: Late Ordovician Mass Extinction Cleared Way for First Fishes

A long-standing mystery in vertebrate evolution – why most major fish lineages suddenly appear in the fossil record tens of millions of years after their presumed origins – is linked to the Late Ordovician Mass Extinction (LOME), according to a new analysis by paleontologists at the Okinawa Institute of Science and Technology. The authors found that this mass extinction event, about 445 to 443 million years ago, triggered parallel, endemic radiations of jawed and jawless vertebrates (gnathostomes) to isolated refugia, reshaping the early history of fish and their relatives.

Reconstruction of the life of Sacabambaspis januarya species of jawless armored fish that lived during the Ordovician period. Image credit: Kaori Serakaki, OIST.

Most vertebrate lineages are first recorded from the Middle Paleozoic, well after their Cambrian origin and Ordovician invertebrate biodiversification events. This delay has often been attributed to poor sampling and long ghost lines.

Instead, Okinawa Institute of Science and Technology paleontologists Wahei Hagiwara and Lauren Sallan suggest that LOME fundamentally reorganized vertebrate ecosystems.

Using newly compiled global databases of Paleozoic vertebrate occurrences, biogeography, and ecosystems, they found that this mass extinction event coincided with the disappearance of the ubiquitous stem-cyclostome conodonts (extinct jawless marine vertebrates), as well as losses among early gnathostomes and pelagic invertebrates.

Subsequently, post-extinction ecosystems hosted the first definitive appearances of most of the major vertebrate lineages of the Paleozoic “Age of Fishes.”

“Although we don’t know the ultimate causes of LOME, we do know that there was a clear before and after the event. The fossil record shows this,” Professor Sallan said.

“We brought together 200 years of Upper Ordovician and Lower Silurian paleontology, creating a new fossil record database that helped us reconstruct the refugium ecosystems,” Dr Hagiwara said.

“From there, we could quantify the genera diversity of the period, showing how LOME led directly to a gradual but dramatic increase in gnathostome biodiversity.” »

LOME itself developed in two pulses during a period marked by prolonged global temperature fluctuations, alterations in ocean chemistry including essential trace elements, sudden polar glaciation, and sea level changes.

These changes have devastated marine ecosystems and created a post-extinction “gap” with low biodiversity. This gap persisted into the earliest Silurian.

Researchers confirm a previously proposed missing vertebrate diversity gap, known as Talimaa’s Gap.

During this period, overall richness remained very low and surviving faunas were almost entirely composed of isolated microfossils.

Recovery was slow: the Silurian period included a recovery interval of 23 million years, during which vertebrate lineages diversified gradually and intermittently.

Most Silurian gnathostome lineages diversified gradually and intermittently during an initial period of otherwise very low overall richness.

Rather than spreading rapidly across ancient oceans, the first jawed vertebrates appear to have evolved in isolation.

Scientists have discovered a high level of endemism among gnathostomes as early as the early Silurian, with diversification occurring in specific, long-lived extinction refuges.

One such refuge was southern China, where the first definitive evidence of jaws appears in the fossil record.

These early jawed vertebrates remained geographically restricted for millions of years.

Renewal and recovery after LOME matched those following similar climatic events like the end-Devonian mass extinction, including prolonged intervals of low diversity and delayed dominance of jawed fishes.

“In what is now southern China, we see the first complete fossils of jawed fish directly related to modern sharks,” Dr Hagiwara said.

“They were concentrated in these stable refuges for millions of years until they evolved the ability to cross the ocean to other ecosystems.”

“By integrating location, morphology, ecology and biodiversity, we can finally see how early vertebrate ecosystems rebuilt themselves after major environmental disturbances,” Professor Sallan said.

“This work helps explain why jaws evolved, why jawed vertebrates ultimately prevailed, and why modern marine life traces back to these survivors rather than earlier forms like conodonts and trilobites.”

The study was published January 9 in the journal Scientific advances.

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Wahei Hagiwara and Lauren Sallan. 2026. The mass extinction triggered the first radiations of jawed vertebrates and their jawless relatives (gnathostomes). Scientific advances 12(2); doi: 10.1126/sciadv.aeb2297