Ever wondered why the seas teem with fish today? It all goes back to a cataclysmic event millions of years ago, the Late Ordovician mass extinction (LOME). This event, which occurred roughly 445 to 443 million years ago, wasn't just a blip in history; it was a major turning point that paved the way for the rise of the fish we know and love. But how did this extinction reshape the early history of fishes? Let's dive in!
The prevailing mystery in vertebrate evolution has always been why major fish lineages appeared suddenly in the fossil record, long after their presumed origins. The research, conducted by paleontologists at the Okinawa Institute of Science and Technology, suggests that the LOME was the key. This extinction event triggered the parallel, endemic radiations of jawed and related jawless vertebrates (gnathostomes) in isolated refugia.
Before the LOME, the Cambrian and Ordovician periods saw the origins of many vertebrates. However, these early forms didn't immediately dominate the seas. This delay has often been attributed to the limitations of the fossil record.
But here's where it gets controversial... Paleontologists Wahei Hagiwara and Lauren Sallan propose that the LOME fundamentally reorganized vertebrate ecosystems. Their research, based on newly compiled global databases of Paleozoic vertebrate occurrences, biogeography, and ecosystems, revealed a direct link. The extinction event coincided with the disappearance of stem-cyclostome conodonts (extinct marine jawless vertebrates), along with losses among early gnathostomes and pelagic invertebrates.
In the aftermath, the ecosystems hosted the first definitive appearances of most major vertebrate lineages of the Paleozoic ‘Age of Fishes.’ "While 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 it,” Professor Sallan stated.
“We pulled together 200 years of Late Ordovician and Early Silurian paleontology, creating a new database of the fossil record that helped us reconstruct the ecosystems of the refugia,” Dr. Hagiwara explained. "From this, we could quantify the genus-level diversity of the period, showing how LOME led directly to a gradual, but dramatic increase in gnathostome biodiversity.”
The LOME itself was a complex event, unfolding in two pulses and marked by significant environmental changes. These included prolonged global temperature fluctuations, alterations in ocean chemistry, sudden polar glaciation, and sea level changes. These changes devastated marine ecosystems, creating a post-extinction ‘gap’ with low biodiversity. This gap persisted into the earliest Silurian.
The researchers confirmed a previously proposed interval of missing vertebrate diversity known as Talimaa’s Gap. During this time, global richness remained very low, and surviving faunas were composed almost entirely of isolated microfossils.
Recovery was slow, with the Silurian period comprising a 23-million-year recovery interval, during which vertebrate lineages diversified gradually and intermittently. Early jawed vertebrates appear to have evolved in isolation, rather than spreading rapidly across ancient oceans.
The scientists found a high level of endemism in gnathostomes from the very beginning of the Silurian, with diversification occurring in specific, long-lasting extinction refugia. One such refugium was South China, where the earliest definitive evidence for jaws appears in the fossil record. These early jawed vertebrates remained geographically restricted for millions of years.
Turnover and recovery following LOME matched those following climatically similar events like the end-Devonian mass extinction, including prolonged intervals of low diversity and delayed dominance of jawed fishes.
“In what is now South China, we see the first full-body fossils of jawed fishes that are directly related to modern sharks,” Dr. Hagiwara said. “They were concentrated in these stable refugia for millions of years until they had evolved the ability to cross the open ocean to other ecosystems.”
“By integrating location, morphology, ecology, and biodiversity, we can finally see how early vertebrate ecosystems rebuilt themselves after major environmental disruptions,” Professor Sallan concluded. “This work helps explain why jaws evolved, why jawed vertebrates ultimately prevailed, and why modern marine life traces back to these survivors rather than to earlier forms like conodonts and trilobites.”
So, what do you think? Does this change your perspective on the history of fish? Do you find it surprising that a mass extinction could be a catalyst for such significant evolutionary change? Share your thoughts in the comments below!
