A few ago 445 million yearsThe Earth experienced one of the most critical moments in its history: a mass extinction that wiped out nearly all of the planet's inhabitants. 85% of marine speciesFar from being the end of the game, that biological disaster opened a completely new chapter for life in the oceans.
An investigation published in the journal Science Advances reveals that, precisely in that scenario of global crisis, Jaw-headed fish appeared and began to dominate, known as gnathostomes. Thanks to the detailed analysis of two centuries of fossil recordsThe scientific team has managed to reconstruct how a climate catastrophe ended up driving the expansion of the first jawed vertebrates.
An unrecognizable planet: this is what Earth looked like in the Ordovician period
On Ordovician periodDuring the period between approximately 486 and 443 million years ago, the planet's appearance was nothing like it is today. The supercontinent GondwanaThe area where we would find South America, Africa, Australia, Antarctica, India, and Madagascar today, concentrated a large part of the emerged landmasses and was surrounded by shallow, temperate seas.
In that scenario, the The poles were practically ice-free and the waters were warm, which favored a huge diversity of marine invertebratesalgae and the first jawless vertebrates. However, this balance was not going to last long: the planet was on the verge of one of the major climate transitions of its geological history.
The data collected by the international team shows that, relatively quickly in geological terms, The climate transitioned from a greenhouse effect to a glacial phase.This abrupt change triggered a chain of events that ended up completely altering life in the shallow oceans surrounding Gondwana.
According to researchers, the cooling was so intense that the The seas behaved like sponges that lose water.The sea level dropped, large coastal areas were exposed, and new landmasses appeared. extensive glaciers over the supercontinent. The result was the disappearance of many marine habitats that, until then, had been true biodiversity hotspots.
In this hostile environment, marine communities were subjected to extreme environmental pressure. The combination of loss of marine platforms, changes in water temperature, and alterations in ocean chemistry This resulted in a collapse of much of the known life in the Ordovician seas.
A mass extinction in two waves
The extinction of the Late Ordovician It was not a single blow, but a complex process that unfolded in two great wavesThe new study, based on fossils from several continents, details how each of these phases impacted marine ecosystems differently.
At PhaseDominated by the expansion of glaciers over Gondwana, the planet transitioned from a warm climate to a markedly cold one. This change drastically reduced sea levels, exposing vast areas of shallow seas and eliminating essential coastal habitats for many species. The most affected ecosystems were precisely those that harbored the greatest diversity of invertebrates and primitive vertebrates.
La second wave It arrived a few million years later, when the climate shifted abruptly again. The melting of the ice sheets caused another rise in sea level and released large volumes of warmer water, poor in oxygen and loaded with sulfur compoundsThe species that had managed to adapt to the cold then faced an environment almost opposite to the one that had allowed them to survive.
This double climatic impact had devastating consequences: a very high percentage of marine species disappeared forever. However, the analyzed fossil records clearly show that, after this collapse, a permanent biological vacuum did not occurbut the beginning of a profound reorganization of ecosystems.
The authors of the study emphasize that, although The causes of that mass extinction are not yet fully understood.It is clear that this marked a turning point in the history of vertebrates. As researcher Lauren Sallan, from the Okinawa Institute of Science and Technology (Japan), explains, it was precisely during this period that Jawed fish began to dominate other groups.
Shelters, isolation and the rise of the gnathostomes
One of the most interesting aspects of the work is the reconstruction of the ecological shelters that allowed certain vertebrates to survive during and after the extinction. Researcher Wahei Hagiwara and his team have compiled a new database from fossils obtained over 200 years of paleontology, focusing on the Late Ordovician and Early Silurian.
These data indicate that, while many groups were wiped out, some The surviving vertebrates were confined to very limited geographical areas.These were truly isolated corners where conditions were somewhat more favorable. In these restricted spaces, competition was sharply reduced due to the disappearance of numerous dominant organisms.
Within that framework ecosystems with many gaps to fill, gnathostomes —the first vertebrates with hinged jaws— found a unique opportunity. With fewer rivals and a large number of empty ecological niches, these fish were able to expand into different marine environments and exploit new food resources.
Scientists compare this situation to the classic case of Darwin's finches in the Galapagos IslandsUpon arriving in an environment with diverse resources and limited competition, finch populations gradually specialized, giving rise to species with peaks of very different shapes depending on their diet. Something similar would have happened with the first jawed fish in those early Silurian refuges.
While Jawed fish were mainly restricted to regions such as southern ChinaTheir jawless relatives continued to evolve in other seas, where they remained abundant for another 40 million years. This parallel evolution suggests that the initial advantage of gnathostomes was not immediate on a global scale, but rather consolidated over time as they colonized new areas.
From catastrophe to ecological reboot
One of the key findings of the study is that the Late Ordovician extinction acted as a kind of "ecological reset"Far from completely erasing the structure of ecosystems, the event allowed other groups of organisms to occupy the niches freed up by conodonts, arthropods and other invertebrates that had been dominant until then.
Research shows that, after the crisis, marine ecosystems reorganized themselves into structures comparable to the previous ones, but populated by different speciesIn other words, the ecological architecture—who occupies what role in the food chain, what types of habitats exist, how energy flows—was reconstructed following similar patterns, albeit with new actors.
This behavior would not be an isolated case. According to the authors, throughout the Paleozoic Similar processes are observed after other major episodes of environmental stress. Each extinction event, associated with climatic or chemical changes in the oceans, would have generated a recurring cycle of "diversity reset" in which some groups disappear and others diversify to take their place.
In this context, jawed fish appear as one of the big evolutionary winners of that crisis. Once adapted to the new marine landscapes and with access to resources previously monopolized by other organisms, gnathostomes were able to expand their distribution and experience remarkable diversification.
What remains an enigma for the scientific community is why, among all the survivors of the extinction, It was precisely the jawed fish that ended up prevailing. in the long term. Although articulated jaws provided a clear advantage for feeding in more varied ways, research is still underway to determine what other ecological and environmental factors contributed to their success.
The diversification of jawed fish and their legacy
As marine life recovered from extinction, gnathostomes began to develop a a wide variety of forms and ways of lifeMany of them adapted to reef environmentsdeveloping more streamlined bodies, different swimming abilities, and, in some cases, alternative and highly specialized oral structures.
These innovations allowed jawed fish to explore a wide range of diets, from mobile prey to sessile organisms, giving them an ecological flexibility that other groups lacked. Over time, this versatility favored the emergence of increasingly diverse lineageswhich would eventually give rise to the large groups of vertebrates today.
Today, the vast majority of vertebrates—including bony fishSharks, amphibians, reptiles, birds, and mammals—all descend from those early gnathostomes that took advantage of the window of opportunity opened after the Late Ordovician extinction. From an evolutionary perspective, the origin of our own lineage ultimately traces back to that time. episode of crisis and ecological reconstruction.
Beyond the purely paleontological interest, this type of study offers relevant clues to understanding how current marine ecosystems may respond to accelerated environmental changesAlthough today's situation has different causes—such as human activity and anthropogenic climate change—geological history shows that major alterations in climate and oceans are often accompanied by profound readjustments in biodiversity.
From Europe, where there is a strong tradition of research in marine paleontology and numerous fossil sites distributed across countries such as Spain, United Kingdom or GermanyThis type of work is followed with particular attention. The data obtained from European outcrops, along with those from Asia, America, and other continents, allow us to construct a global view of how the oceans have changed throughout geological time.
The history of jawed fish illustrates how a A catastrophic event can become the trigger for a new evolutionary stage.The mass extinction 445 million years ago not only led to the disappearance of a large part of marine life, but also paved the way for a previously secondary group of vertebrates to end up playing a leading role in most of the subsequent history of animals with an internal skeleton.
