Ancient Colombian Seabed Reveals 'Biotic Inflection': Evolution Can Turn Without Mass Extinction

Near the town of Villa de Leyva in Colombia, the rock of the Early Cretaceous Paja Formation holds the remains of an ocean that no longer exists: long-necked plesiosaurs, relatives of marine crocodiles, turtles, sharks, crustaceans and more. Most of what has been pulled from the formation is kept at the Centro de Investigaciones Paleontológicas, known as CIP.

Researchers from McGill University and CIP have been studying this material, and they say it records something unusual. The community, which they call the Paja Biota, mixes lineages left over from the Jurassic with groups that were just emerging in the Cretaceous. Older families of animals were nearing the end of their run while newer ones were beginning to spread, and the two shared the same waters.

From that observation, McGill biology professor Hans Larsson and PhD graduate Dirley Cortés have proposed a name for the pattern: a "Biotic Inflection." They define it as an evolutionary turning point in which declining groups close to extinction coexist with newer lineages that are starting to expand and diversify. The idea carries a notable implication. Major shifts in the history of life may not always require a mass extinction to drive them.

Cortés describes the formation as exceptionally rich, with giant marine reptiles alongside turtles, sharks and crustaceans. Together, he says, these animals made up one of the most ecologically complex marine communities found to date. The team had already reconstructed the feeding relationships among the animals in an earlier study. That work revealed a highly intricate food web, with an unusually large variety of apex predators at its top.

The researchers think a particular set of conditions made such a crowded ecosystem possible. The climate was a warm greenhouse, sea levels were high, and coastal waters were rich in nutrients. New ocean connections were also opening between the eastern Pacific and the early Atlantic through the region that is now Colombia. Together, they suggest, these factors gave different groups of animals room to thrive in the same place. Many of those influences still shape biodiversity around the world today.

Larsson calls it a "perfect storm" of environmental factors known to promote ecological complexity, arriving just as ancient lineages were about to vanish and new groups were starting to flourish. "I think we've discovered a new model of global biodiversity change," he said, adding that the next task is to learn how widespread it was in Earth's history and how to quantify its impacts.

The team is careful about what is not yet established. They say more evidence is needed to know how common a Biotic Inflection might have been. That will take additional fossil discoveries, more accurate estimates of fossil ages, and comparisons with geological formations elsewhere in the world to see whether similar transitions occurred.

The work was funded by the Natural Sciences and Engineering Research Council of Canada, the Smithsonian Tropical Research Institute, the Anders Foundation, the 1923 Fund, Gregory D. and Jennifer Walston Johnson, the Fonds de recherche Nature et technologies Québec, and the McGill Biology Department Writing-Year Award 2024-2025. Whether the Paja Formation is a one-off or the first example of a broader pattern now depends on those comparisons.