McGill team proposes a 'Biotic Inflection' for mixed marine fossils in Colombia's Paja Formation

Researchers at McGill University and Colombia's Centro de Investigaciones Paleontológicas say fossils from the Early Cretaceous Paja Formation near Villa de Leyva record a Biotic Inflection: Jurassic marine lineages that later disappeared lived beside groups that later diversified. Hans Larsson and Dirley Cortés frame this as an evolutionary turning point without a mass extinction. The account reached the public through a ScienceDaily release that names neither the journal nor a specimen count. A coined term is not, by itself, a demonstrated mechanism.

The fossils, mostly held at the CIP, include long-necked plesiosaurs, giant pliosaur-grade reptiles, marine crocodile relatives, turtles, sharks, and crustaceans, grouped as the Paja Biota. An earlier feeding-network study by the same researchers reported an unusually large set of apex predators. The release ties that richness to warm greenhouse seas, high sea level, nutrient-rich coasts, and new Pacific-to-Atlantic connections through what is now Colombia. Those factors coincide with the fauna. They are not shown to have caused it. No diversity index, abundance table, or sampling-standardized comparison appears in the public account.

The surprising-twist framing misses older evidence. Tennant and colleagues, in a 2017 Biological Reviews synthesis, already treated the Late Jurassic to Early Cretaceous interval as protracted ecological turnover, not a Big Five mass extinction. Labeling holdovers as approaching extinction reads their later fate backward into the Barremian-Aptian. A formation can also blend animals that never met, because beds accumulate over long spans. Big predators are preferentially collected, so a stack of apex species can exaggerate food-web complexity. Passive environmental tracking remains as plausible as competitive replacement.

Establishing the pattern would require ammonite-zone occurrence matrices, tighter radiometric or biostratigraphic ages, and the same network method on other shelves, notably the Neuquén Basin, with completeness corrections. Until that comparison exists, Biotic Inflection is a label for coexistence during gradual turnover, not proof that major transitions generally skip mass extinctions. Methods note: the public record here is an institutional release about a fossil interpretation, with no stated sample size or formal design; the central limitation is time-averaging plus hindsight about which clades later vanished.