An Ancient Walkway
In the high-altitude desert of Salta Province, the colourful rock layers of the Yacoraite Formation hold a peculiar secret from the Late Cretaceous. This geological unit, dating to the Maastrichtian-Danian ages, documents the final moments of the dinosaurs and the dawn of the Paleocene. Around 70 million years ago, this region was not a series of jagged mountains, but a vast, shallow saline lake system that periodically dried out, creating extensive salt flats. The formation is primarily composed of limestone, calcareous sandstones, and mudstones, which created the perfect medium for preserving the delicate impressions left by long-vanished creatures.
These ancient mudflats were a hub of activity. The full ecosystem is partially preserved, with fossils of fish like Coelodus toncoensis, crocodiles, and turtles found within the layers. The landscape was also crossed by dinosaurs, with tracks from hadrosaurs and sauropods recorded in the formation. But some of the most revealing fossils are not bones, but traces: the footprints of birds that waded through the shallows.
Convergent Evolution in Stone
The bird tracks are known to science as Yacoraitichnus avis. They are tridactyl (three-toed) and show clear evidence of webbing between the digits. What makes these trace fossils remarkable is their age, but their uncanny resemblance to the tracks of modern flamingos. The stride length, the gait pattern, and even traces suggesting filter-feeding behavior in the mud are all consistent with how flamingos move and eat today.
This does not mean flamingos lived during the Cretaceous. The earliest definitive flamingo fossils are from much later periods. The tracks of Yacoraitichnus avis were made by an unrelated group of shorebirds that had evolved the same physical and behavioral adaptations for life in salty, shallow lakes. This is an example of convergent evolution, where different species independently develop similar traits to adapt to similar environments or ecological niches. The fine-grained sediment of the Yacoraite Formation's ephemeral ponds captured this ancient behavior, preserving a snapshot of a lifestyle that would re-emerge millions of years later in a completely different bird lineage. The tracks are often found in association with stromatolites, layered structures built by microbial mats, which further points to a shallow, saline aquatic environment.