The basin's story begins in the Middle Eocene, around 45 million years ago, when the oldest preserved sediments - the Macarao Formation - were laid down in a shallow sea. Foraminifera-rich sandstones and calcareous siltstones, cross-cut by veins of gypsum, record an environment of warm, clear water. Layer by layer, the sea deposited its record: the Siamana Formation's thick carbonates in the Late Oligocene, the Uitpa Formation's calcareous mudstones in the Early Miocene, then the Jimol and Castilletes Formations through the Middle Miocene.

Each formation captures a different world. The Jimol records a fully marine environment around 17 to 16 million years ago. The Castilletes, deposited between roughly 16 and 14 million years ago, shows the transition to a river-delta system where freshwater met the sea. The youngest preserved layer, the Pliocene Ware Formation - formally defined only in 2015 - dates to between 3.5 and 2.8 million years ago. Today the basin sits under a hot semi-arid climate where July temperatures average 30.6 degrees Celsius and rain rarely falls outside October.

What makes the Cocinetas Basin extraordinary is not its geology alone but what the geology preserved. The Castilletes and Ware Formations are densely fossiliferous, yielding an astonishing catalog of ancient life. Giant crocodilians once dominated these waters - relatives of Purussaurus, a predator that could reach 10 meters in length, and Mourasuchus, a flat-headed filter-feeder unlike anything alive today. Ancient turtles of the family Podocnemididae shared the waterways, alongside Chelus colombiana, an extinct relative of the modern mata mata turtle.

The Uitpa Formation has produced fossil sharks: teeth from ancestors of the mako, the hammerhead, and the thresher, as well as specimens attributed to Carcharocles, the lineage that produced the megalodon. Boas slithered through the undergrowth. Fish from families still found in South American rivers today - characids, cichlids, piranhas - left their bones in the sediment alongside marine species, documenting the mixing zone where continent met ocean.

The basin's deepest scientific value lies in its timing. The Cocinetas sits near the Isthmus of Panama, the land bridge that rose from the sea and connected North and South America, triggering the Great American Biotic Interchange - one of the most dramatic episodes of animal migration in Earth's history. For tens of millions of years, South America had been an island continent, its fauna evolving in isolation. When the land bridge formed, animals crossed in both directions. Armadillos, ground sloths, and opossums walked north. Bears, cats, horses, and deer walked south.

The Ware Formation, dating to the Late Pliocene, preserves five distinct families of ground sloths in a single assemblage - an extraordinary diversity spanning two orders of magnitude in body mass. Some of these sloths were the size of large dogs; others rivaled elephants. Yet intriguingly, the species found here are not closely related to the earliest South American immigrants found in North America, suggesting that the interchange was more complex than a simple two-way highway. The Cocinetas fossils are rewriting scientists' understanding of how, when, and which animals made the crossing.

The basin sits entirely within the municipality of Uribia, in the Alta Guajira - the driest region in Colombia, where long droughts are common and the landscape more closely resembles the Sahara than the rainforests most people associate with this country. The Wayuu people have lived here for centuries, navigating a world of scarce water and relentless sun. Their territory extends across both sides of the Colombia-Venezuela border, a boundary they have never fully recognized.

Above the basin to the north, the Serrania de Macuira rises unexpectedly - a mountain range that traps moisture from the trade winds and supports the only dry forests in an otherwise total desert. The national natural park that protects it marks the northern boundary of the Cocinetas Basin. Below the surface, the basin's faults continue eastward beneath the Gulf of Venezuela, connecting this small patch of desert to the tectonic forces that shaped the entire Caribbean margin. The fossils lie scattered through the rock, waiting. Paleontologists have been working here intensively only since the early 2010s, and every field season brings new species to light.