Dig into the right cliff face in the Borbon Basin of northwestern Ecuador and you can hold a megalodon tooth in your hand. The rock is called the Onzole Formation - a geological unit laid down in the Early Pliocene, roughly 5 million years ago, when the coast of what is now Esmeraldas province sat underwater and was being blanketed by shales, mudstones, and sandstones laced with the remains of a sea that no longer exists. Megalodon, Carcharocles megalodon, is the showpiece fossil: the largest shark that ever lived, whose serrated teeth are still being prised out of the Onzole's shoreline outcrops. But megalodon is only one item on a long list. The Onzole Formation has given up dozens of fish species, many of them now extinct, along with gastropods, bivalves, and scaphopods whose shells time has turned into stone.
The formation splits into two parts that record two different undersea environments. The lower Esmeraldas Member is a calcareous shale packed with foraminifera, those tiny single-celled organisms whose shells make up much of the world's chalk and limestone. The shale is tuffaceous, meaning it contains volcanic ash - a reminder that Pliocene Ecuador had the same restless volcanoes it has now. Curiously, the shallow-water fauna in the Esmeraldas Member was deposited at around 1,000 meters depth, carried there by gravity flows that swept faunal material from the shelf around 75 meters down into deeper water. Above it lies the younger Sua Member, a bioturbated silty sandstone laid down in a shallow coastal setting where worms and other burrowing animals reworked the sediment before it hardened. The Onzole therefore preserves two different slices of the Pliocene ocean, stacked one on top of the other.
The shark fauna of the Onzole reads like a who's-who of Miocene and Pliocene predators. Carcharocles megalodon is the giant - a shark that reached at least 15 meters in length, with a bite powerful enough to crush the ribs of whales. Alongside it in the Onzole rocks are tiger sharks (Galeocerdo aduncus), sand tigers (Odontaspis acutissima), lemon sharks (Negaprion eurybathrodon), snaggletooth sharks (Hemipristis serra), and the strange cookiecutter shark Isistius triangulus, which even today takes circular bites out of whales and tuna in the open ocean. Several species of Carcharhinus - requiem sharks - round out the list. The assemblage tells a story. The Pliocene Pacific off northwestern South America was a rich feeding ground, dense with bony fish for the sharks to hunt and with marine mammals whose bones also show up in nearby deposits.
The Onzole Formation's real richness is not megalodon but its bony fish. Paleontologists have identified more than a dozen species from the rocks, many named for the formation or for Ecuador itself. Stellifer onzole takes its name directly from the formation. Diaphus ecuadorensis and Lepophidium borbonensis acknowledge the country and its basin. Cuskeels, hakes, lanternfish, croakers, scorpionfish, flatfishes, porgies, bonnetmouths, grunts, cardinalfishes, gobies, eels, and anchovies all appear in the fish list. Many belong to groups still fished commercially in the same waters today. A few - preserved only as teeth or otoliths, the small ear bones that stay intact long after the rest of the animal has decayed - are known from the Onzole and almost nowhere else. The formation functions as a time capsule of Pacific marine biodiversity at the threshold of the Ice Ages.
Geological formations can feel abstract - lines on a stratigraphic column, names known only to specialists. The Onzole is more than that. It preserves a moment just before megalodon's extinction, when the Pacific off South America was a warmer, different ocean, and it lets paleontologists calibrate how fish lineages in the tropical eastern Pacific responded to the closing of the Isthmus of Panama, which walled off the Atlantic from the Pacific around the time these sediments were being laid down. The Onzole is not studied by mass tourists. It is studied by specialists who travel to coastal Esmeraldas, walk along eroding cliffs, and tap at the shale with geological hammers. What they carry home - teeth, vertebrae, otoliths - builds the story of a sea that existed here five million years ago and then slowly transformed into the one we know.
The formation outcrops in Ecuador's Borbon Basin along the northwest coast, centered near 1.00N, 79.60W. Fossil localities include Punta La Gorda and Punta la Colorada along the Esmeraldas coast. The nearest airport is Esmeraldas (SETN). Recommended viewing altitude is 3,000-6,000 feet AGL along the coast; mid-morning typically offers the clearest visibility before afternoon cloud builds over the coastal mountains.