A Cretaceous lagoon of death
In Brazil's Araripe Basin, layers of limestone from the Early Cretaceous period hold a massive collection of exquisitely preserved fossils. This area, part of the Santana Group, contains two specific fossil beds, or Lagerstätten, known as the Crato and Romualdo Formations. About 110-115 million years ago, this region was a shallow, brackish lagoon connected to the nascent South Atlantic Ocean. The water chemistry of this ancient lagoon was unusual. The bottom layers were likely low in oxygen and high in salt, creating an environment hostile to the bacteria that cause decay.
When fish died and sank to the bottom, they were quickly covered by fine sediment. Minerals in the water, particularly phosphates and carbonates, then began to precipitate around the organic matter. This process formed hard limestone nodules that encased the organisms, preserving their three-dimensional structure before the soft tissues could decompose. This rapid entombment is why paleontologists can find fossils with bones and muscle fibers, gills, skin impressions, and even the contents of their stomachs. The detail is microscopic, revealing cellular structures and, in at least one species, a completely fossilized heart.
The Araripe Basin is now a UNESCO Global Geopark, recognized for its geological and paleontological value. The limestone is quarried for construction and cement, and local workers often split the nodules to reveal the fossils inside.
A menu from 110 million years ago
The fossils of the Santana Group provide a detailed snapshot of a complete Cretaceous ecosystem. Over 25 species of fish have been identified, many with their last meals still preserved in their digestive tracts. This allows scientists to reconstruct predator-prey relationships with unusual certainty.
One common predatory fish, Rhacolepis buccalis, is often found with smaller fish, such as Dastilbe crandalli, in its stomach. Some specimens of Rhacolepis have also been found with hundreds of tiny shrimp in their gut. The preservation is so complete that in 2016, researchers using high-resolution CT scans identified the first-ever fossilized three-dimensional heart inside a Rhacolepis buccalis specimen. The ancient heart was surprisingly complex, featuring five rows of valves in a chamber called the conus arteriosus, a transitional form between primitive fish and more modern ray-finned fish.
Dastilbe crandalli, a relative of the modern milkfish, is one of the most abundant fossils found in the Crato Formation. These smaller fish, typically 25 to 60 millimeters long, were likely subject to frequent mass mortality events, possibly due to changes in water salinity. Larger individuals of Dastilbe were also predators and may have been cannibalistic. Beyond fish, the formations contain a huge diversity of other life, including insects, spiders, scorpions, pterosaurs, turtles, and some of the earliest flowering plants.