A Triassic aquarium in stone
During the Middle Triassic period, between 247 and 237 million years ago, Monte San Giorgio was not a 1,097-meter mountain but a shallow, subtropical marine lagoon on the edge of the Tethys Ocean. The conditions in this basin were perfect for creating a Lagerstätte—a sedimentary deposit that preserves fossils in exceptional detail. The lagoon floor was anoxic, meaning it lacked oxygen. This prevented the decay of dead animals and deterred scavengers. As a result, complete, articulated skeletons were buried and preserved in layers of bituminous shale and dolomite.
The mountain contains a sequence of at least five distinct, highly fossiliferous layers from the Middle Triassic. The most famous is the Besano Formation, also known as the Grenzbitumenzone. These dark, oil-rich shales have yielded a diversity of marine and even terrestrial life. Paleontologists have been systematically excavating the site since 1924, and the total number of collected specimens now exceeds 21,000. These finds are one of the world's most complete pictures of marine life during this chapter of Earth's history.