In 1795, German naturalist Alexander von Humboldt walked through the Jura Mountains along the France, Switzerland border. He noticed carbonate deposits that looked different from the older Triassic rocks nearby. By 1799, he called these layers "Jura limestone" to distinguish them from what came before. The name itself traces back to a Celtic root word meaning "wooded mountain." It passed through Gaulish and Latin before becoming the modern term we use today.
French geologist Alexandre Brongniart took this local observation and turned it into global science in 1829. His book described terrains jurassiques while correlating the Swiss rocks with similar oolitic limestones found in Britain. This publication officially coined the term Jurassic for geological timekeeping. Later, Leopold von Buch divided the period into three parts: Black Jurassic, Brown Jurassic, and White Jurassic based on rock color in Germany.
By 1842, French palaeontologist Alcide d'Orbigny had subdivided the system further using ammonite fossils. His work established ten stages, seven of which remain in use today. Albert Oppel refined these divisions again between 1856 and 1858 by creating biostratigraphic zones. Most modern stages were finally formalized at the Colloque du Jurassique à Luxembourg in 1962.
Ammonites As Global Clocks
Geologists divide the Jurassic Period into three epochs: Early, Middle, and Late. Each epoch contains specific stages defined by the first appearance of certain ammonite species. The Hettangian Stage began around 201.4 million years ago when Psiloceras spelae tirolicum first appeared in Austria's Karwendel Mountains. This boundary was ratified in 2010 at Kuhjoch Pass.
The Sinemurian Stage started roughly 199.5 million years ago with Vermiceras quantoxense appearing near East Quantoxhead in England. Its boundary was set in 2000 within the Blue Lias formation. Moving forward to the Pliensbachian, Bifericeras donovani marked the start around 192.9 million years ago at Robin Hood's Bay in Yorkshire. That boundary received official ratification in 2005.
In France, the Toarcian Stage took its name from the village of Thouars where Alcide d'Orbigny originally studied it in 1842. A boundary defined by Dactylioceras subgenus appears at Peniche, Portugal, which was ratified in 2014. The Aalenian Stage named after a German city began around 174.7 million years ago with Leioceras opalinum showing up in Spain. The Bajocian followed, starting 168.2 million years ago with Hyperlioceras mundum found in Portugal.