Shell-ebrating Colorado’s Ammonites

Picture yourself standing in Colorado 75 million years ago during the Late Cretaceous. Strike that, you’d actually be swimming as Colorado was completely submerged by an inland sea that stretched from Alaska to Mexico. Now imagine swimming through these warm waters with pterosaurs flying overhead and fearsome marine reptiles, like the mosasaur and plesiosaur, swimming below. Also swimming at your feet would have been the less fearsome, but equally remarkable ammonite.  

Ammonites are an extinct group of mollusks, most closely related to squid and octopuses, that built coiled shells reminiscent of ram’s horns. Indeed, the Roman author “Pliny the Elder” named this group of animals in 79 AD for the ram-horn-wearing Egyptian god, Ammun. The earliest representatives of the group, technically called “ammonoids,” evolved around 419 million years ago during the Devonian—that is nearly 40 million years older than the first trees and a whopping 186 million years before the first dinosaurs.  

Later, “true ammonites” evolved some 200 million years ago in the Jurassic Period. Ammonites then flourished in Jurassic and Cretaceous oceans across the globe. That is, until an asteroid the size of Denver crashed into the ocean off the coast of Mexico around 66 million years ago, an event that caused around 75% of all life on Earth to go extinct and most famously included all dinosaurs—except for birds. Despite weathering three previous mass extinctions in Earth’s history, the Cretaceous–Paleogene mass extinction also ended the 350-million-year reign of the ammonites. 

Thankfully, we can study these creatures because they have an incredible shell fossil record. Ammonite shells are made up of a series of chambers that increased in size as the animal grew larger. The size of ammonite and ammonoid shells varied tremendously, as the smallest were less than half an inch in diameter while the largest were six-and-a-half feet—about the same size as the average NBA basketball player. That’s one big ammonite!  

The structure of the shells also changed over time with the walls between shell chambers becoming increasingly complex and, I must say, incredibly beautiful. Interestingly, the soft body parts of ammonites only occupied the newest chamber and were able to pump water out of the other chambers to adjust their buoyancy in the water, much like that of a submarine. 

Until 2021, humans had only recovered the shells of these animals and for the most part the squishy, soft parts of the ammonite body were unknown. Because ammonites are relatives of squid, octopuses and cuttlefish, scientists hypothesized that ammonites also had large eyes, tentacles and a siphon for jetting through the water. Satisfyingly, new evidence from exquisitely preserved ammonite bodies support these general hypotheses, although I’m sure many more amazing ammonite fossils that preserve soft tissue have yet to be uncovered by paleontologists… so discoveries await!  

With the arrival of the new “Jurassic Oceans: Monsters of the Deep” exhibition on March 21, the Denver Museum of Nature & Science is currently in the phase of “all prehistoric oceans all the time,” and ammonites hold a special place in our hearts.  

The collection of ammonites at the Museum is small but mighty, and highlights many of the amazing ammonite species found right here in our backyard. Perhaps our most remarkable ammonite fossils are the 73-million-year-old, two-to-three-foot long Placenticeras costatum specimens, from the Bureau of Land Management’s Kremmling Cretaceous Ammonite Locality in Kremmling, Colo. This site is thought to have been a breeding ground for ammonites because most ammonites found here are female. Perhaps the males departed after spawning and the females, much like their squid and octopus cousins, stuck around to release their clutch of eggs and ultimately perish en masse. Truly a love story for the ages.  

These fascinating and stunning ammonite specimens shells, preserved for millions of years in Colorado’s rocks, tell a story of evolution, extinction and the ever-shifting landscapes found in our small corner of the planet, millions of years before humans walked the Earth. By studying these creatures, we open a window into a thrilling ancient marine past and gain deeper insights into the powerful forces that continue to shape our world today. 

We hope to see you here soon for “Jurassic Oceans: Monster of the Deep” where you can learn even more about our planet’s incredible prehistoric marine past. Learn more about the exhibition, here. 

Get your tickets, here. 

Become a member, here. 

Dr. Gussie Maccracken is Assistant Curator of Paleobotany at the Denver Museum of Nature & Science.