Night of the Giant Ammonites

Natural History, July, 1999 by Kirk R. Johnson

Why did ancient, tire-sized shellfish congregate and then perish en masse? A paleontologist and an artist imagine a romantic scenario.

During the Cretaceous, long-necked plesiosaurs, carnivorous marine reptiles known as mosasaurs, and giant fish and turtles plied the waters of a seaway that covered much of western North America. Stretching from what is now Utah to Missouri and from the Gulf of Mexico to the Arctic Ocean, the Western Interior Sea was also home to ammonites. These mobile, predatory cephalopods--whose modern relatives include the squid, octopus, cuttlefish, and chambered nautilus--ranged from less than a quarter inch to more than two yards in diameter. They filled a variety of marine niches until their demise 65 million years ago. Their fossils are, to my mind, every bit as exciting as those of their vertebrate contemporaries.

Today evidence of a mass death of giant ammonites lies on a gentle sagebrush slope a few miles outside the small town of Kremmling in the Colorado Rockies. It takes most visitors to the site more than a few moments to realize what they are looking at as they wander across a field strewn with four-hundred-pound rocks that look like giant mushrooms. These rocks, left from fossil digs, are the empty halves of concretions that once enclosed fossilized ammonites.

To understand the concretions, you need to wind the clock back to the time of the inland sea, when the Rockies did not exist and the shale that now comprises the hills near Kremmling was hundreds of feet below sea level. In this ancient sea, thousands of ammonites met their end and drifted to the bottom, where scavengers fed on the unexpected bounty. The site was close enough to the shoreline that mud from nearby river mouths sifted over the shells and rapidly buried them. A chemical reaction precipitated calcium carbonate in the mud around the shells, forming the rock-hard concretions, while the surrounding mud was flattened into soft shale, and the ammonites fossilized. The concretions served as heavy stone tombs that saved the fossils of the forty-pound invertebrates from being crushed by the thousands of feet of sediment that were subsequently deposited on top of them. Eventually, erosion caused by the rise of the Rocky Mountains removed the overlying shale and left a grassy slope with the concretion-encased fossils.

The Kremmling site has been known for years; only a few of the fossils, however, have been collected for scientific study. In August 1998, artist Ray Troll, University of Colorado paleontologist Emmett Evanoff, and a crew of volunteers joined me on an expedition to recover ammonite fossils for research and display at the Denver Museum of Natural History. Using a modified metal rake, we probed the soil of the knoll until we heard the characteristic clink of the shallowly buried concretions. We would spend about fifteen minutes digging and brushing before a concretion lay exposed, like a huge egg, at the bottom of the hole. Fortunately for us, the freeze-and-thaw cycles of the Colorado Rockies had already cracked most of the concretions along the plane of weakness defined by the giant shell. With a gentle tap of a crowbar, we could get our fingers under the cracked rock and lift off the top half as you would lift a manhole cover. By systematically testing the ground, we were able to locate and map dozens of ammonites, and by following the layer of concretions along the ridge top, we could see that it extended for several miles.

Like humans, ammonite males and females had physical differences, but ammonite females were much the larger of the two sexes. We found many of the large smooth shells that belonged to female ammonites, sometimes just several feet apart. But we uncovered only a few of the much smaller male shells. Our observations and mapping allowed us to envision the Cretaceous scene: a muddy sea bottom strewn with thousands of dead and dying steel-belted-radial-sized female ammonites. This forced the question of what had caused so many of them to come together and then expire. If we had found a mix of males and females, we would have looked for some environmental catastrophe. Storms, sudden changes in water chemistry, or a rapid depletion of oxygen can all cause mass death in marine environments, but these disasters are not sexually selective. The presence of so many females suggested that these cephalopods had gathered for a reason.

While it is tempting to compare ammonites with modern chambered nautiluses (they bear a resemblance and both have shells), scientific consensus places them closer to squid. Like spawning salmon, many living squid reproduce just once and then perish, sometimes en masse.

Perhaps the hill of giant fossil shells in the Colorado Rockies was the site of a Cretaceous night of mass spawning. Phases of the moon cued the ammonites to congregate; after the males had fulfilled their role, they left the scene. Imagine the sea by the light of the moon and the muffled clatter of thousands of forty-pound female shellfish as they release the next generation in the form of clouds of planktonic embryonic ammonites. Then, with their life cycle complete, they shudder, sink to the seafloor, and begin their trip into the fossil record.