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Giant ammonites from Kremmling, Colorado, were the focus of an advanced class in the DMNS Certification in Paleontology Program in the fall of 1999. In this class, students worked with Dr. Emmett Evanoff. Emmett is a DMNS research associate and an instructor for the Museum's certification program. The purpose of the course was to observe the features of the giant ammonites, such as suture lines, scavenging marks, burrows, and compression cracks.

Right side: Click to enlarge

Left side: Click to enlarge
Ammonites are distant, extinct relatives of modern cephalopods, including the squid, octopus, and nautilus. Although ammonites appear to have the closest affinity to the chambered nautilus, the general consensus among scientists is that they are more closely related to the squid.

Sutures: Click to enlarge
Suture lines
Among the most characteristic features of ammonites (and their straight-shelled cousins, the baculites) are suture lines, the dendritic patterns that form where the internal chamber walls, or septa, connect with the outer shell. Like nautiluses, ammonites had chambered shells. But unlike the thick, smooth septa of a nautilus, chamber walls in ammonites and baculites were corrugated. This corrugation, like that in a cardboard box, gave the ammonites stronger shells that weighed less, required less calcium carbonate, and enabled the animals to grow much faster. Although suture lines may look random at first glance, they are highly organized and enable invertebrate paleontologists to identify species.

Emmett and his students also examined the ammonites for clues to what happened to the animals after they died. The ammonite shells were almost always broken more severely on the side that faced up after their deposition. This suggests that scavengers broke the shells trying to get at the soft tissue of the dead ammonite. Tubeworms and limpets also left their calling cards on the shell walls.

Of the ammonites found near Kremmling, Colorado, the majority were found left-side-up. (If you're wondering which side of an ammonite is the left side, picture the animal facing you. Its shell opening would be on the bottom.) Enough ammonites were found in this position to indicate that this death posture wasn't random, but what caused it is still not clear.

Sexual dimorphism: Click to enlarge
Sexual dimorphism in ammonites is the opposite of that in humans; female ammonites are bigger.
Like humans, ammonites had sexual dimorphism, meaning there were physical differences between males and females. Unlike humans, however, the female ammonites were consistently much larger than males. Females were also far more numerous at the Kremmling site. The disproportionate number of females has led Kirk Johnson and Ray Troll to theorize that the ammonites, like their nearest living relatives (the squid), had just spawned en masse, and the males had just departed from the scene, leaving the females to release their eggs and perish at the bottom of the sea floor. This theory played itself out on a Museum wall in the 1999 exhibition Cruisin' the Fossil Freeway.

For information on visiting the giant ammonite site, see Fossil Site Regulations.


References:
Johnson, Kirk R. 1999. "Night of the Giant Ammonites" Natural History. (June issue)
Johnson, Kirk R. and Richard K. Stucky. 1995. Prehistoric Journey: A History of Life on Earth. Boulder: Roberts Rinehart Publishers.
Ward, Peter D. 1992. On Methuselah's Trail: Living Fossils and the Great Extinctions. New York: W.H. Freeman and Company.
Wolfe, D.G. and J.I. Kirkland. 1999. "The Kremmling Paleontological Resource Area, Middle Park, Colorado" Boulder: University of Colorado Department of Geological Sciences.

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