Invertebrates are among the most diverse multicellular life forms on our planet, and the methods used to collect and study invertebrate fossils vary with the type of fossils in question. In this example, the Denver Museum of Nature and Science focuses on one type of fossil: 73 million-year-old giant ammonites from Kremmling, Colorado.

The ammonite site is about thirteen miles north of Kremmling, and is administered by the Bureau of Land Management (BLM). Although the site is open to scientists and the general public, vehicle access is restricted and the site has no visitors center. Therefore, visitors should contact the BLM office for directions, road conditions, and parking information at 970-724-3437. Please note that visitors are not permitted to collect fossils from the Kremmling site.

Concretions eroding out of the hillside can reveal the fossil horizon.
Ammonites are extinct relatives of modern cephalopods. These ancient mollusks had coiled shells ranging in size from less than a quarter inch to more than six feet in diameter. Like their straight-shelled cousins the baculites, ammonites went extinct at the same time as dinosaurs, 65 million years ago.

During the age of the last dinosaurs, the Western Interior of North America was covered by the Cretaceous Seaway, a shallow sea that advanced and retreated several times. Many species of ammonites lived in the Cretaceous Seaway, and their fossils can be found throughout western North America today.

Dr. Kirk Johnson is the DMNS curator of fossil plants and invertebrates. In August 1998, he was joined by research associate Dr. Emmett Evanoff and Alaskan artist Ray Troll (coauthor of Planet Ocean) to excavate ammonites from a site near Kremmling, Colorado. These fossils became the centerpiece of the Museum's Cruisin' the Fossil Freeway exhibition as well as the subject of an advanced invertebrate paleontology course in 1999.

Exposing concretion: Click to enlarge
Paleontologist unearths a fossil concretion.
Invertebrate fossils are sometimes preserved in concretions, or round, hard rocks. The chemical processes that occur when the animal dies precipitate calcium carbonate in the surrounding mud, causing it to harden quickly. This is a boon for paleontologists who find the fossils millions of years later — the hard concretions often preserve a fossil's shape despite crushing pressure from overlying sediment that may be hundreds of feet thick.

As with vertebrates, prospecting for invertebrates starts with knowing where and how to look.

  • Fossil invertebrates are most easily found in badlands: arid areas with exposed outcrops. Badlands don't support much vegetation, and this means they expose plenty of bare rock that might be rich with fossils. Riverbanks, beaches, and road cuts are also good places to look since erosion is active on fresh outcrops in those spots.
  • Among the three kinds of rocks, igneous (formed from melting rock), sedimentary (eroded by wind or water and later redeposited), and metamorphic (changed by extreme heat or pressure), almost all fossils are found in sedimentary rocks.
  • An area that has produced good fossils in the past is worth visiting again, so paleontologists pay attention to finds recorded by other paleontologists and by the general public.

Split concretion: Click to enlarge
Cracked concretion reveals mollusk fossil.
Before collecting any fossils, paleontologists must pay careful attention to who owns the land and seek the proper permission from the landowner or governing agency.

Fossil-bearing concretions are usually deposited in layers. These layers may erode out of the sediment, leaving clues for paleontologists as to where the fossil horizon is. If the land surface is flat, paleontologists can sometimes search for concretions by probing below the surface with a metal rod and listening for clinking sounds. Following the concretion layer in Kremmling has revealed a fossil horizon extending for several miles.

Finding the fossil horizon is just the first step in a process that involves several disciplines and calls for critical thinking, creativity, and teamwork.


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