Ammonite Fossil Next To Quarter For Scale
Rafting down the upper section of the San Juan River from Sand Island to Mexican Hat, in southeastern Utah, there is a fossil stop near the entrance of Lime Creek, after the river emerges from the canyon. While much of the San Juan River passes through colorful layers of sandstone, the fossils of note are in a dark gray limestone. Here is definite evidence that what is now desert was once covered by marine water. The ancient Paradox Sea inundated a large area in the region and teemed with the marine life whose remains are before us.
I’m not the only one with “paleo-passion” (a term possibly coined by writer Donovan Webster in his article, “The Dino Wars: Who Owns American’s Fossils”, Smithsonian Magazine, April 2009, p. 48-57), but as an earth science educator, I feel protective of even one small trace of a fossil- what’s called a “fossil mold”, an impression made in the substrate. This is the only ammonite fossil I’ve seen in its actual place of formation and so to me it is noteworthy. A thoughtless individual could damage the find, chiseling the mold out of the surrounding matrix, although I can’t imagine the tools or techniques that would be required to do so. It’s possible the true fossil form- the actual body of the animal making the mold- was removed by an eariler collector. In discussing our itinerary on the river, I make sure our guide, Brett LeCompte, working for the Four Corners Outdoor School, stops at the fossil site.
We read and hear so much on our geo-cultural travel study trip to the Colorado Plateau about policies and practices regarding how to respectfully visit Ancestral Puebloan ruins, that I am curious about laws protecting fossils. With a little research, I learn that currently fossils can be taken from private lands with permission of the owner for private collections or auction. Some fossil materials, like petrified wood, in limited quantities, may be taken from public lands without permits. However, additional legislation- the Paleontological Resources Protection Act- has been drafted mandating, “only trained, federally certified professionals be allowed to extract fossils from public lands,” (Webster, p. 56). The intent of the bill is to substanially increase penalties for illegal fossil excavation.
I wonder how much concern there is for ammonities which can be found for sell in fossil stores and on the web. Webster logged onto Ebay and found an ammonite specimen, expecting to go for $3000. Excavating specimens from their resting place eliminates the educational and scientific context, and for me the special significance and value of the fossil.
Ammonite structure
This particular fossil, resulting from ages of compression and cementation, is an invertebrate specimen, a highly developed marine mollusk belonging to the extinct family of ammonites of the cephalopod class. Similar to the shells of snails, cephalapods shells are divided into chambers. Modern members of cephalopods, like the squid and octopus have only two gills, while members of the ammonoid and nautiloid families have four gills. (I can’t imagine gills being distinguishable in a fossil.)
These living animals first appeared during the Silurian Period, approximately 400 million years ago and were abundant and widespread in oceans during the Jurassic and Cretaceous geologic periods. Ammonites are important index fossils, because they can be used to provide a relative date for the rock layer in which they are found as species evolved over time.
The shells of ammonites had hollow chambers separated by walls called septa. A tube called the siphuncle, connected the body with the chambers allowing the animal to use water or air to change its buoyancy in order to rise or drop in the ocean. Only the last and largest chamber was occupied by the living animal. Ammonites vary greatly in size, the largest early forms could be 15 feet. Shell shapes varied as well; some can be snail like and others are uncoiled.
Ammonites fed on plankton and lived exclusively in marine environments and so their presence indicate the location of prehistoric seas. Now, the arid West means little vegetation covers the rocks, making fossils like ammonites easier to find. The limestone and other sedimentary layers of rock deposited in ancient environments are now being exposed, revealing what this corner of the world was once like. For me, this fossil stop is as rich a window into the past as any of the human related historical sites we visit.
Rowing through the Paradox Formation
Travelling along this river by boat, takes you back in time, not only to the time of Ancestral Puebloans, but to a period where the environment was very different. In relation, our modern concerns about how climate change might impact a particular region, seem so anthropomorphically insignificant.
A nice animated image show several perspectives of an ammonite can be found at: http://www.paleodirect.com/ammonites.htm, accessed on 3-27-09.
