We recently returned from a month long road trip in the desert southwest where we were lucky enough to find lots of critters to photograph and we stopped at a number of fossil collecting sites along the way (more on the critters later…).
Our trip was a bit too early in the season to dig in some of the more snowy sites, such as the Green River Formation in Wyoming, where it is possible to find 50 million year old fish fossils deposited in a collection of inter mountain lakes during the Eocene. We did however have much better luck a little further south digging in the Wheeler Shale found in southwestern Utah.
The Wheeler Shale is a 100-200 meter thick calcareous shale layer formation that was deposited during the Cambrian (~500 million years ago). The layer also consists of shaley limestone, mudstone, and flaggy limestone. Select areas within this layer are rich with fossils. While preserved soft bodied organisms can be found (via a type of preservation typically associated with the famous Burgess Shale in Canada), the layer is more well known for preserving numerous trilobite species. Some of these trilobites can be found in excellent shape and occasionally in mortality plates (multiple fossils within one flake of rock matrix).
Trilobite fossils can be found in this formation either as preserved exoskeletons of the organisms themselves or as the mold or impression left behind by the exoskeleton. No fancy tools are necessary to dig for these trilobites, a rock hammer and chisel will suffice just fine. Cleaning them is another matter! It is important to learn where you can and cannot legally dig and collect fossils in the area, but this can be simplified if your short on time by digging at one of the ‘pay quarries’ such as U-Dig near Delta, Utah.
Of the commonly found trilobites in the Wheeler Shale, Peronopsis interstrictais one of the smallest, usually only a few millimeters long, and belonged to the order Agnostina. It is suggested that Agnostina may have been planktonic and associated with deep cold water deposits, however there is still quite a bit of debate on this.
Most of the time, species such as the primitive Elrathia king, Asaphiscus wheeleri, and Bolaspidella housensis are found missing their ‘cheeks’ (also known as librigena), making complete fossils much more exciting to hunt for!
Elrathia kingi deposited slightly above an Asaphiscuc wheeleri impression in the shale.
For more information on where to dig for fossils in North America, check out Fossil Hunting Maps For North America.