At this point we would like to mention some of the best known and most typical trilobite species from Utah, such as Elrathia kingii, Asaphiscus wheeleri, Zacanthoides typicalis or Peronopsis interstrictus (Itagnostus interstrictus). We also want to give an overview of the most important trilobite formations that the fossils contain. These include, for example, the Wheeler Formation, the Marjum Formation, the Weeks Formation or the Pierson Cove Formation. These formations are part of a sequence of sedimentary rocks deposited in a shallow sea in the Middle Cambrian. Here is a table showing the sequence of Middle Cambrian formations of Utah:

Formation	Age (million years)	Typical trilobites	Image
Weeks	505 - 500	Cedaria minor
Marjum	500 - 497	Modocia typicalis
Wheeler	497 - 490	Elrathia kingii
Pierson-Cove	490 - 485	Elrathia marjumi

Here on the table you can also see that the lithostratigraphic formations (rock units) partially overlap in time. Pierson Cove Formation corresponds partial to the Wheeler Formation and the Marjum Formation. As a result, the trilobite Elrathia kingii also occurs in the Pierson Cove Formation. And vice versa, albeit much less frequently. The transition between the species Elrathia marjumi and Elrathia kingii is sometimes diffuse. Elrathia marjumi is much rarer been found than Elrathia kingii and differs from it by the pleural segment fused in the pygidium. In addition, Elrathia marjumi often tends to have 14 body segments. However, it should be noted here that the genus Elrathia has a certain variability in the number of body segments. For Elrathia kingii there are typically 13 pleural segments. From Utah, all possible variants of these features could actually be found here. Also all possible numbers of 12 to 15 segments, as well as with and without the fussed pleural segment in pygidium. This is quite a special occurrence, as the transition of one species to another can be somewhat been observed here. Elrathia marjumi and Elrathia kingii occur intermittently in parallel. However, Elrathia kingii is no longer known in the younger Marjum Formation.