Late Triassic Mystery Fossil #1 is...

...the posterior portion of the right squamosal bone of a phytosaur. Because of the robust nature of these elements they are often preserved even when the rest of the skull is not. In fact, they are a very common fossil in the Chinle Formation. This is actually fortunate because the morphology of the posterior process of the squamosal is diagnostic and therefore the entire skull does not need to be recovered to determine the presence of a taxon. The specimen to the upper left belongs to Pseudopalatus pristinus and is characterized by being extremely thickened and knoblike (down in the photo is posterior), with many surfaces for muscle attachment. Phytosaur evolution is characterized by changes in the "post temporal" arcade, including depression and narrowing of the supratemporal fenestrae as well as a posterior elongation and mediolateral widening of the squamosal process. The common preservation and recovery of isolated squamosals in concert with their diagnostic nature make them excellent biostratigraphic index fossils and thus they should be collected whenever found. The picture below shows a complete upper portion of a skull of P. pristinus with the position of the squamosal fragment above (from a different individual) outlined.