PRELIMINARY WORK ON THE ONTOGENIES OF TRILOBITES FROM THE WEEKS FORMATION (CAMBRIAN; UTAH)

The Weeks Formation (House Range, Utah) is an important Cambrian (Guzhangian stage) site for its fauna of well-preserved and diverse, articulated trilobites. The articulated trilobites are only part of the story—there are also trilobite sclerites that are preserved in silica and recoverable via acid digestion. The acid residues derived from the Weeks Formation contain a diverse set of juvenile trilobites. Matching these juvenile stages with their corresponding articulated adult stages has the potential to provide a suite of new characters that will be useful in deciphering the evolutionary relationships of Cambrian trilobites.

The trilobite fauna described from the Weeks Formation is composed of 20 known species (plus 17 species of agnostid arthropods). Sorting and matching the juvenile stages with their corresponding adult stages is still in progress, but the ontogenetic sequences of several species have become fairly well-known. For instance, the genal and occipital spines of Norwoodia boninoi Robison & Babcock, 2011 are present early in the meraspid phase. N. boninoi also has a fine cephalic covering of minute tubercles, and the preglabellar field is much shorter than in holaspids. Other trilobites with tentatively identified juvenile stages include Menomonia sp., Ithycephalus stricklandi Robison & Babcock, 2011, as well as an unassigned agnostid. Curiously, the common articulated trilobites Cedaria and Modocia have not yielded any obvious sclerites yet.