SYSTEMATIC REVISION OF MARJUMAN STAGE (MIAOLINGIAN SERIES: GLOBAL GUZHANGIAN STAGE) TRILOBITES FROM THE LINCOLN PEAK FORMATION, EASTERN NEVADA, WITH NEW SILICIFIED COLLECTIONS

Trilobites from the Laurentian Marjuman Stage (middle Cambrian) have been intensely studied since the discovery of rich fossiliferous units in the Great Basin of the western United States by Charles Walcott and others in the nineteenth century. New collections from localities in the Lincoln Peak Formation of eastern Nevada have yielded abundant secondarily silicified remains that provide a wealth of morphological detail. Acid preparation of limestone containing silicified fossils allows for the total recovery of specimens, effectively eliminating mechanical sampling bias. The present work is focused on material recovered from two localities, one near Cleve Creek and one near Patterson Pass in the Schell Creek Range of eastern Nevada.

The Lincoln Peak Formation contains a diverse assemblage of trilobites belonging to a total of twenty-two genera representing four families of known ordinal affinity and six of unknown ordinal affinity. Many of the species are poorly known from other localities with other modes of preservation, but previously undocumented aspects of their anatomy and growth are revealed by study of silicified specimens.

For example, no species of Pemphigaspis Hall, 1863, had previously been reported with specimens that preserve the visual surface. Intact silicified specimens retain the visual surface and show that it has a large area in proportion to the main body of the librigena. The pleural field of the holaspid pygidium is characteristically inflated and it can now be demonstrated that this morphology develops from a more typical one in smaller silicified specimens. Two distinct species, assigned to the genus Carinamala Palmer, 1962, are reported. C. springensis n. sp. is distinguished from C. pattersonpassensis n. sp. by its longer pygidium and lack of posteromedian glabellar node. The rostral plate is documented for both species and is shown to be fan-shaped in dorsal profile with extreme transverse narrowing posteriorly. The ventral morphology of several new and existing species is also described and high-resolution images have been produced to illustrate specimens in detail.

In short, this new silicified material from eastern Nevada provides important information on the taxonomic makeup, development, and morphology of Marjuman trilobites.