Abstract

Holmiids are a major component of the trilobite fauna in the middle part of the Montezuman Stage, the lowermost stage of the Cambrian of Laurentia with a continuous trilobite record. They are less common in the upper Montezuman where nevadiids predominate and other trilobite families appear. New material from western Nevada allows a revision of these holmiids. Palmettaspis parallelaFritz, 1995 and Palmettaspis cometes (Fritz, 1995) are assigned to a new genus, Montezumaspis, based primarily on thoracic characters, including a prominent axial spine on the thorax. A second new genus, Grandinasus, is based on Esmeraldina argenta (Walcott, 1910), in which the L3 lobe is incorporated in an expanded anterior lobe (LA) of the glabella. New species described are Esmeraldina elliptica, Grandinasus auricampus, G. patulus, Holmiella falx, and H. millerensis. The Laurentian taxa appear to be more primitive than the holmiids of Baltica by having simple, unfurrowed ocular lobes that are separated from the LA by an axial furrow. Three informal biofacies are suggested with a holmiid-dominated biofacies in a deep subtidal dysoxic environment, and two nevadiid-dominated biofacies in shallower subtidal environments.

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