Intensive sampling of the House Formation, Ibex area, western Utah, has revealed a wealth of unsuspected trilobite species diversity. At the Lava Dam North section, 59 in situ horizons were sampled for silicified trilobites, and comparable collections were made at several other sections in the area. Correlative faunas from the lower Garden City Formation were sampled at Hillyard Canyon and Franklin Basin, Bear River Range, southern Idaho.

These collections allow the recognition of at least ten stratigraphically successive and distinct trilobite assemblages in the House Fm., paving the way for substantial refinement of the trilobite biostratigraphy. The Garden City Formation contains only three of these ten assemblages, and fossiliferous horizons can now be correlated precisely with the Ibex sequence.

These new data reveal three major features of trilobite paleobiology, which may apply widely to the Tremadocian of the western Laurentian craton: 1) the rate of species turnover accelerates through the formation, and in the upper part, replacement of congeneric species occurs at stratigraphic intervals of less than 5 m; trilobite evolutionary rates in the upper House Fm. are among the highest ever documented. 2) Trilobite alpha diversity, assessed using large sample sizes and rarefaction, increases steadily through the formation, from 3-4 species in the lower intervals to 12 or more species in the highest assemblages. 3) Revised trilobite systematics demonstrate the presence and stratigraphic persistence of multiple trilobite clades (including many new genera) through most of the House Fm. All became globally extinct in a mass extinction event marked by the "Paraplethopeltis Zone" in the upper House. No lineages crossed this event, and succeeding faunas in the lower Fillmore Fm. are unrelated to the extinguished clades. The trilobite extinction event preceded the well-known Midcontinent Province conodont mass extinction, which occurs in the lowest Fillmore.