178-2 Accelerated and Wholesale Taxonomic Turnover Before and After an Obliterating Early Ordovician Trilobite Mass Extinction

Session: Paleontology, Diversity, Extinction, Origination

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Authors:

Abstract:

The shallow water Early Ordovician trilobite faunas of the (then) northern margin of Laurentia have been shown to feature brief stratigraphic durations punctuated by sudden episodes of nearly complete species level taxonomic turnover.  This pattern can be documented throughout the entire Early Ordovician, based on rich secondarily silicified samples, primarily from the Ibex region of western Utah.  In many cases turnover can plausibly be associated with significant sequence stratigraphic phenomena but in many others no major stratigraphic signal is obvious.  The tempo of turnover is also broadly variable.  Through much of the approximately 650 meters of sampled section, faunas most often persist in stasis on the order of low tens of meters.  In some intervals, though, turnover is much more rapid, and on a meter scale.  Superimposed on this pattern of lockstep turnover are two major trilobite mass extinctions, each of which obliterated the existing dominant groups and completely changed the taxonomic composition of benthic faunas.  The younger event, which took place near the end of the Stairsian Age (Tremadocian) was only discovered during extensive field sampling for the overall megaproject, and it has been difficult to identify and sample continuously exposed strata capturing the extinction.  Well exposed strata were finally identified at sections C and D in the southern House Range in the Ibex region and samples were made of all possible horizons that might yield silicified trilobites.  Section C spans the entire Stairsian Stage and for most of its thickness, turnover is at the typical scale of tens of meters: the lower 145 m include eight distinct trilobite assemblages.  In the approximately 30 m below the extinction event (which is located between 172.4 m and 174.5 m), no less than five completely distinct assemblages occur in rapid succession.  Post-extinction turnover is even more rapid, with seven distinct assemblages replacing each other in the lowest 22 m of the Tulean.  Following this, beginning with the first horizon of the Psalikilus spinosum Zone 24 m above the extinction, the more typical low tens of meters pace of turnover is resumed.  The cause of the extinction, which only a single trilobite genus survived, remains under study, but it took place during an interval of almost unprecedentedly rapid, wholesale faunal turnover.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10408

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