Meeting Abstract
The Cretaceous-Paleogene (K-Pg) mass extinction is a well-studied mass extinction event, but vertebrate diversity patterns surrounding the event are not fully understood. Two main competing hypotheses are: (1) a sudden and rapid loss of many species at the K-Pg boundary (ca. 66 Ma) caused by a Bolide impact (Chicxulub); and (2) vertebrate biodiversity declines prior to and at the K-Pg boundary due to multiple factors (e.g., sea levels, volcanism, bolide impact). We examine biodiversity of Chondrichthyes from freshwater deposits of the Hell Creek Formation (HC), Garfield County, Montana. We analyze diversity dynamics using 426 fossil teeth from seven vertebrate microfossil localities stratigraphically distributed throughout the HC. Using raw taxonomic, standing, subsampled analytic rarefied, and shareholder quorum subsampling (SQS) richnesses, our study shows that raw richness increased from six species in the lower third of the HC to a peak of eight in the middle third followed by a decline in the upper third (five species). This stepwise decline in diversity is mimicked in SQS and standing richnesses, also. Our estimates include the occurrences of four new shark species based on morphologically distinct teeth. We also report on the first sand tiger shark from the HC, indicating a freshwater incursion by this, otherwise, marine taxon. This decline in diversity more closely resembles stepwise patterns shown for amphibians (aquatic) than sudden extinction patterns for mammals or lizards (terrestrial), indicating that the aquatic vertebrates from the HC likely had multiple causal factors affecting their biodiversity, such as proximity of the Western Interior Seaway.
