TRACE FOSSIL EVIDENCE FOR DWARFISM ACROSS THE K-PG BOUNDARY, BRAGGS, ALABAMA AND BRAZOS RIVER, TEXAS

Most major mass extinctions during the Phanerozoic induced changes in ecological and anatomical parameters within endobenthic communities, which are often reflected in their traces. We herein report a decrease in burrow size in Thalassinoides-dominated ichnoassemblages across the K-Pg boundary in well studied shallow-marine sections at Braggs, Alabama and along the Brazos River and Cottonmouth Creek tributary in Falls County, Texas. Across the K-Pg boundary at Braggs, diameters (D_Th) of simple maze Thalassinoides structures from similar, recurring depositional facies abruptly decrease by 22% (mean K=13.1 mm, mean Pg=10.2 mm; n=26). At the Cottonmouth Creek site, Thalassinoides isp. occurring above the previously reported negative δ¹³C shift and the first occurrence of Danian planktonic foraminifera are 17% smaller in diameter (mean K=21.5 mm, mean Pg=17.9 mm; n=53) than those excavated and filled prior to deposition of a cross-bedded, ejecta-bearing sandstone complex commonly interpreted as the Chicxulub ‘event deposit’. The Cretaceous and Paleogene burrows were preserved in similar lithologies, suggesting the reduction in size is not attributed to sedimentological factors. At both localities, up-section trends in D_Th are statistically significant (a <0.05, non-parametric Kruskal-Wallis test). All statistical parameters display a negative shift, which indicates a directional reduction in size, rather than an artifact of decreased variance. Since trace-fossil diameter is a proxy for tracemaker body size, a reduction in D_Th above the K-Pg boundary likely reflects dwarfing within the post-extinction endobenthic community. These ichnological trends are similar to those recently documented at more impact-distal sites along the Atlantic Coastal Plain in New Jersey. Dwarfing during the early recovery stages of the end-Cretaceous mass extinction, as recorded by crustacean ichnofossils, appears to be widespread.