The end-Cretaceous (K/Pg) mass-extinction event is the most recent and well-understood of the "Big Five" and triggered establishment of modern terrestrial ecosystem structure. Despite the depth of research into this event, our knowledge of upper Maastrichtian terrestrial deposits globally relies primarily on assemblage-level data limited to a few well-sampled formations in North America, the Hell Creek and Lance formations. These assemblages disproportionally affect our interpretations of this important interval. Multiple investigations have quantified diversity patterns within these assemblages, but the potential effect of formation-level size-dependent taphonomic biases and their implications on extinction dynamics remains unexplored. Here, the relationship between taphonomy and body size of the Hell Creek and Lance formation dinosaurs and mammals are quantitatively analyzed. Small-bodied dinosaur taxa (< 70 kg) are consistently less complete, unlikely to be articulated, and delayed in their description relative to their large-bodied counterparts. Family-level abundance (particularly skeletons) is strongly tied to body mass, and the relative abundance of juveniles of large-bodied taxa similarly is underrepresented. Mammals show similar but non significant trends. The results are remarkably similar to those from the Campanian-aged Dinosaur Park Formation, suggesting a widespread strong taphonomic bias against the preservation of small taxa, which will result in their seemingly depauperate diversity within the assemblage. This taphonomically skewed view of diversity and abundance of small-bodied taxa amidst our best late Maastrichtian samples has significant implications for understanding speciation and extinction dynamics (e.g., size-dependent extinction selectivity) across the K/Pg Boundary.

Supplemental Table 1 – Data on number of dinosaur collections and occurrences for Maastrichtian-aged geological formations globally. Data downloaded from the Paleobiology Database on 2020-08-27. Data URL: http://paleobiodb.org/data1.2/occs/strata.csv?datainfo&rowcount&base_name=Dinosauria&max_ma=72.1&min_ma=66 Supplemental Table 2 - Dinosaur faunal list for the Hell Creek Formation (and Lance) as well as accompanying data. Each species includes data on: date of description, reference specimens, skeletal completeness, taphonomic mode, body mass estimate, body mass estimate method. Sheet 1: Hell Creek - Taxonomic Split, Sheet 2: Hell Creek - Taxonomic Lump, Sheet 3: Hell Creek and Lance - Taxonomic Split, Sheet 4: Hell Creek and Lance - Taxonomic Lump. Sheet 5: Taxonomic Split/Lump references. See Methods section of paper for details. Supplemental Table 3 - Summary data from the six major surveys of dinosaur richness and relative abundances within the Hell Creek Formation, or Hell Creek Formation and coeval deposits. See Methods section of paper for details. Supplemental Table 4 - Data for specimen-level intraspecific dinosaur abundances as a factor of body mass/size for Edmontosaurus (sheet 1), Tyrannosaurus (sheet 2), and Triceratops (sheet 3). Data include specimen number and size metric. See Methods section of paper for details. Supplemental Table 5 - Mammal faunal list for the Hell Creek Formation as well as accompanying data. Each species includes data on: body mass estimate, mass estimate method, reference specimen, taphonomic mode, and skeletal completeness. See Methods section of paper for details. Supplemental Table 6 - Correlations (Spearman rank correlation) of body mass with relative abundance of dinosaur families based on published survey data.