title: Dinosaur extinction: closing the '3 m gap'.
abstract: Modern debate regarding the extinction of non-avian dinosaurs was ignited by the publication of the Cretaceous-Tertiary (K-T) asteroid impact theory and has seen 30 years of dispute over the position of the stratigraphically youngest in situ dinosaur. A zone devoid of dinosaur fossils reported from the last 3 m of the Upper Cretaceous, coined the '3 m gap', has helped drive controversy. Here, we report the discovery of the stratigraphically youngest in situ dinosaur specimen: a ceratopsian brow horn found in a poorly rooted, silty, mudstone floodplain deposit located no more than 13 cm below the palynologically defined boundary. The K-T boundary is identified using three criteria: (i) decrease in Cretaceous palynomorphs without subsequent recovery, (ii) the existence of a 'fern spike', and (iii) correlation to a nearby stratigraphic section where primary extraterrestrial impact markers are present (e.g. iridium anomaly, spherules, shocked quartz). The in situ specimen demonstrates that a gap devoid of non-avian dinosaur fossils does not exist and is inconsistent with the hypothesis that non-avian dinosaurs were extinct prior to the K-T boundary impact event.
authors.affiliation: Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA. tyler.lyson@yale.edu
authors.forename: Tyler R
authors.initials: TR
authors.lastname: Lyson
authors.forename: Antoine
authors.initials: A
authors.lastname: Bercovici
authors.forename: Stephen G B
authors.initials: SG
authors.lastname: Chester
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Sargis
authors.forename: Dean
authors.initials: D
authors.lastname: Pearson
authors.forename: Walter G
authors.initials: WG
authors.lastname: Joyce
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001842:Bone and Bones; D025061:Dinosaurs / Q000033:anatomy & histology / Q000145:classification*; D053476:Extinction, Biological; D005580:Fossils; D019015:Geologic Sediments; D006733:Horns / Q000033:anatomy & histology; D009009:Montana