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.

Figure 1.

(a) Geological map showing the placement of the two localities discussed in the text. (b) Photo of the Camel Butte K–T boundary section showing the position of the palynologically defined K–T boundary and the in situ non-avian dinosaur (Ceratopsidae) brow horn. Abbreviations: Kp: Pierre Shale Fm., Kf: Fox Hills Fm., Kh: Hell Creek Fm., Tl: Fort Union Fm., Tp: Slope Fm., Tb: Bullion Creek Fm., QTu: Quaternary and upper Tertiary, Qor: Holocene. Geographic coordinates are given as a 10 km UTM grid.

Figure 2.

Sedimentological, palynological and vertebrate palaeontological data from the Mud Buttes (no. 171) and Camel Butte (no. 190) K–T boundary sections. The non-avian dinosaur specimen was found 13 cm below the Camel Butte K–T boundary section (see §4 for justification of the placement of the boundary). Arrows indicate levels where palynological samples were collected. Appearance data (first—FAD; last—LAD) for vertebrates are indicated for the Camel Butte section. Abbreviations: Ir: iridium anomaly; PU1: Puercan 1.