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.

Dinosaur morphological diversity and the end-Cretaceous extinction

“…The prime example is hadrosauroids, which underwent a significant late Campanian-Maastrichtian disparity decrease in North America but a possible increase in Asia (which, although insignificant, is consistently found when all four metrics are subject to rarefaction). Almost all work on Late Cretaceous dinosaur diversity and abundance has focused either on pooled global datasets or the North American record, which is understandable given the relatively limited number of quality Campanian-Maastrichtian dinosaur fossils from other continents and the detailed stratigraphic understanding of the well-sampled Hell Creek Formation [4][5][6][7][8] . It may be, however, that the North American record represents a local anomaly.…”

“…Despite 30 years of intensive research, a fundamental question remains: were dinosaurs undergoing a long-term decline before intensive volcanism and the Chicxulub bolide impact in the latest Cretaceous, or did these contingencies of Earth history strike down dinosaurs during or near their prime (at a time when their global biodiversity was stable or even increasing) [3][4][5][6][7][8][9][10][11][12][13][14][15] ?…”

“…Finally, understanding the global picture of Late Cretaceous dinosaur evolution may be complicated by a traditional focus on the North American record 22 , particularly the abundance and distribution of dinosaurs within a single formation (Hell Creek Formation). This is understandable, as the Hell Creek is one of the few units that globally preserves a dinosaurdominated ecosystem and a precisely located Cretaceous-Palaeogene boundary [4][5][6][7][8] , but may be only partially informative, if there were region-specific trends in biodiversity. It also cannot reveal patterns that occurred over longer timescales (for example, the last 12 million years of the Cretaceous).…”

A snapshot into the terrestrial ecosystem of an exceptionally well-preserved dinosaur (Hadrosauridae) from the Upper Cretaceous of North Dakota, USA

“…The sediments of the Hell Creek Formation preserve an abundant fossil fauna, including some of the last non-avian dinosaurs to have roamed the earth (Pearson et al, 2001;Lyson et al, 2012). The nonavian dinosaur fauna present in the Hell Creek Formation disappears at the CretaceousePaleogene boundary (e.g.…”

“…The nonavian dinosaur fauna present in the Hell Creek Formation disappears at the CretaceousePaleogene boundary (e.g. Lyson et al, 2012 and references therein) reflecting the global extinction of species connected to this event where global food webs were disrupted (Vajda et al, 2001(Vajda et al, , 2004Ocampo et al, 2006: Chin et al, 2013.…”