Vertebrate biostratigraphy of the Hell Creek Formation in southwestern North Dakota and northwestern South Dakota

Abstract

Field surveys of the Upper Cretaceous Hell Creek Formation in southwesternNorth Dakota since 1986 have produced a total of 10 124 specimens from 42 vertebrate microsites and an additional 41 dinosaur skulls or partial skeletons or skulls from separate sites representing 61 taxa of vertebrates dominated by fish, dinosaurs, turtles, and crocodilians. Common elements of this diverse fauna occur to within 2.37 m of the Cretaceous-Tertiary (K-T) boundary. The stratigraphically highest fossil in the study is a partial ceratopsian skeleton 1.77 m below the K-T boundary in the basal Fort Union Formation. All dinosaurs that occur at more than two sites also occur at the highest level that yielded more than 500 specimens (8.40 m below the K-T boundary). The fine-grained uppermost 2 m of the Hell Creek is nearly devoid of all fossils, including taxa known to occur in the overlying formation. The absence of channel deposits in this part of the formation may be the reason for the absence of fossil localities. The presence of marine-tolerant taxa suggests that the study area may have been adjacent to a previously unidentified seaway of latest Cretaceous age. Rarefaction analysis indicates no evidence for a decline in vertebrate diversity through the formation or dinosaurian diversity in the 3 m below the K-T boundary. Our results are not compatible with gradual vertebrate extinction at the end of the Cretaceous.

Mass extinction of lizards and snakes at the Cretaceous–Paleogene boundary

“…Discussions of diversity have tended to focus on dinosaurs, but whether they experienced a minor decline during the Maastrichtian (9) or did not (10,42,43), the persistence of an exceptionally diverse lizard and snake fauna just before the K-Pg extinction argues against the idea that late Maastrichtian terrestrial ecosystems were stressed before the Chicxulub impact. The available evidence suggests instead that extinctions among the squamates were abrupt and coincided with the K-Pg boundary, supporting the hypothesis that the Chicxulub asteroid impact was the sole cause of the end-Cretaceous mass extinction.…”

“…In particular, because many genera persisted for 10 My or longer (Fig. 6), and because vertebrate faunas showed little turnover in the late Maastrichtian (42,43) it is unlikely that a significant number of these species became extinct in the short intervals separating these localities from the K-Pg boundary.…”

Choristoderes and the freshwater assemblages of Laurasia

“…Champsosaurus has a long slender rostrum whereas that of Leidyosuchus-Borealosuchus is wider and has been described as 'generalised' (Brochu, 2001). In the Hell Creek Formation (Maastrichtian) of southwestern North Dakota and northwestern South Dakota, crocodiles with a wider range of rostral shapes are found with Champsosaurus (Pearson et al, 2002). The closest match is the longirostrine gavialoid Thoracosaurus (e.g.…”

No post-Cretaceous ecosystem depression in European forests? Rich insect-feeding damage on diverse middle Palaeocene plants, Menat, France

“…Jablonski & Raup 1995;Alroy 1999;Pearson et al 2002;MacLeod et al 2006;Benton & Harper 2009). Among the many consequences was substantial turnover for North American plants and herbivorous insects (Tschudy et al 1984;Wolfe & Upchurch 1986;Johnson et al 1989;Hotton 2002;Labandeira et al 2002a,b;Nichols 2002;Wilf & Johnson 2004), expressed in a general pattern of low-diversity floras throughout the 10 Myr of the Palaeocene and a nearly complete loss of specialized insect-feeding damage on leaves until the last approximately 1 Myr of the Palaeocene Labandeira et al 2002a,b;Wilf et al 2006;Currano et al 2008).…”