PaleoBios 38:1–11, August 19, 2021 PaleoBios OFFICIAL PUBLICATION OF THE UNIVERSITY OF CALIFORNIA MUSEUM OF PALEONTOLOGY NATHAN VAN VRANKEN & CLINT A. BOYD (2021). The first in situ collec- tion of a mosasaurine from the marine Breien Member of the Hell Creek Formation in south-central North Dakota, USA. Cover: An adult 15 m long mosasaurine mosasaur (either Mosasaurus or Prognathodon) investigates a subadult hadrosaur (Ed- montosaurus) in its attempt to cross the deeper waters of the receding Western Interior Seaway. Artwork by Christopher DiPiazza Citation in situ collection of a mosasaurine from the marine Breien Member of the Hell Creek Formation in south-central North Dakota, USA. PaleoBios, 38. ucmp_paleobios_54460. doi: https://doi.org/10.5070/ P938054460.: Van Vranken, N. and Boyd. 2021. The first The first in situ collection of a mosasaurine from the marine Breien Member of the Hell Creek Formation in south-central North Dakota, USA NATHAN E. VAN VRANKEN1* and CLINT A. BOYD2 1STEM Division, Potomac State College, Keyser, West Virginia, USA [email protected] 2North Dakota Geological Survey, Bismarck, North Dakota, USA [email protected] The upper Maastrichtian Breien Member situated within the lower portion of the Hell Creek Formation in south-central North Dakota records one of the last transgressions of the Western Interior Seaway (WIS) during the terminal Cretaceous. A fragmentary articular-prearticular complex and isolated verte- bra belonging to a mosasauroid were recovered in 2016 from sandstones and mudstones deposited in a nearshore marine paleoenvironment within the southern arm of the bisected WIS. The medially-rotated retroarticular process on the articular-prearticular complex, the shape of the glenoid fossa, along with the morphology of the isolated vertebra, facilitate a conservative referral to a large-bodied mosasaurine such as Mosasaurus or Prognathodon. The rocks of the Breien Member provide paleontologists a unique glimpse of intracontinental marine ecosystems immediately prior to the end of the Cretaceous Period. This discovery provides additional evidence that the latest Maastrichtian marine fauna is a continuation of the fauna preserved in the underlying Fox Hills Formation and that the marine faunal turnover that gave rise to the subsequent Cannonball Sea fauna recorded in Paleocene rocks in North Dakota occurred at the Cretaceous-Paleogene boundary. Keywords: biogeography, youngest occurrence, morphology, squamate, marine INTRODUCTION to become extinct at the end of the Cretaceous (Alegret Swimming in the epeiric Western Interior Seaway (WIS) et al. 2012, Slattery et al. 2015). and along the continental margins of North America dur- The mosasaur fossil record from upper Maastrichtian ing the Late Cretaceous (Turonian to Maastrichtian) were deposits in North America largely comes from the Atlan- diverse members of the clade Mosasauroidea (hereafter tic and Gulf Kiernan referred to as mosasaurs), a group of marine-adapted, 2002, Lindgren and Schulp 2010, Gallagher et al. 2012). carnivorous squamates with a world-wide distribution Within the intracontinental Coastal Plains and region, Pacific the Margin mosasaur ( fossil (Bardet et al. 2014). This clade of marine reptiles displays record from the WIS is better sampled from Campanian a 30 Ma evolutionary history, during which they adapted units, such as the Pierre Formation, whereas specimens to different marine (and non-marine: Makádi et al. 2012) from Maastrichtian units are comparatively rare and environments and, for several lineages, attained large typically consist of highly fragmentary material (Driscoll body sizes (Bardet et al. 2014, Polcyn et al. 2014, Harrell et al. 2019). The youngest mosasaur occurrences from and Perez-Huerta 2015). The collapse of the marine food the WIS are mostly from the Fox Hills Formation and chain through the effects of a ‘Strangelove Ocean’ and an include a partial skull of Mosasaurus hoffmannii Mantell - (1829) from north-central South Dakota (Harrell and sion of the WIS may have caused most of the remaining Martin 2015), a single vertebrae from an indeterminate Mesozoicover-acidification marine megafauna,of the waters including along with the mosasaurs,the regres mosasaur from southeastern South Dakota (Cicimurri et al. 1999), and isolated teeth and vertebrae from North Dakota referred to Mosasaurus dekayi Bronn (1838) *author for correspondence Citation: in situ collection of a mosasaurine from the marine Breien Member of the Hell Creek Formation in south-central North Dakota, USA. PaleoBios, 38. ucmp_paleobios_54460. DOI: https://doi.org/10.5070/P938054460Van Vranken, N. and Boyd. 2021. The first Permalink: https://escholarship.org/uc/item/8v08w2d6 Copyright: Published under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC-BY-NC-SA) license. 2 PALEOBIOS, VOLUME 38, AUGUST 2021 (=Mo. hoffmannii [Street 2016]) and ?Plioplatecarpus sp. outcrop led to the recovery of a partial mosasaur articu- (Getman 1994, Hoganson et al. 2007). lar-prearticular complex (NDGS 2729) in situ within the Overlying the Fox Hills Formation in North Dakota po- are the largely terrestrial deposits of the Hell Creek sition of the surface-collected vertebra. Here, we describe Formation, impeding study of the latest Maastrichtian Breien Member, confirming the original stratigraphic mosasaur fossil record within the WIS. Thus far only a the Breien Member of the Hell Creek Formation and dis- single, indeterminate mosasaur tooth is reported from cussthese their first implicationsconfirmed, non-dental for the persistence mosasaur of remains large-bodied from the upper-most portion of the Hell Creek Formation (De- mosasaurs within the WIS during the latest Maastrichtian Palma et al. 2019 in North America. of that specimen from a putative surge deposit related to the Chicxulub: impactsupplement at the fig. K-Pg S3), boundarybut the recovery raises MATERIALS AND METHODS the possibility that it was either transported to that location from a marine environment or was reworked Geologic Setting from older rocks. In south-central North Dakota two The Breien Member is the only formal subdivision of brief, latest Maastrichtian transgressions of the WIS are the Hell Creek Formation recognized in North Dakota recorded within the Hell Creek Formation: the lower (Murphy et al. 2002). It records one of the last trans- Breien Member and the upper Cantapeta Tongue (Ho- gressions of the WIS into the Dakotas during the late ganson and Murphy 2002 Maastrichtian Hoploscaphites nebrascensis Zone (Hogan- fossils are currently unknown from the latter unit, a son and Murphy 2002 mix of terrestrial, brackish,: fig. and 13). marine Although vertebrate vertebrate taxa sandstones, siltstones, and mudstones were deposited in are documented from the Breien Member (Hoganson interdistributary tidal channels,). These bays,fine-grained and estuaries glauconitic along and Murphy 2002). With one possible exception, no the eastern edge of the prograded Sheridan Delta (sensu mosasaur fossils are currently documented from the Gill and Cobban 1973). Recorded outcrops of the Breien Breien Member. Leonard (1912) reported the recovery Member are restricted to Morton, Emmons, and Sioux Counties in the south-central portion of North Dakota Mosasaurus sp., collected loose on the surface of either (Frye 1967, 1969, Hoganson and Murphy 2002). A type theof two Fox mosasaurHills Formation teeth, outcrops identified or by the C. overlying W. Gilmore Lance as Formation “Ceratops Beds” (=Hell Creek Formation in (Laird and Mitchell 1942), but Frye (1969) designated this study) along the Cannonball River in south-central asection type section was not in identified the northern when portion it was originally of Sioux Countynamed North Dakota. Rocks of the Breien Member, which was not near the town of Breien, North Dakota based on personal recognized at the time, are also exposed at that location communications with those latter authors. Further study between three and four meters above the Fox Hills-Hell revealed a high degree of lateral and vertical variability Creek Formation contact (measured section 1 in Laird in the lithology of the Breien Member, prompting the and Mitchell [1942]). Given that stratigraphic relation- description of three reference sections (Hoganson and ship, the possibility that those teeth were eroded from Murphy 2002) to supplement the description of the the overlying Breien Member and were subsequently type section (Frye 1969). In all, more than two dozen collected on the surface of the Fox Hills Formation cannot measured sections of the Breien Member are described be discounted. Although we consider that scenario to be across south-central North Dakota (Laird and Mitchell unlikely, the provenance of those specimens should be 1942, Frye 1964, 1969, Hoganson and Murphy 2002). treated with caution as the teeth were not collected in Thus, the new fossil locality (NDGS L301) can be put situ unlike the specimens described in this report. into context relative to previously published measured In 2016, a private landowner brought an isolated sections and paleontological localities (Fig. 1). This site vertebra collected loose on the surface of the Hell Creek is situated approximately three miles southwest of the Formation into the North Dakota Geological Survey for Stumpf Composite reference section (Hoganson and Mur- NDGS 2728) phy 2002 suggested it is likely a pygal vertebra from a mosasaur. the Fort Rice measured section (Hoganson and Murphy