Geology and taphonomy of a unique tyrannosaurid bonebed from the upper Campanian Kaiparowits Formation of southern Utah: implications for tyrannosaurid gregariousness.

Alan L Titus,Eric M Roberts,Daigo Yamamura,Abigail K Lukacic,Katja Knoll,Jonathan E Ginouves,Celina A Suarez,Ian J Glasspool,Joseph J.W Sertich

doi:10.7717/peerj.11013

Abstract

Tyrannosaurids are hypothesized to be gregarious, possibly parasocial carnivores engaging in cooperative hunting and extended parental care. A tyrannosaurid (cf. Teratophoneus curriei) bonebed in the late Campanian age Kaiparowits Formation of southern Utah, nicknamed the Rainbows and Unicorns Quarry (RUQ), provides the first opportunity to investigate possible tyrannosaurid gregariousness in a taxon unique to southern Laramidia. Analyses of the site’s sedimentology, fauna, flora, stable isotopes, rare earth elements (REE), charcoal content and taphonomy suggest a complex history starting with the deaths and transport of tyrannosaurids into a peri-fluvial, low-energy lacustrine setting. Isotopic and REE analyses of the fossil material yields a relatively homogeneous signature indicating the assemblage was derived from the same source and represents a fauna living in a single ecospace. Subsequent drying of the lake and fluctuating water tables simultaneously overprinted the bones with pedogenic carbonate and structurally weakened them through wet-dry cycling. Abundant charcoal recovered from the primary bone layer indicate a low temperature fire played a role in the site history, possibly triggering an avulsion that exhumed and reburied skeletal material on the margin of a new channel with minimal transport. Possible causes of mortality and concentration of the tyrannosaurids include cyanobacterial toxicosis, fire, and flooding, the latter being the preferred hypothesis. Comparisons of the RUQ site with other North American tyrannosaur bonebeds (Dry Island-Alberta; Daspletosaurus horneri-Montana) suggest all formed through similar processes. Combined with ichnological evidence, these tyrannosaur mass-burial sites could be part of an emerging pattern throughout Laramidia reflecting innate tyrannosaurid behavior such as habitual gregariousness.

Highlights

Monodominant large theropod dinosaur bonebeds are rare in the geologic record
All carbonate samples were digested in phosphoric acid at room temperature and analyzed via a gas bench II attached to a Thermo Advance Plus isotope ratio mass spectrometer (IRMS) at the University of Arkansas Stable Isotope Laboratory (UASIL)
This is corroborated by the heavy REE (HREE)-depleted signature of Rainbows and Unicorns Quarry (RUQ) fossils, which is typical of high suspension load or colloid-rich fluvial environments like floodplains (Herwartz et al, 2013; Rousseau et al, 2015), indicating rare earth elements (REE) infusion of fossils and peds was essentially complete prior to incorporation into the channel setting of units 4 and 5, which would have enriched the bones with HREEs (Trueman, 1999)

Summary

Introduction

Monodominant large theropod dinosaur bonebeds are rare in the geologic record. Published examples include the Lower Jurassic Dilophosaurus wetherilli type locality (Welles, 1954), the Upper Jurassic Cleveland-Lloyd Allosaurus fragilis-dominated site in central Utah (Madsen, 1976; Gates, 2005; Peterson et al, 2017), the middle Cretaceous Mapusaurus locality in Neuquen, Argentina (Coria & Currie, 2006), the Upper Cretaceous MAD05-42 Majungasaurus quarry in Madagascar (Ratsimbaholison, Felice & O’Connor, 2016), the middle Cretaceous tyrannosauroid Yutyrannus site in China (Xu et al, 2012), and two tyrannosaurid sites from North America (Fig. 1): the Dry Island Buffalo Jump Albertosaurus sarcophagus site in the lower Maastrichtian Horseshoe Canyon Formation of Alberta, Canada (Eberth & Currie, 2010), and the Daspletosaurus horneri site (TA 1997.002) in the upper Campanian portion of the Two Medicine Formation of central Montana (Currie et al, 2005; Carr et al, 2017). Perhaps the most controversial inference made from the Dry Island site is that it evidences habitual gregarious behavior including cooperative hunting (Currie, 1998; Currie & Eberth, 2010), something previously hypothesized only for tyrannosaur’s smaller paravian cousins (Maxwell & Ostrom, 1995) Such monotaxic or monodominant bonebeds have historically been one of the few accepted arguments for gregariousness in fossil species (Currie & Eberth, 2010). Attritional traps such as tar seeps (Stock, 1972), quicksand/mires (Kirkland et al, 2016), or sinkholes (Martin & Gilbert, 1978) can accumulate non-time associated individuals

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