Elsevier

Cretaceous Research

Volume 125, September 2021, 104865
Cretaceous Research

Short communication
Amber from a Tyrannosaurus rex bonebed (Saskatchewan, Canada) with implications for paleoenvironment and paleoecology

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Abstract

The Frenchman Formation (Maastrichtian) exposed within Chambery Coulee in southwestern Saskatchewan, Canada, has yielded exceptional vertebrate fauna specimens, including one of the largest known Tyrannosaurus rex individuals. The site has excellent preservation of a rich and diverse paleobotanical assemblage containing amber, which is analyzed in detail and compared to other sources of paleoecological data herein. Fourier-Transform Infrared (FTIR) spectroscopy of the fossil resin suggests a botanical source among the Cupressaceae (cypress trees). The ecological requirements of the crocodilians imply a mean annual temperature of 16 °C, which is slightly higher than previous estimates based on paleobotanical analyses. Stable isotope analyses of amber in the deposit provided insight into the environment and the ecological conditions at the time of resin secretion. The δD values showed a marine enrichment that suggests proximity to oceanic waters, and by extension, strong influence of the Western Interior Seaway in the region during the latest Cretaceous. The carbon isotope composition might also indicate that a potential ecological stress affected the trees such as a drought. These results highlight the interest of including amber in bonebed studies: even in the absence of identifiable inclusions, valuable paleoenvironmental, paleoecological, and even paleogeographic clues can be derived from chemical analyses of fossil resins.

Introduction

The Frenchman Formation (Maastrichtian) is exposed throughout much of Chambery Coulee near Eastend, Saskatchewan, Canada (Fig. 1), and this unit has yielded the largest specimen of Tyrannosaurus rex in the world (Tokaryk and Bryant, 2004; Persons IV et al., 2019). Being relatively complete (65%), this individual named “Scotty”, represents a major contribution to the study of the taxon. As such, the site is commonly referred to as the “Scotty bonebed”. During the course of excavating this specimen, a relatively diverse vertebrate fossil assemblage was also unearthed from the site. However, the presence of a rich botanical assemblage in close association with the vertebrate material is perhaps the most informative paleoecological component of this dig (e.g., McIver, 2002; Bamforth et al., 2014). Amber analysis adds an additional facet to the interpretation of this flora and its habitat.

Up to now, bonebed amber deposits have received little attention in the literature (e.g., McKellar et al., 2019; Cockx et al., 2020). In addition, only a few amber deposits have been recorded from the Maastrichtian, including deposits from: Canada, France, Italy, Jamaica, Spain and the U.S.A. (Iturralde-Vinent, 2001; Nel et al., 2004; Delclòs et al., 2007; McKellar et al., 2008; DePalma et al., 2010; Davies et al., 2014). Consequently, little is known about the paleoforests and soft-bodied terrestrial organisms in these settings. Preliminary results show that amber represents a new source of paleontological data in the study of bonebed deposits—amber chemistry and fossil content provide unique insights into the paleoecological and paleoenvironmental conditions of the paleoforests, and preserve faunal components that would otherwise be overlooked in the fossil record (Davies et al., 2014; McKellar et al., 2019; Cockx et al., 2020).

Section snippets

Geological setting

The Maastrichtian period is represented in Saskatchewan by five formations: the marine shales of the Bearpaw Formation, the fluvial-deltaic cross-bedded sands of the Eastend Formation, the kaolinized sands and clays of the Whitemud Formation, the bentonitic Battle Formation, and the fluvial Frenchman Formation. This latter formation comprises cross-bedded sands, locally cemented by calcium carbonate into ledges and concretions interbedded with clays (Natural Resources Canada, 2018). These sands

Stable isotopes

Amber from the deposit exhibited δ13C values (Table 1) ranging from −24.2‰ to −20.6‰ with a mean of −22.1‰ (standard deviation of 1.3‰). With respect to other Upper Cretaceous deposits, the Scotty bonebed amber appears to be enriched in 13C even though a similar variability is observed (Fig. 3). The mean δ13C values recorded in other Campanian–Maastrichtian deposits of Alberta range from −24.8‰ to −22.8‰ with standard deviations between 0.87 and 1.64 (McKellar et al., 2008; Davies et al., 2014;

Paleoclimatic reconstructions based on the fossil assemblage

Paleotemperature estimates based on the fossil assemblage provide a reference point for comparisons with data derived from amber. The objective is to determine the value of fossil resin as a paleoclimate proxy when compared to these better-established and independent data sources. Over the past decades, Chambery Coulee has yielded a large and diverse fossil assemblage including abundant plant remains. Based on a review of the sedimentology and the plant assemblage of the site, McIver (2002)

Conclusions

The Scotty bonebed deposit provides an exceptional opportunity to document the past environment and climate of southwestern Saskatchewan during the Maastrichtian. Although no inclusions were found in the amber, chemical analyses of the fossil resin offer insight into the state of local paleoforest. FTIR spectroscopic analyses suggest that the botanical source belong to the Cupressaceae family. An enrichment in 13C of the resin (with respect to the Genesee amber) might indicate that a potential

Acknowledgements

This work was supported by the Natural Sciences and Engineering Research Council of Canada [Discovery Grant number 2015-00681]; the Friends of the Royal Saskatchewan Museum; and Mitacs, through the Mitacs Accelerate Program. We thank two anonymous reviewers for their valuable comments and suggestions to improve the manuscript; T. Tokaryk, W. Long and RSM field teams for samples; and the landowners for site access.

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