Background
Dinosaur Biology and Evolution
Dinosaurs are a clade of archosaur reptiles (the group containing crocodilians, pterosaurs, and dinosaurs, including birds), which gained prominence and worldwide distribution throughout the Mesozoic. Dinosaurs are traditionally split into Saurischia, subdivided into theropods and sauropods, and Ornithischia, subdivided into marginoceophalians, ornithopods, and thyreophorans (Fig. 2) (Baron et al., 2017). Theropods, sauropods, and early ornithischians originated in the Late Triassic, and reached a worldwide distribution in the Early Jurassic (Bronzati et al., 2019). By the Late Jurassic, sauropods, thyreophorans, and ornithopods were the dominant large herbivores on Earth, with near identical ecosystems found in Laurasia and Gondwana (Barrett, 2014). Dinosaurs were the primary terrestrial group during the Mesozoic but despite the long history of studies, many questions about the ecology and biology of these animals remain (Benson, 2018).
Over the last 20 years, stable isotope geochemistry has become a common tool for paleontological questions that cannot be answered from body fossils alone (e.g., Fricke et al., 2011; Hassler et al., 2018; Owocki et al., 2020). Stable isotopes do not decay significantly through time and can be incorporated into an animal’s body from the environment. The ratios of these different isotopes can provide information about the source of these elements in an animal’s body and can be preserved through fossilization. For example, stable isotope geochemistry has been used to reconstruct diet and niche partitioning (Figs. 3, 4) (Fricke and Pearson, 2008; Hassler et al., 2018; Martin et al., 2018; 2022), migratory patterns (Fricke et al., 2011), and paleoclimate (Cullen et al., 2020; Owocki et al., 2020). These isotopes can also be compared between sites to identify differences in populations (such as migration or dietary changes) that might not otherwise be identifiable (Fricke et al., 2009). Different isotope systems preserve distinct information; therefore, a suite of stable isotope tools will be employed to answer questions that cannot be addressed with fossil morphology alone.
Dinosaurs constantly shed and grow new teeth during their lives, with tooth formation taking 1–2.5 years (Erickson, 1996). This means annual cycles are recorded and is one possible reason why the isotope ratios in dinosaur teeth vary (Stanton Thomas and Carlson, 2004). Studies specifically target annual variation in tooth d18O to assess yearly temperature variation (Owocki et al., 2020) or migration (Fricke et al., 2011). Constant tooth shedding also means that differences can be compared between young and old individuals, as the teeth reflect their diets over the course of weeks (Fricke et al., 2011) to years (Owocki et al., 2020) rather than a lifetime, as is the case of mammals (Hassler et al., 2018). While no study has explicitly targeted these differences using in dinosaurs, these data could provide novel insight for theories about niche differentiation with ontogeny (changes in niche as an organism grows) (Woodward et al., 2020).
Diagenesis
In order to utilize stable isotopes for paleoecology, the possibility of diagenesis must be addressed. Diagenesis is the post burial alteration of material, which can obscure original isotopic information (Fricke and Pearson, 2008).The mineral phase of bone and enamel is hydroxyapatite (Ca10(PO4)6(OH)2), herein referred to as bioapatite, which is what is preserved during fossilization (Hassler et al., 2018). In bioapatite, diagenesis is caused by the flow of fluids through sediments causing exchange of biological and non-biological isotopes, as well as radiation and high temperature (Zazzo et al., 2004). Diagenesis is not a function of time (Fortner, 2019) so studies can be performed regardless of a deposit’s age. While many parts of an animal may fossilize, tooth enamel is dense and has lower porosity than bone or dentine, making it less susceptible to chemically altering fluids (Owocki et al., 2020) and thus reducing the potential effect of diagenesis (Kirsanow et al., 2008; Kohn and McKay, 2012). Therefore, enamel is the best material to sample when studying biological signatures and thus will be targeted in this work.
No standardized method to determine diagenesis in fossil material has been developed or has not become widespread if it has. The extent of diagenesis can be measured in several ways. Early methods relied on the preservation of fine microstructures (Kolodny et al., 1996) or scanning electron microscope (SEM) imagery (Fortner, 2019) to determine fossil alteration. Other studies use rare earth elements (REE) or measurements of La/Yb or La/Sm ratios (Hassler et al., 2018). Fricke et al. (2011) and Amiot et al. (2015) suggested that if isotope values follow a pattern similar to those in living organisms, the original biological signature must be preserved. The assessment of REEs will be a significant component of this project, as it will be our primary methods for assessing diagenesis. We will also look for elements like U, Na, Fe, Mg, and F which were used to assess diagenesis in the teeth of a fossil bear (Galiova et al., 2013).
Previous Fossil Studies Using LA-ICP-MS