Abstract
There is broad consensus that many bones of extinct vertebrates contain Rare Earth Elements (REE) and other trace minerals that have been incorporated and enriched into the fossil during diagenesis. During fossilization, apatite minerals in vertebrate bones recrystallize from metastable biogenic forms to thermodynamically more stable inorganic apatite minerals while incorporating REEs from their environment. More than ~95% of the REE in fossil bones are diagenetically incorporated postmortem and bones in different environments have different and distinct REE signatures, generally viewed to be controlled by sedimentological and taphonomic processes. The REE pattern in fossils is generally stable and the normalized concentration patterns provide unique “fingerprints” or signatures that have been used for various geological and paleontological investigations. The discovery of embryos and their adults at the same site, a previously unrecorded occurrence in the fossil record, allowed us to compare for the first time the relationship between REE concentrations in the same taxon at widely differing ontogenetic stages. Here we compare REE patterns in bones from two distinct sites in Yunnan, China, both preserving embryonic and adult bones of Early Jurassic sauropodomorph dinosaurs. These dinosaurs are closely related to each other and close in geological age, and their bones show very similar REE patterns. However, the embryonic bones have significantly lower levels of total REEs when compared to their adult counterparts. We attribute the tenfold difference to the level of ossification of the bones and the dramatically lower concentration of apatites in the embryonic than in the adult bones. We conclude that the ontogenetic stage of fossil materials can have a significant impact on REE concentrations, and discuss the impact of these results on future work.
Highlights
Rare Earth Element (REE) concentration in fossilized bone is generally considered to be a post-mortem phenomenon (e.g., Bertram et al, 1992; Trueman and Benton, 1997; Williams, 1998; Trueman, 1999, 2013; Patrick et al, 2002; Trueman et al, 2003; Herwartz et al, 2013; McCormack et al, 2015)
Since fossil bone can be very heterogeneous in nature, Rare Earth Elements (REE) concentration could vary significantly in various parts of the individual fossil bone
Embryonic bones are known to have much lower levels of ossification than the adult bones of the same species (Reisz et al, 2005). This difference is striking in the case of Sauropodomorpha dinosaurs, in which the embryonic bones are characterized by very rapid growth (Reisz et al, 2013), and by the presence of Primary Vascular Cavities (PVC) that occupy more than 50% of the cortex of the femur
Summary
Rare Earth Element (REE) concentration in fossilized bone is generally considered to be a post-mortem phenomenon (e.g., Bertram et al, 1992; Trueman and Benton, 1997; Williams, 1998; Trueman, 1999, 2013; Patrick et al, 2002; Trueman et al, 2003; Herwartz et al, 2013; McCormack et al, 2015). Jurassic exposures (Figure 1) in Yunnan Province have yielded the remains of both embryos and adults of the same taxon (Reisz et al, 2013), Lufengosaurus (Figure 2), at the same site These are very rare occurrences in the fossil record of terrestrial vertebrates, with a combination of similar taxa of similar age, both represented by the adult and embryonic bones. The hydroxyapatite crystal lattice (hexagonal space group P63/m) can accommodate various other ions, and carbonate ions CO32− can readily replace the hydroxyl ions OH− or orthophosphate ions PO43−, leading to carbonated apatite type A and type B, respectively, as confirmed by SR-FTIR (Reisz et al, 2013; Lee et al, 2017) and neutron scattering In this respect, carbonatoapatite-B is closest to the actual bone mineral (Loong et al, 2000). Several previous studies suggest that the REE composition of fossils reflects the environment of early diagenesis in both terrestrial and marine settings (e.g., Henderson et al, 1983; Staron et al, 2001; Metzger et al, 2004; Patrick et al, 2004; Martin et al, 2005; Kaflak et al, 2006)
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