USING RAMAN SPECTROSCOPY TO INFER COMPOSITIONAL DIFFERENCES BETWEEN FOSSIL AND MODERN LIZARD OSTEODERMS

Bone shows a remarkable variability in shape and microstructure. It serves as storage primarily for Calcium and Phosphorous and forms the structural framework in vertebrates. Beyond this, bone also forms defense structures and ornamentations, like frills, horns, and antlers. The most widespread bony defense structures are osteoderms, a type of bone forming in between the dermis and epidermis layers of the skin found in many groups of living and extinct organisms. Although we have an overall idea of the nature of bone – a compound of inorganic bone mineral and organic collagen – and understand bone microstructure, the exact chemistry of the bone mineral itself is not fully known. This is especially true for osteoderms, since they are not of interest in human medicine – where the majority of investigations in bone mineral composition are undertaken. After encountering a suspicious shiny outer layer on an osteoderm of a fossil glyptosaurine lizard (Squamata, Anguidae), we decided to investigate its composition using Raman spectroscopy to test its possible compositional affinity to tooth enamel and compare it to osteoderms of modern squamates.

Analysis of a fossil glyptosaurine osteoderm from the Eocene Santiago Formation, California, using µRaman spectroscopy did not reveal similarities to that of tooth enamel. This somewhat contradicts a previously reported enameloid – osteodermine – for the same group of extinct anguid lizards. Instead, the bone mineral most closely resembles hydroxylapatite. We compared the results for the fossil osteoderm to the Raman spectrum of a modern anguid lizard – Elgaria multicarinata (the Southern Alligator Lizard). The main differences between the modern and fossil specimens are overall higher peak intensities in the fossil and presence of a peak corresponding to Amine in Elgaria. This has to be expected as organics are removed from the bone mineral by fossilization and diagenesis. The increased peak intensity can be attributed to remineralization of the bone mineral during the same processes.

This shows that the long held belief that bone does not change significantly through geologic processes holds true for the composition of its mineral component. Although remineralization occurs, there seems to be no significant exchange of substances.