Lithium isotopes in Palaeozoic stem-tetrapod bioapatite: Preservation, controls, ecology and oceanographic insights

Major ecological changes in vertebrate evolution through geological times include transitions between aquatic and terrestrial environments. However, temporality and modalities of these transitions are poorly known, relying on fossil discoveries and their sedimentary context. Recently, the stable isotopic composition of lithium (δ⁷Li) in mineralised tissues of modern vertebrates has been proposed to finely discriminate aquatic environments. This new proxy has strong potential applications in the fossil record to better understand key ecological changes such as vertebrate terrestrialisation during the Late Devonian. However, the impact of diagenetic processes on the preservation of the original vertebrate δ⁷Li has never been studied to assess the usefulness of this isotopic system. Here we report the first in-depth analysis of δ⁷Li values  of mineralised fossil vertebrate tissues, as well as of major and minor elements. The specimens come from the Late Devonian outcrop of Miguasha, known to have preserved stem-tetrapods corresponding to the first steps of the initial vertebrates terrestrialisation. We highlight the good preservation of the majority of the δ⁷Li values  with the identification of a diagenetically altered endmember which constitutes a new reference framework to assess the preservation of δ⁷Li in the vertebrate fossil record. δ⁷Li variations of the well-preserved specimens are not controlled by biological factors, but by the environment setting as highlighted by strong correlations between contribution of detrital inputs on the chemical content of fossil bioapatites and their δ⁷Li values. Consequently, lithium isotope composition of vertebrate bioapatite can be used to finely reconstruct past aquatic environments and vertebrate palaeoeocology. The δ⁷Li value range of 20 ‰ to 23 ‰ in tetrapodomorphs indicates that they preferentially lived in the most distal and marine-influenced part of the estuary, confirming their euryhalinity and the complex environmental context of the vertebrate terrestrialisation. Tetrapodomorphs and placoderms have δ⁷Li comprised between 20 ‰ and 23 ‰, which are considered potentially valuable proxies for the δ⁷Li of the contemporaneous oceans.