During the Eocene, world climate experienced rapid and intense global warming, reaching a peak during the Palaeocene–Eocene Thermal Maximum (PETM), 56 my ago. The warmest global climate of the past 66 my occurred during the early Eocene epoch (about 56 to 48 mya) when megathermal floral elements, including palms, reached Antarctica. The increase in temperatures led to a rise in sea level, turning Europe into an archipelago. Data regarding the early Eocene herpetofaunas are scant, but the locality of Dormaal in Belgium represents one of the rare exceptions. The lizards consist of gekkotans, acrodontan and pleurodontan iguanians, anguimorphs such as glyptosaurines and the varanid Saniwa. These groups are believed to be thermophilic, and their appearance in this high latitude locality indicates that the tropics were expanded during this time. Some of these records also represent first appearances of these clades in Europe. Among them, a new iguanian taxon is represented by a unique tooth morphology – the teeth are bifurcated – indicating a specialization on trophic resources. However, because terrestrial ecosystems changed substantially during the Palaeogene, this might have caused higher extinction risk relative to generalists (e.g., the iguanian Geiseltaliellus). Understanding this geological epoch is relevant for present global climate change, including sea level rise, as well as the expansion of distribution of thermophilic taxa, including parasites that cause serious infectious diseases such as malaria.
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Diagenesis has been recognized for decades to significantly alter the trace elements biogenic signatures in fossil tooth enamel and bone that are routinely used for paleobiological and paleoenvironmental reconstructions. This signature is modified during diagenesis according to a complex continuum between two main processes, addition and substitution. For an additive-like, or early diagenesis, the trace elements biogenic profiles can be restored by leaching secondary minerals, but this technique is inefficient for a substitutive-like, or extensive diagenesis for which secondary trace elements are incorporated into the biogenic mineral. This scheme is however unclear for Ca, the major cation in tooth enamel and bone hydroxylapatite, whose stable isotope composition (δ44/42Ca) also conveys biological and environmental information. We present a suite of leaching experiments for monitoring δ44/42Ca values in artificial and natural fossil enamel and bone from different settings. The results show that enamel δ44/42Ca values are insensitive to an additive-like diagenesis that involves the formation of secondary Ca- carbonate mineral phases, while bone shows a consistent offset toward 44Ca-enriched values, that can be restored to the biogenic baseline by a leaching procedure. In the context of a substitutive-like diagenesis, bone exhibits constant δ44/42Ca values, insensitive to leaching, and shows a REE pattern symptomatic of extensive diagenesis. Such a REE pattern can be observed in fossil enamel for which δ44/42Ca values are still fluctuating and follow a trophic pattern. We conclude that Ca isotopes in fossil enamel are probably not prone to extensive diagenesis and argue that this immunity is due to the very low porosity of enamel that cannot accommodate enough secondary minerals to significantly modify the isotopic composition of the enamel Ca pool.
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