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Evolution of herbivory in vertebrates: developing combined isotope (Ca, Sr) and dental surface texture analysis as deep time diet proxies

Periodic Reporting for period 3 - VERTEBRATE HERBIVORY (Evolution of herbivory in vertebrates: developing combined isotope (Ca, Sr) and dental surface texture analysis as deep time diet proxies)

Reporting period: 2019-09-01 to 2021-02-28

Food is a major factor in the evolution of vertebrates. This projects overall objectives are to constrain the evolution of herbivory (plant feeding) and trophic interaction of extinct vertebrates at different spatiotemporal scales by analysing their teeth with isotopic and dental wear techniques. A new approach using combined stable Ca and Sr isotopes as well as 3D dental surface texture (3DST) analysis will be developed and applied to fossil teeth of mammal-ancestors and dinosaurs. The isotopic composition of tooth enamel records information about an animal’s diet over long time periods, while wear on the tooth surface tracks short-term food abrasion. These diet proxies will be calibrated on extant vertebrates with known diets including animals from controlled feeding experiments designed to simulate diet and trophic level switches as well as wild animals from modern ecosystems. Both Ca isotopes and enamel surface textures have a high preservation potential in fossil teeth. Moreover, these methods enable micro-sampling of enamel for Ca isotope and non-destructive 3DST analysis. For the first time, Ca isotope and 3DST analysis will be combined to reconstruct the diet of extinct vertebrate taxa and their trophic level in fossil food webs. Feeding hypotheses usually rely on tooth and skeletal morphology, however, this new multi-proxy approach will provide a versatile toolset to independently test existing morphology based hypotheses, leading to fundamental new insights into the palaeoecology, dietary flexibility and niche partitioning of fossil vertebrates. Beyond the field of palaeontology these dietary proxies will be broadly applicable in archaeology, anthropology and ecology. Exploring dietary traits and trophic relationships in fossil food webs is fundamental for understanding radiation and extinction events and may also provide new insights into the dietary habits and evolution of our human ancestors.
In the first reporting period, controlled animal feeding experiments of rats, guinea pigs, quails, bearded dragons and green iguanas (ongoing) were performed to grow animal (hard-)tissue samples and/or to take in vivo dental imprints under sedation for 3D dental surface texture (3DST) analysis. The animals were fed custom designed pelleted fodders with different plant and animal matter (animal-, bone- or insect- meal). To some pelleted fodders, different types of mineral dust (loess, quartz sand, volcanic ash) of different grain sizes (clay to sand) were added as external abrasives to simulate tooth wear by soil ingestion. Additionally, different types of natural plant- and animal-fodders (fruit-, vegetable-, seed-mix, insects, day chicks) were fed to rats. Guinea pigs were fed plants rich and poor in phytolith content, both in fresh and dried state, to assess differences in dental wear. The thousands of tissue samples generated in the feeding experiments have been archived for ongoing and future analysis. Sample preparation (lopholisation, cryogenic homogenisation, digestion, etc.) for isotopic analyses is ongoing. All rat and guinea pig skulls have been macerated and dental surface texture analysis has been performed on their cheek teeth. For the first time, the 3DST of teeth from modern wild reptiles with different dietary traits were measured to establish a reference dataset for dietary reconstruction in non-chewing taxa such as fossil synapsids and dinosaurs. Mechanical and chemical alteration experiments of modern mammal, crocodile and shark teeth were performed to assess the robustness of these dietary proxies in enamel during sediment transport and fossilisation processes. Preservation potential of original 3DST on mammal and shark teeth was assessed by sediment-water tumbling experiments. We determined that diet-related differences in 3DST between browsers and grazers is still preserved after tumbling in fine-grained sediments although some alteration of surface roughness parameters occurred. The chemical stability of elephant enamel was tested by experimental in vitro alteration in isotopically enriched acidic solution at different temperatures (30°C and 90°C). Only the outermost ~0.2 mm enamel portion displayed alteration while the innermost enamel preserved diet related isotope compositions. In contrast, dentin was completely altered within weeks. Guinea pigs fed with fresh versus dried plants of different silica content (lucerne < grass < bamboo) display significant differences in 3DST between fresh- and dry-grass feeders, which has important implication for dietary reconstructions of herbivores. First magnesium and zinc isotope analysis of soft tissues from the feeding experiment indicate diet-related differences. Calcium isotope analysis of modern mammals display different isotope compositions between carnivores and herbivores but also insectivores. First fossil applications to avian and non-avian dinosaurs revealed that the Eocene flightless groundbird Gastornis, traditionally viewed as a giant terror bird, was a herbivore, similar to a Moa, and not a bone-crushing carnivore like T-Rex and other theropod dinosaurs.
This project will provide a new deep-time diet toolbox of combined Ca and Sr isotope as well as 3D dental surface texture analysis, which is applicable to a single tooth. Its application to fossil teeth will provide new insights into the evolution of plant-feeding among mammal-ancestors and dinosaurs as well as trophic niche partitioning in ancient food webs.
Dietary proxy tool box: combined Ca isotope and enamel surface texture analysis.