Calcium Isotopic Evidence for Vulnerable Marine Ecosystem Structure Prior to the K/Pg Extinction

International audienceThe collapse of marine ecosystems during the end-Cretaceous mass extinction involved the base of the food chain [1] up to ubiquitous vertebrate apex predators [2–5]. Large marine reptiles became suddenly extinct at the Cretaceous-Paleogene (K/Pg) boundary, whereas other contemporaneous groups such as bothremydid turtles or dyrosaurid crocodylomorphs, although affected at the familial, genus, or species level, survived into post-crisis environments of the Paleocene [5–9] and could have found refuge in freshwater habitats [10–12]. A recent hypothesis proposes that the extinction of plesiosaurians and mosasaurids could have been caused by an important drop in sea level [13]. Mosasaurids are unusually diverse and locally abundant in the Maastrichtian phosphatic deposits of Morocco, and with large sharks and one species of elasmosaurid plesiosaurian recognized so far, contribute to an overabundance of apex predators [3, 7, 14, 15]. For this reason, high local diversity of marine reptiles exhibiting different body masses and a wealth of tooth morphologies hints at complex trophic interactions within this latest Cretaceous marine ecosystem. Using calcium isotopes, we investigated the trophic structure of this extinct assemblage. Our results are consistent with a calcium isotope pattern observed in modern marine ecosystems and show that plesiosaurians and mosasaurids indiscriminately fall in the tertiary piscivore group. This suggests that marine reptile apex predators relied onto a single dietary calcium source, compatible with the vulnerable wasp-waist food webs of the modern world [16]. This inferred peculiar ecosystem structure may help explain plesiosaurian and mosasaurid extinction following the end-Cretaceous biological crisis

Precise analysis of calcium stable isotope variations in biological apatites using laser ablation MC-ICPMS

Laser ablation (LA) is potentially an interesting technique to measure natural variations (delta Ca-44/42) of calcium isotopes in calcium-rich minerals because it allows spatial resolution and avoids micro-sampling and consecutive wet chemistry. We developed a matrix-match sample/standard normalization method and used an Excite 193 nm Photon Machines LA system coupled to a Neptune plus MC-ICPMS to measure delta Ca-44/42 variations in enamel apatite. First, high precision delta Ca-44/42 solution mode (SOL) analyses were performed on a series of 5 crystalline igneous apatite and 6 modern tooth enamel samples, which were micro-sampled using a MicroMill device. The delta Ca-44/42 isotopic values ranged evenly between -0.60 and +0.60(sic) (per amu). Second, we sintered by means of a spark plasma sintering technique the bone ash SRM1400 standard and two synthetic apatites (doped or not with Sr). The Ca isotope compositions using LA were measured in the samples in the raster mode along 600 x 85 mu m profiles and bracketed with the SRM1400 standard. We obtained very good agreement between SOL and LA measurements, i.e. delta Ca-44/42(LA) vs. delta Ca-44/42(SOL) slope of 0.960 x 0.091 (2SE, R-2 = 0.971) and null offset at origin (0.012 +/- 0.084, 2SE). For all samples, residual values to the 1 : 1 slope were \textless= 0.1& (per amu). However, an unexplained and constant 0.13& offset occurred when considering the Ca-43/42 ratio, suggesting an uncorrected isobaric interference on Ca-43 in the LA mode. We also noticed that the doubly charged strontium (Sr) interference correction is of crucial importance for accurate matching between LA and SOL measurements. In the SOL mode, Sr is discarded by ion chromatography leading to typical Sr-87(2+)/Ca-44(+) ratios of 10(-5) to 10(-6). In the LA mode, this ratio can exceed 10(-3). We show that the value set for the Sr-87/Sr-86 ratio is of importance to correct the Sr interference, and that optimized residuals to the 1 : 1 slope are obtained using a Sr correction that takes into account a mass fractionation factor for doubly charged Sr distinct from that of Ca. We found that deciduous teeth enamel is depleted of Ca heavy isotopes by about 0.35-0.40(sic) (per amu) compared to wisdom teeth enamel, a shift compatible with a transition from a milk based diet to a plant and meat based diet

Data from: Non-traditional isotope perspectives in vertebrate palaeobiology

The recent development of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) notably in the disciplines of earth sciences, now allows the precise measurement of isotope ratios, even at low concentration. Non-traditional isotope systems, such as alkaline earth (Ca, Mg) and transition (Cu, Fe, Zn) metals are now being measured in a variety of biological tissues, including bone and teeth. Although our understanding of the environmental and biological mechanisms behind the fractionation of such elements is still in its infancy, some of these isotopes are suspected to fractionate along the food chain, as has been reported in the literature for calcium, magnesium and zinc. Other geochemical methods, such as concentration analyses, permit a prior assessment of diagenesis in the fossils and such an approach indicates that in some circumstances, not only enamel but also dentine or bone can preserve its original biogenic composition. The aims here are to: review the current knowledge surrounding these various isotopic tools; address their potential preservation in biological apatite; and provide the palaeobiologist with a guide to the different toolkits available, including a discussion of their potential applications in vertebrate palaeobiology with a case study involving two mammal assemblages from the Pleistocene of Europe

SuppFig4-Jaurens sampling 2-2

Figure S4. Photographs of samples analysed for this study; Ursos arctos, Coelondonta antiquitatis, Rangifer tarandus, Bison priscus; Jaurens, Corrèze, France

Reconstructing Pleistocene Australian herbivore megafauna diet using calcium and strontium isotopes

Isotopes in fossil tooth enamel provide robust tools for reconstructing food webs, which have been understudied in Australian megafauna. To delineate the isotopic composition of primary consumers and understand dietary behaviour at the base of the food web, we investigate calcium (Ca) and strontium (Sr) isotope compositions of Pleistocene marsupial herbivores from Wellington Caves and Bingara (New South Wales, Australia). Sr isotopes suggest small home ranges across giant and smaller marsupial herbivores. Ca isotopes in Pleistocene marsupial herbivores cover the same range as those in modern wombats and placental herbivores. Early forming teeth are depleted in heavy Ca isotopes compared to late-forming teeth of a given individual, suggesting a weaning signal. Distinct Ca compositions between taxa can be interpreted as dietary niches. Some niches conform to previous dietary reconstructions of taxa, while others provide new insights into niche differentiation across Australian herbivores. Combined with the small roaming ranges suggested by Sr isotopes, the Ca isotope niche diversity suggests rich ecosystems, supporting a diversity of taxa with various diets in a small area

SuppFig3-Jaurens sampling 1-2

Photographs of samples analysed for this study; Canis lupus, Crocuta crocuta, Panthera spelaea, P. pardus, Ursos arctos; Jaurens, Corrèze, France

A simplified protocol for measurement of Ca isotopes in biological samples

International audienc

SuppFig1-mass frac line supp

Fig. S1. Triple isotope plot (δ44/42Ca versus δ43/42Ca in ‰) of all data analysed in this study. Regression line is Y = 0.487 (± 0.022) X – 0.024 (± 0.030) with R2 = 0.974. This is in agreement with mass dependent fractionation slope of 0.507 according to exponential law

Calcium isotopic ecology of Turkana Basin hominins

International audienceDiet is a major driver of hominin evolution, but most of the geochemical evidence relies on carbon isotopes (δ 13 C). Here, we report enamel stable calcium isotope (δ 44/42 Ca) values against δ 13 C values for several hominins and co-existing primates in the Turkana Basin area, circa 4 to 2 Ma. Australopithecus anamensis clusters with mammal browsers, Kenyanthropus platyops is distinct from A. anamensis in foraging into more open environments and the coexisting Theropithecus brumpti encompasses both the grazer and omnivore/carnivore domains. Early Homo is remarkable for its wide distribution in δ 44/42 Ca values, possibly reflecting omnivorous and opportunistic preferences. Paranthropus boisei is uniquely distributed in the δ 13 C versus δ 44/42 Ca iso-space being distinct from all other hominins from the Turkana Basin area as well as from the co-existing Theropithecus oswaldi. Several hypotheses are explored to discuss the unique δ 44/42 Ca values of Paranthropus boisei including significant differences observed with δ 44/42 Ca values recently reported for P. robustus from South Africa, questioning the monophyly of this genus

SuppFig2-Ca vs Mn

Fig. S2. Manganese plotted against δ44/42Ca values for bone and enamel samples measured in this study