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The carbon isotope composition of tooth enamel from all five mosasaur taxa known to date from the type Maastrichtian (southeast Netherlands, northeast Belgium) has been analysed. Differences in enamel δ 13 C values between taxa suggest resource partitioning. Body size and δ 13 C value correlate surprisingly well, with larger taxa having δ 13 C values more depleted in the heavier 13 C isotope.

Primary marine and terrestrial carbonates consist predominantly of two polymorphs of calcium carbonate: calcite and aragonite. The relative abundance of these minerals has varied over 100 myr timescales (Sandberg, 1983;Zhang et al., 2020) due to changes in water chemistry and surface temperature (Adabi, 2004; Balthasar & Cusack, 2014). Despite being a common mineral at the Earth's surface, aragonite is metastable at surface temperatures and pressures and only becomes the more stable crystalline arrangement with significant substitution of ions (Carlson, 1980) and/or at elevated pressure at burial depths (Hacker, 2005). Because of this metastability, aragonite is more susceptible to chemical alteration, and thus its preservation is considered an indicator of pristine geochemistry (Stahl & Jordan, 1969). There is considerable variability in the response of different aragonite materials to alteration processes, which are related to differences in chemical composition and porosity (Pederson et al., 2020). The alteration typically involves the dissolution of aragonite and reprecipitation of the more stable polymorph calcite (Bischoff & Fyfe, 1968), a process termed neomorphism (Folk, 1965).

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To address questions regarding the evolutionary origin, radiation and dispersal of the genus Homo, it is crucial to be able to place the occurrence of hominin fossils in a high-resolution chronological framework. The period around 2 Ma (millions of years ago) in eastern Africa is of particular interest as it is at this time that a more substantial fossil record of the genus Homo is first found. Here we combine magnetostratigraphy and strontium (Sr) isotope stratigraphy to improve age control on hominin-bearing upper Burgi (UBU) deposits in Areas 105 and 131 on the Karari Ridge in the eastern Turkana Basin (Kenya). We identify the base of the Olduvai subchron (bC2n) plus a short isolated interval of consistently normal polarity that we interpret to be the Pre-Olduvai event. Combined with precession-forced (~20 kyr [thousands of years]) wet-dry climate cycles resolved by Sr isotope ratios, the magnetostratigraphic data allow us to construct an age model for the UBU deposits. We provide detailed age constraints for 15 hominin fossils from Area 131, showing that key specimens such as cranium KNM-ER 1470, partial face KNM-ER 62000 and mandibles KNM-ER 1482, KNM-ER 1801, and KNM-ER 1802 can be constrained between 1.945 ± 0.004 and 2.058 ± 0.034 Ma, and thus older than previously estimated. The new ages are consistent with a temporal overlap of two species of early Homo that can be distinguished by their facial morphology. Further, our results show that in this time interval, hominins occurred throughout the wet-dry climate cycles, supporting the hypothesis that the lacustrine Turkana Basin was a refugium during regionally dry periods. By establishing the observed first appearance datum of a marine-derived stingray in UBU deposits at 2.058 ± 0.034 Ma, we show that at this time the Turkana Basin was hydrographically connected to the Indian Ocean, facilitating dispersal of fauna between these areas. From a biogeographical perspective, we propose that the Indian Ocean coastal strip should be considered as a possible source area for one or more of the multiple Homo species in the Turkana Basin from over 2 Ma onwards.

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