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Understanding and mitigating the e ects of our ongoing biodiversity crisis requires a deep-time perspective on how ecosystems recover in the aftermath of environmental catastrophes. The mass extinction event at the Cretaceous/Paleogene (K/Pg) boundary (ca. 66 Ma) represents a natural laboratory wherein the tempo and mode of biotic recovery can be studied with high chronostratigraphic resolution. Although the morphological evolution of mammals across this event has been reconstructed from skeletal remains, the exact nature of any changes in dietary preference remains unknown. A primary goal here is to fill this gap by investigating how ecological preferences of mammals, reflected by diet, changed from the Late Cretaceous, when they shared landscapes with dinosaurs, to the earliest Paleogene, when they did not. To accomplish this, carbon and oxygen isotope ratios of fossil tooth enamel (bioapatite) were measured using laserablation mass spectrometry in order to infer animal diet and drinking water sources, which vary depending on the niche occupied by an animal. Fossil teeth were collected from two sites located within 400 meters of one another within the West Bijou Creek field area of the Denver Basin, one 9 meters (~128 ky pre-K/Pg) below the boundary (teeth from ceratopsian and hadrosaurid dinosaurs and the multituberculate mammal Mesodma, as well as gar fish scales), and the other 4 meters (~57 ky post-K/Pg) above (Mesodma teeth and gar fish scales). Carbon isotope ratios (δ13C) of Mesodma tooth enamel vary significantly across the K/Pg boundary, with Late Cretaceous teeth having lower and more variable δ13C (-10.1 to -16.4‰, n=4) and early Paleocene teeth having higher and less variable δ13C (-5.3 to 9.0 ‰, n=5), the latter being similar to values for Late Cretaceous dinosaurs. These results suggest Mesodma had very di erent dietary behaviors following the extinction event, presumably a result of the disappearance of non-avian dinosaurs as well as 57% of North American plants, both of which made new food sources and niches available to them. These results also hint at a decoupling of behavioral change from morphological change, at least in the case of Mesodma, over 10 ky timescales. Isotopic analysis of teeth from other Late Cretaceous and earliest Paleogene mammalian taxa is ongoing and will hopefully allow for more detailed interpretations of ecological change across the K/Pg extinction event in the Denver Basin.
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How teeth record and attenuate seasonal signals
Variability of oxygen isotopes in environmental water is recorded in tooth enamel, providing a record of seasonal change, dietary variability, and mobility. Physiology dampens this variability, however, as oxygen passes from environmental sources into blood and forming teeth. We showcase two methods of high resolution, 2-dimensional enamel sampling, and conduct modeling, to report why and how environmental oxygen isotope variability is reduced in animal bodies and teeth. First, using two modern experimental sheep, we introduce a sampling method, die-saw dicing, that provides high-resolution physical samples (n = 109 and 111 sample locations per tooth) for use in conventional stable isotope and molecular measurement protocols. Second, we use an ion microprobe to sample innermost enamel in an experimental sheep (n = 156 measurements), and in a Pleistocene orangutan (n = 176 measurements). Synchrotron and conventional μCT scans reveal innermost enamel thicknesses averaging 18 and 21 μm in width. Experimental data in sheep show that compared to drinking water, oxygen isotope variability in blood is reduced to 70–90 %; inner and innermost enamel retain between 36 and 48 % of likely drinking water stable isotope range, but this recovery declines to 28–34 % in outer enamel. 2D isotope sampling suggests that declines in isotopic variability, and shifted isotopic oscillations throughout enamel, result from the angle of secretory hydroxyapatite deposition and its overprinting by maturation. This overprinting occurs at all locations including innermost enamel, and is greatest in outer enamel. These findings confirm that all regions of enamel undergo maturation to varying degrees and confirm that inner and innermost enamel preserve more environmental variability than other regions. We further show how the resolution of isotope sampling — not only the spatial resolution within teeth, but also the temporal resolution of water in the environment — impacts our estimate of how much variation teeth recover from the environment. We suggest inverse methods, or multiplication by standard factors determined by ecology, taxon, and sampling strategy, to reconstruct the full scale of seasonal environmental variability. We advocate for combined inverse modeling and high-resolution sampling informed by the spatiotemporal pattern of enamel formation, and at the inner or innermost enamel when possible, to recover seasonal records from teeth.
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- Award ID(s):
- 2420088
- PAR ID:
- 10563293
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Archaeological Science
- Volume:
- 175
- Issue:
- C
- ISSN:
- 0305-4403
- Page Range / eLocation ID:
- 106148
- Subject(s) / Keyword(s):
- Dicing microsampling Enamel biomineralization Innermost enamel Oxygen isotopes Seasonality and paleoecology
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jas.2024.106148
