AbstractSedimentary rocks exposed at Dob’s Linn, Scotland, have significantly influenced our understanding of how life evolved over the Ordovician to Early Silurian. The current interpreted chronostratigraphic boundary between the Ordovician and Silurian periods is a Global Boundary Stratotype Section and Point (GSSP), calibrated to 443.8 ± 1.5 Ma (Hirnatian–Rhuddanian age), based on biostratigraphic markers, radioisotopic dates and statistical modelling. However, challenges arise due to tectonic disturbances, complex correlation issues and the lack of systematic dating in Ordovician–Silurian stratigraphic sections. Here, hundreds of zircon grains from three metabentonite ash horizons were dated using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). A subset of the grains were re-analyzed using Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS). We present a high-precision CA-ID-TIMS 238U-206Pb weighted mean date of 440.44 ± 0.55/0.56/0.72 Ma (±analytical/with tracer/with U-decay constant) for the Coronagraptus cyphus biozone. However, the study reports younger, and in certain cases, older LA-ICP-MS zircon dates within the Coronagraptus cyphus, Akidograptus ascensus and Dicellograptus anceps zones, suspected as being influenced by Pb loss and LA-ICP-MS matrix mismatch. The study reports concerns about the suitability of Dob’s Linn as a GSSP section and examines various LA-ICP-MS maximum depositional age (MDA) approaches, suggesting the use of the TuffZirc date and the youngest mode weighted mean (YMWM) as suitable MDA calculations consistent with CA-ID-TIMS results.

The Tissint strewn field has produced over 16 kg of material that has minimal terrestrial weathering and/or contamination. Tissint, along with 16 other incompatible trace element (ITE)-depleted shergottite specimens with igneous crystallization ages spanning 327-2403 Ma, were ejected together from Mars 1.1 m.y. ago. Despite the geochemical similarities of fragments from the Tissint strewn field, there are reported discrepancies in age determinations from different fragments that raise the possibility that the strewn field may be heterogenous. There are also questions about whether the shock ejection event incorporated martian soil components into impact glass, and the sources of radiogenic Sr and Pb that have been measured from leachate fractions in previous studies. An impact melt-rich fragment of Tissint was analyzed by LA-ICPMS for rare-earth element (REE) and highly siderophile element (HSE) concentrations and Pb isotopic compositions. Leachate and residues from 8 specimens representing separate individual fragments collected from the strewn field were analyzed for Rb-Sr. Unleached fractions of the 8 specimens were also analyzed for Sm-Nd and Lu-Hf. The measured REE and HSE concentrations of impact melt glass and associated sulfide measured by LA-ICPMS are consistent with bulk rock compositions of Tissint and show no evidence for incorporation of more ITE-enriched martian surface components. Measured Pb isotopic compositions confirm that the impact melt glass and associated sulfide contain no evidence for incorporation of more radiogenic materials than the Pb compositions inherited from the primary magma. In situ Pb isotopic data from sulfide likely represents the most robust method for constraining initial Pb isotopic compositions of shergottites whereas approaches that rely on leaching and digestion may not remove all mineral and/or crack surface contaminants. Rubidium-strontium analyses of the 8 Tissint specimens indicate that labile components hosting HCl-soluble Rb and Sr are not in isotopic equilibrium with the igneous assemblage and that the washed residues are in isotopic equilibrium with the igneous assemblage. The Sr isotopic compositions of the leachate are within the range of "more ITE-enriched' depleted shergottites, perhaps indicating sources from the igneous pile on Mars. The radiogenic Sr component could represent crack and mineral surface coatings of volatilized materials derived from nearby depleted shergottite rock units during the impact ejection process but are not radiogenic enough to represent ITE-enriched crust or mantle components. The Lu-Hf isotopic data from the specimens indicate no evidence of contamination or element mobility, whereas the Rb-Sr and Sm-Nd isotopic systems show evidence for element mobility and potential mixing with an isotopic component not in equilibrium with the igneous phases. The calculated ages using data compiled fromBrennecka et al. (2014), and Grosshans(2013) for Lu-Hf, Rb-Sr, and Sm-Nd are 571 +/- 84 Ma, 590 +/- 49 Ma and 559 +/- 39 Ma, respectively. These data indicate thatthe specimens analyzed here are cogenetic and the Tissint strewn field appears to be homogeneous.(c) 2022 Elsevier Ltd. All rights reserved.

Dob’s Linn (Scotland) is a location that has significantly influenced our understanding of how life evolved over the Ordovician to early Silurian. The current chronostratigraphic boundary between the Ordovician and Silurian periods is a Global Boundary Stratotype Section and Point (GSSP) at Dob’s Linn calibrated to 443.8±1.5 Ma, partly based on biostratigraphic markers, radiometric ages, and statistical modeling. Graptolites are used here as relative dating markers. We dated hundreds of zircon grains extracted from defined metabentonites from six horizons exposed at Dob’s Linn using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Each zircon was imaged using cathodoluminescence, and most show igneous zoning with minimal alteration. Sample locations range from 42 meters above to 5 meters below the recognized GSSP for the Ordovician-Silurian. Samples were responsibly collected and analyzed for paleontology and geochemistry in other work. Overall, many 238U-206Pb zircon ages from the section are significantly younger than expected. The youngest zircon in sample DL7, located 5 meters below the GSSP, yielded a 238U-206Pb age of 402±12 Ma (±2s, 5% disc). Nineteen spots on zircons from this sample are younger than the presently assigned GSSP age, including more concordant results of 426±8 Ma (0.8% disc) and 435±5 Ma (0.2% disc). The youngest zircon in sample 19DL12, < 1 m below the GSSP, is 377±8 Ma (2% disc) with a more concordant age of 443±7 Ma (0.6% disc). A sample located directly on the GSSP (19DL09) yields 327±5 Ma (0.8% disc). Eight spots on zircons from this sample are also younger than the presently assigned GSSP age. We also dated two samples (DL24 and BRS23) 8 meters above the GSSP, and the youngest, most concordant zircon ages in these samples are 400±11 Ma (5% disc) and 421±9 Ma (0.4% disc), respectively. Overall, the U-Pb ages would re-assign the Dob’s Linn chronostratigraphic section to Silurian-Devonian. The young age results could be attributed to Pb loss due to hydrothermal alteration during the Acadian and Alleghenian orogenies. Future work will implement Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS) to obtain accurate U-Pb dating and evaluate the potential effects of Pb loss.

In contrast to the ‘propaganda’ of the currently dominant vertebrates (i.e. us), arthropods not amphibians were the first land animals. And like many another great advance, they first appeared in Scotland. The first land animals were ‘millipedes’, which evolved with the first true land plants at the edges of Scottish mountain lakes about 425 Ma. From then on, more elaborate and complex plant‐arthropod land communities evolved incredibly rapidly, and spread to lowland marshes, taking only 40 Myr to reach complex forest grade communities by 385 Ma. We recently dated some of the sediments enclosing these fossils to give a more precise age for the communities, a study still in progress, which indicates that the oldest land animal is a ‘millipede’ from Kerrera, Oban, Scotland.

The Ludlow Bone Bed (Welsh Basin) is a critical stratigraphic horizon and contains a rich assemblage of fish scales. Units above provide insights into the early evolution of animal and plant life. The bed has not yet been radioisotopically dated. Here, we report 207 secondary ion mass spectrometry (SIMS) ages from 102 zircon (ZrSiO 4 ) grains from the Ludlow ( n = 2) and stratigraphically higher Downton ( n = 1) bone beds. SIMS ages are middle Ordovician (471.6 ± 20.7 Ma) to late Devonian (375.7 ± 14.6 Ma, 238 U– 206 Pb, ±1σ analytical uncertainty). Cathodoluminescence images show that the youngest ages appear affected by alteration. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology was utilized to improve precision. Detrital zircon grains from Downton yield 424.91 ± 0.34/0.42/0.63 Ma and from Ludlow 424.85 ± 0.32/0.41/0.62 Ma ( n = 5 each, 238 U– 206 Pb, ±2σ analytical, tracer or systematic uncertainty). These ages provide a maximum deposition age. Results overlap the basal Přídolí age (423.0 ± 2.3 Ma) in its stratotype (Požáry Section, Reporyje, Prague, Czech Republic). The Ludlow Bone Bed marks the base of the local Downton Group, which has previously been correlated with the base of the Přídolí Series. The CA-ID-TIMS ages are older than those for other land arthropod-bearing sediments, such as the Cowie Harbour Fish Bed and Rhynie Chert. Supplementary material: An Excel file containing detailed information on the SIMS analyses, a figure showing calibration curves for AS3 standards sputtered over sessions 1 and 2, and a figure showing CA-ID-TIMS U–Pb age data (concordia and weighted mean plots) are available at https://doi.org/10.6084/m9.figshare.c.5087031

Major advances in planetary science will be driven by determining absolute ages of geologic units on multiple bodies in the inner Solar System in the next decade.Absolute ages calibrate bodyspecific chronologies and create a framework for understanding Solar System formation, the effects of impact bombardment on life, and the evolution of planets and their interiors.We recommend that the Decadal Survey:• Support Mars Sample Return to ensure the return to Earth of a set of carefully-chosen, well-documented samples for geochronology and other critical studies of Mars' evolution and past habitability.• Prioritize sample-return missions from planetary bodies in the inner Solar System that would answer fundamental questions about the timing of major events in planetary history.• Advocate for sustained funding for in situ dating instruments to continue to raise their technology readiness levels for future exploration of planetary surfaces.• Include a Solar System chronology mission on the New Frontiers mission list, setting the science goals of such a mission but allowing flexibility in how those goals are accomplished.• Reemphasize the importance of an investment strategy for laboratory instrumentation and curation sufficient to provide for both replacement of existing capacity and development of new capabilities.• Recommend that NASA partner with NSF on their Decadal geochronology initiative to leverage NASA's investments in infrastructure supporting planetary geochronology.

Molecular clock calculations suggest a late Cambrian (similar to 500 Ma) divergence of myriapod classes. Yet, the earliest myriapods only appear in the latest Silurian (similar to 425 Ma). 75 million years later; though correlation with the standard marine-based geological time scale is difficult. We radiometrically dated (U/Pb method) zircons in sediments at 3 sites in the UK associated with the supposed earliest millipedes. Kerrera, with only extinct kampecarid myriapods from temporary playa lake deposits in a semi-arid intermontane basin is 425 Ma old. Ludlow, with kampecarid myriapods and the first trigonotarbid arachnids (with book lungs), from transported material in ripple troughs at the base of a semi-arid delta complex, is 420 Ma. Cowie, with the supposed earliest air-breathing millipede (with spiracles), from a temporary playa lake in an intermontane basin complex, is 414 Ma old. The more diverse land arthropod fauna of the Rhynie Chert hot spring deposit, also in an intermontane basin, is 407 Ma old. A rapid radiative evolution from simple intermontane lake margin communities through more evolved lowland communities took only about 20 Ma to reach complex forest grade communities by the Middle Devonian Givetian (about 385 million years).

SR, SM-ND and LU-HF ISOTOPE SYSTEMATICS. S. E. Suarez, T. J. Lapen, M. Righter, B. L. Beard, A. J. Irving Department of Earth and Atmospheric Sciences, University of Houston, 312 Science & Research Building 1, Houston TX 77204 (sesuare2@central.uh.edu) Department of Earth and Space Sciences, University of Washington, Seattle WA Department of Geoscience, University of Wisconsin-Madison, Madison, WI

Barringer Meteorite Crater (a.k.a. Meteor Crater), Arizona, is one of the youngest and best preserved impact craters on Earth. For that rea-son, it provides a baseline for similar craters formed in the geologic past, formed elsewhere in the Solar Sys-tem, and illuminates the astronomical and geological processes that produce them. The crater has not, how-ever, escaped erosion completely. While Shoemaker [1] mapped a breccia with fallback components inside the crater, he did not locate it beyond the crater rim. He only found remnants of that type of debris in re-worked alluvium [1; see also 2]. Fallback breccia and any base-surge deposits have, thus, been missing components in studies of material ejected beyond the transient crater rim.

The oldest-known air-breathing land animal is the millipede Pneumodesmus newmani, found in the Cowie Harbour Fish Bed at Stonehaven, Scotland. Here we report the youngest, most concordant 238U-206Pb zircon age from ash below the fish bed of 413.7±4.4 Ma (±2σ), whereas the youngest age from a tuffaceous sandstone above the fish bed is statistically indistinguishable at 414.3±7.1 Ma. The Cowie Harbour Fish Bed thus appears to be lowermost Devonian (Lochkovian), contrary to the previously accepted mid-Silurian age based on palynomorphs from adjacent exposures. This has implications for the evolutionary timetable of land colonization, as the Cowie ages overlap late Lochkovian zircon ages reported elsewhere for andesite below the nearby (~50 mi) Rhynie Chert, which has more advanced terrestrial biota. The results postdate the possible late Silurian Ludford Lane locality in Shropshire, England. Pneumodesmus newmani is thus not the earliest air-breathing land animal, unless the Ludford Lane locality is younger than presently assigned.