Timing the Extant Avian Radiation: The Rise of Modern Birds, and the Importance of Modeling Molecular Rate Variation

Unravelling the phylogenetic relationships among the major groups of living birds has been described as the greatest outstanding problem in dinosaur systematics. Recent work has identified portions of the avian tree of life that are particularly challenging to reconstruct, perhaps as a result of rapid cladogenesis early in crown bird evolutionary history (specifically, the interval immediately following the end-Cretaceous mass extinction). At face value this hypothesis enjoys support from the crown bird fossil record, which documents the first appearances of most major crown bird lineages in the early Cenozoic—in line with a model of rapid post-extinction niche filling among surviving avian lineages. However, molecular-clock analyses have yielded strikingly variable estimates for the age of crown birds, and conflicting inferences on the impact of the end-Cretaceous mass extinction on the extant bird radiation. This uncertainty has often been ascribed to a patchy avian fossil record, but the possibility of model misspecification in molecular divergence time analyses represents an important and relatively underexplored alternative hypothesis. Here, we highlight the necessity of further developing and using models that account for co-ordinated variation in rates of molecular evolution across a phylogeny (e.g. molecular early burst) as a means of assessing support for a rapid post-Cretaceous radiation of crown birds. We discuss how relationships between life-history and substitution rates can mislead divergence time studies that do not account for directional changes in substitution rates over time, and suggest that these effects might have caused some of the variation in existing molecular date estimates for birds. We suggest multiple paths forward that could help resolve this and similar conflicts within other major eukaryotic clades

Phylogenetic definitions for Caprimulgimorphae (Aves) and major constituent clades under the International Code of Phylogenetic Nomenclature

Phylogenetic nomenclature, a system of taxonomic nomenclature in which taxon names are defined based on phylogenetic relationships, has been widely adopted in recent decades, particularly by vertebrate palaeontologists. However, formal regulation of this taxonomic system had been non-existent until the recent implementation of the International Code of Phylogenetic Nomenclature (PhyloCode). To fulfil the requirements of the PhyloCode, we explicitly establish phylogenetic definitions that we recommended in a recent phylogenetic study on the avian taxon Caprimulgimorphae (which includes nightjars, potoos, frogmouths, swifts, hummingbirds, and others) and many of its major constituent subclades. Two new names are coined: Sedentaves (for the smallest crown clade uniting Steatornis and Nyctibiidae) and Letornithes (for the smallest crown clade uniting Podargidae and Daedalornithes). We also briefly review the fossil record and diagnostic morphological apomorphies of caprimulgimorph clades for which relevant information is available

Metamorphic evolution of the Great Slave Lake shear zone

The Paleoproterozoic Great Slave Lake shear zone (GSLsz) is a crustal‐scale strike‐slip structure, with a total length >1000 km and a width of ~25 km, that separates the Archean Rae and Slave cratons. The range of metamorphic rocks now exposed at the surface encompasses granulite facies mylonite through to lower‐greenschist facies ultramylonite and cataclasite, providing a potential type example of fault‐zone structure in the middle and lower crust. However, the metamorphic evolution of the units remains poorly quantified, hindering detailed structural and tectonic interpretations. Here, we use phase equilibria modelling and thermobarometry to determine the metamorphic conditions recorded by pelitic, mafic, and felsic GSLsz mylonites. Samples from the entire range of granulite–greenschist facies units preserve evidence for nested clockwise pressure–temperature paths that are consistent with a single orogenic cycle. Our findings indicate that the northern Rae margin underwent pervasive crustal thickening with peak pressures in metasedimentary rocks reaching ~1.1 GPa. The crustal thermal gradient at the onset of thickening was ~650ºC GPa‐1, whereas, the final stages of equilibrium recorded by fine‐grained matrix minerals in all samples collectively define a metamorphic field gradient of ~1000ºC GPa‐1. Deformation microstructures are consistent with the main phase of dextral shear having been synchronous with or following peak metamorphism. The history of metamorphism and exhumation of the GSLsz is consistent with the Sibson‐Scholz model for shear zones, with a narrowing of the deforming zone and the progressive overprinting of higher‐grade assemblages during exhumation through shallower crustal levels

Deep Earth explorers

The Cambridge Deep Earth Seismology group has an exhibition at the Sedgwick Museum, Cambridge, aimed at increasing understanding of our planet and changing perceptions of geophysics – and geophysicists. Group members Jennifer Jenkins, Jess Bartlet and Sanne Cottaar tell us more

Methods of Studying Early Theropod Flight

The study of early theropod flight involves avialans as well as other pennaraptorans. It requires the study of anatomy that is familiar to the modern ornithologist, but also very different and alien. Early theropod flight therefore necessitates study methods that can incorporate what we know about sophisticated powered and unpowered flight in living birds while being mindful of the differences between them and the earliest theropod "yers. In this chapter we will survey key methods and approaches, covering their best-practice applications along the timeline of early theropod flight evolution and priorities for future method development

Shear dispersion in a porous medium. Part 1. Fixed-extent immiscible intrusion.

The dispersion of tracer through a liquid injected into a confined aquifer with vertically varying permeability is studied theoretically. The injected fluid is buoyant and of high viscosity relative to the original fluid in the aquifer, which causes the nose region of the flow, where the thickness of the injected fluid is less than the thickness of the aquifer, to advance with constant velocity and fixed shape. Behind the nose, tracer is sheared at early times owing to the vertically varying permeability. At later times, cross-aquifer diffusion homogenises the tracer distribution, which becomes independent of depth but spreads longitudinally in this shear dispersion regime, which leads to much faster spreading than by diffusion alone. As tracer diffuses symmetrically in the longitudinal direction, it eventually reaches the nose. Subsequently, the nose acts as a no-flux boundary and the concentration profile transitions to a half-Gaussian, with the maximum concentration at the front. The centre of mass of the tracer spreads backwards relative to the fixed nose at a rate proportional to [D(T −T 0 )] 1/2 where D is the dispersion coefficient and T 0 is a time offset owing to the adjustment to shear dispersion and the interaction with the nose. The initial release of tracer may not be vertically uniform owing to the heterogeneity and we show that this can lead to the centre of mass of tracer initially advancing faster than the mean flow. We consider the implications of our results on tracer migration in sedimentary layers

Late Miocene to late Pleistocene geomagnetic secular variation at high northern latitudes

We report a palaeomagnetic study of Icelandic lavas of late Miocene to late Pliocene age to test the geocentric axial dipole hypothesis at high northern latitudes. Cores were sampled from 125 sites in the Fljótsdalur valley in eastern Iceland, and hand samples were taken for 17 new incremental heating 40Ar/39Ar age determinations. 96 per cent of the cores were oriented using both a Brunton compass and a sun compass. Comparison of the magnetic and sun azimuths reveals deviations of ±5°, ±10° and ±20°, respectively, for 42, 16 and 3 per cent of the data points, indicating that core sampling intended for palaeosecular variation (PSV) studies at high northern latitudes should be oriented by sun. A total of 1279 independent specimens were subjected to AF- and thermal-demagnetization for palaeodirectional analysis, and well-grouped site mean directions were obtained for 123 sites of which 113 were found to be independent sites. Applying a selection criteria of k > 50 and N ≥ 5 (Nmean = 9.5), we obtain a combined grand mean direction for 46 normal and 53 reverse (for VGPlat > ±45°) polarity sites of declination = 5.6° and inclination = 77.5° that is not significantly different from that expected from a GAD field. The corresponding palaeomagnetic pole position (VGPlat = 86.3°N, VGPlon = 21.2°E, dp/dm = 4.0°/4.3°) is coincident with the North Pole within the 95 per cent confidence limits. An updated age model is constructed based on the 40Ar/39Ar ages, showing that the majority of the Fljótsdalur lavas fall within 2–7 Ma. We combine the Fljótsdalur data with existing data from the nearby Jökuldalur valley. The 154 palaeodirections are well-dispersed between 1 and 7 Ma and constitute a high-quality data set for PSV analysis. Our results partly support previous conclusions of a generally higher dispersion during reverse polarity intervals. However, when comparing our Matutayma data with Brunhes age data from Jan Mayen, we find no evidence of a higher VGP scatter during the Matuyama as previously suggested. When comparing our VGP scatter to the two commonly used models for VGP dispersion: Model G and TK03, we find a good fit for all 1–7 Ma VGP scatter data SB(1–7) to Model G, whereas SB(1–7) is not fitted by TK03, even when considering the uncertainty of SB(1–7). We also find that all VGP scatter estimates, except that for the Gilbert subset, are consistent with Model G, while the discrepancy with TK03 is generally larger

Joint Inversion of High‐Frequency Receiver Functions and Surface‐Wave Dispersion: Case Study in the Parnaíba Basin of Northeast Brazil

We assess the performance of the joint inversion of receiver functions (RF) and surface‐wave dispersion in the characterization of the sedimentary package comprising the Parnaíba basin. This procedure is routinely utilized in passive‐source crustal studies to retrieve S‐wave velocity variations with depth, and has seldom been used with higher‐frequency datasets to investigate fine sedimentary structure. The Parnaíba basin is a Paleozoic cratonic basin composed of five supersequences, accumulating ∼3.5  km of sedimentary rocks interbedded by Late Cretaceous diabase sills. The dataset used for this research was acquired between 2015 and 2017 through deployment of 10 short‐period and one broadband seismic stations distributed along an approximately 100‐kilometer‐long linear array in the center of the basin. The deployment was carried out under the Parnaíba Basin Analysis Project, a multi‐institutional and multidisciplinary effort funded by BP Energy do Brasil. High‐frequency RFs (⁠f<4.8  Hz⁠) were calculated from deconvolution of teleseismic P waveforms (⁠30°<Δ<90°⁠) after rotation into the great‐circle path, whereas high‐frequency dispersion curves (0.25–2 Hz) were obtained through multiple filter analysis of empirical Green’s functions developed from cross‐correlation (ZZ component) and stacking (six months) of time–frequency‐normalized ambient seismic noise recordings. S‐wave velocity–depth profiles down to ∼5  km depth were developed through an iterative, linearized joint inversion approach. Comparison to independent active‐source seismic profiles overlapping with our passive‐source seismic line reveals the inverted velocity models successfully retrieve sedimentary thickness (top of the Cambrian), sedimentary velocity structure, and depth to the Cenozoic sedimentary sequence. In addition, high‐velocity zones at depths ranging from 1.5 to 2.5 km are observed in the inverted velocity–depth profiles, which are interpreted as due to the Late Cretaceous sills interbedding the basin’s sedimentary rocks. The relative low cost of our approach makes it ideal for basic characterization of relatively unknown sedimentary basins

The reactive transport of Li as a monitor of weathering processes in kinetically limited weathering regimes

Analytical solutions to reactive-transport equations describing the evolution of Li concentrations and isotopic ratios are presented for one-dimensional flow paths where reaction stoichiometry is constant along the flow path. These solutions are considered appropriate for chemical weathering in rapidly eroding catchments. The solutions may be described by two dimensionless numbers; 1) a Damkӧhler number describing the product of reaction rate and fluid residence time, and 2) a net partition coefficient which describes the fraction of Li re-precipitated in secondary minerals as the product of a fluid-secondary mineral partition coefficient and the mass fraction of secondary mineral precipitates. In settings where water entering flow paths is dilute, Li concentrations will increase along the flow path until they reach a limiting value determined by the net partition coefficient. Simultaneously, 7Li/6Li isotopic ratios will increase to a limiting value of the source rock ratio minus the secondary mineral-fluid Li-isotopic fractionation factor. Waters with Li-isotopic ratios in excess of this limiting value must have evolved with a change of reaction stoichiometry and/or partition coefficient along the flow path such that at some point net removal of Li to secondary minerals exceeds that supplied by dissolution of primary minerals. The modelling shows that the multiple controls on chemical weathering rates (temperature, rainfall, erosion rate, hydrology) cannot be inferred from Li concentration and isotopic ratio data alone which only provide two independent constraints. Caution should be exercised in interpretation of oceanic Li records in terms of potential climatic variables. The model is illustrated by a set of Li concentration and isotopic ratio measurements on river waters and bed sands in the Alaknanda river basin which forms the headwaters of the Ganges. This illustrates how values of the Damkӧhler number and net partition coefficient can be used to trace weathering processes. Water samples from catchments with similar lithologies and climates scatter along contours of approximately constant net partition coefficient, reflecting similar reaction stoichiometries, but with more variable Damkӧhler numbers reflecting variations in flow path length, fluid flux and/or reaction rate. Samples from the lower, warmer and less rapidly eroding catchments have high 7Li/6Li isotopic ratios with lower Li concentrations and must reflect at least a two-stage weathering process where reaction stoichiometry and/or Li fluid-mineral partition coefficients change along the flow path so that net Li is removed in the later stages

Field-response of magnetic vortices in dusty olivine from the Semarkona chondrite

Recent paleomagnetic studies have constrained the strength and longevity of the magnetic field generated by the solar nebula, which has broad implications for the early evolution of the solar system. Paleomagnetic evidence was recorded by nanoscale iron inclusions in olivine crystals in the Semarkona LL 3.0 chondrite. These dusty olivines, have been shown to be credible carriers of ancient magnetic remanence. The small scale of the iron inclusions presents several challenges for defining their fundamental magnetic properties. Here we present the first correlative study of the response of these magnetic structures under applied laboratory fields. Results show that the majority of particles are in a single-vortex state and exhibit stable magnetic behavior in applied fields up to 200 mT. Experimental observations using Lorentz microscopy and magnetic transmission X-ray microscopy are shown to compare well with the results of finite-element micromagnetic simulations derived from 3D models of the particles obtained using electron tomography. This correlative approach may be used to characterize the fundamental magnetic behavior of many terrestrial and extraterrestrial paleomagnetic carriers in the single- to multi-vortex size range, which represent the vast majority of stable magnetic carriers in rocks and meteorites. Plain Language Summary: Some of the first solid materials to form in the solar system have been brought to Earth by meteorites. They contain tiny metallic inclusions which record information about the magnetic fields at the earliest stages of our solar sys- tem’s history. Understanding these magnetic fields, and how they are recorded by metallic particles, provides very important information for understanding how our solar system formed and evolved. We have studied some of these particles to image their magnetic structure using microscopes which allow us to see structures a billionth of a metre in size. We have developed a new technique using X-rays to image how the magnetic structure in these particles changes when we apply different magnetic fields in the laboratory. We have also been able to reproduce our results using computer simulations of the magnetic behavior of the particles. This is the first study that has imaged the magnetic structure of these particles under applied fields. We find that the particles are very stable; even under magnetic fields thousands of times stronger than Earth’s, the particles still don’t change the magnetic structure they had in the early solar system. Keypoints: • We show the first experimental results demonstrating the stability of dusty olivines under applied magnetic fields. • We use a novel combination of electron and X-ray microscopy techniques combined with micromagnetic simulations. • We confirm previous results that dusty olivines are capable of recording magnetic fields from the solar nebula