The concentration of the bio-limiting nutrient element, phosphorus (P), in seawater is important for primary marine productivity and the evolution of life on geological time scales. The molar percentage of P/Fe in banded iron formations (BIF) and iron oxide-rich chemical sediments is a good proxy for the first-order approximation of seawater P concentration. Bio-available concentration of phosphorus in Precambrian, especially during the late Palaeoproterozoic Era (2.0–1.8 Ga), is poorly constrained. We evaluated the P/Fe ratios of iron-rich rocks from the late Palaeoproterozoic Chilpi Group, Bastar Craton, Central India. The bulk rock molar percentage of P/Fe ratios of the Chilpi rocks vary between 0.11 and 1.17 (average 0.51 ± 0.3), and the average of EPMA spot analysis P/Fe molar ratio is 0.32 ± 0.4; both have values similar to Archaean BIFs of the world. The observed low molar ratio is not an artefact of contamination from terrestrial sources, diagenetic alterations or high-temperature hydrothermal inputs; it indicates the deposition from phosphorus-lean seawater. The modelled P/Fe molar ratio in the Chilpi Group suggests that the concentration of phosphorus in the shallow marine environment was less than 0.12 μM. The low level of phosphorus concentration in seawater during the late Palaeoproterozoic Era is interpreted to be a consequence of the low primary production during a period of low atmospheric oxygen content, which might have impeded the evolution of eukaryotes.

A remarkable specimen of the actinopterygian fish Pachycormus macropterus from the Early Jurassic (Toarcian) Posidonienschiefer Formation of Germany exceptionally preserves an unusually large ammonite inside its gut. The ammonite was swallowed by the fish, likely by accident, and represents the first direct evidence for an actinopterygian fish consuming an ammonoid. Exceptional aragonite preservation of the conch retaining partial nacreous lustre, combined with only minor acid etching of the shell, strongly indicates that the ammonite was ingested immediately prior to and was directly responsible for the fish’s death. The fish’s stomach provided a microenvironment protecting the aragonite from chemical dissolution.

The glaciogenic nature of the Yudnamutana Subgroup was first recognized over a century ago, and its global significance was recognized shortly after, with the eventual postulation of a global Sturtian Glaciation and Snowball Earth theory. Much debate on the origin and timing of these rocks, locally and globally, has ensued in the years since. A significant corpus of research on the lithology, sedimentology, geochronology and formal lithostratigraphy of these sequences globally has attempted to resolve many of these debates. In the type area for the Sturtian Glaciation, South Australia’s Adelaide Superbasin, the lithostratigraphy and sedimentology are well understood; however, formal stratigraphic nomenclature has remained complicated and contested. Absolute dates on the stratigraphy are also extremely sparse in this area. The result of these longstanding issues has been disagreement as to whether the sedimentary rocks of the Yudnamutana Subgroup are truly correlative throughout South Australia, and if they were deposited in the same time span recently defined for Sturtian glacial rocks globally, c. 717 Ma to c. 660 Ma. This study presents a large detrital zircon study, summarizes and compiles existing global geochronology for the Sturtian Glaciation and revises the formal lithostratigraphic framework of the Yudnamutana Subgroup. We show equivalence of the rocks that comprise the revised Sturt Formation, the main glaciogenic unit of the Yudnamutana Subgroup, and that it was deposited within the time span globally defined for the Sturtian Glaciation.

We present U–Pb dates from peridotitic pyrope-rich garnet from four mantle xenoliths entrained in a kimberlite from Bultfontein, South Africa. Garnet dates magmatic emplacement due to the high mantle residence temperatures of the source material prior to eruption, which were most likely above the closure temperature for the pyrope U–Pb system. We determine a U–Pb date of 84.0 ± 8.1 Ma for the emplacement of the Bultfontein kimberlite from garnet in our four xenolith samples. The date reproduces previous dates obtained from other mineral-isotope systems (chiefly Rb–Sr in phlogopite). Garnet can be dated despite extremely low concentrations of U (median ∼0.05 μg/g), because concentrations of common Pb are often low or non-detectable. This means that sub-concordant garnets can be dated with moderate precision using very large laser-ablation spots (130 μm) measured by quadrupole inductively coupled plasma – mass spectrometry (LA-Q-ICP-MS). Our strategy demonstrates successful U–Pb dating of a U-poor mineral due to high initial ratios of U to common Pb in some grains, and the wide spread of isotopic compositions of grains on a concordia diagram. In addition, the analytical protocol is not complex and uses widely available analytical methods and strategies. This new methodology has some advantages and disadvantages for dating kimberlite emplacement versus established methods (U-based decay systems in perovskite and zircon, or Rb- or K-based systems in phlogopite). However, this method has unique promise for its potential application to detrital diamond prospecting and, more speculatively, to the dating of pyrope inclusions in diamond.

Hydrofractures, or hydraulic fractures, are fractures where a significantly elevated fluid pressure played a role in their formation. Natural hydrofractures are abundant in rocks and are often preserved as magmatic dykes or sills, and mineral-filled fractures or mineral veins. However, we focus on the formation and evolution of non-igneous hydrofractures. Here we review the basic theory of the role of fluid pressure in rock failure, showing that both Terzaghi’s and Biot’s theories can be reconciled if the appropriate boundary conditions are considered. We next discuss the propagation of hydrofractures after initial failure, where networks of hydrofractures may form or hydrofractures may ascend through the crust as mobile hydrofractures. As fractures can form as a result of both tectonic stresses and an elevated fluid pressure, we address the question of how to ascertain whether a fracture is a hydrofracture. We argue that extensional or dilational fractures that formed below c. 2–3 km depth are, under normal circumstances, hydrofractures, but at shallower depth they may, but must not be hydrofractures. Since veins and breccias are often the products of hydrofractures that are left in the geological record, we discuss these and critically assess which vein structures can, and which do not necessarily, indicate hydrofracturing. Hydrofracturing can suddenly and locally change the permeability in a rock by providing new fluid pathways. This can lead to highly dynamic self-organization of crustal-scale fluid flow.

Horseshoe crabs as a group are renowned for their morphological conservatism punctuated by marked shifts in morphology associated with the occupation of non-marine environments and have been suggested to exhibit a consistent developmental trajectory throughout their evolutionary history. Here, we report a new species of horseshoe crab from the Ordovician (Late Sandbian) of Kingston, Ontario, Canada, from juvenile and adult material. This new species provides critical insight into the ontogeny and morphology of the earliest horseshoe crabs, indicating that at least some Palaeozoic forms had freely articulating tergites anterior to the fused thoracetron and an opisthosoma comprising 13 segments.

The Niagara Escarpment is a geological feature located in southern Ontario, Canada, and the northeastern United States, comprising highly fractured sandstone, shale and carbonates deposited during the Ordovician and Silurian periods. Differential erosion of the strata has generated a steep cliff face which bisects the city of Hamilton, Ontario. Geological fractures are widespread in the escarpment and result in the formation of unstable blocks of rock subject to erosion through rockfall. This presents structural stability issues of concern due to the proximity of the escarpment to urban infrastructure. We quantify and analyse fracture networks in the escarpment using a combined field- and numerical-modelling-based approach. The location, orientation and aperture of fractures were documented from local outcrops using scanline and area survey methods. Clusters of poles describing the orientation of geological discontinuities were identified in spherical projections, defining three sets: (1) a sub-vertical stratabound set striking N–S, (2) a sub-vertical stratabound set striking E–W, and (3) a set parallel to horizontal sedimentary bedding planes which we infer controlled sub-vertical fracture geometry. Discrete fracture network modelling of fracture sets highlights their high degree of connectivity, and contribution to local geohazards, and quantifies their role in controlling fluid flow through escarpment strata, which is dependent on fracture aperture. Additionally, bedding planes have the potential to act as free surfaces, facilitating stress conditions in which approximately cuboid blocks are produced, and increasing the risk of rockfalls. We conclude that fractures present a first-order control on the fluid-flow properties and stability of the Niagara Escarpment.

For almost 150 years, megascopic structures in siliciclastic sequences of terminal Precambrian age have been frustratingly difficult to characterize and classify. As with all other areas of human knowledge, progress with exploration, documentation and understanding is growing at an exponential rate. Nevertheless, there is much to be learned from following the evolution of the logic behind the biological interpretations of these enigmatic fossils. Here, I review the history of discovery as well as some long-established core members of widely recognized clades that are still difficult to graft on to the tree of life. These ‘orphan plesions’ occupy roles that were once held by famous former Problematica, such as archaeocyaths, graptolites and rudist bivalves. In some of those cases, taxonomic enlightenment was brought about by the discovery of new characters; in others it required a better knowledge of their living counterparts. Can we use these approaches to rescue the Ediacaran orphans? Five taxa that are examined in this context are Arborea (Arboreomorpha), Dickinsonia (Dickinsoniomorpha), Pteridinium plus Ernietta (Erniettomorpha) and Kimberella (Bilateria?). With the possible exception of Dickinsonia, all of these organisms may be coelenterate grade eumetazoans.

Fieldwork forms the basis of geoscience studies. However, field activities present limitations for people with mental or physical impairments. This aspect can preclude participation in field trips by certain groups of students or limit their experience. In recent years, new types of supporting material and the development of accessible field trips have been a step forward towards the reduction of barriers to inclusion and equal opportunity. In the present work, normal practices of field teaching and potential solutions (and their limitations) to foster inclusion and accessibility to fieldwork are presented.

Spinosaurus aegyptiacus was probably the largest predatory dinosaur of the Cretaceous period. A new study shows that it was a semiaquatic hunter. The function of Spinosaurus’ huge dorsal ‘sail’ remains unsolved, however. Three hypotheses have been proposed: (1) thermoregulation; (2) humpback storage; or (3) display. According to our alternative hypothesis, the submerged sail would have improved manoeuvrability and provided the hydrodynamic fulcrum for powerful neck and tail movements such as those made by sailfish or thresher sharks when stunning or injuring prey. Finally, it could have been employed as a screen for encircling prey underwater.