M. D. Brasier

Early Cambrian continental reconstructions

Climatically-related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early Cambrian world. Laurentia, Baltica and Siberia appear to have formed a continental group which rifted apart from each other prior to 600 Ma. The consolidation of much of Gondwana probably occurred (in the Pan-African orogeny) well before the Cambrian, though the assembly of several east Asian terranes is still speculative. The archaeocyathan reefs of Siberia, southern Europe and Morocco, with their bigotinid trilobite fauna, suggest that these areas were adjacent to each other and at low latitudes. Avalonia had close faunal links with western Gondwana, but lacked bigotinids and archaeocyathans and may have been situated off west Africa and Florida. During the early Cambrian, there was a northward migration of Laurentia towards the Equator as its separation from Baltica and Siberia increased, and a southward movement of Gondwana.

Nutrient-enriched waters and the early skeletal fossil record

This paper examines the role of stratified, oxygen-depleted and nutrient-enriched water masses (NEW) in transforming the fossil record over the Precambrian-Cambrian boundary interval. The following may serve as markers for the encroachment of NEW over low-latitude cratons: trace-element enriched black shales, ‘stone coals’ and hydrocarbon source rocks, phosphorites, phosphatized shells and organic matter, sponge spicule cherts, early appearance of silica and phosphatic skeletons, appearance of Anabarites and Coleoloides biofacies, peak diversity of phosphatic species, occurrence of black shale lagerstätten and light δ13C values of carbonates. Nutrient-depleted waters (NDW) are inferred over inner cratonic areas, and nutrient-starved waters (NSW) may well have developed over deeper waters at times through the later Cambrian, related to the effects of sea-level rise and salinity stratification. Carbon isotopic fluctuations are suggested to record the history of these water masses, with major changes in carbon burial and palaeoproductivity. The latter may well imply episodes of ‘greenhouse’ climate, with globally raised levels of carbon dioxide and relatively high ocean temperatures.

Giving the early fossil record of sponges a squeeze.

Twenty candidate fossils with claim to be the oldest representative of the Phylum Porifera have been re‐analysed. Three criteria are used to assess each candidate: (i) the diagnostic criteria needed to categorize sponges in the fossil record; (ii) the presence, or absence, of such diagnostic features in the putative poriferan fossils; and (iii) the age constraints for the candidate fossils. All three criteria are critical to the correct interpretation of any fossil and its placement within an evolutionary context. Our analysis shows that no Precambrian fossil candidate yet satisfies all three of these criteria to be a reliable sponge fossil. The oldest widely accepted candidate, Mongolian silica hexacts from c. 545 million years ago (Ma), are here shown to be cruciform arsenopyrite crystals. The oldest reliable sponge remains are siliceous spicules from the basal Cambrian (Protohertzina anabarica Zone) Soltanieh Formation, Iran, which are described and analysed here in detail for the first time. Extensive archaeocyathan sponge reefs emerge and radiate as late as the middle of the Fortunian Stage of the Cambrian and demonstrate a gradual assembly of their skeletal structure through this time coincident with the evolution of other metazoan groups. Since the Porifera are basal in the Metazoa, their presence within the late Proterozoic has been widely anticipated. Molecular clock calibration for the earliest Porifera and Metazoa should now be based on the Iranian hexactinellid material dated to c. 535 Ma. The earliest convincing fossil sponge remains appeared at around the time of the Precambrian‐Cambrian boundary, associated with the great radiation events of that interval.

Charnia and sea pens are poles apart

Charnia from the Ediacara biota is here examined in terms of its growth and development. The Ediacara biota comes from the critical period of evolution just before the Cambrian Explosion and is key to our understanding of the origin of animal life. We show that Charnia cannot be related to the modern cnidarian group the sea pens (Pennatulacea) with which it has for so long been compared, as generative zones cannot be homologized between these forms.

Towards a morphospace for the Ediacara biota

Abstract The Ediacara biota of the late Neoproterozoic is justly famous as a biological puzzle. Studies of Ediacaran biology have commonly used analogy with living organisms as a cipher for the decoding of biological affinity, and consequently the life mode and habit. Here, we discuss the problems of using such analogous reasoning and put forward our alternative approach, that of using Morphospace Analysis for the study of growth, form and phylogeny. This tool, we suggest, has the potential to be used for testing the unity of an evolutionary clade, such as ‘rangeomorphs’ and ‘dickinsoniomorphs’. Preliminary data from the members of the Ediacara biota do indeed show such a unity within our preliminary morphospace model (all k values are low). This method reveals no clear relationships, between these forms and more recent biological groups such as the sea pens or the Foraminifera.

Earth System Evolution and Early Life

Abstract It has become accepted in recent years that the fossil record can preserve labile tissues. We report here the highly detailed mineralization of soft tissues associated with a naturally occurring brain endocast of an iguanodontian dinosaur found in c. 133 Ma fluvial sediments of the Wealden at Bexhill, Sussex, UK. Moulding of the braincase wall and the mineral replacement of the adjacent brain tissues by phosphates and carbonates allowed the direct examination of petrified brain tissues. Scanning electron microscopy (SEM) imaging and computed tomography (CT) scanning revealed preservation of the tough membranes (meninges) that enveloped and supported the brain proper. Collagen strands of the meningeal layers were preserved in collophane. The blood vessels, also preserved in collophane, were either lined by, or infilled with, microcrystalline siderite. The meninges were preserved in the hindbrain region and exhibit structural similarities with those of living archosaurs. Greater definition of the forebrain (cerebrum) than the hindbrain (cerebellar and medullary regions) is consistent with the anatomical and implied behavioural complexity previously described in iguanodontian-grade ornithopods. However, we caution that the observed proximity of probable cortical layers to the braincase walls probably resulted from the settling of brain tissues against the roof of the braincase after inversion of the skull during decay and burial. Supplementary material: Information regarding associated fossil material, and additional images, can be found at https://doi.org/10.6084/m9.figshare.c.3519984