J. Javier Álvaro

Palaeogeographical controls on the Cambrian trilobite immigration and evolutionary patterns reported in the western Gondwana margin

Abstract Southward drifting of the western Gondwanan margin during the Cambrian has been demonstrated by means of both palaeomagnetic methods and lithological indicators of climate (such as carbonates and evaporites). Recent improvements in biostratigraphical correlations permit an enhanced understanding of the climatic and palaeobiogeographical constraints that controlled the distribution of Cambrian benthic communities. Palaeogeographical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interaction between migration, speciation and extinction rates. The variability of the documented biogeographical patterns is directly related to species diversity, in which wider distribution coincides with transgressive trends and subsequent connection of neighbouring platforms. Early Cambrian trilobite faunas show a high degree of both substrate control and endemicity, although transgressions led to parallel shifts in faunal compositions. By contrast, Mid-Cambrian trilobite faunas are relatively uniform across western Gondwana, and latest Mid- and Late Cambrian associations document influence of an increased similarity with Asian trilobite faunas.

High‐latitude pelmatozoan–bryozoan mud‐mounds from the late Ordovician northern Gondwana platform

Mud-mound complexes identified within the early to middle Ashgill Cystoid Limestone Formation of northeastern Spain are the first fossil build-ups to be described in the high latitude north-facing margin of Gondwana. Mud-mound complexes comprise individual lenticular mounds (composed of floatstones, cephalopod-rich mudstones and cementstones), flanks and intermound deposits (including pelmatozoan packstones and floatstones). The small mounds are mainly composed of bryozoans, cystoids and crinoids, and were developed on outer ramp environments. Mound initiation depended upon the stabilization and colonization of densely packed lenses of pelmatozoan-rich sediments. In a mid-ramp setting, pelmatozoan–bryozoan meadows were episodically degraded by common wave- and storm-induced processes, the development of semi-consolidated substrates, and the periodic influx of terrigenous material. Finally, during the Hirnantian regression, the Iberian mixed (carbonate–siliciclastic) platform was exposed to subaerial conditions sufficiently for erosion and karstification to occur. From a palaeogeographical point of view, the pattern of the Ashgill Iberian platform deposition is characterized by episodic exclusion of carbonates from most nearshore environments by a shoreline source of siliciclastic sediments. A similar interpretation to that made on the Iberian Cystoid Limestone Formation, in terms of gradual proximality–distality changes, is proposed for comparable facies types in Ashgill limestones described in southwestern Europe.

Sedimentary patterns across the Lower–Middle Cambrian transition in the Esla nappe (Cantabrian Mountains, northern Spain)

In the carbonate platforms of the western Gondwana margin, the extinction recorded at the Lower–Middle Cambrian boundary is accompanied by a profound change in the style of carbonate deposition. The Lancara Formation of the Esla nappe (Cantabrian Mountains, northern Spain) contains a distinct sedimentary turnover due to a combination of tectonism, eustatic fluctuations, and immigration and colonization of new benthic communities, such as the youngest archaeocyathan assemblage of the entire Iberian Peninsula. During latest Early Cambrian times, a regressive trend is recorded in the Lancara Formation. This regression was recorded on a peritidal-dominant, homoclinal ramp that is topped by a tectonically induced discontinuity (D1). The latter surface marks the beginning of a last prograding, regressive tendency recorded on an intra-shelf ramp with ooidal/bioclastic shoals protecting archaeocyathan-microbial patch reefs. The overlying discontinuity (D2) corresponds to a major erosive unconformity, which coincides with the Lower–Middle Cambrian boundary in the Cantabrian Mountains. The subsequent, long-term, earliest Middle Cambrian rise in relative sea-level allowed deposition of low-relief, bioclastic shoals bearing a diverse and cosmopolitan assemblage of benthic fauna. Finally, the previous evolution is bounded by a third discontinuity (D3), which marks the beginning of a rhythmic sedimentation indicative of a major phase of tectonic breakdown and drowning of platforms recognised throughout southwestern Europe. Two associations of calcimicrobes occur in the latest Early Cambrian regressive trend of the Lancara Formation: (i) Proaulopora and Subtiflora are identified in peritidal, high-energy settings, lacking self-supported structures, whereas (ii) intergrowths of Epiphyton, Renalcis and Girvanella encrusted branching colonies and solitary archaeocyaths in protected (back-shoal) patch reefs. The latest Early Cambrian regression is correlated in southwestern Europe in both siliciclastic (Iberian Chains and Ossa–Morena) and carbonate-dominant platforms (Cantabrian Mountains, Montagne Noire and Sardinia). Its tops are recognised as diachronous unconformities ranging in age from early Bilbilian to the Bilbilian–Leonian or Lower–Middle Cambrian boundary.

Episodic development of Cambrian eocrinoid-sponge meadows in the Iberian Chains (NE Spain)

SummaryThe Lower-Middle Cambrian transition in the Iberian Chains (NE Spain) records a transgressive deposition from peritidal to open-shelf environments. This overall trangressive trend took place across an interval of syndepositional tectonic processes. which have been illustrated during late Bilbilian to Leonian times.The scarcity of building organisms in this areas seems to be directly related to a rhythmic influence of fine terrigenous decantation, which inhibited and interrupted the carbonate production. Only in the more proximal and quiet environments, the episodic establishment of stromatolitic boundstones is evidence.Eocrinoid-sponge meadows were developed in shallow subtidal environments, and were completed by the presence of trilobites, inarticulate brachiopods, hyoliths and Chancelloriid sclerites. The biggest accumulation rates associated to these complex meadow communities seem to be reflected by differential synsedimentary faulting processes.

Global Cambrian trilobite palaeobiogeography assessed using parsimony analysis of endemicity

Abstract Palaeobiogeographical data on Cambrian trilobites obtained during the twentieth century are combined in this paper to evaluate palaeoceanographic links through c. 30 myr, once these arthropods biomineralized. Worldwide major tectonostratigraphic units are characterized at series intervals of Cambrian time and datasets of trilobite genera (629 for Cambrian Series 2, 965 for Cambrian Series 3, and 866 for the Furongian Series) are analysed using parsimony analysis of endemicity. Special attention is given to the biogeographical observations made in microcontinents and exotic terranes. The same is done for platform-basinal transects of well-known continental margins. The parsimony analysis of endemicity analysis resulted in distinct palaeogeographical area groupings among the tectonostratigraphic units. With these groupings, several palaeobiogeographical units are distinguished, which do not necessarily fit the previously proposed biogeographical realms and provinces. Their development and spatial distributions are broadly controlled by Cambrian palaeoclimates, palaeogeographical conditions (e.g. carbonate productivity and anoxic conditions) and ocean current circulation. Supplementary material: Global dataset of Cambrian Epoch 2 (A), Cambrian Epoch 3 (B) and the Furongian Epoch (C) trilobite genera are provided at: http://www.geolsoc.org.uk/SUP18669

Major geodynamic and sedimentary constraints on the chronostratigraphic correlation of the lower-middle Cambrian transition in the western Mediterranean region

The sedimentary rocks of the lower-middle Cambrian transition in the western Mediterranean region have recorded a superposition of extensive tectonic, volcanic, epeirogenic, and eustatic events that led to a complex sequence framework that, in some cases, makes detailed chronostratigraphic correlations difficult. This paper summarizes and updates the relationships between event stratigraphy, fluctuations of relative sea level, setting of major stratigraphic discontinuities, unconformities, and condensed levels, and succession of benthic community replacements displayed by outcrops located in the Iberian Peninsula (Iberian, Cantabrian and Córdoba platforms), Moroccan Atlas (Souss Basin), and the Montagne-Noire and Sardinian platforms. The resulting mosaic of inter-related geodynamic processes is correlated by trilobite, archaeocyath-and acritarch-based chronostratigraphic scales, taking as reference for the base of the Middle Cambrian in West Gondwana the immigration of paradoxidid trilobites.

Trilobite diversity patterns in the Middle Cambrian of southwestern Europe: a comparative study

Abstract This paper describes the pattern of trilobite diversity throughout the Middle Cambrian in two fossiliferous basins of the western Gondwana margin: the Iberian Chains (northeastern Spain) and the Montagne Noire (southern France). The documented fluctuations of species diversity allow us to recognize: (1) a substantial extinction event recorded in latest Early Cambrian times (named Valdemiedes event), which separates the culmination of a widespread decline of trilobites and the stepwise immigration of cosmopolitan invaders; (2) a major trilobite radiation occurring in the earliest Leonian and culminating across the Caesaraugustian/Languedocian transition, in which a peak in diversity of trilobites and carpoids was close to a major flooding surface; (3) a major reduction of trilobite taxa across the early/middle Languedocian transition related to a well-documented regressional trend; and (4) a second immigration of trilobite fauna in the late Languedocian which coincided with trangressional pulses and the establishment of suitable shaly substrates. In addition, we summarize the known trilobite occurrences from the Middle Cambrian of southwestern Europe. Maps of the distribution of some relevant taxa (genera and species) over the western Gondwana margin are documented.

The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid.

The earliest fossil echinoderms have, until now, come almost exclusively from North America and are represented by few taxa, all of which have a radiate body plan. Here we report the discovery of two new echinoderm faunas from the early part of the Cambrian of Morocco (West Gondwana). The former represents the oldest echinoderm fauna from Gondwana, approximately equivalent in age to those from North America, and the latter the oldest diversified fauna from Gondwana. In both cases, the appearance of well-preserved echinoderms coincides with a change in palaeogeographic regime. The presence of four markedly different echinoderm body plans in these earliest faunas indicates that considerable diversification had already taken place by 510 Ma. Yet all share the same distinctive biomineralized skeleton that, based on the fossil record and ocean geochemistry, probably evolved just 10-15 my earlier. This suggests that a rapid rate of morphological divergence took place during the initial stages of echinoderm evolution.

Microbially mediated phosphatization in the Neoproterozoic Doushantuo Lagerstätte, South China

Detailed petrographic and geochemical studies of pore-filling cements and replaced (epigenic) membranes of some organic-walled microorganisms preserved in the Neoproterozoic Doushantuo Lagerstatte (China) are based on an energy dispersive system (EDS) of elemental mapping attached to backscattered MEB. Their microcrystalline apatite (collophane) occurs predominantly as phosphatized microbial pseudomorphs: phosphatic crusts of chasmolithic bio-films (or microstromatolites) and globular clusters occur lining intraparticular porosities. Three major pre-compactional cement types are identified on algal thalli, embryos and vase-shaped microfossils: epigenic collophane (by replacement of membranes), polyphasic filling of collophane and crystalline apatite and fluorapatite within intraparticular porosities (some of them microbially mediated), and latest carbonate precipitation filling the remaining intraparticular porosities (preserved at present as dedolomitized calcite). P2O5 concentration has a clear peak in the microbial pseudomorphs and uniformly low concentrations elsewhere, whereas MgO, Na2O, K2O and SiO2 show a slight increase in concentration through successive cements. These geochemical trends evidence a slight change of pore fluids accompanied by a final rise in pH, presumably following the main interval of decay, which favoured precipitation of CaCO3 postdating that of apatite.

PELMATOZOAN ECHINODERMS AS COLONIZERS OF CARBONATE FIRMGROUNDS IN MID-CAMBRIAN HIGH ENERGY ENVIRONMENTS

Abstract New echinoderm holdfast discoveries from Gondwana demonstrate that pelmatozoans have been cementers able to attach to carbonate firmgrounds since the basal middle Cambrian. Echinoderms were thus colonizing shallow, high-energy environments well before the appearance of the first true carbonate hardgrounds in the Furongian. Morphological innovations and adaptations to firmground media ( = substrates) were first developed in softground, clayey, offshore environments where echinoderms cemented to shell fragments. This preadaption allowed echinoderms to quickly and effectively exploit the newly emerging hardground habitats in the Furongian to Early Ordovician.