Search

The fossil record indicates a major turnover in marine phytoplankton across the Ediacaran–Cambrian transition, coincident with the rise of animal-rich ecosystems. However, the diversity, affinities and ecologies of Cambrian phytoplankton are poorly understood, leaving unclear the role of animal interactions and the drivers of diversification. New exceptionally preserved acritarchs (problematic organic-walled microfossils) from the late early Cambrian (around 510 Ma) reveal colonial organization characterized by rings and plates of interconnected, geometrically arranged cells. The assemblage exhibits a wide but gradational variation in cell size, ornamentation and intercell connection, interpreted as representing one or more species with determinate (coenobial) colony formation via cell division, aggregation and growth by cell expansion. An equivalent strategy is known only among green algae, specifically chlorophycean chlorophytes. The fossils differ in detail from modern freshwater examples and apparently represent an earlier convergent radiation in marine settings. Known trade-offs between sinking risk and predator avoidance in colonial phytoplankton point to adaptations triggered by intensifying grazing pressure during a Cambrian metazoan invasion of the water column. The new fossils reveal that not all small acritarchs are unicellular resting cysts, and support an early Palaeozoic prominence of green algal phytoplankton as predicted by molecular biomarkers. [ABSTRACT FROM AUTHOR]

The euarthropod Luohuilinella deletres sp. nov. is described from rare material from the Chengjiang biota, Cambrian Series 2, Stage 3, of Yunnan Province, China. Phylogenetic analysis recovers a xandarellid affinity for L. deletres , representing only the fifth described species of this clade. L. deletres possesses a head shield that is about one-fifth of the total body length and a trunk with 30 tergites, the reduced anterior-most tergite and terminal three tergites lacking pleural elongations. Anteriorly situated notches in the head shield are associated with stalked eyes, in contrast to the more posterior, enclosed eye slits present in Xandarella. Posterior to the antennae there are at least 11 pairs of biramous appendages preserved, including three pairs in the head. The morphology of the midline gut of L. deletres , in which lateral, unbranched diverticula are wider towards the front of the body, is a characteristic also found in various trilobites. The dorsoventrally flattened exoskeleton suggests a benthic or nektobenthic mode of life for L. deletres , as for other trilobitomorphs, and it likely used its well-developed anteriorly positioned eyes for searching out food, either to scavenge or to find prey. [ABSTRACT FROM AUTHOR]

Chancelloriids are an extinct group of spiny Cambrian animals of uncertain phylogenetic position. Despite their sponge-like body plan, their spines are unlike modern sponge spicules, but share several features with the sclerites of certain Cambrian bilaterians, notably halkieriids. However, a proposed homology of these 'coelosclerites' implies complex transitions in body plan evolution. A new species of chancelloriid, Allonnia nuda, from the lower Cambrian (Stage 3) Chengjiang Lagerstätte is distinguished by its large size and sparse spination, with modified apical sclerites surrounding an opening into the body cavity. The sclerite arrangement in A. nuda and certain other chancelloriids indicates that growth involved sclerite addition in a subapical region, thus maintaining distinct zones of body sclerites and apical sclerites. This pattern is not seen in halkieriids, but occurs in some modern calcarean sponges. With scleritome assembly consistent with a sponge affinity, and in the absence of cnidarian- or bilaterian-grade features, it is possible to interpret chancelloriids as sponges with an unusually robust outer epithelium, strict developmental control of body axis formation, distinctive spicule-like structures and, by implication, minute ostia too small to be resolved in fossils. In this light, chancelloriids may contribute to the emerging picture of high disparity among early sponges. [ABSTRACT FROM AUTHOR]

Exceptionally preserved 'Burgess Shale-type' fossil assemblages from the Cambrian of Laurentia, South China and Australia record a diverse array of non-biomineralizing organisms. During this time, the palaeocontinent Baltica was geographically isolated from these regions, and is conspicuously lacking in terms of comparable accessible early Cambrian Lagerstätten. Here we report a diverse assemblage of small carbonaceous fossils ( SCFs) from the early Cambrian (Stage 4) File Haidar Formation of southeast Sweden and surrounding areas of the Baltoscandian Basin, including exceptionally preserved remains of Burgess Shale-type metazoans and other organisms. Recovered SCFs include taxonomically resolvable ecdysozoan elements (priapulid and palaeoscolecid worms), lophotrochozoan elements (annelid chaetae and wiwaxiid sclerites), as well as 'protoconodonts', denticulate feeding structures, and a background of filamentous and spheroidal microbes. The annelids, wiwaxiids and priapulids are the first recorded from the Cambrian of Baltica. The File Haidar SCF assemblage is broadly comparable to those recovered from Cambrian basins in Laurentia and South China, though differences at lower taxonomic levels point to possible environmental or palaeogeographical controls on taxon ranges. These data reveal a fundamentally expanded picture of early Cambrian diversity on Baltica, and provide key insights into high-latitude Cambrian faunas and patterns of SCF preservation. We establish three new taxa based on large populations of distinctive SCFs: Baltiscalida njorda gen. et sp. nov. (a priapulid), Baltichaeta jormunganda gen. et sp. nov. (an annelid) and Baltinema rana gen. et sp. nov. (a filamentous problematicum). [ABSTRACT FROM AUTHOR]

Ostracods are tiny bivalved crustaceans with a fossil record extending into rocks of the Lower Ordovician. They occupy almost all aquatic environments today, from the ocean abyssal planes to damp forest leaf litter. Their stratigraphical record suggests they had diversified into a wide range of marine and non-marine habitats already during the Palaeozoic. Through the Ordovician, ostracods are mostly known from marine shelf depositional settings. These are mostly podocope ostracods that appear to have had a benthic mode of life like their modern counterparts; myodocope ostracods, though known from the Ordovician, likely became pelagic only in the Silurian. As they are considered benthic, and possessed no pelagic larval stage, Ordovician podocope ostracods have been widely used as key biogeographical index species for much of the early Palaeozoic. A fundamental question in the oceanographic evolution of ostracods is: when did a psychrosphere evolve (a fauna inhabiting cool waters below the thermocline)? A psychrospheric ostracod fauna in the Ordovician would question some of their biogeographic utility, given that such taxa might have a much wider dispersal capability than more shallow shelf faunas. Here we describe a new ostracod fauna from a palaeotropical South China plate setting, preserved in Upper Ordovician mudstones and siltstones from northern Vietnam. The fauna contains taxa endemic to the South China palaeoplate, but also yields several taxa at the generic level that are known from European and North American Ordovician settings. We discuss whether these latter taxa might be indicative of a more widely dispersed deeper marine psychrospheric Ordovician ostracod fauna, and the implications this would have on traditional biogeographic models. We also discuss other possibilities for these apparently more cosmopolitan taxa, including homeomorphy, previously unknown palaeogeographical connections, and the possibility of pelagic podocope taxa. [ABSTRACT FROM AUTHOR]

Nidelric pugio gen. et sp. nov. from the Cambrian Series 2 Heilinpu Formation, Chengjiang Lagerstätte, Yunnan Province, China, is an ovoid, sac-like metazoan that bears single-element spines on its surface. N. pugio shows no trace of a gut, coelom, anterior differentiation, appendages, or internal organs that would suggest a bilateral body plan. Instead, the sac-like morphology invites comparison with the radially symmetrical chancelloriids. However, the single-element spines of N. pugio are atypical of the complex multi-element spine rosettes borne by most chancelloriids and N. pugio may signal the ancestral chancelloriid state, in which the spines had not yet fused. Alternatively, N. pugio may represent a group of radial metazoans that are discrete from chancelloriids. Whatever its precise phylogenetic position, N. pugio expands the known disparity of Cambrian scleritome-bearing animals, and provides a new model for reconstructing scleritomes from isolated microfossils. [ABSTRACT FROM AUTHOR]

Diverse carbonaceous microfossils, including exceptionally preserved remains of non-biomineralizing metazoans, are reported from a basal middle Cambrian interval of the Kaili Formation (Guizhou Province, China). The application of a gentle acid maceration technique complements previous palynological studies by revealing a larger size-class of acritarchs, a richer assemblage of filamentous microfossils, and a variety of previously unrecovered forms. Metazoan fossils include Wiwaxia sclerites and elements derived from biomineralizing taxa, including chancelloriids, brachiopods and hyolithids, in common with previously studied assemblages from the early and middle Cambrian of Canada. In addition, the Kaili Formation has yielded pterobranch remains and an assemblage of cuticle fragments representing "soft-bodied" worms, including a priapulid-like scalidophoran. Our results demonstrate the wide distribution and palaeobiological importance of microscopic "Burgess Shale-type" fossils, and provide insights into the limitations and potential of this largely untapped preservational mode. [ABSTRACT FROM AUTHOR]

The Early-Middle Cambrian Mount Cap Formation, NWT, hosts a diverse range of exceptionally preserved fossils. Like the celebrated Burgess Shale of British Columbia, the Mount Cap contains carbonaceous compression fossils of animals that lacked mineralized hard parts, as well as the fully articulated skeletons of shelly taxa. Its unique importance, however, lies in exceptional carbonaceous preservation at a microscopic scale. Acid-extracted microfossils from the 'Little Bear biota' of the Mackenzie Mountains reveal important details of problematic groups including chancelloriids and hyolithids, and provide direct evidence of Cambrian diets in the form of faecal strings. A complementary microfossil assemblage from the subsurface of the Colville Hills region contains an extraordinary diversity of exquisitely preserved arthropod cuticle, and constitutes the oldest known record of complex crown-group crustaceans. We discuss the wider significance of the Mount Cap fossils, and describe some new forms that point to the potential for future discoveries. [ABSTRACT FROM AUTHOR]