Machaeridians are Palaeozoic armoured annelids

The systematic affinities of several Palaeozoic skeletal taxa were only resolved when their soft-tissue morphology was revealed by the discovery of exceptionally preserved specimens. The conodonts provide a classic example, their tooth-like elements having been assigned to various invertebrate and vertebrate groups for more than 125 years until the discovery of their soft tissues revealed them to be crown-group vertebrates. Machaeridians, which are virtually ubiquitous as shell plates in benthic marine shelly assemblages ranging from Early Ordovician (Late Tremadoc) to Carboniferous, have proved no less enigmatic. The Machaeridia comprise three distinct families of worm-like animals, united by the possession of a dorsal skeleton of calcite plates that is rarely found articulated. Since they were first described 150 years ago machaeridians have been allied with barnacles, echinoderms, mollusks or annelids. Here we describe a new machaeridian with preserved soft parts, including parapodia and chaetae, from the Upper Tremadoc of Morocco, demonstrating the annelid affinity of the group. This discovery shows that a lineage of annelids evolved a dorsal skeleton of calcareous plates early in their history; it also resolves the affinities of a group of problematic Palaeozoic invertebrates previously known only from isolated elements and occasional skeletal assemblages

Large colonial organisms with coordinated growth in oxygenated environments 2.1Gyr ago

International audienceThe evidence for macroscopic life during the Palaeoproterozoic era (2.5-1.6Gyr ago) is controversial. Except for the nearly 2-Gyr-old coil-shaped fossil Grypania spiralis, which may have been eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6-1.0Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45-2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5Gyr later, in the Cambrian explosion