Microdictyon

Abstract: Marine animals related to Recent onychophorans form a significant component in Cambrian faunas. Twelve characters are analysed for homologies in the seven best known Cambrian onychophorans. New morphological evidence and homology analyses for several characters indicate an anteroposterior reversal of Hallucigenia and Microdictyon. Proposed expansion of the trunk in Microdietyon during compaction is rejected. A jaw is tentatively identified in Onychodictyon. The shape of the annulations and the disposition of the tenth leg pair in Aysheaia are reinterpreted, and the suggestion of two somites to the first appendage pair is rejected. A suggested morphocline may mirror the phylogeny of the group. The taxonomic confusion surrounding the supposed radiolarian family Eoconchariidae is cleared

Abstract: N2 fixation (C2H2 reduction) was associated with several species of macroalgae on a coral reef near Grand Bahama Island. The highest rates were associated with Microdictyon sp. (Chlorophyceae) and Dictyota sp. (Phaeophyceae). Extensive mats of filamentous blue-green algae, not heterotrophic bacteria, were the N2 fixing agents: in experiments with samples of Microdictyon sp., the activity was lightdependent and not stimulated by organic compounds under either aerobic or anaerobic conditions. Assays in situ, at 20 m depth, and on shipboard, gave similar rates of N2 fixation; the cyanophytes presumably have pigment adaptations to function in blue light. The maximum rate of N2 fixation, associated with Microdictyon sp., was 3.8 μg N fixed g dry weight-1 h-1. Coral-reef communities flourish in nutrientimpoverished waters, and therefore any input of nitrogen is probably important in stabilizing such ecosystems.

Abstract: An onychophoran-like fossil animal, Paucipodia inermis gen. et sp. nov., is described from the Lower Cambrian Chengjiang lagerstatte in Yunnan, China. The animal was soft-bodied, and possessed six pairs of unjointed legs, each having two claws distally. The body was finely annulated, lacked trunk plates, and apparently terminated posteriorly with the last leg pair, without preserved evidence of a posterior trunk extension. Paucipodia widens the morphological range hitherto known in Cambrian lobopodians, notably regarding leg number and presence or absence of plates. Its morphology also provides support for the recently suggested anteroposterior orientation of Hallucigenia and Microdictyon .

Abstract: Isolated lobopodian plates are reported from Early Cambrian strata at five localities in southern China. A wide variety of morphologies is represented, reflecting a considerable diversification within the phylum at this time. The new taxon Microdictyon jinshaense is erected and new observations are recorded on established taxa, based on examination of more than 600 well-preserved plates; irregular patterns of node distribution and the presence of large spines are documented on some taxa for the first time. Rare specimens in which two plates are conjoined, with a larger plate underlying a smaller one, are interpreted as showing a new sclerite emerging underneath its predecessor, which has not yet been moulted. These specimens confirm the process of ecdysis in the lobopodians and contribute to a fuller understanding of the ontogeny of these organisms. A functional hypothesis that suggests that the plates were complex visual structures is refuted; it is possible that they were sites of muscle attachment, but a protective role is regarded as more plausible.