Ediacaran microbial colonies

Enigmatic discoidal fossils are common in Neoproterozoic sedimentary sequences and in the stratigraphic record pre-date the first appearance of diverse Ediacaran fossil assemblages. Termed 'medusoids', these Neoproterozoic discoidal fossils have generally been interpreted as coelenterate-grade organisms implying a radially symmetrical body plan for ancestral eumetazoans. Analysis of exceptionally preserved discoidal fossils from the White Sea area, however, indicates that most of these discoidal forms represent colonial microbes. Localized pyritization, for example, reveals the presence of a conspicuous filamentous substructure in Ediacaria , whereas concentric rings, radial sectors and central structures in Cyclomedusa and Paliella compare directly with Recent microbial colonies growing in a nutritionally heterogeneous environment. At least some Ediacaran discoids can be compared with extant concentric ring-shaped microbial colonies that grow in hypersaline microbial mats. Insofar as most of the remaining record of Ediacaran discoids can be attributed to the holdfast structures of non-radiate modular organisms, there is no support from the fossil record for identifying a radiate ancestry for the Metazoa.     

ENIGMATIC WORM-LIKE ORGANISMS FROM THE UPPER DEVONIAN OF NEW YORK: AN APPARENT EXAMPLE OF EDIACARAN-LIKE PRESERVATION

Abstract:  The supposed polychaete annelid Protonympha salicifolia , from the Upper Devonian (Frasnian) of New York State, USA, is redescribed. P .  salicifolia has a bilateral and segmented body, but appears to have a 'mattress-like' construction with blade-like extensions along the margins. The affinities of P. salicifolia remain unresolved, but a proposed relationship to the annelids is unlikely. The preservation of the three known fossils, as mouldic imprints in sandstones, is strongly reminiscent of the circumstances associated with Ediacaran fossilization, and as such is an anomalous occurrence of such soft-part preservation in Phanerozoic sediments. Material associated with these enigmatic fossils has been referred to Palaeochaeta devonica , and also compared with the annelids. Such an assignment is also rejected. These fossils appear to be arthropodan, and are possibly myriapods. A supposed example of Protonympha (' P '.  marcellensis ) from the Middle Devonian of New York State is now excluded from this genus, and it may be a crustacean.     

Maintenance of species boundaries despite rampant hybridization between three species of reef fishes (Hexagrammidae): implications for the role of selection

Hybridization and introgression are antagonistic to the process of speciation. If hybrids are viable and backcross, introgression will slow or prevent the evolution of reproductive isolation between populations. If species hybridize upon secondary contact, introgression will ultimately erode species boundaries. If hybridization is rampant and gene flow is high, the proportion of hybrids will approach values expected with random mating, and introgression will overcome species boundaries, homogenizing populations. Alternatively, if species are genetically distinct, the proportion of hybrids would be significantly less than expected with random mating, gene flow would be interrupted by selection against hybrids, and inviability or sterility of F ₁ hybrids or backcrosses would be expected. Here, we investigate a system with characteristics expected by both of these opposing scenarios. Hybridization occurs between three species of reef fishes in the genus Hexagrammos at unexpected high frequencies in a zone of distributional overlap. Backcrossed individuals are detected, indicating F ₁ hybrids are viable and capable of reproducing. Yet, these species are genetically distinct at multiple loci. To study this apparent paradox, we estimate the relative proportions of hybrids, patterns of symmetry, inviability, and cytonuclear disequilibria using one mitochondrial and two nuclear markers. We invoke selection against hybrids, at various life history stages, in the maintenance of species boundaries in this system.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 135–147.