Wall ultrastructure in three species of the dispersed spore Emphanisporites from the Early Devonian
ABSTRACT Emphanisporites is a widely dispersed and well recognized spore genus that ranges from the Late Silurian throughout most of the Devonian. In this contribution, we present basic ultrastructural details for three species in the genus; E. rotatus, E. annulatus, and E. schultzii (with one specimen possibly grading into E. erraticus). Under the TEM, spores are variable – even within species – with respect to presence of laminae, orientation of laminae, presence of localized spongy layers, presence and prominence of equatorial thickenings, and overall exospore thickness, which ranges from 1.5 to 5.0 μm. There are also common elements of the spore ultrastructure, including: spongy zones at the base of sutures, and ornamentation integral to exospores (as opposed to perispore or para-exospore). Two features that are present in some of the specimens may offer insights into the vexing question of the affinities of the parent plants of these spores. Some specimens have both a single homogeneous lamina at the exospore surface, and a structure we are calling a “pseudosuture”, which is a zone of weakness, similar to a proximal suture, that occurs at the spore equator. Such a feature is known among extant plant spores only among the hornworts. When considering all the ultrastructural evidence together, and comparing it to that known for all extant plant spores, a case can be made for production by both hornworts and lycopsids. Thus, we conclude that the emphanoid condition (possessing proximate radially disposed muri) likely arose in response to some as yet unknown selective factor, and that members of various groups adopted the form by convergence.
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ABSTRACT: The John Williams Index of Palaeopalynology (JWIP) is the result of the lifetime’s work of Dr John E. Williams. Housed at the Department of Palaeontology of The Natural History Museum (NHM) in London, the JWIP is publically available and provides probably the most comprehensive fully cross-referenced catalogue on palaeopalynology in the world. It has 23,350 references to fossil palynomorph genera or species as of February 2012. Since its inception in 1971, every publication in the collection referring to a fossil palynomorph genus or species has been critiqued by John E. Williams. Each item is given an accession number and appropriately referenced within the JWIP using index cards which are sorted alphabetically. Once added to the main reference subindex, further entries are completed for four themed subindexes. The first three of these are sets of cards on the three major palynomorph groups (acritarchs/dinoflagellate cysts, chitinozoa and pollen/spores), 26 stratigraphical intervals and 17 geographical areas. The fourth themed subindex is where each palynomorph taxon has a card (or cards) listing all the records of that species in the literature within six categories (acritarchs, dinoflagellate cysts, chitinozoa, fungal spores, pollen/spores and miscellaneous). Due to the sustained and meticulous recording of data since 1971, users can therefore search the database by major palynomorph group, species, age and/or geographical region. The comprehensive and cross-referenced nature of the JWIP means that researchers can readily identify key publications on, for example, specific palynomorph types over a particular interval in a prescribed area. The JWIP is currently entirely analogue, but the NHM is currently evaluating potential strategies for digitisation.Palynology 01/2012; 36(2):224-233. · 1.35 Impact Factor
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ABSTRACT: • Plants have utterly transformed the planet, but testing hypotheses of causality requires a reliable time-scale for plant evolution. While clock methods have been extensively developed, less attention has been paid to the correct interpretation and appropriate implementation of fossil data. • We constructed 17 calibrations, consisting of minimum constraints and soft maximum constraints, for divergences between model representatives of the major land plant lineages. Using a data set of seven plastid genes, we performed a cross-validation analysis to determine the consistency of the calibrations. Six molecular clock analyses were then conducted, one with the original calibrations, and others exploring the impact on divergence estimates of changing maxima at basal nodes, and prior probability densities within calibrations. • Cross-validation highlighted Tracheophyta and Euphyllophyta calibrations as inconsistent, either because their soft maxima were overly conservative or because of undetected rate variation. Molecular clock analyses yielded estimates ranging from 568-815 million yr before present (Ma) for crown embryophytes and from 175-240 Ma for crown angiosperms. • We reject both a post-Jurassic origin of angiosperms and a post-Cambrian origin of land plants. Our analyses also suggest that the establishment of the major embryophyte lineages occurred at a much slower tempo than suggested in most previous studies. These conclusions are entirely compatible with current palaeobotanical data, although not necessarily with their interpretation by palaeobotanists.New Phytologist 07/2011; 192(1):266-301. · 6.74 Impact Factor