Journal Article A reconsideration of cf. Psilophyton princeps (Croft & Lang, 1942), a zosterophyll widespread in the Lower Old Red Sandstone of South Wales Get access DIANNE EDWARDS, DIANNE EDWARDS F.L.S. 1Department of Geology, University of Wales, P.O. Box 914, Cardiff CF13'YE Search for other works by this author on: Oxford Academic Google Scholar PAUL KENRICK, PAUL KENRICK 2Paléobotanique et Paléopalynologie, Université de Liège, 7, Place du XX Août, B 4000 Liège, Belgium Search for other works by this author on: Oxford Academic Google Scholar LEEDS M. CARLUCCIO LEEDS M. CARLUCCIO 3Department of Biological Sciences, Central Connecticut State University, New Britain, Connecticut 06050, U.S.A. Search for other works by this author on: Oxford Academic Google Scholar Botanical Journal of the Linnean Society, Volume 100, Issue 4, August 1989, Pages 293–318, https://doi.org/10.1111/j.1095-8339.1989.tb01723.x Published: 28 June 2008 Article history Received: 01 August 1988 Accepted: 01 February 1989 Published: 28 June 2008
Abstract Modern cryptogamic ground covers (CGCs), comprising assemblages of bryophytes (hornworts, liverworts, mosses), fungi, bacteria, lichens and algae, are thought to resemble early divergent terrestrial communities. However, limited in situ plant and other fossils in the rock record, and a lack of CGC‐like soils reported in the pre‐Silurian sedimentological record, have hindered understanding of the structure, composition and interactions within the earliest CGCs. A key question is how the earliest CGC‐like organisms drove weathering on primordial terrestrial surfaces (regolith), leading to the early stages of soil development as proto‐soils, and subsequently contributing to large‐scale biogeochemical shifts in the Earth System. Here, we employed a novel qualitative, quantitative and multi‐dimensional imaging approach through X‐ray micro‐computed tomography, scanning electron, and optical microscopy to investigate whether different combinations of modern CGC organisms from primordial‐like settings in Iceland develop organism‐specific soil forming features at the macro‐ and micro‐scales. Additionally, we analysed CGCs growing on hard rocky substrates to investigate the initiation of weathering processes non‐destructively in 3D. We show that thalloid CGC organisms (liverworts, hornworts) develop thin organic layers at the surface (<1 cm) with limited subsurface structural development, whereas leafy mosses and communities of mixed organisms form profiles that are thicker (up to ~ 7 cm), structurally more complex, and more organic‐rich. We term these thin layers and profiles proto‐soils. Component analyses from X‐ray micro‐computed tomography data show that thickness and structure of these proto‐soils are determined by the type of colonising organism(s), suggesting that the evolution of more complex soils through the Palaeozoic may have been driven by a shift in body plan of CGC‐like organisms from flattened and appressed to upright and leafy. Our results provide a framework for identifying CGC‐like proto‐soils in the rock record and a new proxy for understanding organism–soil interactions in ancient terrestrial biospheres and their contribution to the early stages of soil formation.
Many compendia at the species, genus and family levels document the fossil record, but these are not standardized, nor usually critical in content, and few are available on the World Wide Web. The sampling of the available record is good for organisms with fossilizable parts, but preservational constraints on the entire morphology, life history and geographical distribution lead to difficulties in recognizing and naming species. We recommend abandoning some of the palaeontological species concepts such as chronospecies and stratospecies, and we advocate species recognition based on unique combinations of characters. The compilation of species lists is extremely time consuming, and given the inherent problems we suggest that compilation of generic lists is a more achievable goal because genera are recognized by definitive morphological characters. In calculating taxon duration, care must be taken to distinguish between mono-, para- and polyphyletic groups, the first being the only reliable unit for use in calculating diversity curves. We support the inclusion of fossils into classifications based on Recent organisms, but we recognize some of the problems this may pose for standard Linnaean classifications. Web-based taxonomy is the way forward, having the advantages of speed and currency of information dissemination, universal access with links to primary literature and increasingly sophisticated imagery. These advantages over conventional outlets will only be realized with careful Web design and a commitment to maintenance.
Specific micro-weathering features and biochemically derived residues formed by living organisms can be used as biomarkers to infer the presence of biological communities within sedimentary units of ancient ecosystems. We examined basaltic soil minerals from modern cryptogamic ground covers (CGCs) in Iceland and compared these with two early Paleozoic fossil systems. Nine biologically mediated weathering features (BWFs) were identified in modern soils including micron-scale surface trenching and penetrative tunnels, which are attributed to the actions of bacteria, fungi and exudates. Specific BWFs are associated with Fe residues, and with Fe-rich bio-precipitated nodules. Further, putative comparable features and Fe enrichment are identified in palaeosols from the late Silurian (Llansteffan, south Wales) and the Early Devonian (Rhynie chert, Scotland). Although we are cautious about attributing biological affinity to individual isolated features, our results demonstrate the potential of using multiple BWF types as a collective together with their chemical signatures as new proxies to understand community structure and interactions in early terrestrial ecosystems. This new information is the first evidence of interactions between ancient CGC-like organisms and substrate or soil inorganic components in the fossil record, and demonstrates the ability of CGC-like biospheres to contribute to mineral weathering, soil development and biogeochemical cycling during the early Paleozoic. Supplementary material: Fieldwork geomorphological information and triplot SEM-EDS data are available at https://doi.org/10.6084/m9.figshare.c.4373717
Summary Current ideas on the evolution of alternation of generations in land plants are reviewed in the context of important recent advances in plant systematics and the discovery of remarkable new palaeobotanical evidence on early embryophyte life cycles. An overview of relationships in major groups of green plants is presented together with a brief review of the early fossil record as a prelude to discussing hypotheses of life cycle evolution. Recent discoveries of life cycles in the early fossil record are described and assessed. The newly discovered gametophyte and sporophyte associations are based on exceptionally well‐preserved material from the Rhynie Chert, Scotland (Middle Devonian: 380–408 Myr) and compression fossils from other Devonian localities. These data document diplobiontic life cycles in plants at the ‘protracheophyte’ and early tracheophyte level of organization. Furthermore, the early fossils have a more or less isomorphic alternation of generations, a striking departure from life cycles in extant embryophytes. This unexpected similarity between gametophyte and sporophyte calls for a cautious approach in identifying ploidy level in early groups. Viewed in a systematic context, the neontological and palaeontological data contribute towards the formulation of a coherent hypothesis of life cycle evolution in major, early embryophyte groups. Evidence from extant groups strongly supports a single direct origin of the diplobiontic life cycles of land plants from haploid, haplobiontic life cycles in ancestral ‘charophycean algae’. The interest of the new palaeobotanical data lies in its relevance to life cycle evolution at the restricted level of vascular plants rather than at the more general level of embryophytes (vascular plants plus ‘bryophytes’). The occurrence of morphologically complex, axial gametophytes in early vascular plants is consistent with the moss sister‐group proposed in some cladistic analyses. Similarities of moss gametophytes to fossils in the vascular plant stem‐group are discussed, and it is argued that the late appearance of mosses in the macrofossil record may be due to the problem of recognizing stem‐group taxa. The new palaeobotanical evidence conflicts with previous hypotheses based on extant groups that interpret morphological simplicity as the plesiomorphic condition in the gametophytes of vascular plants. These new data indicate that a significant elaboration of both gametophyte and sporophyte occurred early in the tracheophyte lineage, and that the gametophytes of extant ‘pteridophytes’ are highly reduced compared to those of some of the earliest ‘protracheophytes’. Vestiges of this early morphological complexity may remain in the gametophytes of some extant groups such as Lycopodiaceae.
Pyrite-permineralized fruits and seeds from the London Clay Formation (Ypresian; England) in the NHMUK are stored in silicone oil to retard decay processes. X-ray micro-computed tomography (micro-CT) has revealed internal morphology for multiple holotypes (including severely cracked and encrusted specimens) scanned in the protective fluid. Silicone oil alone has a similar X-ray attenuation to parts of the specimens, causing minor uncertainty for digitally rendered surfaces, but key systematic characters are readily visualized. Further work is needed to optimize visualization of fine-scale cellular detail. Labelling and segmentation to visualize important structures is achievable with these micro-CT datasets. However, manual labelling of individual slices is required, and defining boundaries between features can be difficult due to differential pyritization and silicone oil permeation. Digital sections through specimens can be made in any orientation and digital locule casts can be produced for studies in virtual taphonomy. These achievements have been accomplished with minimal risk to specimens, which remained in silicone oil and were studied within the museum. The datasets provide a potentially permanent record of at-risk specimens, can be made widely available to researchers unable to visit the collections and to other interested parties, and they enable monitoring for future conservation.
We document xylem structure and hydraulic properties in the earliest woody plant Armoricaphyton chateaupannense gen. nov. & sp. nov. based on c. 407-million-year-old fossils from the Armorican Massif, western France. The plant was small, and the woody axes were narrow and permineralized in pyrite (FeS2). We used standard palaeobotanical methods and employed propagation phase contrast X-ray synchrotron microtomography (PPC-SRμCT) to create three-dimensional images of the wood and to evaluate its properties. The xylem comprised tracheids and rays, which developed from a cambium. Tracheids possessed an early extinct type of scalariform bordered pitting known as P-type. Our observations indicate that wood evolved initially in plants of small stature that were members of Euphyllophytina, a clade that includes living seed plants, horsetails and ferns. Hydraulic properties were calculated from measurements taken from the PPC-SRμCT images. The specific hydraulic conductivity of the xylem area was calculated as 8.7 kg m−1 s−1 and the mean cell thickness-to-span ratio (t/b)2 of tracheids was 0.0372. The results show that the wood was suited to high conductive performance with low mechanical resistance to hydraulic tension. We argue that axis rigidity in the earliest woody plants initially evolved through the development of low-density woods.
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