Exceptional preservation in Lower Devonian coalified fossils from the Welsh Borderland: a new genus based on reniform sporangia lacking thickened borders

Reniform sporangia, comprising two equal valves and containing retusoid spores, recovered from Lower Old Red Sandstone strata of Devonian age ( micrornatus-newportensis Spore Biozone: lower Gedinnian lower Lochkovian) on North Brown Clee Hill in the Welsh Borderland are placed in Resilitheca salopensis gen. et sp. nov. Conventional compression fossils from Targrove, Ludlow of fertile axes showing isotomous branching with limited overtopping are considered conspecific because the terminal reniform sporangia contain the same spores. Spore ultrastructure is described using scanning and transmission electron microscopy. Sections show faint traces of lamellae. Particles associated with spores and sporangium wall are compared with the globules of pteridophytes and Ubisch bodies of angiosperms, and related to the development of the sporangium. The new plants are compared with Cooksonia caledonica Edwards known only from impressions, and with Renalia Gensel showing far more pronounced pseudomonopodial branching.     

Exceptional preservation in Lower Devonian coalified fossils from the Welsh Borderland: a new genus based on reniform sporangia lacking thickened borders

Reniform sporangia, comprising two equal valves and containing retusoid spores, recovered from Lower Old Red Sandstone strata of Devonian age (micrornatus-newportensis Spore Biozone: lower Gedinnian lower Lochkovian) on North Brown Clee Hill in the Welsh Borderland are placed in Resilitheca salopensis gen. et sp. nov. Conventional compression fossils from Targrove, Ludlow of fertile axes showing isotomous branching with limited overtopping are considered conspecific because the terminal reniform sporangia contain the same spores. Spore ultrastructure is described using scanning and transmission electron microscopy. Sections show faint traces of lamellae. Particles associated with spores and sporangium wall are compared with the globules of pteridophytes and Ubisch bodies of angiosperms, and related to the development of the sporangium. The new plants are compared with Cooksonia caledonica Edwards known only from impressions, and with Renalia Gensel showing far more pronounced pseudomonopodial branching.     

New insights into early land ecosystems: a glimpse of a lilliputian world

Small, relatively uncompressed, very fragmentary plant remains (mesofossils) are described from a Silurian (P ídolí) and a Lower Devonian (Lochkovian) locality in the Welsh Borderland. Excellent cellular preservation provides characters leading to the demonstration of diversity in plants of simple gross morphology and allows deliberations on functional anatomy (e.g. of stomata), and reproductive biology (including development and dehiscence of sporangia). A survey of in-situ spores is presented, and preliminary comparisons made with dispersed spore assemblages especially in relation to reconstruction of vegetation on local and regional scales. The earliest body fossils of unequivocal terrestrial arthropods isolated from the same locality as the P ídolí plants suggest that the decomposer/microherbivore/predator soil and litter communities found in the Lower and Middle Devonian extend back at least into the Silurian. Evidence for plant—animal interaction in the Lower Devonian comes from spore-dominated coprolites believed to have been produced by litter-feeding myriapods.     

A new genus for isolated bivalved sporangia with thickened margins from the Lower Devonian of the Welsh Borderland

A new taxon Sporathylacium salopense gen. et sp. nov . is based on small isolated coalified sporangia from the Lower Devonian (Lochkovian: micrornatus—newportensis spore zone) from the Welsh Borderland. The sporangia have two equal valves with multi-layered walls and thickened borders, and contain trilete crassitate, non-curvaturate isospores that are completely covered by a microgranular ornament with possible distal verrucate/murornate structures. They differ from zosterophyll sporangia in details of the presumed dehiscence zone extending around the entire free convex margin, particularly in the presence of a wedge of amorphous material between the valves, and in spore characters. Absence of any information relating to water-conducting cells prevents further assignment within the embryophytes. Spherical bodies associated with spores and a resilient sporangial lining are compared with similar structures in extant free-sporing plants and with Ubisch bodies. The mode of sporangial dehiscence involving anatomical modifications of the valve margins and the novel wedge of tissue which connects them remains speculative.     

The role of mid-palaeozoic mesofossils in the detection of early bryophytes

Recently discovered Silurian and Devonian coalified mesofossils provide an additional source of data on early embryophytes. Those reviewed in this paper are considered of some relevance to understanding the early history of bryophytes while highlighting the difficulties of recognizing bryophytes in often very fragmentary fossils. The first group comprises sporophytes in which terminal sporangia contain permanent dyads and tetrads. Such spores (cryptospores) are similar to those found dispersed in older Ordovician and Silurian strata, when they are considered evidence for a land vegetation of embryophytes at a bryophyte grade. The phylogenetic significance of plants, where the axes associated with both dyad- and tetrad-containing sporangia are branching, a character state not found in extant bryophytes, is discussed. The second group comprises axial fossils, many with occasional stomata, in which central conducting strands include G-type tracheids and a number of novel types of elongate elements not readily compared with those of any tracheophyte. They include smooth-walled, evenly thickened elongate elements as well as those with numerous branching +/- anastomosing projections into the lumen. Some of the latter bear an additional microporate layer, but the homogenized lateral walls between adjacent cells are never perforate. Such cells, which occur in various combinations in central strands, are compared with the leptoids and hydroids of mosses, hydroids of liverworts and presumed water-conducting cells in coeval Lower Devonian plants such as Aglaophyton. It is concluded that lack of information on the chemistry of their walls hampers sensible assessment of their functions and the affinities of the plants. Finally, a minute fossil, comprising an elongate sporangium in which a central cylindrical cavity containing spores and possible elaters terminates in a complex poral dehiscence apparatus, is used to exemplify problems of identifying early bryophytes. It is concluded that further progress necessitates the discovery of pre-Upper Silurian fossils with well-preserved anatomy, as well as a re-evaluation of criteria used to assess existing and new Devonian fossils for bryophyte affinity.     

FURTHER OBSERVATIONS ON ZOSTEROPHYLLUM LLANOVERANUM FROM THE LOWER DEVONIAN OF SOUTH WALES

Zosterophyllum llanoveranum was first described by Croft and Lang in 1942. This account presents the anatomy of the axes. Details of the sporangium wall in the vicinity of the dehiscence line are reported and the spores are described. The plant is compared with other members of the genus Zosterophyllum and also with those Devonian plants possessing exarch protosteles and lateral sporangia, which Banks has placed in the subdivision Zosterophyllophytina.     

Lower Devonian coalified sporangia from Shropshire: Salopella Edwards & Richardson and Tortilicaulis Edwards

A diverse assemblage of coalified sporangia from Lochkovian/Gedinnian, fluvial rocks imicrornatus-newportensis Spore Biozone) contains fusiform forms assignable to Salopella Edwards & Richardson and Tortilicaulis Edwards. Both show bifurcation within some sporangia. In specimens assigned to Salopella cf. marcensis , longitudinal dehiscence produces two equal valves revealing spores resembling Aneurospora , although trilete marks may be difficult to distinguish. The spores are in two forms with distal ornament of either coarse or fine coni, each occurring in separate sporangia. A further sporangium of Salopella shape contains distally apiculate, hilate cryptospores. Elongate sporangia exhibiting spiralling of superficial cells and sometimes gross twisting, are placed in a new species of Tortilicaulii, T. offaeus. Anatomical data include details of irregularly thickened epidermal cells, sporangial wall and in situ spores. The latter are trilete, equatorially thickened and highly distinctive, because the entire exospore surface is covered with grana, a feature not common in dispersed spores of this age. The nomenclatural and curatorial problems associated with these small, coalified fossils, whose anatomical examination by scanning electron microscope necessitates elimination of gross morphology, are discussed.     

Sporangia containing Scylaspora from the Lower Devonian of the Welsh Borderland

ABSTRACT. Bulk maceration of Early Devonian (Lochkovian) deposits from the Welsh Borderland has yielded eight specimens of a new type of sporangium characterized by its elongate shape and distinctive spores. The specimens have been examined using scanning electron, transmission electron and light microscopy. The elongate sporangia occur isolated and are fragmented to varying degrees. They contain trilete spores that possess a proximal surface with shallow murornate ornament and a distal surface that is laevigate. The spores belong to the dispersed spore genus Scylaspora , and this is the first report of such spores in situ . Ultrastructural studies demonstrate that the spore walls are bilayered with a lamellate inner layer overlain by an essentially homogeneous outer layer. There is little or no associated extra-exosporal material. To date this is the earliest reported example of lamellate wall ultrastructure in trilete spores. Models of spore wall development are suggested in the light of evidence provided by spore wall ultrastructure. Detailed comparisons of the characters preserved in the fossils (morphological, anatomical and ultrastructural), with those in other fossil and extant plants, currently shed little light on the phylogenetic relationships of these specimens, primarily due to the paucity of comparable data. It is proposed that the plant is probably of vascular status, but in the absence of evidence for vascular tissue, it must be classified as rhyniophytoid.     

Fertile pinnules of Danaeites rigida Gu and Zhi (Marattiales) from the Upper Permian of south China

Recently collected specimens of Danaeites rigida Gu and Zhi from the Upper Permian of south China have been subjected to detailed morphological investigations in order to reveal features of their fertile pinnules. The specimens are preserved as compression/impressions and possess pecopteroid-type pinnules with a single row of synangia on either side of the pinnule midvien. Individual synangia are bilaterally symmetrical, sessile, and their bases are embedded in the tissues of the pinnules. Synangia possess 18–24 sporangia that are laterally fused to one another throughout their entire length. Sporangial dehiscence is through a longitudinal slit and sporangia contain trilete spores with a granular ornamentation, referable to the dispersed spore Cyclogranisporites. This combination of characters is unique in specimens preserved by compression/impression although they are similar to those known in permineralized marattialean fertile fronds. However, anatomical details salient to the identification of these permineralized taxa are not identifiable in impression/compression fossils such as Danaeites. The taxonomic and phylogenetic implications of these findings are considered in detail and we conclude that Danaeites has closer links with Marattia than with Danaea.     

A new genus of Early Devonian plants with novel strobilar structures and vegetative appendages from the Posongchong Formation of Yunnan,China

A new plant, Dibracophyton acrovatum gen. et sp. nov., is described from the Lower Devonian (Pragian) Posongchong Formation of Wenshan District, southeastern Yunnan, China. The plant has creeping axes from which arise vegetative and fertile axes. The vegetative axes helically bear lateral dichotomous appendages with curved or round tips. The fertile axes possess terminal strobili with numerous fertile units arranged in irregular helices. Each fertile unit consists of a stalked long-elliptical sporangium, with dehiscence into two equal valves, and two discrete long-ovate bracts covering sporangium from above–below directions. A new genus and species is thus established based on these characters and is temporarily regarded as incertae sedis of Tracheophyta, although it perhaps closes to the barinophytes in affinity. Detailed comparisons with other plants having a similar architecture, i.e., sporangia closely associated with modified vegetative structures, are made. The discovery of D. acrovatum further enriches the composition of the Posongchong flora and demonstrates great morphological disparity of the Early Devonian vascular plants.     

Spore wall ultrastructure in the early lycopsid Leclercqia (Protolepidodendrales) from the Lower Devonian of North America: Evidence for a fundamental division in the lycopsids

Documenting the morphology and ultrastructure of spores from known Silurian-Devonian plants clarifies organization and probable affinities of dispersed spores and contributes to analyses of evolutionary changes and phylogenetic relationships in early plants. In this study of fossil in situ spores from the early protolepidodendralean lycopsid Leclercqia, we identified new characters including an additional synapomorphy of the ligulate lycopsid clade. A detailed light (LM), scanning electron (SEM), and transmission electron microscope (TEM) analysis of spores from two species of Leclercqia from the Lower Devonian (Emsian) of New Brunswick, eastern Canada, L. andrewsii and L. complexa, shows both are homosporous, yielding spores belonging to the dispersed spore form taxon Acinosporites lindlarensis. Important features of wall ultrastructure include the presence of a paraexospore, peculiar exospore-derived, peg-like structures located in the gap between the outer exospore/inner paraexospore, and multilamellate regions in the interradial areas of the proximal surface. Similar interradial multilamellate regions occur in other ligulate lycopsids (fossil and extant). This character is probably a further synapomorphy for the ligulate lycopsid clade, within which heterosporous lycopods form a monophyletic group. These data suggest the ligule and interradial multilamellate region appeared prior to heterospory.     

A diverse assemblage of early land plants from the Lower Devonian of the Welsh Borderland

FANNING, U., EDWARDS, D. & RICHARDSON, J. B., 1992. A diverse assemblage of early land plants from the Lower Devonian of the Welsh Borderland. Nine rhyniophytoid taxa are described from an early Gedinnian locality ( middle micromatus-newportensis spore Biozone) near Ludlow, England. They include Cooksonia pertoni, C. hemisphaerica, C. cambrensis, Tortilicaulis transwalliensis and three new taxa, Salopella marcensis sp. nov., Uskiella reticulata sp. nov. and Tarrantia salopensis gen. et sp. nov. Isolated sporangia of reniform shape, and those subtended by short lengths of axis, contain spores of Apiculiretusispora type and may belong to C. caledonica or Renalia . Morphologically distinctive forking, terminal sporangia lacking identifiable spores are not placed in a new taxon, because evidence based on in situ spores from elsewhere suggests they may belong to Salopella . Structures previously interpreted as clusters of sporangia of Yarravia- type are shown to be ± globular sporangia longitudinally split into valves. Sterile axes are dominated by smooth forms; although rare examples possess small enations. Tracheids have not been seen in axes of fertile specimens nor in sterile coalified compressions. A single pyrite permineralization contains tracheids of zosterophyll type. The assemblage demonstrates diversity among rhyniophytoids in the early Devonian and the existence of low vegetation 'alongside' the much larger zosterophyll dominated type.     

Autochory in ferns,not all spores are blown with the wind

Dispersal is a key process in plant population dynamics. In ferns, two successive vectors are needed: the sporangium catapulting mechanism, and wind or gravity. However, some rock ferns have a growth habit that suggests a kind of autochory by placing spores on the rock surface. Moreover, some ferns show modifications of the sporangial dehiscence. To determine the role of growth habit in spore dispersal, we checked the sporangial opening mechanism and explored the spatial distribution of plants on the walls. The presence of spores of Asplenium celtibericum, a rupicolous fern, in the rock surface was checked. In addition, its sporangial dehiscence, plant size and position in the wall were analysed. Spores and indehiscent sporangia were present on walls at each sampling moment. Their highest number was found close to the plants. There was a positive correlation between crack width and plant size. However, most plants occupy the upper half of the cliffs. The growth habit of A. celtibericum is instrumental to deposit the spores over the neighbouring rock surface, thus enhancing the probability of spores to find suitable crevices for germination. Furthermore, dispersal of indehiscent sporangia might promote intergametophytic mating, and the modified sporangial opening mechanism extends the dispersive period.     

ULTRASTRUCTURE OF DISPERSED AND IN SITU SPECIMENS OF THE DEVONIAN SPORE RHABDOSPORITES LANGII: EVIDENCE FOR THE EVOLUTIONARY RELATIONSHIPS OF PROGYMNOSPERMS

Abstract: The spore Rhabdosporites (Triletes) langii (Eisenack) Richardson, 1960 is abundant and well preserved in Middle Devonian (Eifelian) ‘Middle Old Red Sandstone’ deposits from the Orcadian Basin, Scotland. Here it occurs as dispersed individual spores and in situ in isolated sporangia. This paper reports on a detailed light microscope (LM), scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis of both dispersed and in situ spores. The dispersed spores are pseudosaccate with a thick walled inner body enclosed within an outer layer that was originally attached only over the proximal face. The inner body has lamellate/laminate ultrastructure consisting of fine lamellae that are continuous around the spore and parallel stacked. Towards the outer part of the inner body these group to form thicker laminate structures that are also continuous and parallel stacked. The outer layer has spongy ultrastructure. In situ spores preserved in the isolated sporangia are identical to the dispersed forms in terms of morphology, gross structure and wall ultrastructure. The sporangium wall is two‐layered. A thick coalified outer layer is cellular and represents the main sporangium wall. This layer is readily lost if oxidation is applied during processing. A thin inner layer is interpreted as a peritapetal membrane. This layer survives oxidation as a tightly adherent membranous covering of the spore mass. Ultrastructurally it consists of three layers, with the innermost layer composed of material similar to that comprising the outer layer of the spores. Based on the new LM, SEM and TEM information, consideration is given to spore wall formation. The inner body of the spores is interpreted as developing by centripetal accumulation of lamellae at the plasma membrane. The outer layer is interpreted as forming by accretion of sporopollenin units derived from a tapetum. The inner layer of the sporangium wall is considered to represent a peritapetal membrane formed from the remnants of this tapetum. The spore R. langii derives from aneurophytalean progymnosperms. In light of the new evidence on spore/sporangium characters, and hypotheses of spore wall development based on interpretation of these, the evolutionary relationships of the progymnosperms are considered in terms of their origins and relationship to the seed plants. It is concluded that there is a smooth evolutionary transition between Apiculiretusispora‐type spores of certain basal euphyllophytes, Rhabdosporites‐type spores of aneurophytalean progymnosperms and Geminospora‐/Contagisporites‐type spores of heterosporous archaeopteridalean progymnosperms. Prepollen of basal seed plants (hydrasperman, medullosan and callistophytalean pteridosperms) are easily derived from the spores of either homosporous or heterosporous progymnosperms. The proposed evolutionary transition was sequential with increasing complexity of the spore/pollen wall probably reflecting increasing sophistication of reproductive strategy. The pollen wall of crown group seed plants appears to incorporate a completely new developmental mechanism: tectum and infratectum initiation within a glycocalyx‐like Microspore Surface Coat. It is unclear when this feature evolved, but it appears likely that it was not present in the most basal stem group seed plants.     

The development of early terrestrial ecosystems

Summary

In this review of terrestrialization by plants and animals in the early Phanerozoic, the classical idea of a major mid-Palaeozoic event is discarded in favour of gradual colonization over a long time period. Four phases of colonization of the land by plants are recognized but their limits are often difficult to define. The first, of microbial mats comprising prokaryotes and later photosynthesizing protists (algae) but with no direct fossil evidence, extends from the Precambrian and may persist in environments unsuitable for colonization by higher plants and animals today. The second, based on microfossils (spores and cuticles) possibly from plants of bryophyte aspect (if not affinity) started in the Ordovician (c. 460 Ma ago) and ended in the Lower Devonian, but was overlapped by the third phase beginning early in the Silurian (c. 430 Ma). This consisted of small plants of axial organization with terminal sporangia probably allied to the tracheophytes. The advent of taller vascular plants of varied organization around the Silurian — Devonian boundary (c. 420–400 Ma) signalled the final pioneering phase — that of major adaptative radiations on land, culminating in the appearance of extant groups, in changes in reproductive strategy and in the development of complex vegetation structure. The animal record is sparser than that of the plants, but suggests an early land fauna in the mid-Palaeozoic which differed from later terrestrial assemblages in lacking herbivores, with the first direct fossil evidence for land animals in the late Silurian.     

A new microphyllous plant from the Lower Devonian of Yunnan Province, China

Sterile and fertile shoots of a microphyllous vascular plant are described from the Lower Devonian Posongchong Formation (? upper Siegenian), Yunnan Province, southern China and placed in the new taxon Halleophyton zhichangense gen. et sp. nov. The sterile axes are characterized by conspicuous, contiguous swollen rhomboidal to hexagonal leaf bases and relatively broad spatulate laminae. The relief of the leaf bases is not so pronounced in the fertile areas, which bear numerous cauline sporangia with two equal valves. In vegetative characters the new fossils, although smaller and more fragmentary, resemble Drepanophycus gaspianus but in the absence of information on the fertile parts of the latter and because of the controversy surrounding the concept of the genus Drepanophycus, we decided to erect a new genus. Its relationship to the lycophytes sensu stricto is equivocal as exact details of the relationship of sporangium with leaf and sporangium vascularization remain unknown.     

ULTRASTRUCTURAL STUDIES OF IN SITU DEVONIAN SPORES: BARINOPHYTON CITRULLIFORME

Each sporangium in the Upper Devonian taxon Barinophyton citrulliforme contains both microspores and megaspores. Microspores range up to 50 μm in diam and possess a homogeneous sporoderm characterized by an outer separable layer. The sporoderm of the megaspores (up to 900 μm) is constructed of sporopollenin units that are loosely arranged in the outer portion of the wall, and that give the megaspore wall a spongy organization. Ultrastructural evidence suggests that the small spores were not abortive megaspores, but that both spore types were functional. The spores of this plant, as well as other Devonian spores that show less dramatic size differences, are suggested as demonstrating a phase in the evolution of heterospory where sex determination was established in spores within the same sporangium prior to the evolution of micro- and megasporangia.     

Ultrastructure of some Cambrian palynomorphs from the Bright Angel Shale, Arizona, USA

Palynomorphs with complex resistant walls have been recovered from several Cambrian deposits in the continental United States. Those recovered from the Bright Angel Shale of Arizona typically preserve both a primary wall, and an outer envelope (synoecosporal wall) that encloses multiple spore-like bodies within. At least three distinct types of primary walls, are recognized with the TEM: 1) a unilaminate wall with a smooth inner surface and a sculptured outer surface, 2) a wall of three unornamented laminae of very uniform thickness, and, 3) a thicker wall with multiple thin, lightly-staining layers embedded in a darker matrix. This third type of primary wall bears a strong resemblance to those of certain Lower Devonian hilate cryptospore monads from the Welsh Borderlands. No extant algae produce spores with walls as thick or as complex, suggesting that these Cambrian palynomorphs were the desiccation-resistant spores of cryptogams belonging to the charophyte–embryophyte lineage. Multilaminate spore walls, which are characteristic of some extant liverworts and Paleozoic cryptospores, may have evolved via the fusion of separate, multiple laminae. This appears to be the primitive plant sporoderm type, but it may have evolved asynchronously with respect to the evolution of the embryophytic development of the sporophyte in land plants.     

Ultrastructure of laevigate hilate spores in sporangia and spore masses from the Upper Silurian and Lower Devonian of the Welsh Borderland

Spore masses and isolated sporangia, containing laevigate hilate cryptospores attributable to the dispersed taxon Laevolancis divellomedia sensu lato, have been recovered on bulk maceration of Upper Silurian (Pridoli) and Lower Devonian (Lochkovian) deposits from the Welsh Borderland. Detailed morphological, anatomical and ultrastructural analysis, using light microscope, scanning electron microscope and transmission electron microscope techniques, reveals subtle differences between the specimens and they can be grouped into five distinct types. The different groups are distinguished principally by using sporangia-spore mass characteristics, presence or absence of extra-exosporal material and nature of spore-wall ultrastructure. Of the groups, one has a uniformly homogeneous exospore and the other four groups have a bilayered exospore. In the former the spores lack extra-exosporal material and occur in a discoidal sporangium. Of the bilayered groups, two have exospores of homogeneous composition but with the two layers differing in electron density. They occur in discoidal sporangia and spore masses and are distinguished on the presence or absence of extra-exosporal material and differences in the widths of the two layers. Finally, two bilayered groups possess a lamellate inner layer, but vary in presumed sporangial shape. Elongate sporangia have spores with concentric continuous lamellae, lacking further ultrastructure. In contrast, spores from a discoidal spore mass have white-line-centred, presumably tripartite, lamellae which are laterally discontinuous, overlapping and irregularly spaced. These findings, which suggest that morphologically similar spores were produced by a number of plant taxa, have important implications regarding the assessment of early land-plant diversity. The affinities of hilate cryptospore-producing plants are unknown and problematic, particularly as no extant non-angiosperm plants produce dyads, other than through meiotic irregularity, and spore-sporangial characters have no exact counterpart in coeval plants. Studies of specimens with in situ hilate cryptospores suggest that they derive from rhyniophytoids, i.e. plants that resemble the simplest of vascular plants but lack evidence of vascular tissue, although hilate cryptospore-containing examples show no axial branching. It might be argued, based on evidence from spore wall ultrastructure, that some of the plants have more in common with lycopsids and filicopsids than bryophytes, a surprising finding bearing in mind the stratigraphic distribution of hilate cryptospores-dyads and inferences that the producers were bryophyte-like. Detailed studies of wall structure in the hilate cryptospores permit consideration of spore wall development. It is suggested that extra-exosporal material derives from a tapetum and is thus produced by the diploid sporophyte. The white-line-centred lamellae in a single specimen provide the earliest evidence for the presence of such structures in early land plant spores and provide further evidence that sporopollenin deposition on such structures is the most primitive mode of sporopollenin deposition among land plants.