William A. DiMichele

Stratigraphic and interregional changes in Pennsylvanian coal-swamp vegetation: Environmental inferences

Abstract Quantitative analysis of Pennsylvanian coal-swamp vegetation provides a means of inferring organization and structure of communities. Distribution of these communities further provides inferences about environmental factors, including paleoclimate. Our observations are based on in situ, structurally preserved peat deposits in coal-ball concretions from 32 coal seams in the eastern one-half of the United States and from several seams in western Europe and on spore assemblages from more than 150 seams. There were three times of particularly significant and nearly synchronous vegetational changes in the Midcontinent and Appalachian coal regions during the Pennsylvanian Period. Each was different in kind and magnitude. The first marked changes occurred during the early part of the Middle Pennsylvanian with the fluctuating decline in the high level of lycopod dominance. The abundance of cordaites increased. There was a rise in the occurrences of the lycopod herbs to form intercalated marshlands and an overall increase in floral diversity. Changes ensuing from this time also include shifts in dominant species of lycopod trees and a sustained rise in abundance and diversity of tree-fern spores. The next significant time of change was during the middle part of the Middle Pennsylvanian, representing both a culmination of earlier trends and expansions of cordaites in the Midcontinent where there was a maximum change in species without net loss of diversity. Tree ferns and medullosan pteridosperms attained subdominant levels of abundance and diverse lycopod species dominated except in the Atokan-Desmoinesian transition of the Midcontinent. The third and sharpest break occurred near the Middle—Late Pennsylvanian boundary when extinctionsof the dominant, coal-swamp lycopods allowed development of tree-fern dominance. The Late Pennsylvanian coal swamps apparently were colonized or recolonized mainly by species from outside coal swamps rather than by the survivor populations of the Middle Pennsylvanian swamps. Paralleling the changes in floras through the Pennsylvanian are changes in preservational aspects of the peat. These include a decline in shoot/root ratios from approximately 1 to

Paleobotanical and paleoecological constraints on models of peat formation in the Late Carboniferous of Euramerica

Abstract The dominant plants of the Late Carboniferous lowland tropics were taxonomically and structurally distinct from those of any later time periods. Dominance was distributed among lycopsids, ferns, sphenopsids, pteridosperms and cordaites, and each of these groups had distinctive and different ecological preferences and amplitudes. Peat-forming habitats were dominated by lycopsids throughout the Westphalian, with a significant cordaitean element in the middle Westphalian; during the Stephanian tree ferns were dominant, following major extinctions near the Westphalian-Stephanian transition. Each of the major plant groups had distinctive architectures and tissue composition. Trees contributed up to 95% of the peat biomass and tree forms of lycopsids. Psaronius and Medullosa lack good modern analogues. The cordaites were the only woody plant group to contribute significantly to peat, and then only during the mid-Westphalian. Structurally wood-like lycopsid bark is the major “woody” tissue encountered in most Westphalian coals. Tree ferns and pteridosperms were largely parenchymatous in construction; the stigmarian root systems of lycopsids also were largely parenchymatous. The tissue structure of these dominant plant suggests the need for extreme caution in the inference of mire ecological conditions or vegetational structure from coal petrographic data. Peat formed under arborescent ferns or pteridosperms, or peat repeatedly exposed to decay and rerooting by stigmarian root systems of lycopsids, would have a distinctly non-woody signature and yet would have formed in a forested environment. A summary is presented of the autecology and synecology of mire plants, emphazing the structural framework provided by lycopsids during the Westphalian. Certain constraints in the links between peat biomass and miospore palynology are discussed in terms of over-representation, under-representation and non-representation. The formulation of Smiths four-phase hydroseral model is discussed and compared with more recent data available from plant paleoecology. The current debate over an ombrotrophic vs. rheotrophic origin of Late Carboniferous peats relies in large part on paleobotanical data, almost entirely palynological, in combination with petrographic analyses. Ecological studies of miospores and of coal-ball and compression macrofossils, and the linkage of miospores to source plants, permit the re-evaluation of mire successional models. Evidence for tree lycopsids, sphenopsids, pteridosperms and cordaites suggests growth mainly in rheotrophic mires. Tree ferns are likely candidates for growth in domed mires, although evidence is ambiguous and some tree ferns clearly grew under rheotrophic conditions. Densospores, produced by at least Sporangiostrobus lycopsid subtrees, have been considered diagnostic of ombrotrophic conditions; abundant evidence refutes this simplistic interpretation and suggests broad ecological amplitudes for densospore producers, including growth under rheotrophic conditions. Although plant fossils alone can not resolve most of the major debates in modern coal geology, paleobotany does contribute significantly to our understanding of ancient mires. An approach combining paleobotanical data with petrography, sedimentology and geochemistry, on a case by case basis, is most likely to produce a clear picture.

Comparative Ecology and Life-History Biology of Arborescent Lycopsids in Late Carboniferous Swamps of Euramerica

The comparative ecologies of Diaphorodendron, Lepidodendron, Lepidophloios, Paralycopodites (= Anabathra), and Sigillaria in Late Carboniferous coal swamps serve as a context for assessing life cycles and exploring possible structure-function relations. The distinctive aspects of the «lycopsid tree habit» in lepidodendrids are emphasized as part of the arborescent reproductive architecture of relatively short-lived (10-15 years) plants (...)

HETEROSPORY: THE MOST ITERATIVE KEY INNOVATION IN THE EVOLUTIONARY HISTORY OF THE PLANT KINGDOM

1 In aggregate, past discussions of heterospory and its role in the alternation of generations are riddled with ambiguities that reflect overlap of terms and concepts. Heterospory sensu lato can be analyzed more effectively if it is fragmented into a series of more readily defined evolutionary innovations: heterospory sensu stricto (bimodality of spore size), dioicy, heterosporangy, endospory, monomegaspory, endomegasporangy, integumentation, lagenostomy, in situ pollination, in situ fertilization, pollen tube formation, and siphonogamy (Tables 1, 2, Figs 1, 13). Current evidence suggests that the last five characters are confined to the seed‐plants. 2 The fossil record documents repeated evolution of heterosporous lineages from anisomorphic homosporous ancestors. However, interpretation is hindered by disarticulation of fossil sporophytes, the difficulty of relating conspecific but physically independent sporophyte and gametophyte generations in free‐sporing pteridophytes, the inability to directly observe ontogeny, and the rarity of preservation of transient and/or microscopic reproductive phenomena such as syngamy and siphonogamy. Unfortunately, the rarely preserved phenomena are often of far greater biological significance than corresponding readily preserved phenomena (e.g. heterospory versus dioicy, heterosporangy versus endospory). 3 In most fossils gametophyte gender can only be inferred by extrapolation from the morphology of the sporophyte and especially of the spores. This is readily achieved for species possessing high‐level heterospory, when the two spore genders have diverged greatly in size, morphology, ultrastructure and developmental behaviour. However, the earliest stages in the evolution of heterospory, which are most likely to be elucidated in the early fossil record of land‐plants, also show least sporogenetic divergence. It is particularly difficult to distinguish large microspores and small megaspores from the large isospores of some contemporaneous homosporous species (Figs 3–6 a, g). Heterospory is best identified in fossils by quantitative analysis of intrasporangial spore populations. 4 The spatial scale of the differential expression of megaspores and microspores varies from co‐occurrence in a single sporangium (anisospory) to different sporophytes (dioecy) (Figs 6–8). Studies of the relative positions of the two spore morphs on the sporophyte, and of developmentally anomalous terata (Fig. 9), demonstrate that gender is expressed epigenetically in both the sporophyte and gametophyte. Hormonal control operates via nutrient clines, with nutrient‐rich microenvironments favouring femaleness; megaspores and microspores compete for sporophytic resources. External environments can also influence gender, particularly in free‐living exosporic gametophytes. 5 The evolution of heterospory was highly iterative. The number of origins is best assessed via cladograms, but no current phylogeny includes sufficient relevant tracheophyte species. Also, several extant heterosporous species differ greatly from their closest relatives due to high degrees of ecological specialization and/or saltational evolution; extensive molecular data will be needed to ascertain their correct phylogenetic position. Current evidence suggests a minimum of 11 origins of heterospory, in the Zosterophyllopsida (1: Upper Devonian), Lycopsida (1: Upper Devonian), Sphenopsida (?2: Lower Carboniferous), Pteropsida (?4: Upper Cretaceous/Palaeogene) and Progymnospermopsida (?3: Upper Devonian/Carboniferous). The arguably monophyletic Gymnospermopsida probably inherited heterospory from their progymnospermopsid ancestor (Table 3, Figs 11–13). No origin of heterospory coincides with the origin of (and thus delimits) any taxonomic class of tracheophytes. The actual number of origins of heterospory is probably appreciably higher, exceeding that of any other key evolutionary innovation in land‐plants and offering an unusually good opportunity to infer evolutionary process from pattern. 6 Heterospory reflects the convergent attainment of similar modes of reproduction in phylogenetically disparate lineages. Nature repeated this experiment many times, whereas true phylogenetic synapomorphies evolve only once and require a unique causal explanation. Cladograms also offer the best means of determining the sequence of acquisition of heterosporic phenomena within lineages, here exemplified using the lycopsids (Fig. 10). Comparison of such sequences among lineages can potentially allow generalizations about underlying evolutionary mechanisms. Current evidence (albeit inadequate) indicates broadly similar sequences of character acquisitions in all lineages, though they differ in detail. Some logical evolutionarily stages were temporarily bypassed. Other lineages appear to have acquired two or more characters during a single saltational evolutionary event. Heterosporic phenomena can also be lost during evolution. Although no complete reversals to homospory have been documented, this could reflect unbreakable developmental canalization of heterospory rather than selective advantage relative to homospory. Several extant species refute widely held assumptions that certain phenomena, notably heterospory and dioicy, are reliably positively correlated. Moreover, some fossils are likely to possess combinations of heterosporic characters that are not found in their extant descendants. Fossil data have played a crucial role in understanding both the number of origins of heterospory and the ensuing patterns of character acquisition. 7 Although non‐adaptive evolutionary events are likely in at least some lineages, the highly iterative nature of heterospory and similar patterns of character acquisition in different lineages together suggest that its evolution was largely adaptively driven. However, many previous adaptive models of heterosporic evolution confused pattern and process, and paid insufficient attention to the role of the environment as a passive filter of novel morphotypes. Linear gradualistic models were imposed on the data, often intercalating hypothetical intermediates where desired. 8 The evolution of heterospory is best understood in terms of inherent antagonism between the sporophytic and gametophytic phases of the life history for control of sex ratio and reproductive timing. Control is achieved directly by the gametophyte, via gametogenesis, and indirectly by the sporophyte, via sporogenesis and the ability to determine to varying degrees the environment in which the gametophyte undergoes sexual reproduction. Increasing levels of heterospory (particularly the acquisition of endospory) compress the heteromorphic life history, as the increasingly dominant sporophyte progressively co‐opts the sex determination role of the gametophyte. The resulting life history is more holistic, effectively streamlining evolution by offering only a single target for selection. 9 However, by wresting control of sex ratios from the gametophyte, the ability of the sporophyte to respond rapidly to environmental changes decreases. This competitive weakness is greatest for heterosporous species possessing exosporic but obligately unisexual gametophytes (epitomized by the pteropsid Platyzoma*). It can be alleviated in endosporic species by occupying favourable environments (e.g. the aquatic Salviniales and Marsileales), switching to an apomictic mode of reproduction (thereby incurring inbreeding depression; e.g. many selaginellaleans), or acquiring more complex pollination biologies (thereby by‐passing the environment as a selective filter: the seed‐plants). 10 Lineages differ greatly in the maximum number of heterosporic characters that were acquired by their most derived constituent species. Several Devono‐Carboniferous lineages reached the level of reducing numbers of functional megaspores to one per sporangium (Figs 7 e, f, 8, 13), but only the putatively monophyletic gymnospermopsids broke through this apparent barrier to acquire the increasingly complex pollination biology that characterizes modern seed‐plants. 11 Many theories have been proposed to explain the remarkable success (both in terms of species diversity and ecological dominance) of seed‐plants. The majority focus on characters that are absent from the earliest seed‐plants (the Devono‐Carboniferous lyginopterid pteridospermaleans), which were no more reproductively sophisticated than other penecontemporaneous lineages possessing advanced heterospory (particularly the most derived lycopsids, equisetaleans and progymnospermopsids). Reliable pollination was a key reproductive breakthrough, though the sophisticated economic‐vegetative characters inherited by the earliest seed‐plants from their putative progymnospermopsid ancestors were probably equally important in ensuring their success in water‐limited habitats. 12 With the exception of some ecologically specialized pteropsids, known heterosporous lineages originated during a relatively short period in the Upper Devonian and Carboniferous (Fig. 11). They exploited a window of opportunity that existed before niches became too finely partitioned and saturated with seed‐plant species. This non‐uniformitarian ecology renders negligible the probability of new heterosporous lineages becoming established today, even though ‘hopeful monsters’ possessing ‘incipient heterospory’ are probably constantly being generated from homosporous parents.

Arborescent lycopod reproduction and paleoecology in a coal-swamp environment of late Middle Pennsylvanian age (herrin coal, Illinois, U.S.A.)

Abstract The arborescent lycopods dominated many coal-swamp plant communities of the Middle Pennsylvanian. A relatively small number of important species occurred in coal swamps, each with distinctive ecological requirements reflected in their reproductive biology. The major genera in the late Middle Pennsylvanian age Herrin Coal of the Illinois Basin were Lepidophloios, Lepidodendron sensu L. scleroticum and L. dicentricum, and Paralycopodites, which are dealt with in this study. Sigillaria, and Lepidodendron sensu L. hickii (true Lepidodendron), were minor parts of the vegetation. The species of these genera conform to three ecological strategies. Opportunists include Lepidophloios hallii, Lepidodendron dicentricum, and Lepidodendron hickii. These species had determinate, dendritic crowns and each tree apparently reproduced during a short, unrepeated interval late in determinate growth. They grew in areas that were disturbed or with high abiotic stress. Paralycopodites brevifolius was a colonizing species, rapidly occupying sites where peat formation was irregular, and perhaps locally disturbed by clastic influx. Paralycopodites had straight trunks with rows of deciduous lateral branches; cones were borne at the tips of the branches conferring individuals with continuous, high levels of reproduction. Displacement from sites occurred as edaphic conditions changed. Lepidodendron scleroticum and Sigillaria were site occupiers. Lepidodendron scleroticum trees individually produced the most massive wood and periderm of coalswamp lycopod species, and also had deciduous lateral branches and low but continuous reproductive output. Locally, L. scleroticum was very abundant, and such areas occurred irregularly. None of the lycopod trees had vegetative reproduction. Some Sigillaria may have been apomictic, although the major circumstantial evidence supporting this is from Upper Pennsylvanian specimens of Mazocarpon oedipternum.

THE PENNSYLVANIAN-PERMIAN VEGETATIONAL TRANSITION: A TERRESTRIAL ANALOGUE TO THE ONSHORE-OFFSHORE HYPOTHESIS

An analysis of 68 floras from the Pennsylvanian and Early Permian of Euramerica reveals distinct patterns of environmental distribution. Wetland assemblages are the most commonly encountered floras from the Early and Middle Pennsylvanian. Floras from drier habitats characterize the Permian. Both wetland and dry‐site floras occur in the Late Pennsylvanian, but floristic overlap is minimal, which implies strong environmental controls on the distributions of the component species. Drier habitats appear to be the sites of first appearance of orders that become prominent during the Late Permian and Mesozoic. Higher taxa originated in physically heterogeneous, drier habitats, which were geographically marginal throughout most of the Pennsylvanian. They then moved into the lowlands during periods of climatic drying in the Permian, replacing older wetland vegetation. This pattern is analogous to the marine onshore‐offshore pattern of origination and migration. The derivation of Mesozoic wetland clades from the Permian dry‐lowland vegetation completes the parallel. The similarities of the marine and terrestrial patterns suggest that the combination of evolutionary opportunity, created by physical heterogeneity of the environment, and migrational opportunity, created by changing extrinsic conditions, may be underlying factors that transcend the specifics of organism and environment.

Experimental cladistic analysis of anatomically preserved arborescent lycopsids from the carboniferous of Euramerica : an essay on paleobotanical phylogenetics

This evolutionary cladistic analysis of the arborescent (wood-producing) lycopsids, an exclusively fossil group of vascular plants, is confined to the strongest available data: anatomically preserved fossils that have been painstakingly reconstructed into conceptual whole plants. Ten Carboniferous genera are represented by 16 species: four pseudoherbs/«shrubs» and 12 of the arboreous (tree-sized) species that epitomize the Pennsylvanian coal swamps of Euramerica. The 69 vegetative and 46 reproductive characters are described in detail; several key terms are redefined and homologies reassessed (...)

Climate and vegetational regime shifts in the late Paleozoic ice age earth

The late Paleozoic earth experienced alternation between glacial and non-glacial climates at multiple temporal scales, accompanied by atmospheric CO2 fluctuations and global warming intervals, often attended by significant vegetational changes in equatorial latitudes of Pangaea. We assess the nature of climate-vegetation interaction during two time intervals: middle-late Pennsylvanian transition and Pennsylvanian-Permian transition, each marked by tropical warming and drying. In case study 1, there is a catastrophic intra-biomic reorganization of dominance and diversity in wetland, evergreen vegetation growing under humid climates. This represents a threshold-type change, possibly a regime shift to an alternative stable state. Case study 2 is an inter-biome dominance change in western and central Pangaea from humid wetland and seasonally dry to semi-arid vegetation. Shifts between these vegetation types had been occurring in Euramerican portions of the equatorial region throughout the late middle and late Pennsylvanian, the drier vegetation reaching persistent dominance by Early Permian. The oscillatory transition between humid and seasonally dry vegetation appears to demonstrate a threshold-like behavior but probably not repeated transitions between alternative stable states. Rather, changes in dominance in lowland equatorial regions were driven by long-term, repetitive climatic oscillations, occurring with increasing intensity, within overall shift to seasonal dryness through time. In neither case study are there clear biotic or abiotic warning signs of looming changes in vegetational composition or geographic distribution, nor is it clear that there are specific, absolute values or rates of environmental change in temperature, rainfall distribution and amount, or atmospheric composition, approach to which might indicate proximity to a terrestrial biotic-change threshold.

Incised channel fills containing conifers indicate that seasonally dry vegetation dominated Pennsylvanian tropical lowlands

The idea that the Pennsylvanian tropical lowlands were temporally dominated by rainforest (i.e., the Coal Forest) is deeply ingrained in the literature. Here we challenge two centuries of research by suggesting that this concept is based on a taphonomic artifact, and that seasonally dry vegetation dominated instead. This controversial finding arises from the discovery of a new middle Pennsylvanian (Moscovian) fossil plant assemblage in southeast Illinois, United States. The assemblage, which contains xerophytic walchian conifers, occurs in channels incised into a calcic Vertisol below the Baker Coal. These plants grew on seasonally dry tropical lowlands inferred to have developed during a glacial phase. This xerophytic flora differs markedly from that of the typical clubmoss-dominated Coal Forest developed during deglaciation events. Although preserved only very rarely, we argue that such xerophytic floras were temporally as dominant, and perhaps more dominant, than the iconic Coal Forests, which are overrepresented in the fossil record due to taphonomic megabias. These findings require the iconography of Pennsylvanian tropical lowlands to be redrawn.

AN EARLY PERMIAN FLORA WITH LATE PERMIAN AND MESOZOIC AFFINITIES FROM NORTH-CENTRAL TEXAS

Abstract Early Permian (late Leonardian Series) plant assemblages from King, Knox, and Stonewall Counties of North-Central Texas are dominated by seed plants, some apparently congeneric with taxa heretofore known only from the Late Permian or the Mesozoic. Conifers are the dominant elements, including one or more species of Ullmannia, Pseudovoltzia liebeana, both known from the Late Permian Zechstein flora of Germany and England, Podozamites sp., characteristic of the Mesozoic, and Walchia sp., abundant in Early Permian floras. Locally common are Taeniopteris cf. eckardtii, a Zechstein species, an unidentified plant represented by pinnule-like laminae with fine parallel veins, similar to pinnules of some Mesozoic cycads, and calamite stems. Rarely encountered are leaf fragments of the Paleozoic ginkgophyte Dicranophyllum, flabellate ginkgophyte leaves, leaves with a broad midvein and narrow, fimbriate lamina, and Wattia, typical of the Early Permian. Associated with these foliar remains are ovulate reproductive structures including the presumed cycad megasporophyll Dioonitocarpidium, known only from the Mesozoic, a voltzialean cone scale similar to Swedenborgia, and a variety of seeds, some remarkably similar to Agathis, of Cretaceous age. The assemblage includes only rare scraps of foliage and seeds possibly attributable to the pteridophyllous elements (gigantopterids, callipterids, and ferns) that dominate the Permian. The fossil plants occur in multistorey, fining-upwards, tidal-channel deposits that also include pelecypods and fragmentary palaeoniscoid fish. The occurrence of derived lineages in xeric habitats during the Early Permian indicates that some supposed Mesozoic groups actually preceded and survived the end-Permian extinction, reappearing in basinal lowlands during the mid-Mesozoic.