Vegetative Characters, Growth Habit and Microsporangiate Strobilus of Lycopsid Minostrobus chaohuensis
Late Devonian Minostrobus chaohuensis is one of the earliest monosporangiate-strobilate isoetaleans. Based on new material of this plant, the vegetative axis and microsporangiate strobilus are studied in detail, and the whole plant knowledge is summarized. The vegetative axis is isotomously branched. The stem is up to 55 mm in diameter with helically arranged leaf cushions. Stems and thick branches bear long fusiform leaf cushions and interareas with vertical linear ornamentations. A ligule pit, oblanceolate leaf scar, and vascular bundle scar appear on the leaf cushion. Distal axes have persistent lanceolate leaves and rhombic leaf bases. The microsporangiate strobilus is cylindrical in shape, possesses sporophyll with alate pedicel and long triangular lamina, uniseriate sporangial wall, subarchesporial pad inside the sporangium, and microspore with cingulum. Based on comparisons with other isoetaleans, the usage of the terms “leaf cushion” and “leaf base” is discussed, and Minostrobus chaohuensis is considered as a tree-like lycopsid. It suggests that arborescent isoetaleans with monosporangiate strobili had appeared and diversified in the Late Devonian. The multi-dichotomous branching system of Minostrobus provides new data on the evolution of growth architecture in rhizomorphic lycopsids.
Kossoviella timanica Petrosjan emend. from the Upper Devonian of North Timan: morphology and spore ultrastructure
ABSTRACT The morphology of sterile and fertile structures (terminal strobili) of the Upper Devonian heterosporous lycopsid Kossoviella timanica Petrosjan 1984 from northern Russia (North Timan) is re-described: the axes are dichotomously branched; sterile leaves are narrow with smooth margins; the transition from sterile axes to strobili is gradual; the strobili are narrow and cylindrical, occasionally dichotomously branched; sporophylls are long, lanceolate, with crenulated margins; megasporangia with thin, mostly destroyed, sporangium walls contain one or two tetrads of large megaspores without a gula; numerous microspore tetrads are present in the microsporangia; both mega- and microspores are cavate, with a two-layered sporoderm; the outer layer of the sporoderm of both mega- and microspores consists of a net of intertwined cylindrical elements; the inner layer of the megaspore sporoderm is a basal lamina; and the inner homogeneous layer of the microspore sporoderm is split into multilamellate zones near the arms of the proximal scar. A comparison between abortive and fertile megaspores, some of which apparently were not completely mature, allows us to hypothesise that the enlargement and lateral stretching of structural units of the sporoderm, and the spaces between them, took place during the final stages of ontogenesis of megaspores along with the additional accumulation of amorphous sporopollenin. Both layers of the megaspore sporoderm, as well as the cavity between them, developed early in the ontogenesis. Although Kossoviella timanica was certainly a unique Late Devonian plant, it bears some resemblance to the Givetian heterosporous, bisporangiate lycopsid Yuguangia ordinata in having dichotomously branching axes, sporophylls with spiny margins and strobili with proximal megasporangia and distal microsporangia. Kossoviella timanica is also similar to the Famennian bisporangiate lycopsid Bisporangiostrobus harrisii in lacking a ligula and in having dichotomously branching strobili with proximal megasporangia and distal microsporangia.
Palaeobotanical systematics for the phylogenetic age: applying organspecies, form­species and phylogenetic species concepts in a framework of reconstructed fossil and extant whole­plants
A typical vascular land-plant consists often to twelve definable organs. Most plant fossils have been disarticulated into their component organs, which must therefore be correlated if the fossil plant is to be understood holistically and compared with its presumed descendants in the extant flora. The resulting conceptually reconstructed whole-plants are the crown jewels of palaeobotany, as they permit full morphological comparison with extant plants and provide templates that guide through reciprocal illumination further attempts at reconstruction. Each of the three lines of evidence facilitating whole-plant reconstruction (association/dissociation, morphological similarity and organic connection) yields only a probability statement that the organs in question have been successfully correlated. Disarticulation means (1) that phenotypic variation can be studied only at the level of individual organs, and (2) that in order to be distinguished from all other kinds of the same organ, an organ must bear a unique morphological character state (autapomorphy). In our terminology, each definable kind of organ is an organ-species. The few organ-species perceived as possessing autapomorphies (and thus as unique) are termed autapospecies, whereas the remaining organ-species characterise more than one whole-plant species and hence are termed form-species. The distinction between autapospecies and form-species is dependent on the range of taxa sampled and is wholly character-based; the age of the fossils under comparison is irrelevant, and the state of preservation is relevant only through its influence on the range of characters that can realistically be scored. None of the many other species concepts recognised by (palaeo)biologists is applicable to fossil plants. Our ability to apply this phylogenetically-inspired approach and terminology to formal taxonomy has been increasingly compromised by modifications to successive editions of the International Code ofBotanical Nomenclature, culminating in the 2001 and 2006 Codes which reduced palaeobotanical provision to the fatally over-generalised 'morphotaxon' concept. A more conceptually rigorous, palaeobotanically informed revision of the Code, placing nomenclature more clearly in the service of taxonomy, would strengthen the crucial roles of reconstructed plants within palaeobotany and of palaeobotany within 21st Century science.