Frost survival of plants

Establishing the timescale of early land plant evolution is essential for testing hypotheses on the coevolution of land plants and Earth's System. The sparseness of early land plant megafossils and stratigraphic controls on their distribution make the fossil record an unreliable guide, leaving only the molecular clock. However, the application of molecular clock methodology is challenged by the current impasse in attempts to resolve the evolutionary relationships among the living bryophytes and tracheophytes. Here, we establish a timescale for early land plant evolution that integrates over topological uncertainty by exploring the impact of competing hypotheses on bryophyte-tracheophyte relationships, among other variables, on divergence time estimation. We codify 37 fossil calibrations for Viridiplantae following best practice. We apply these calibrations in a Bayesian relaxed molecular clock analysis of a phylogenomic dataset encompassing the diversity of Embryophyta and their relatives within Viridiplantae. Topology and dataset sizes have little impact on age estimates, with greater differences among alternative clock models and calibration strategies. For all analyses, a Cambrian origin of Embryophyta is recovered with highest probability. The estimated ages for crown tracheophytes range from Late Ordovician to late Silurian. This timescale implies an early establishment of terrestrial ecosystems by land plants that is in close accord with recent estimates for the origin of terrestrial animal lineages. Biogeochemical models that are constrained by the fossil record of early land plants, or attempt to explain their impact, must consider the implications of a much earlier, middle Cambrian-Early Ordovician, origin.

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The timescale of early land plant evolution

Phylogenetic analysis of seed plants and the origin of angiosperms

We present a numerical cladistic (parsimony) analysis of seed plants plus progymnosperms, using characters from all parts of the plant body, outgroup comparison, and a method of character coding that avoids biases for or against alternative morphological theories. The robustness of the results was tested by construction of alternative trees and analysis of subsets of the data. These experiments show that although some clades are strongly supported, they can often be related to each other in very different but nearly equally parsimonious ways, apparently because of extensive homoplasy. Our results support Rothwell’s idea that coniferopsids are derived fromCallistophyton- like platyspermic seed ferns with saccate pollen, but the hypothesis that they evolved fromArchaeopteris- like progymnosperms and the seed arose twice is nearly as parsimonious. Meyen’s division of seed plants into radiospermic and primarily and secondarily platyspermic lines is highly unparsimonious, but his suggestion that ginkgos are related to peltasperms deserves attention. Angiosperms belong among the platyspermic groups, as the sister group of Bennettitales,Pentoxylon, and Gnetales, and this “anthophyte” clade is best related toCaytonia and glossopterids, although relationships with other combinations of Mesozoic seed fern taxa are nearly as parsimonious. These results imply that the angiosperm carpel can be interpreted as a modified pinnate sporophyll bearing anatropous cupules (=bitegmic ovules), while gnetalian strobili are best interpreted as aggregations of highly reduced bennettitalian flowers, as anticipated by Arber and Parkin and Crane. Our most parsimonious trees imply that the angiosperm line (though not necessarily all its modern features) extended back to the Triassic, but a later derivation of angiosperms from some species ofCaytonia or Bennettitales, which would be nearly as parsimonious, should also be considered. These results raise the possibility that many features considered key adaptations in the origin and rise of angiosperms (insectpollinated flowers, rapid reproduction, drought tolerance) were actually inherited from their gymnospermous precursors. The explosive diversification of angiosperms may instead have been a consequence of carpel closure, resulting in increased speciation rates due to potential for stigmatic isolating mechanisms and/or new means of dispersal. DNA sequencing of extant plants and better information on anatomy, chemistry, sporophyll morphology, and embryology of Bennettitales and Caytoniales and the morphological diversity of Mesozoic anthophytes could provide critical tests of relationships.

Seed plant phylogeny and the origin of angiosperms: An experimental cladistic approach

An Analysis of the Flora of Southern Africa: Its Characteristics, Relationships, and Orgins

The distribution of conifer and taxad genera in time and space

Evolution in cordaites and conifers

Untersuchungen zur Stammesgeschichte der Coniferales und Cordaitales

SUMMARY

1
The morphology of the female conifer cones has long been a matter of dispute. In 1900 an account of the theories put forward so far was given by Worsdell. After a brief characterization of the situation at that time, the present article deals with the later history of the subject.

2
At the turn of the century there were rivalling concepts of the nature of the so-called ovuliferous scale in the conifers: (a) the Excrescence or Ligular theory of Sachs-Eichler, (b) the general Brachyblast theory of Braun, Caspary, celakovský and others, (c) van Tieghem's modification of the Brachyblast theory, and (d) the Foliolar theory of Delpino-Penzig.

3
In the subsequent three decades the discussion proceeded on the same or similar lines. The Excrescence theory retained a strong position until the end of the nineteen-twenties, but, as before, some morphologists professed the general Brachyblast theory. Herzfeld and Wettstein considered that the axillary conifer strobilus had one or more reduced carpels (megasporophylls) which were used up in the formation of terminal ovules, as well as an ‘ovuliferous scale’ consisting of secondary outgrowths from the strobilar axis. In Goebeľs opinion these outgrowths were instead produced by the megasporophylls. Doubts were expressed of the unity of the true conifer group, but Eames showed that the apparently widely different female cones of the Pinaceae and Araucariaceae are homologous, and that the Araucariaceae, Taxodiaceae and Podocarpaceae exhibit complete transitions by fusion and reduction from types with distinctly compound strobilar units–each with an ‘ovuliferous scale’ in the axil of a bract–to others of the most simple form. In contradistinction to the true conifers, the genus Taxus has no compound strobilus, and its ovule is a direct continuation of the axis of the fertile short shoot (Dupler).

4
In the first half of the period after 1930 opinions differed as much as ever, although the general Brachyblast theory now prevailed over the Excrescence theory and other concepts. Chadefaud believed the ‘ovuliferous scale’ and the bract to represent between them a carpel derived from a prototype analogous to the pinnate megasporophyll of Cycas. Hirmer interpreted the ‘ovuliferous scale’ and the bract as formed by a serial splitting of one single member. Lanfer supported Goebeľs views of the terminal position of the conifer ovules on reduced megasporophylls, and of the nature of the ‘ovuliferous scale’. In Hagerup's opinion the female cones of most true conifers are compound and have a short secondary axis developed axillary to each bract. This axis was supposed to carry two transversal prophylls, fertile (megasporangial) or sterile, and a varying number of sterile leaves; and the megasporophyll, with a megasporangium on its upper side, to constitute the integument of the ovule, and be homologous to a lycopod sporophyll.

5
Florin (1938-45) found that the Palaeozoic cordaites and conifers furnished the principal clue to the interpretation of the true conifer cones of Mesozoic and more recent age. Primarily, the fertile seed-scale complex in the axil of each bract was a radially symmetrical short shoot (strobilus) with several sterile scales and one to a few uniovulate megasporophylls; the ovules were terminal in position. The later types of cones have arisen by the reduction and transformation of this primitive organization, which in the majority of cases has differentiated the strobilus into a proximal fertile part facing the cone axis and a distal sterile part (‘ovuliferous scale’), while its anterior sector facing the bract became totally suppressed. Exceptionally, no ‘ovuliferous scale’ at all was developed, and the strobilus became wholly fertile. The ovular integument is a continuation of the megasporophyll, and appears to arise out of two transversal primordia at its apex. The taxads differ from the true conifers by their simple strobili being placed axillary on reduced vegetative shoots. Their ovules are seated terminally on the strobilar axis itself; megasporophylls are accordingly absent. The living and extinct genera which have previously as a rule been considered coniferous represent therefore two separate subdivisions of the gymnosperms–the true conifers, and the taxads.

6
Earlier concepts of the morphology of the female cones of the conifers, and of the nature of the integument of their ovules, are in part or wholly untenable.

Rudolf Florin

Papers

The distribution of conifer and taxad genera in time and space

Evolution in cordaites and conifers

Untersuchungen zur Stammesgeschichte der Coniferales und Cordaitales

The female reproductive organs of conifers and taxads

On Jurassic taxads and conifers from north-western Europe and eastern Greenland