Saturday, 7 December 2019

gymnosperm phylogeny

gymnosperm phylogeny diagram

It is a gymnosperm phylogenetic tree plant, while earlier anatomy phylogeny groups represent a variety of morphologies and structures. While the above gymnosperm and angiosperm divisions represent the distinctive adaptive types, their phylogenetic relations are to be inferred on the basis of the homology of their respective morphological structures. Discrimination of homology and homoplasy is a much-debated problem of apparently no once and for all solution.

The cladistic techniques provide a cumulative assessment of homoplasy at most. Ho­mology is more often inferred from phylogeny than vice versa, while phylogeny is based on oblique reasoning that is rooted in the contemporary evolutionary philosophy. Insofar as phylogeny primarily conveys our understanding of homology, it cannot be more objective than the latter. The objective elements in it are chronological relationships as well as morphological links, that is, the intermediate forms that show character states midway in the polarity gradient or the mosaic forms that combine the typical characters of different taxonomic units. A phylogenetic reconstruction would then start with assigning the operational units - gymnosperm orders in our case - to successive geological age groups reflecting their first appearances in the fossil record. The relations within and between the age groups would then be inferred on the basis of intermediate and/or mosaic forms.

Members of a single age group are likely to have independent origins or, if con­nected by intermediate forms, a common origin, that is, their relations are fraternal rather than the progenitor. On the other hand, the members of successive age groups, if connected by intermediate or mosaic forms, are likely to have progenitor relations. Presently, the chronological relations between the major gymnosperm units are mostly based on a sufficiently high stratigraphic resolution and are fairly reliable. Discovery of linking forms may seem accidental but, since there were intermediate habitats and dis­ crept evolution rates for different functional units, the intermediate and mosaic forms ought to be found.

According to their first appearances in the fossil record, the gymnosperm orders can be assigned to the following age groups:

(1) Late Devonian - Early Carboniferous: Hydrospermales, Lagenostomales, Trigonocarpales (Lyginopteridales, Medullosales of the axial anatomy classification);

(2) Mid-Carboniferous: Callistophytales, Cordaitales, Walchiales;

(3) Late Carboniferous - Permian: Glossopteridales, Vojnovskyales, Gigantopteridales, archaic  Eltaspermales (call Pteris), archaic Coniferales (Voltziaceans);

(4) Early Mesozoic: advanced Peltaspermales, Corystospermales, Nilssoniales, Ginkgoales, Czekanowskiales, Caytoniales, Bennettitales, archaic Gnetales (Protognetaleans, such as  Dinophyton)',

(5) Late Mesozoic: Pentoxylales, Cycadales, modem Coniferales, modem Gnetales, their derived angiospermous forms, such as Eoantha,  etc.

The gymnosperms are thought to be rooted in Progymnosperms of which both the heterosporous archaeopteryx and the homosporous Aneurophytes are considered as potential ancestors. Moreover, there were herbaceous plants of simpler axial anatomy but having elaborate cupule-like sporangial clusters, as in Lenlogia. Early seed plant diversity is insufficiently known, however, for establishing the progenitor relations otherwise but in a very general form. The possibility of the gymnosperm anatomy and seeds originating in different lineages then combined by horizontal gene transfers in the Devonian plant communities cannot be excluded.

In the age group (1),

the Hydrospermales with 2-many-ovulate cupules, sometimes containing sporangia, appeared somewhat earlier than the other two or­ders. That this type of cupules might have been primary is scarcely deducible from their morphology: it is just a geochronological fact. The Hydrospermales merge with the Lagenostomales - their respective axial organs are assigned to a single order, Lyginopteridales, while the ovules are of the same basic type and are linked to the Trigonocarpales by the forms with an inter­ mediate Callamopytialean-medullosalean anatomy, such as the Late Mississippian Questor.

Both the Lagenostomales and Trigonocarpales show a tendency of a not quite syn­ Chronus transition of the pollen receptive function from the nucellar apex with its excessively developed pollen trapping and secretory structures, Lagenostome and salpinx, to the integumental micropyles, with the lobed micropyles as a transitional stage. They differ, however, in the relative development of the inner, integumental, and the outer, cupular, coats of their ovules, a variation that can be traced up to the present-day angiosperms. While in the Lagenostomales the micropyle was primarily integumental, in the Trigonocarpales it might have been of a cupular origin, with the primary integu­ introduced and adnate to the nucellus, the latter thus transformed into a chimeric structure assuming the vascular system of integumental origin, hence non-homologous to the nucelli of the Lagenostomalean line.

The fraternal phylogenetic relations of Lagenostomales and Trigonocarpales imply that the ovules borne on fern-like leaves, a critical feature of pteridosperms, appeared independently in both groups in the course of their parallel evolution. Such parallel developments of the fern-like foliage on the basis of systemic branch systems in spore plants and early gymnosperms might have been urged by demand of medium photosynthetic surfaces in response to either a decreased ultraviolet radiation rate or the build-up of the structural complexity, canopy formation and the ensuing competition for light in the early plant communities, or both. In distinction from the exposed Hydrospermalean cupules, the leaf-borne cupules were less effective in wind pollination. The concomitant development of the medium Synangiate polliniferous structures and the secretory apical structures of the early ovules suggest pollination by arthropods finding shelter in the foliage and not yet engaged in the leaf-cutting activi­ties.

In the age group (2)

the Callistophytales embodied a mosaic of forms combining an essentially Lyginopteroid morphology of their vegetative parts and synangia with the Platyspermic Cardiocarpoid ovules and true pollen also resembling some Cordaitalean morphotypes.  Among chordates, Mesoxylon is likewise a linking form with the eustele formed of leaf traces, as in megaphyllous plants, and with compound pollen cones producing pure pollen. The seeds were, however, Cardiocarpoil, like in the advanced forms that acquired a sympodial eustele and true pollen grains. The Walchiales are obviously related to chordates, their coniferous foliage corresponding to the lower leaves of the latter. 

Neither morphological nor chronological evidence suggests derivation of the age group (2) gymnosperm orders from each other. The evidence is rather in favor of their common Lagenostomalean ancestry. Remarkably, these three orders have in parallel de­veloped the Anasulcate pollen on the basis of spore-like pure pollen, as well as the Platyspermic seeds derived from Radiospermic precursors, while the compound Strobili have appeared in two of them. True pollen has independently arisen also in the Trigonocarpales, a geologically older age group (1) order that underwent a consider­ able modernization in the late Paleozoic. A synapomorphic group based on this obvi­ously derived character would include the advanced members both of Trigonocarps and Cordaites, as well as Callistophytes, otherwise the most plesiomorphic order of the age group (2).

Another common tendency in the age group (2) was the loss of cupules either by reduction or by fusion with the integument, or the cupules might have lost their iden­tity in replacing the primary integuments that became incorporated in the nucelli, thus giving rise to Crassinucellate ovules. In effect, the age group (2) seed plants were ren­ dered true gymnosperms replacing the prevailingly Chlamydospermous age group (1) pteridosperms.

In the age group (3), 

the Permian calipers can be considered as transitional be­ tween the Callistophytes of the age group (2) and the typical Triassic Peltasperms. Their leaves and pollen organs are quite similar to those of Callistophytes, the pollen grains are of the same morphotype,  while the seed organs of the Autunia type, with their scaly Ovuliphores bearing two sessile, Platyspermic, inverted ovules, are ob­viously a variation of the Callospermarion theme.

The same can be said of a typical coniferous seed-scale. In other characters, conifers are similar to both Callistophytes and archaic Peltasperms, clustering with the latter as their sister-group. The in the archaic forms com­ pound pollen cones, tree pollen, and laminar seed-scales are against their close affinities with Wallachians, while the compound strobilation of Ovuliphores, shared with the latter group, as well as with Cordaites, appears a parallel development.

At the opposite morphological pole, the Peltasperms are linked to the gigantopterids through such intermediate forms as Tinsley, Russelites, Protoblechnum,  etc. Both Peltasperms and gigantopterids show a tendency to a consecutive fusion of the pinnules and pinnae resulting in a simple angiosperm-like leaf with a hierarchical areolate venation.

The Glossopterid stem anatomy and ovulate structures are not yet unambiguously interpreted rendering their phylogenetic position uncertain. Or the ambiguity might be caused by polyphyletic origins. The protostelic stems and the dichotomously branched pollen organs with terminal sporangial clusters are archaic features found in the most primitive Lyginopterids and suggestive of a fraternal relation to Callistophytes. The derived characters, notably the saccate pollen grains and Platyspermic ovules, might have evolved in parallel with the Callistophytes, Cordaites, and Peltasperms. The leaves appear phyllodes, as in coordinates, and some leaf forms  {Euryphyllum, Noeggerathiopsis)  are actually quite similar to Cordaites,  the similarity extending also to Vojnovskyaleans, the Angaridan counterpart of Glossopterids.

Arberia,  the most primitive ovuliferous organ of Glossopteridales, is here interpreted as a somewhat flattened, profusely branched shoot. Some of the derived ovuliferous struc­tures might have arisen as modifications of Arberia. Their fusion to sub­tending bracts finds its parallel in conifers, the persistent bracts apparently assisting in dispersal in both cases. However, in the advanced Glossopteridales, a prevailing tendency seems to have been a shortening of the ovuliferous shoot resulting in a radial arrange­ment of the ovules in the capitulate or campanulate, sometimes discoid, clusters. This tendency was paralleled by the Vojnovskyaleans. Remarkably, both groups had Protosaccate taeniae pollen grains shared also with Peltasperms and some early coni­fers. A variety of Permian insects fed on such pollen. Polliniferous insects might have both promoted the parallelism by mediating horizontal gene transfers and initiate entomophile, the latter explaining also the capitulate ovuliferous structures of amassed small ovules confronting a pollination vector with a brush of adpressed micropyles.

Characteristic of age group (3) are specialized laminar Ovuliphores - seed-scales -developed in parallel in most of its members, although not necessarily homologous. Inpeltasperms and perhaps also in conifers, they have modified foliar Ovuliphores of the age group (1) pteridosperms, while in gigantopterids they might have been secondarily flattened fertile shoots, partly lacking lamina in aberrant forms, such as  Physmatocycas.

In the age group (4), 

The Nilssonialeans are linked to the Peltasperms through their ovuliferous organs (Beania)  that are basically identical to Futuna.  Previously assigned to cycads, they differ in the Leptocaul life-form with dimorphic shoots, simple leaves and the ovules abaxial on the heads of the longly stalked Ovuliphores. The pollen cones are similar to those of the extant cycads but differ from those of Mesozoic cycads, such as Cycandra that produced the much more elaborate Multi-sporangiate synangia. The simpler eusporangiate half-synangia is thus derived in the modern cycads. The Czekanowskiales are similar to Nilssonias in the life-form characters including the deciduous spur-shoots, while their bivalved cupules seem derivable from a pair of mar­ginally fused peltasperm-type peltae.

The Corystosperms are linked to Pentoxyleans and cycads on the basis of their essen­tially medullosalean stem anatomy,  and to Ginkgoaleans on the basis of their similarly paired ovules embedded in the short collar-like cupules conceivably re­ducible to the "collars" of the extant Ginkgo.  In addition, the flabellate dissected leaf morphotypes typical of the Mesozoic Ginkgoleans occur Dicroidium,  the Corystosperm foliage. There is also a marked chromatographic similarity between Corystosperms and Ginkgoaleans extending to Centoxyleans. The Cycadalean stem anatomy with characteristic girdling leaf traces first appeared in Antarcticycas of a Corystosperm-dominated Triassic flora.

The extant Gingko is markedly different from the extant cycads in the life-form and is usually allied with coniferous gymnosperms rather than with cycadophytes. How­ ever, Ginkgoales share with the latter the general structure of both pollen and seed Strobili the pollen and ovule morphologies, as well as many features of reproductive biology, including the pollen tube, sperm, archegonial and embryological similarities. In the Mesozoic Ginkgoales ovules,  the nucelli were free, beaked and strongly cutinized, thus more cycad-like than in the extant species. Interpretation of the characters shared by cycads and ginkgo as symplesiomorphies attesting to their primitiveness is hardly warranted, for both the modem Ginkgoales and the modern cycads appeared rather late in the geological history of gymnosperms. In them, the medium ovules and their correlated reproductive features are recent developments lacking in their early Mesozoic precursors. Contrary to a widely held opinion, derived states in plant morphology are not necessarily those due to reduction. Sometimes they are those due to magnification.

In the rest of the age group (4), 
The relatedness of benefits and Gnetales is attested by their basically similar ovuliferous structures with orthotropous ovules enclosed in the cupules formed of bracteoles or their homologous interspinal scales. The bennet-Italian receptacles with an apical corona of sterile scales correspond to the shortened binodal Strobili of extant Gnetum Scandens, with the distal node reduced. The Bennett Italian ovules, Radiospermic with residual nucellar vascular bundles, are essentially Trigonocarpalean, at the same time resembling Gnetum in the peculiar cutinized flange at the base of the micropyle tube. The Triassic Dinophyton,  a peculiar Protognetalean form, showed a 3-ribbed nucellus as in Pachytesta,  a Trigonocarpales ovule. Notably, some medullosaleans had a decussate leaf arrangement character­istic of cetaceans. At the same time, the Hirmerellaceae, another Protognetalen Mesozoic group, seems to have been being related io Majonica,  a Permian coniferous.

The Caytoniales produced compound leaves with GlossopterisA'ikQ leaflets occa­sionally showing a decussate arrangement betraying their caulomic origin. Their pollen organs are essentially glossed period while the cupules are fused to their residual sub­tending bracts (lips) and seem derivable from the bracteate Glossopteridales cupules, perhaps by arrested development at a juvenile stage when some of the cupules were closed, like those described by Gould & Delevoryas (1977).

Characteristic of the age group (4) gymnosperms, irrespective of their phylogenetic position, are the deciduous short shoots, the prevailingly simple, entire or pinnatifid, often phyllodes leaves, the saccate pollen grains, sometimes with residual Sacchi or folds betraying their origin from saccate forms, and the strobilate ovuliferous organs, while the leafy Oulipo's, so widespread in the Paleozoic, is totally lacking. The latter change might have been due to the appearance of leaf-cutting insects or foliophagous tetrapods, or both.

In the mid-Cretaceous, the age group (4) gymnosperms either vanished or under­ went a radical modernization. The nearly synchronous appearance of fleshy seeds in cycads, ginkgo, and some conifers, of fleshy cupules in cytopenias, of fleshy receptacles in benefits and of fruit-like cones in Pentax lens might have been related to zoochory that was then fostered by a rapid spread of birds and multituberculates. This factor, perhaps in conjunction with the spread of ovule-sucking insects, might have been impelled better protection of ovules either by the armor of closed peltate cone scales, as in cycads and the concomitantly appearing modern spinaceous, taxodiaceous and cupressaceous conifers, or by the newly formed cupules that reappeared in several phylogenetic lines leading to the angiospermous Leptostrobus,  Claytonia,  Dirhopalostachys,  Basia,  Eoantha,  etc. At this stage, angiosperms have been added to the seed plant diversity.

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