Box Palynology

Palynomorphs, fossil pollen and spores, provide evidence about ancient paleoenvironments, often when other fossils are absent, and they are key tools in biostratigraphy (see pp. 26-32). Fossil pollen and spores have proved to be essential in understanding the biostratigraphy of the Late Paleozoic, Mesozoic and Cenozoic, especially in terrestrial rock sequences. Pollen analysis is also a routine part of studies of Quaternary paleoenvironments, especially those studied by archeologists. They can sometimes be used in correlating marine and non-marine rocks, because pollen and spores are easily blown from land out over lakes, rivers and shallow seas.

Palynomorphs differ from most other microfossils because of their chemical composition. They are not usually mineralized, but their polymerized, organic outer coat (exine) is extremely durable. This coat is resistant to nearly all acids, so hydrofluoric acid is used to dissolve surrounding sand grains and leave the pollen for microscopic examination.

Some spores show bilateral symmetry (Fig. 18.13). The proximal pole is marked by the germinal aperture, which may be a rectilinear slit (monolete condition) or it may have a triad of branches (trilete condition). The laesurae are the contact scars with neighboring spores, commonly converging at a point or commissure. When extracted in a dispersed form from sediments, an arbitrary size distinction is applied that classifies spores above 200 |im as megaspores and smaller ones as microspores.

Pollen grains are usually smaller, ranging in size from 20 to 150 |im. Inaperturate spores lack a germinal aperture (Fig. 18.13). A single aperture at the distal pole characterizes the gymnosperms, the monocotyledons and primitive dicotyledons. Acolpate or asulcate pollen grains lack an obvious germinal aperture. Many pollen grains, however, such as those of pine and spruce, are saccate, with both a body or corpus and vesicles or sacci. The terms colpus and sulcus are often used for similar depressions or furrows; strictly speaking, the sulcus refers to a furrow not crossing the equator of the pollen. Monosulcate pollen with a single distal sulcus that developed during a series of meioses is typical of gymnosperms and monocotyledon angiosperms. The tricolpate pattern, seen in dicotyledon angiosperms, has three germinal apertures or colpi arranged with triradiate symmetry.

Virtually all fossil pollen and spores are identified and classified on the basis of the morphology of the resistant outer wall or exine. As is the case with a number of other palynomorph groups, only a parataxonomy is possible - that is a "form system" that does not reflect evolution. In one scheme, the palynomorphs are grouped together into "turma" categories; thus spores belong to the Anteturma Sporites and pollen in the Anteturma Pollenites. However, the pollen and spores of plants are often quite distinctive, and they can be used on their own to infer the presence of families, genera, and even species.

Exploration geologists frequently describe the shapes of pollens and spores with a code that describes the exine structure, germinal aperture, outline, shape, size and ornament. Spores (S) are classified on laesurae (scar) type: c, trilete; a, monolete; b, dilete; 0, lacking laesurae. Pollen grains (P) are classified on colpation or sulcation type: a, monocolpate; c, tricolpate; 0, lacking colpation.

ektexine endexine-

ektexine endexine-

intine

Monocolpate

Monoporate

Trilete (3-slit)

Syncolpate

Saccate

Inaperturate polar eq.

Bulomus

Gramineae

Sphagnum

Pedicularis

Nymphoides

Eriocaulon

QDQ5>

Pinus

Potamogeton

Dyads

Tetrads

Polyads

Scheuchzeria

Ericaceae

Typha

Ericaceae

Mimosa

Orchidaceae

Figure 18.13 Basic morphology and terminology of spores and pollen, shown in polar and equatorial (eq.) views.

Through time, different palynomorphs came and went (Fig. 18.14). These can generally be matched with the broad outlines of plant evolution. The oldest spores are from the Ordovician (see Box 18.1). Spore diversity increased through the Silurian, when some 15 sporomorphs have been reported, including so-called cryptospores that lack monolete or trilete markings, and are commonly found in monads, dyads and tetrads, often with an outer membranaceous envelope. Smooth-walled forms dominated assemblages until the end of the Early Silurian. Some simple spore types with trilete markings may come from bryophytes, but most were probably from tracheophytes. In rare cases, spores may be found in direct association with plants, such as numerous specimens of Ambitosporites in the sporangia of some Cooksonia (see Fig. 18.7).

intine

Devonian and Carboniferous palynofloras were much more diverse (Fig. 18.15). The plants of the Lower Devonian Rhynie Chert (see Box 18.3) were all homosporous, producing a single kind of spores, and having sculpture and spines. Monolete and trilete spores from lycopsids appeared during the Devonian. Heterospory, the property of having microspores and megaspores, arose independently in plants at least 11 times beginning in the Late Devonian. Lycopsid megaspores with a variety of wall sculpture appeared at this time. The seed ferns of the Carboniferous produced monolete pollen, and the conifers predominantly saccate pollen with a distal aperture. Monocolpate pollen, typical of the cycads and ginkgos, was supplemented, during the Carboniferous and Permian, by both polyplicate and saccate grains. During the Permian, spores are less common than the more dominant saccate pollens that continued through the Triassic. Gymnosperms continued to dominate the floras of the Early Jurassic (Fig. 18.16), including mono-saccates from Cordaitales, disaccates from some Coniferopsida, monosulcates from Bennettitaleans, Cycadales and Ginkgoales, polyplicates from Gnetales, and inaperturates from other Coniferopsida.

Palynomorphs changed dramatically in the Cretaceous with the radiation of the angiosperms. Angiosperm pollen has a double outer wall, and the seeds also have a double protective casing. The first undoubted angiosperm pollen grains are reported from the Lower Cretaceous where morphs such as Clavatipollenites are oval and monosulcate. During the Cretaceous the monosulcate condition was supplemented by the tricolpate, in for example Tricolpites. Monocotyledon pollen is monosulcate and bilaterally symmetric, and dicotyledon pollen has both furrows and pores.

Read more about palynology from links given at http://blackwellpublishing.com/paleobiology/.

Latest Devonian Palynology

Figure 18.15 Some Devonian and Carboniferous spore taxa: (a) Retusotriletes, (b) Retusispora, (c) Spinozonotriletes, (d) Raistrickia, (e) Emphanisporites, (f) Grandispora, (g) Hystricosporites, (h, i) Ancyrospora, and (j) Auritolagenicula. Magnification x400 (a-d, f, i), x750 (e), x90 (g), x125 (h), x40 (j). (Courtesy of Ken Higgs.)

Figure 18.15 Some Devonian and Carboniferous spore taxa: (a) Retusotriletes, (b) Retusispora, (c) Spinozonotriletes, (d) Raistrickia, (e) Emphanisporites, (f) Grandispora, (g) Hystricosporites, (h, i) Ancyrospora, and (j) Auritolagenicula. Magnification x400 (a-d, f, i), x750 (e), x90 (g), x125 (h), x40 (j). (Courtesy of Ken Higgs.)

Cordaitales Classification
Figure 18.16 Some Jurassic spore and pollen taxa: (a, b) Klukisporites, (c) Dettmanites, (d) Dictyophyllidites, (e) Retusotriletes, (f) Callialasporites, (g) Classopolis, (h) Podocarpidites, and (i) Protopinus. Mgnification x400 for all. (Courtesy of Ken Higgs.)

horsetails, lycopsids and more primitive plants such as mosses continued to diversify, especially in damp situations, and they continued their successful evolution without the " benefit" of seeds.

Seed ferns_

The seed ferns, or "pteridosperms", have been regarded traditionally as a major gymnosperm class, but they share no unique characters, and it is clear that they are a paraphyletic or perhaps polyphyletic assemblage of gymno-sperms of varied affinities. Pteridosperms were important components of Late Paleozoic and Mesozoic floras.

The Carboniferous and Permian seed ferns belong to a variety of groups, such as the Medullosales, which looked superficially like tree ferns, but bore ovules and pollen. Another group of Late Paleozoic seed ferns, the Glos-sopteridales, include Glossopteris (Fig. 18.17), a 4 m-tall tree with radiating bunches of tongue-shaped leaves. This seed fern was the key member of the famous Glossopteris flora that characterized Gondwana, the southern hemisphere continents, from the Pennsylva-nian to Late Permian (see p. 42). The Glos-sopteridales existed through the Triassic, and a number of other groups of seed ferns of uncertain affinities radiated during the Triassic and Jurassic.

Plant ecology of the coal measures_

Early reconstructions of Carboniferous vegetation tended to show crowds of ferns, horsetails, tree ferns and clubmosses growing in dense profusion around vegetation-filled lakes. However, detailed studies have shown that the floodplain vegetation consisted almost exclusively of clubmosses such as Lepidoden-dron and Sigillaria, with rare examples of horsetails such as Calamites. Seed ferns,

Permian Fern Glossopteris
Figure 18.17 The seed fern Glossopteris, a 4 m-tall tree, from the Late Permian of Australia. (Based on Delevoryas 1977.)

conifers and ferns were adapted to drier conditions, and they occupied elevated locations such as levees, the banks of sand thrown up along the sides of rivers. There are hints of extensive dry upland vegetations during the Carboniferous, but the fossil record of these is barely preserved. Surprisingly, some of the best preservation came about through huge forest fires (Box 18.5).

Towards the end of the Carboniferous, the floodplain vegetation of Europe and North America changed, probably as a result of slight drying of the environment. The club-mosses were replaced to some extent by the dryland ferns and seed ferns. These boggy habitats virtually disappeared in Europe and North America by the end of the Carbonifer ous, but persisted to the end of the Permian in China.

Conifers

Conifers are the most successful gymno-sperms, having existed since the Pennsylva-nian, and being represented today by over 550 species. Living conifers include the tallest living organisms of all time, the Coastal Redwood of North America, Sequoia sempervirens, which can reach over 110 m tall and an estimated 1500 tonnes. Conifers have a variety of adaptations to dry conditions, including their narrow, needle-like leaves with thick cuticles and sunken stomata, all adaptations to minimize water loss. The tough needles also escape freezing in cold polar winters. The seeds are contained in tough scales grouped in spirals into cones, usually borne at the end of branches, while the pollen-producing cones are usually borne on the sides of branches.

The Cordaitales of the Carboniferous and Permian are a distinctive group of early conifers which had strap-shaped, parallel-veined leaves (Fig. 18.19). Some Cordaitales were tree-like, and bore their leaves, sometimes up to 1 m long, in tufts at the ends of lateral branches. The Voltziales, of Pennsylvanian to Jurassic age, are represented by abundant finds of leaves and cones. The cones show a variety of structures, some with a single fertile scale at the tip, showing apparent intermediate stages to the cones of modern conifers, where all or most scales are fertile.

Modern conifers radiated in the Late Trias-sic and Jurassic, possibly from ancestors among the Voltziales. The main families -Podocarpaceae (southern podocarps), Taxa-ceae (yew), Araucariaceae (monkey puzzle), Cupressaceae (cypresses, junipers), Taxodia-ceae (sequoia, redwood, bald cypress), Ceph-alotaxaceae and Pinaceae (pines, firs, larches) - are distinguished by leaf shape and features of the cones. Podocarps and yews do not have cones.

Diverse gymnosperm groups_

Compared to the conifers, the other gymno-sperm groups did not radiate so widely. The ginkgos are represented today by one species, Ginkgo biloba, the maidenhair tree, a native

0 0

Post a comment