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I. Modern ferns
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Earlier we looked at Paleozoic fern diversity and studied such groups
as the Stauropteridales, Zygopteridales, Marattiales and Paleozoic Filicales.
Most of the plants we looked at earlier are extinct today, except for the
Marattiales, which have some living relatives like Marattia and Angiopteris,
and, of course, the modern Filicalies, which include most of the ferns
present in today's flora. We will also look at the Ophioglossales, a small
homosporous family, and at the heterosporous water ferns traditionally
placed in Marsilieles and Salviniales. In this lab we will look at fossil
representatives of primarily modern fern groups.
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A. Ophioglossales
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The Ophioglossales are a small order of homosporous ferns that include
the three genera Ophioglossum, Botrychium, and Helminthostachys.
This group is characterized by homosporous, eusporangiate plants that bear
their sporangia in a fertile spike. While Ophioglossum ("Moonwort")
has a simple, entire frond, Botrychium ("Grape fern") has a highly
dissected pinnately organized frond. In contrast, the frond of Helminthostachys
is palmately compound. Look briefly at the herbarium sheet of Botrychium.
Although the fossil record of this group is scarce, excellent fossils of
Botrychium have recently been reported from the Paleocene of Alberta,
Canada. See text for details (p. 235-236).
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B. The Modern Filicales
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The Filicales are the so-called "true ferns" and represent the major
groups of ferns today. Filicaleans are homosporous and bear a relatively
small number of spores in reduced sporangia called leptosporangia. Leptosporangia
are stalked and have an annulus, a group of larger, thicker-walled cells
on a portion of their surface. It is the position of the annulus, the arrangement
of the sori (clustered sporangia) and the presence or absence of accessory
structures that provide the means for classification of the filicaleans.
We will look at a some Pennsylvanian filicalean ferns and then a few representatives
of filicalean ferns that have extensive fossil records. Many of
the modern filicalean fern families have a long fossil record. We will
consider several of them.
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1. Osmundaceae
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The Osmundaceae are a group of primitive filicalean ferns that are
found in anatomically preserved state as far back as the Permian. Numerous
species of Osmunda, Osmundicaulis, and several other genera
have been described based on variations in anatomical detail of petiole
and rhizome anatomy. Look first at herbarium sheets and photographs
of the Osmundaceae. This family includes Osmunda cinnamomea
(Cinnamon fern), O. claytoniana (Interrupted fern), and O.
regalis (Royal fern). Now look at a transverse section of the Osmunda
rhizome. This fern produces numerous leaves closely together, such
that there are numerous leaf, as well as root traces visible in the axis.
This stem has an ectophloic siphonostele, and contains a large amount of
sclerenchyma in the pith and cortex, which stains red or purple in prepared
slides. Note the protoxylem strands that are mesarch, mixed in with the
rest of the primary xylem. Fossils of the Osmundaceae are found in anatomically
preserved state as far back as the Permian. Numerous species of Osmunda,
Osmundicaulis, and several other genera have been described based
on variations in anatomical detail of petiole and rhizome anatomy. Most
of the fossil material that has been described for this group is of vegetative
material. Is there a problem in using this type of material to talk about
evolution in this group?
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Look at the specimens of osmundaceous fossil stems in the lab.
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| The genus Tempskya represents petrified trunks of "false stems"
that are actually composed of a large number of individual rhizomes that
grow together. Tempskya is found in Cretaceous strata in North America
and is an extinct form. Look at the sections of Tempskya.
Can you identify individual axes? |
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2. Gleicheniaceae
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| The Gleicheniaceae are primarily tropical ferns that have long creeping
rhizomes and large fronds that can get to be up to 10 meters long, and
include the genera Gleichenia and Dicranopteris. The fronds
characteristically are forked with foliar buds occurring at the junctures.
Stems of Gleichenia are protosteles with a mixed pith. Sori commonly
have 2 -4 large sporangia and lack indusia. The annulus is transverse to
oblique. Generally this family is considered to be a fairly primitive group
that is related to fossil forms of Jurassic and Cretaceous, and possibly
Pennsylvanian age. Look at the herbarium sheets of the gleicheniaceous
Malaysian fern Dicranopteris. Compare with the fossil foliage of
Gleichenia from the Cretaceous of Utah. |
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3. Matoniaceae
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The Matoniaceae, including the extant genus Matonia and the
Triassic/Jurassic fossil form Phlebopteris are today tropical plants
from Indonesia, New Guinea and Borneo. The fronds are forked and have pinnatifid
organization. The rhizome has concentric vascular cylinders with amphiphloic
phloem. Sori are composed of relatively few, large sporangia that are protected
by a conspicuous peltate (umbrella-like) indusium. The annulus is oblique,
stalks are short, and sporangia in a given sorus mature simultaneously.
Spores are smooth and triangular with a conspicuous trilete mark. Look
at the fossil foliage of Matondium. The Matoniaceae are
thought to have an evolutionary relationship to the Gleicheniaceae, on
the basis of reproductive structures.
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4. Dennstaeditioideae/Asplenioideae
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The genus Onoclea is placed within this complex which is characterized
by ferns with erect to long, creeping stems with trichomes, scales or both.
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Compare the herbarium sheet of extant Onoclea to the photographs
of fossil Onoclea found in the Paleocene of Alberta, Canada.
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C. The heterosporous water ferns: Marsilieles
and Salvineales
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1. Marsileales
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The Marsileales are comprised of three genera, Marsilea, Regnellidium
and Pilularia. All of these plants live along pond or lake margins
and produce a creeping rhizome that bears the fronds and roots at nodes.
Marsilea has a 4-lobed pinnule and resembles a 4-leaf clover, while
in Regnellidium the frond is 2-lobed and Pilularia lacks
a laminate portion to its frond and has a reduced, grass-like leaf. Rhizomes
are amphiphloic siphonosteles, with both inner and outer phloem, frequently
zonate cortex, with an inner, more compact zone and an outer aerenchymatous
zone, and may have a sclerotic pith. The reproductive structures of the
Marsileales are called sporocarps: these are small bean-like structures
borne singly or on branches at the base of fronds, and are produced during
adverse conditions, as water availability becomes scarce. They are interpreted
as being homologous to either a single pinna or an entire frond. When they
are moistened, sporocarps open to produce an elongate, gelatinous structure
called a sorophore that bears a row of sori, each in an indusium. The
fossil record of the Marsileales is not very extensive. However, in a recent
publication by Skog and Dilcher, plants of this group have been described
from the Cretaceous of Kansas.
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2. Salvineales
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| In contrast to the Marsileales, the Salvineales are completely floating
plants. There are two genera, Azolla and Salvinea. Salvinea
has a rhizome that bears leaves in whorls of three, with two of each whorl
being thick, flattened photosynthetic leaves that have a thick pad of hairs
on their abaxial surface that maintain them as floating plants, and a third
leaf that is highly dissected, submerged and acts as a root. There are
no true roots in this plant. In Azolla, the leaves are spoon-shaped
and fit into one another, and true roots are present. In these two genera
the reproductive structures are sporocarps that are basically modified
indusia. Within each sorus, either the megasporangium with its one functional
megaspore, or a cluster of microsporangia with numerous microspores differentiates,
but not both. Along with the spores, tissue derived from the tapetum forms
an accessory tissue that produces massulae or "floats". As the spores emerge
from their sporangia the attached floats act to maintain them as floating
units. Floats containing embedded masses of microspores often have tiny
hook-like projections called glochidia that hook onto the megaspore allowing
for fertilization of the archegonia within the megagametophyte tissue within
the megaspore. Azolla also harbors the blue-green nitrogen fixing
alga Anabaena in a symbiotic relationship, and thus has been economically
important, especially to rice production. |
| The distinctive megaspores, as well as whole plants of Azolla,
have been found rather extensively in a number of Cretaceous and Tertiary
fossil deposits. Look at the herbarium sheets of Azolla and compare
to fossils of from British Columbia. |
Since the text was published, an interesting aquatic fern has been
described that has features of both Marsileales and Salvineales and suggests
that these two orders are more closely related than was previously thought.
See the paper on Hydropteris by Rothwell and Stockey.
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II. The Origin of Modern Conifer Families
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| The families of modern conifers are delimited primarily on the following
characters: leaf arrangement and morphology, degree of fusion of ovulate
bract-scale complex, number of ovules per scale, number of pollen sacs
per microsporophyll, and type of pollen. Below is a brief summary of each
family and its fossil record. |
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1. Araucariaceae:
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| This southern hemisphere family is represented today by the genera
Araucaria (Norfolk Island pine) and Agathis (monkey puzzle
tree). These plants are either dioecious or monoecious trees with opposite
or spirally arranged leaves that are linear to broad. Pollen cones have
5-20 pendant microsporangia per microsporophyll while seed cones contain
one ovule per fused bract-scale complex. Pollen is nonsaccate. |
| Araucarian conifers have a rich fossil record, extending from at least
the Triassic, on the basis of reproductive structures. The genus Araucarites,
in particular, represents fossil specimens of fused bract-scale complexes
similar to those of extant Araucaria. (See the diagram on p. 425).
As is discussed in the textbook, wood and leaf fossils with features similar
to modern Araucariaceae may point to earlier origin but cannot be unequivocally
placed in this group. Permineralized Jurassic cones are well known and
contain young embryos. |
| Although the araucarians are restricted today to the southern hemisphere,
fossil evidence from the Mesozoic demonstrates that they were widespread
throughout the world from the Triassic through the Cretaceous. Their present
day distribution probably dates back to the Tertiary. |
Look briefly at the Jurassic Araucaria cone and at the paper
by Stockey on the history of the araucarians.
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2. Podocarpaceae:
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Podocarpus, Dacrydium, Phyllocladus are genera
in this second, primarily southern hemisphere family. These plants are
mostly dioecious trees or shrubs with spirally arranged, scalelike, linear
or broad leaves. Pollen cones have 2 microsporangia per microsporangium
while seed cones have one ovule maturing per ovulate structure that is
conelike or modified into several small scales and a subtending bract.
The ovule may be borne on fleshy "receptacle" (epimatium) at maturity.
Pollen is saccate.
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| In contrast to the araucarians, which had a wide distribution in the
Mesozoic, the podocarps have always been essentially southern hemisphere
plants. Podocarps are first recognized in the Triassic, and become extensive
in the Jurassic of Gondwana. Vegetative and reproductive structures referred
to the genus Rissikia from Australia and Africa are podocarpaceous,
as are several Jurassic forms. Recently a podocarpaceous stem has been
described from the Triassic of Antarctica. |
| Look at the photographs of Rissikia in the book by White (p.
160-2).
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3. Pinaceae:
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| The Pinaceae are the best known and largest family of extant conifers,
and are probably the most familiar to you because they are a predominantly
northern hemisphere group well represented in the current flora. This family
is represented by the genera Pinus (pine), Pseudotsuga (Douglas
fir), Abies (fir), Picea (spruce), Cedrus (cedar).
These plants are monecious trees with leaves that are spirally arranged,
linear or needlelike. Pollen cones in this group have 2 microsporangia
per microsporophyll while seed cones bear 2 ovules per woody cone scale,
and have scales and bracts that are free from one another. Pollen is bisaccate. |
| The fossil record of the Pinaceae extends from at least the Cretaceous
based on anatomically preserved cones. However, a number of Triassic and
Jurassic compressed fossils that are less completely known have also been
suggested as members of the Pinaceae. In the Cretaceous, numerous pinaceous
cones are found anatomically preserved. Although a few can be assigned
to the modern genus Pinus, most are placed in the fossil genus Pityostrobus
because they combine characters not found in modern taxa. Other modern
pinaceous genera evolved during the Tertiary. |
| Look at specimens of Pinus ovulate cones, stems and leaves from
the petrified Princeton Chert, and at the compressed Miocene pine needles
from Creede, Colorado. |
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4. Taxodiaceae:
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| The Taxodiaceae are another good example of a family of plants that
was widely distributed and common in the Cretaceous and Tertiary, that
has a much more restricted distribution today. This family is represented
by Taxodium (bald cypress), Metasequoia (dawn redwood); Sequoia
(redwood), Sequoiadendron (giant redwood), and Cryptomeria.
These plants are monecious trees with leaves that are spirally arranged
or opposite, needlelike or linear. Pollen cones have 2-9 microsporangia
per microsporophyll while seed cones bear 2-9 ovules per scale and have
scales and bracts that are joined. Pollen is nonsaccate. |
| The fossil record of the Taxodiaceae extends from the Jurassic to the
Recent. Fossil representatives are plentiful especially in the Cretaceous
when Metasequoia was widespread and represents the dominant foliage
type in many localities, particularly in northwestern North America. |
Look at compressed Metasequoia foliage from the Cretaceous of
Alaska and at the paper by Serbet and Stockey on cones from Drumheller,
Alberta.
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A fossil family, the Cheirolepidaceae, which may be transitional
between the Voltziales and Taxodiaceae is important from the Triassic through
the Cretaceous in Europe and Argentina. This family is a good example of
the greater degree of conifer diversity that occurred during the Mesozoic.
We know this group was widespread and ecologically important because of
both the megafossil record and its distinct pollen, Classopollis,
which rivals the angiosperms in complexity of pollen wall structure.
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5. Cupressaceae:
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| Cupressus, Juniperus (juniper), Thuja (hemlock)
are included in this family. These plants are monoecious or dioecious trees
and shrubs with leaves that are either opposite or whorled and usually
scalelike. Pollen cones have 3-6 (+) microsporangia per microsporophyll
while seed cones have 2-many ovules borne on fused bract-scale complex.
Pollen is nonsaccate. |
| The fossil record of cupressaceous plants is less detailed than for
a number of the families discussed above. One problem is that it is difficult
to determine whether some foliage is cupressaceous or taxodiaceous based
on whether leaves are borne decussately or helically. This family is at
least as old as the Jurassic, with extant genera recognizable by the Upper
Cretaceous. |
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6. Cephalotaxaceae:
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| This family is represented by the genus Cephalotaxus. These
plants are mostly dioecious trees or shrubs with spirally arranged to two-ranked,
needlelike leaves. Pollen cones contain 3-8 pollen sacs per microsporophyll
and seed cones are decussately arranged with 2 ovules per bract. Only one
ovule matures into an olive-like seed. Pollen grains are nonsaccate. |
| The fossil record of this group is relatively small, but extends from
the Jurassic to the Recent. While in extant Cephalotaxaceae single ovules
develop from cones, fossil relatives such as Palissya show numerous
ovules on the bract-scale complex. See text for details. |
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7. Taxales
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| A. Taxaceae: Taxus (yew), Torreya; Trees or shrubs dioecious;
leaves spirally arranged, linear or needlelike; 2-8 microsporangia per
peltate microsporophyll, one ovule at maturity on ovuliferous branch enclosed
in a fleshy aril; pollen nonsaccate. |
Taxaceous plants are first reported from the Jurassic. Their relationships
to the transition conifers are less clear than are those of other families.
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