Origin of gymnosperms

Angiosperms form the most dominant group of plants with at least
253,300 species (Thorne, 2007). Angiosperms are also found in a far greater
range of habitats than any other group of land plants. The phylogeny of
angiosperms is a much-debated subject, largely because of very poor records of
the earliest angiosperms. These earliest angiosperms probably lived in habitats
which were not best suited for fossilization.

ORIGIN OF ANGIOSPERMS

With the exception of conifer forest and moss-lichen tundra,
angiosperms dominate all major terrestrial vegetation zones, account for the
majority of primary production on land, and exhibit extensive morphological
diversity. Unfortunately, much less is known about the origin and early
evolution of angiosperms, resulting in a number of different views regarding
their ancestors, the earliest forms and course of evolution. The origin of
angiosperms may be conveniently discussed under the following considerations.

Angiosperms form a distinct group of seed plants sharing a unique
combination of characters. These important characters include carpels enclosing
the ovules, pollen grains germinating on the stigma, sieve tubes with companion
cells, double fertilization resulting in triploid endosperm, and highly reduced
male and female gametophytes. The angiosperms also have vessels. The pollen
grains of angiosperms are also unique in having non-laminate endexine and
ectexine differentiated into a foot-layer, columellar layer and tectum. The
angiosperm flower typically is a hermaphrodite structure with carpels
surrounded by stamens and the latter by petals and sepals, since insect
pollination prevails. Arbuscular mycorrhizae are also unique to angiosperms.
The vessel elements of angiosperms typically possess scalariform perforations.
There may be individual exceptions to most of these characters. Vessels are
absent in some angiosperms (Winteraceae) while some gymnosperms have vessels
(Gnetales). The flowers are unisexual without perianth in several Amentiferae,
which also exhibit anemophily. In spite of these and other exceptions, this
combination of characters is unique to angiosperms and not found in any other
group of seed plants.

The time of origin of angiosperms is a matter of considerable
debate. For many years, the earliest well-documented angiosperm fossil was
considered to be the form-genus Clavitopollenites. Couper, (1958) described
from Barremian and Aptian strata of Early Cretaceous of southern England (132
to 112 mya-million years), a monosulcate pollen with distinctly sculptured exine,
resembling the pollen of the extant genus Ascarina. Brenner and Bickoff
(1992) recorded similar but inaperturate pollen grains from the Valanginian (ca
135 mya) of the Helez formation of Israel, now considered to be the oldest
record of angiosperm fossils (Taylor and Hickey, 1996).

 The number and diversity of
angiosperm fossils increased suddenly and by the end of the Early Cretaceous
(ca 100 mya) period major groups of angiosperms, including herbaceous
Magnoliidae, Magnoliales, Laurales, Winteroids and Liliopsida were well
represented. In Late Cretaceous, at least 50 per cent of the species in the
fossil flora were angiosperms. By the end of the Cretaceous, many extant
angiosperm families had appeared. They subsequently increased exponentially and
constituted the most dominant land flora, continuing up to the present. The
trail in the reverse direction is incomplete and confusing. Many claims of
angiosperm records before the Cretaceous were made but largely rejected.
Erdtman (1948) described Eucommiidites as a tricolpate dicotyledonous
pollen grain from the Jurassic which had bilateral symmetry instead of the
radial symmetry of angiosperms and granular exine with gymnospermous laminated
endexine. This pollen grain was also discovered in the micropyle of seeds of
the female cone of uncertain but clearly gymnospermous affinities. Several
other fossil pollens from the Jurassic age attributed to Nymphaeaceae ultimately
turned out to be gymnosperms. In the last few years fossils of Archaefructus
from Upper Cretaceous (nearly 124 mya) of China, with clearly defined
spirally arranged conduplicate carpels enclosing ovules, a feature not reported
in earlier angiosperms. The fruit is a follicle. This is considered to be the
oldest record of angiosperm flower.

Several vegetative
structures from the Triassic were also attributed to angiosperms. Brown (1956)
described Sanmiguilea leaves from the Late Triassic of Colorado and
suggested affinity with Palmae. A better understanding of the plant was made by
Cornet (1986, 1989), who regarded it as a presumed primitive angiosperm with
features of monocots and dicots. Although its angiosperm venation was refuted
by Hickey and Doyle (1977), Cornet (1989) established its angiosperm venation
and associated reproductive structures. Our knowledge of this controversial
taxon, however, is far from clear. Marcouia leaves (earlier described as
Ctenis neuropteroides by Daugherty, 1941) are recorded from the Upper
Triassic of Arizona and New Mexico. Its angiosperm affinities are not clear. Harris
(1932) described Furcula from the Upper Triassic of Greenland as
bifurcate leaf with dichotomous venation. Although it seems to approach dicots
in venation and cuticular structure, it has several non-angiospermous
characters including bifurcating midrib and blade, higher vein orders with relatively
acute angles of origin (Hickey and Doyle, 1977). Cornet (1993) has described Pannaulika,
a dicot-like leaf form from Late Triassic from the Virginia-North Carolina
border. It was considered to be a three-lobed palmately veined leaf. The
associated reproductive structures were attributed to angiosperms but it is not
certain that any of the reproductive structures were produced by the plant that
bore Pannaulika. Taylor and Hickey (1996), however, do not accept its angiosperm
affinities, largely on the basis

            Ancestry of
angiosperms is perhaps one of the most controversial and vigorously debated
topics. In the absence of direct fossil evidence, almost all groups of fossil
and living gymnosperms have been considered as possible ancestors. Some authors
even suggested the Isoetes origin of monocotyledons because the plant has a
superficial resemblance with onion, although no trace of seed habit. The various
theories have revolved around two basic theories, viz., the Euanthial theory and the Pseudanthial theory of angiosperm origin. Some other theories projecting herbaceous
ancestry for the angiosperms have also recently received attention, making the question
of ancestry of angiosperms rather more ambiguous:

Euanthial Theory

            Also known as
Anthostrobilus theory, Euanthial theory was first proposed by Arber and Parkins
(1907). According to this theory, the angiosperm flower is interpreted as being
derived from an unbranched bisexual strobilus bearing spirally arranged ovulate
and pollen organs, similar to the hermaphrodite reproductive structures of some
extinct bennettitalean gymnosperms. The carpel is thus regarded as a modified
megasporophyll

            The bisexual
flower of Magnoliales has been considered to have evolved from such a
structure. Also agreeing with this general principle, various authors have
tried to identify different gymnosperm groups as possible angiosperm ancestors:
Cycadeoidales (Bennettitales) The group, now better known as Cycadeoidales,
appeared in the Triassic and disappeared in the Cretaceous. Their potential as
angiosperm ancestors was largely built upon the studies of Wieland (1906, 1916).
Lemesle (1946) considered the group to be ancestral to angiosperms, primarily
because of the hermaphrodite nature of Cycadeoidea, which had an
elongated receptacle with perianth-like bracts, a whorl of pollen-bearing
microsporophylls surrounding the ovuliferous region having numerous. The plant
was believed to look like cycads with a short compact trunk and a crown of pinnate
compound leaves.

            It was earlier suggested that
the microsporophylls opened at maturity but the subsequent studies of Crepet
(1974) showed that microsporophylls were pinnate, and distal tips of pinnae
were fused, the opening of the region was not structurally possible, and they
later disintegrated internally. The ovules were terminal in contrast to their
position in carpels of angiosperms.

Sporne (1971) suggested possible links between Cycadales and
angiosperms in the palm-like habit of Cycadales. The ovules being borne on
leaf-like megasporophylls. Trends in the reduction of sporophyll blade as seen
in various species of Cycas. Although it may be difficult to assume
Cycadales as ancestral to angiosperms.

            Commonly
associated with the Englerian School, the theory was first proposed by Wettstein
(1907), who postulated that angiosperms were derived from the Gnetopsida,
represented by Ephedra, Gnetum and Welwitschia (formerly
all placed in the same order Gnetales).

            The group shows
more angiosperm characteristics than any other group of living or fossil
gymnosperms. These include the presence of vessels, reticulate dicot-like
leaves (Gnetum), male flower with perianth and bracts, strong
gametophyte reduction, and fusion of the second male gametophyte with the
ventral canal nucleus. Ephedra resembles Casuarina in habit.
Wettstein homologized the compound strobili of Gnetales with the inflorescences
of wind-pollinated Amentiferae. Showy insect pollinated bisexual flowers of Magnolia
as pseudanthia derived by aggregation of unisexual units, the carpel thus
representing a modified branch (Stachyosporous origin of carpel).

 Gnetopsida is a very young
group. But this theory has been strongly supported by Young (1981), who
challenged the view that first angiosperms were vesselless and assumed that
vessels were lost in several early lines. Muhammad and Sattler (1982) found
scalariform perforations in vessel-elements of Gnetum, suggesting that
angiosperms may be derived from Gnetales after all. Ephedra is generally considered to be the most primitive of the three
living genera of gnetopsids. Cornet believes that Archaestrobilus possessed
characters that may be plesiomorphic even for Ephedra.

Anthocorm
Theory

            This theory is a
modified version of the pseudanthial theory and was proposed by Neumayer (1924).
According to this theory, the angiosperm flower has several separate origins
(i.e. angiosperms are polyphyletic). In most Magnoliidae and their
dicotyledonous derivatives, they are modified pluriaxial systems (holanthocorms).
They originated from

the gnetopsids through the Piperales.

            The Gonophyll theory was developed
by Melville, largely on the basis of a study of the venation pattern. He derived
angiosperms from Glossopteridae, which formed important elements in the flora
of Gondwanaland. He further derived angiosperm flower from gonophyll (a fertile
branching axis adnate to a leaf).