Evolutionary Success of early seed plants
Evolutionary Success of early seed plants
The salient characteristic of seed reproduction is that fertilization occurs within the confines of the ovule. It doesn't require standing water as a medium for free-swimming sperm. The evolutionary and ecological consequence is that seed plants could extend further from water edge. This ability would facilitate the diversification of seed plants from the Carboniferous through to the present day. However, these ultimately successful seed plants apparently had rather humble origins. The early seed plants at Red Hill and elsewhere were relatively modest plants that may not have exceeded 50 cm in height. Moreover, they seemed to have achieved their first successes as pioneer species.lSeed plant fossils at Red Hill were most commonly found in layers with abundant charcoal fragments. Layers above and below the charcoal layers had few if any seed plants but were typically rich with the ancient "fern" Rhacophyton. Similarly, deltaic deposits in West Virginia contained dense and monotypic mats of seed plants in one layer and dense, monotypic mats of Rhacophyton in the next. In other words, the seed plants rapidly colonized disturbed habitats (recently burned sites at Red Hill and the leading edges of postgrading deltaic lobes in West Virginia), but they were soon succeeded by Rhacophyton.lThe first indication of seed plant dominance occurs in a latest Devonian site in southwest Ireland. The site was probably a well-drained floodplain with relatively little sedimentation. Seed plants become more diverse, abundant and widespread during the Lower Carboniferous, and come to dominate levees, floodplains and drier upland habitats by the Late Carboniferous.The earliest seed plants (e.g., Aglosperma, Archaeosperma, Elksinia, Lagenostoma, Lyginopteris and Moresnetia) are sometimes collectively known as lyginopterids. Most of these are known only from their ovules and associated organs (i.e., cupules, adjacent stems and possibly Sphenopteris-like foliage). Nonetheless, they exhibited a wide variety of ovule forms. Their record extends from the (Famennian) Late Devonian through the Early Pennsylvanian (Middle Carboniferous). Most are believed to have been either small bushes or vines.Other important groups of seed plants appeared during the Carboniferous. The Pteridosperms, or seed ferns, is a polyphyletic group of seed plants with fern-like foliage. Different authorities have included a variety of plants within the seed ferns, including the lyginopterids and some Mesozoic forms. One clade of seed ferns, the Medullosaceae (e.g., Medullosa), were abundant trees in Carboniferous floodplains and extend well into the Permian.The Cordiatales (e.g., Cordiates) were also important floodplain trees and shrubs from the Late Carboniferous and Permian. Glossopterids (e.g., Glossopteris) were abundant in the Permian forests of Gondwanaland; the biogeographic distribution of Glossopteris provided crucial early evidence of continental drift.Conifers (e.g., pine, spruce, redwood) and cycads (e.g., Cycas and Leptocycas) also first appeared in the Late Carboniferous. Cycads were abundant and diverse from the Permian through much of the Mesozoic, but have since declined. They are represented today by only about 100 tropical to subtropical species. Conifers came to dominate many tropical to boreal forest ecosystems during the Permian and Mesozoic. Despite being largely displaced by flowering plants in the Cretaceous and Cenozoic, conifers continue to dominate a variety of temperate and boreal ecosystems.Ginkgoes appeared in the Triassic, reached their maximum diversity in the Jurassic and are represented today by a single species (Ginkgo biloba) surviving primarily as a valued ornamental. Angiosperms (flowering plants) appeared in the Lower Cretaceous and came to dominate most terrestrial ecosystems. Most of the vascular plants living today are angiosperms.
EARLY
EVIDENCE FOR LAND INHABITATION
1) Banded and smooth tubular elements.Elements with an annular or spiral banding embedded in a lighter wall. Widths range from 11-31 m; lengths up to 200 m. Clusters of tubes or parallel pairs are common.Smooth-walled elements are abundant, the ends of which appear to be jagged suggesting that these elements were longer. Mean width is 21 µm, lengths up to 45 µm.2) Cuticular and membranaceous fragments (one cell and several cells thick).lCellular sheets contain upwards of 200 cells, the shape of which are equidimensional and often orbicular.l3) Alete and trilete spores and tetrads.Trilete spores (35-41 µm), alete spores (14-30 µm), and sphaeromorphs up to 41 µm in diameter.4) Small septate hyphal filaments and filamentous mats.No evidence of true vascular tissues has been recovered.One of the earliest inhabitors of land may have been the lichen, an organism that is the symbiotic relationship between a green alga and a fungus. These organisms today colonize bare rock surfaces and are responsible for the breakdown of lithic materials via biochemical weathering. The earliest known lichen has recently been reported from the Early Devonian Rhynie Chert.
EXTINCTION; Definitions:
The salient characteristic of seed reproduction is that fertilization occurs within the confines of the ovule. It doesn't require standing water as a medium for free-swimming sperm. The evolutionary and ecological consequence is that seed plants could extend further from water edge. This ability would facilitate the diversification of seed plants from the Carboniferous through to the present day. However, these ultimately successful seed plants apparently had rather humble origins. The early seed plants at Red Hill and elsewhere were relatively modest plants that may not have exceeded 50 cm in height. Moreover, they seemed to have achieved their first successes as pioneer species.lSeed plant fossils at Red Hill were most commonly found in layers with abundant charcoal fragments. Layers above and below the charcoal layers had few if any seed plants but were typically rich with the ancient "fern" Rhacophyton. Similarly, deltaic deposits in West Virginia contained dense and monotypic mats of seed plants in one layer and dense, monotypic mats of Rhacophyton in the next. In other words, the seed plants rapidly colonized disturbed habitats (recently burned sites at Red Hill and the leading edges of postgrading deltaic lobes in West Virginia), but they were soon succeeded by Rhacophyton.lThe first indication of seed plant dominance occurs in a latest Devonian site in southwest Ireland. The site was probably a well-drained floodplain with relatively little sedimentation. Seed plants become more diverse, abundant and widespread during the Lower Carboniferous, and come to dominate levees, floodplains and drier upland habitats by the Late Carboniferous.The earliest seed plants (e.g., Aglosperma, Archaeosperma, Elksinia, Lagenostoma, Lyginopteris and Moresnetia) are sometimes collectively known as lyginopterids. Most of these are known only from their ovules and associated organs (i.e., cupules, adjacent stems and possibly Sphenopteris-like foliage). Nonetheless, they exhibited a wide variety of ovule forms. Their record extends from the (Famennian) Late Devonian through the Early Pennsylvanian (Middle Carboniferous). Most are believed to have been either small bushes or vines.Other important groups of seed plants appeared during the Carboniferous. The Pteridosperms, or seed ferns, is a polyphyletic group of seed plants with fern-like foliage. Different authorities have included a variety of plants within the seed ferns, including the lyginopterids and some Mesozoic forms. One clade of seed ferns, the Medullosaceae (e.g., Medullosa), were abundant trees in Carboniferous floodplains and extend well into the Permian.The Cordiatales (e.g., Cordiates) were also important floodplain trees and shrubs from the Late Carboniferous and Permian. Glossopterids (e.g., Glossopteris) were abundant in the Permian forests of Gondwanaland; the biogeographic distribution of Glossopteris provided crucial early evidence of continental drift.Conifers (e.g., pine, spruce, redwood) and cycads (e.g., Cycas and Leptocycas) also first appeared in the Late Carboniferous. Cycads were abundant and diverse from the Permian through much of the Mesozoic, but have since declined. They are represented today by only about 100 tropical to subtropical species. Conifers came to dominate many tropical to boreal forest ecosystems during the Permian and Mesozoic. Despite being largely displaced by flowering plants in the Cretaceous and Cenozoic, conifers continue to dominate a variety of temperate and boreal ecosystems.Ginkgoes appeared in the Triassic, reached their maximum diversity in the Jurassic and are represented today by a single species (Ginkgo biloba) surviving primarily as a valued ornamental. Angiosperms (flowering plants) appeared in the Lower Cretaceous and came to dominate most terrestrial ecosystems. Most of the vascular plants living today are angiosperms.
l
Ordovician
& Early Silurian Terrestrial PlantslStrother et al. 1996 (Geology, v. 24, p. 55-58)
have studies the Middle Ordovician shales of Saudi Arabia.
l
lNearshore marine setting from which palynomorphs (acritarchs and cryptospores) and phytoclasts recovered.Spore-like tetrads (fundamental character of embryophytes), dyads and cuticle-type
fragments with cell clusters.Dyads
are known from Cooksonia and Rhynia-type sporangia (Fanning et al., 1991). They argue for a
terrestrial origination for this assemblage; plants at a bryophyte-grade of development
(permanently bound tetrads). No extant algae are known to produce either trilete spores or resistant-walled, tetrahedrally arranged spore tetrads. Earliest
trilete spore in the Ashgillian (latest Ordovician). Taylor
(1995) has shown that the ultrastructure of Dyadospora (Ashgillian Ohio) has a lamellated spore wall that is similar to modern
hepatics.
lRemains
of what have been interpreted to be the earliest known vascular land plants
appear in the mid-Silurian (Wenlockian). This evidence consists of
spores, recovered from shallow marine environments, similar to those isolated
from early land plants. Spore diversity by Late Silurian time is greater than
that known from megafossil evidence. This may be due to
spore release from vegetation living on better-drained soils, evidence for the
parent plant not having been preserved.lEvidence for early land plants is
first encountered in fluvial sediments (Massanutten Sandstone) of the Early Silurian (Llandoverian) of Virginia. The PASSAGE CREEK
ASSEMBLAGE is comprised of:
1) Banded and smooth tubular elements.Elements with an annular or spiral banding embedded in a lighter wall. Widths range from 11-31 m; lengths up to 200 m. Clusters of tubes or parallel pairs are common.Smooth-walled elements are abundant, the ends of which appear to be jagged suggesting that these elements were longer. Mean width is 21 µm, lengths up to 45 µm.2) Cuticular and membranaceous fragments (one cell and several cells thick).lCellular sheets contain upwards of 200 cells, the shape of which are equidimensional and often orbicular.l3) Alete and trilete spores and tetrads.Trilete spores (35-41 µm), alete spores (14-30 µm), and sphaeromorphs up to 41 µm in diameter.4) Small septate hyphal filaments and filamentous mats.No evidence of true vascular tissues has been recovered.One of the earliest inhabitors of land may have been the lichen, an organism that is the symbiotic relationship between a green alga and a fungus. These organisms today colonize bare rock surfaces and are responsible for the breakdown of lithic materials via biochemical weathering. The earliest known lichen has recently been reported from the Early Devonian Rhynie Chert.
l
Evidence in Paleosols
lPaleosols (ancient soils) are a record of
land surfaces and the vegetation that shaped their character. Plants that
colonized these soils can be interpreted from root traces and soil (paleosol) profile (once diagenetic effects have been accounted
for). Retallack has proposed two terms:Polsterland - well-drained soils dominated
by multicellular plants that lack roots and
rhizomes;Brakeland - soils that support herbaceous
plants with rhizomes, runners, or fibrous roots to describe soils in which early
terrestrial plants (and extant mosses, lichens, and liverworts) lived.Polsterlands are believed to exist in the
Late Ordovician of Nova Scotia and Pennsylvania (which also include abundant
burrows, but no root traces). More evidence exists for the identification of brakelands. Brakelands are found as early as Late Silurian and
continue into the Carboniferous. Evidence of macroinvertebrate bioturbation exists in these oxidized calcareous soils.lPinnatiramosus from China (Cai et al. 1996) believed to be Silurian due to
regional stratigraphic correlations. The
"rooting" structures are 2 meters below the Silurian/Permian
unconformity.Rooting
structures are pinnate, cross-cut bedding and are found horizontal along
bedding surfaces. Vascular cells are interpreted from casts of the internal
anatomy.Cai et al. argue that these are SILURIAN
vascular plants and, hence, the level of evolution of plant clades is higher than previously
thought. Approximately >40 MY of time between Silurian/Permian; similar to
that of present/Eocene. Possible that Silurian sediments were not lithified and Permian plants rooted.
Rooting structures, then, may represent Permian vegetation and not Silurian. Cooksonia has been found in the Late
Silurian of Bolivia (Morel et al.
1995) at 50o - 60o South Latitude indicating that it has spread to high
latitudes soon after its appearance in the Wendlockian. It is the 4th Silurian record from Gondwana: Pridoli of Libya & Pridoli of Bohemia.
lSeveral environmental changes are noted to occur in the early
Silurian (435 Ma)-
Autotrophs reproduced and increased in
diversitylProduction
of oxygen that exceeded consumption - formation of ozone layer providing
protection Decrease
in tectonic activity - new sizeable and stable near shore environments Local
movement of the earth's crust causing cyclical emergence and subsidence in the
littoral zone - littoral inhabitants exposed to longer periods of drying out Plants
increasing rate of weathering of rocks and contributing to the formation of
soils (e.g., fossil lichens have been reported from the Devonian-aged Rhynie chert - 400 million years old; Taylor et al. 1995)
l
Extinction: The loss of a species from the
biota; the failure of a taxonomic group to produce direct descendants, causing
its worldwide disappearance from the record at a given point.Extirpation: The loss of a species from a
significant portion of its rangeEndemic: A species restricted to a
defined geographic area
lExtinction
as a natural process
lEach species has a finite
lifetime probably 99% of all organisms that have existed are now extinct Most species exist 2-5 million
years Extinction can result in an
available ecological niche (where an organism lives and its behavior in that place) to be occupied by
other species
l
lKinds of extinction
lBackground extinction: The continuous, low-level rate
of extinctionlMass extinction: A large loss of species in a
brief geological period of time lCretaceous
extinction when the dinosaurs disappeared some 65 million
years ago (mya)Numerous major extinctions have
occurred (5 or 6) each resulting in a fundamental change of the biota (e.g.,
rise of flowering plants at the end of the Jurassic some 130 mya)Significant
extinctions occur about every 26 million years
lCauses of mass extinction
lClimate:
Changes in the climate always results in changes in the biota; sudden (in
geologic time) and profound changes nearly always result in mass extinction
events. Gradual changes usually result in a displacement of the biota but not
necessarily mass extinction Pleistocene
glaciation [good but slow to load!] (over
the last 2.5 my) resulted in significant extinction of grazing animals in North
America and Eurasia, but not in Africa and portions of South AmericalEastern
deciduous forests pushed into eastern Mexico so that most of the flora
survived; montane forests in southern California
and Arizona were extirpated during the Holocene (last 10,000 y) as the climate
warmed after the glacial era
Geologic events: Dramatic flooding, extensive volcanic
activity, major tetonic shifts (e.g., continental drift,
widespread volcanic eruptions), etc. can all resulted in global or near-global
extinction events
Increases or decrease in sea levelslVolcanic and fire-induced high
altitude air pollution Continental drift and island formation
Meteorite: Impact of large or numerous
meteorites. [See When the Sky Fell by Philippe Claeys, a thorough summary] Meteorite: Impact with earth can
cause increase in dust at high elevationlEnd
of Cretaceous probably caused by meteorite hitting the earth near Yucatan;
iridium layer found at same level all around the earth
lHuman: The
single largest cause of extinction presentlylAbout
2400 species disappear dailyAbout
10000 new species described annually
lExtinction rate now greater than that at the end of the
Cretaceous Period
References
References
Jensen, WA & Salisbury, FB.
Botany: An ecological Approach. Wadsworth Publishing Company inc., California.
Gorge B. Johnson 2000. The living
world. Second edition
Murray W. Nabors 2004. Introduction to
Botany. Pearson Benjamin Cummings. University of Mississippi
Berg, LR. 1997. Introductory Botany: Plants, People
and the Environment. St Pettersburg Junior college.
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