EARLY LAND PLANTS
The oldest known body fossils of land plants are
Silurian in age. Fossil root traces of land plants are known back in the
Ordovician. The Devonian was the key time interval during which land
plants flourished and Earth experienced its first “greening” of the land. The earliest land plants were small and
simple and probably remained close to bodies of water. By the Late
Devonian, land plants had evolved large, tree-sized bodies and the first-ever
forests appeared.
Baragwanathia from the Lower Devonian of Victoria, southeastern Australia.
This is the oldest known lycopod plant and one of the oldest known land plant
body fossils. Lycopods are an important and well known component of the
Paleozoic land plant fossil record. Baragwanathia occurs in the
Upper Silurian to Middle Devonian of Australia and Canada, principally from
impressions and carbonized compressions, but some specimens are
permineralized. Baragwanathia was a decent-sized plant, with
upright axes (stems) up to 7 cm wide, but did not get too big - it was a
herbaceous plant that probably stood less than ~1 m high. Numerous elongated
leaves (microphylls) attached to the axis in a helical pattern.
Individual leaves reached up to 4 cm long.
Classification: Plantae, Lycophyta, Drepanophycales
(FMNH PP48927, Field Museum of Natural History,
Chicago, Illinois, USA)
Archaeosigillaria (a.k.a. Clwydia) from the Devonian at
Gilboa, southeastern New York State, USA.
This distinctive fossil plant is also a lycopod, the
best-known plant group in the Paleozoic fossil record. This plant was a
relatively small, herbaceous lycopod that had simple, bifurcate
branching. The leaves (microphylls) were small and generally needle-like
(see the spine-like structures alongside the stems in the above specimens), and
were attached to the stem (axis) in a helical pattern.
Classification: Plantae, Lycophyta, Protolepidodendrales
(FMNH P21749, Field Museum of Natural History,
Chicago, Illinois, USA)
Colpodexylon deatsii Banks, 1944 from the Oneonta Formation (lower Upper
Devonian) of Sullivan County, southeastern New York State, USA (FMNH PP 33689,
Field Museum of Natural History, Chicago, Illinois, USA). Colpodexylon
is another lycopod plant, like Archaeosigillaria (above). Each
dimple on the stem is a leaf attachment star. The leaves were trifurcate
(three-branched) and very thin & elongated, like modern pine needles.
Leaves were attached to the stem in a subtle helical/spiral arrangement.
Classification: Plantae, Lycophyta, Protolepidodendrales
Aneurophyton from the Devonian at Gilboa, southeastern New York State, USA.
These are branches from an extinct group of vascular plants, the
aneurophytalean progymnosperms. Progymnosperms have characteristics of
seed plants (they have lots of gymnosperm-like wood - secondary xylem) and
characteristics of ferns (fern-like reproductive structures that produce
spores). Aneurophyton is a progymnosperm that has non-flattened
branches that emerge from the main axis of the plant in a helical or decussate
fashion. The photosynthetic structures in Aneurophyton are not
true leaves - they are usually referred to as “pre-leaves”. This plant
produced spores having one morphology but a range of sizes (70 to 150 μ in
the same plant).
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Aneurophyton has been reported from the Devonian of Europe, Russia, and America.
Classification: Plantae, Progymnospermophyta, Aneurophytales
(FMNH P21741, Field Museum of Natural History,
Chicago, Illinois, USA)
Archaeopteris cf. Archaeopteris halliana from the Devonian of Quebec,
southeastern Canada (left) & Archaeopteris from the Devonian
of Pennsylvania, USA (right).
These are branches with leaves from another
progymnosperm plant. In the 1960s, the leaf/frond genus Archaeopteris
was recognized to be part of the same plant as the wood genus Callixylon.
Archaeopteris had 3 to 4 feet diameter trunks (axes) and formed shrubby
to tree-sized plants, possibly similar to modern conifers. The leaves
(they are recognized as true leaves in this form) are flattened and range in
shape from spatulate to fan-shaped to dissected. Species of Archaeopteris
have been reported from the Devonian of North America, Eurasia, and Australia.
Classification: Plantae, Progymnospermophyta, Archaeopteridales
(FMNH PP16302 (above left) & FMNH UP 2054 (above
right), Field Museum of Natural History, Chicago, Illinois, USA)
Chaleuria cirrosa Andrews et al., 1974 from the upper Lower Devonian of
New Brunswick, southeastern Canada (FMNH PP 33636, Field Museum of Natural
History, Chicago, Illinois, USA). Chaleuria is the
oldest known plant that has heterospory - its sporangia produced spores of two
discrete size ranges. Heterospory became a widespread reproductive
feature in land plants by the Late Devonian. Chaleuria was
initially interpreted as a progymnosperm (see above), but it has since been
considered to have uncertain affinities (Plantae incertae sedis).