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The
oldest conifer fossil so far discovered is Swillingtonia
denticulata, which dates from the Carboniferous of
c.310 Ma.
While
the conifers were present from this time onwards, it was
not until the Triassic (245-208 Ma) that they radiated
out and probably occupied the drier parts of the planet.
The eight conifer families which still exist today (Pinaceae,
Podocarpaceae, Taxaceae, Cupressaceae, Taxodiaceae,
Araucariaceae and Cephalotaxaceae) appear in the fossil
record from this time.
Conifers
are typified by a pyramidal growing form in many
species, as well as needle-like or scale-like and often
waxy leaves and tracheids which are often arranged into
rings with resin-canals.
The
pollen of conifers has a pollen tube which delivers
sperm cells directly to the eggs and this makes the
Coniferales more advanced that the Cycadales and
Ginkgoales.
A
common fossilised conifer is Utrechia which grew
to about 5 metres in height. It had a resemblance to the
modern-day Araucaria heterophylla or the Norfolk
Island Pine which is a native of Australia. The leaves
of this species were needle-shaped and microphyllous.
This species, like many conifers, produced separate male
and female cones on the one plant meaning that the
species was monoecious. The evolutionary origins of the
Coniferales is uncertain at present.
The
Triassic also saw the development of the forked-frond
seed ferns. The first to be found in the fossil record
was Dicroidium callipteroides of New South Wales,
Australia, which emerged approximately 200 Ma. This
species had repeated forking in its fronds unlike later
fork-fronded Triassic seed ferns which only had a single
fork. The fork-fronded seed ferns were more advanced
than the earlier seed ferns that grew alongside the Glossopterids.
A
fossilised Dicroidium frond.
There
was significant diversity in the Dicroidium flora,
and as is the case for the Glossopterids, there is
considerable difficulty in naming some species. The
reproductive organs of Dicroidium bear
resemblance to those of the Glossopterids,
and some scientists believe they could be descended from
them.
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