made up a significant portion of the Mesophytic flora.
The Mesozoic is sometimes referred to as the "age
of cycads". Cycadophytes
are gymnosperms that have a superficial resemblance to
the flowering palms. Cydadophyte trunks range from
short and squat to
with a protective layer of persistent leaf bases. Leaves
can be scale-like or in the form of fern-like fronds.
One living and one extinct order are recognized within
both living and extinct belong to the order Cycadales.
Cycads range from the Permian to recent times. All members
of Cycadales are dioecious (they have male and female plants).
Living cycads have cones at the apex of the stem. Pollen
are small and compact. Male gametes
carried by the pollen are flagellated, a primitive characteristic
biloba (Taylor, Taylor, and Krings,
2009, p. 706). Leaf traces arise from the vascular cambium
and spiral upward
the stem before entering the leaf (girdling leaf traces).
Cycadales have a eustele stem. In cross-section a massive
central core of pith is surrounded by vascular
tissue. An outer layer of bark encases the vascular cambium
and pith. Pinnate, frond-like leaves form a rosette at
the top of the trunk. The upper
stem is clothed in scale leaves, which eventually form
a fibrous sheath around the stem. Leaf bases covering
the trunk provide a firm outer layer.
Charmorgia and Lyssoxylon are
two important fossils found in the Chinle formation of
Arizona that belong to Cycadales (Tidwell, 1998, pp 196-197).
Many Mesozoic species had slender branching
forms. Extant short, squat cycad forms
did not appear until the Tertiary (Willis & McElwain,
2002, p. 135).
extinct order Cycadeoidales or Bennettitales range
from the Triassic to the late Cretaceous. In the U.S. this
order is referred to as Cycadeoidales, while in Europe
it is known as Bennettitales. Members of this group bear
a striking resemblance to true cycads.
they were classified as cycads; however, a number of features
have been observed that suggest they are a separate taxonomic
group. In some classification schemes the cycadeoids are
placed in a separate division Cycadeoidophyta.
Cycadeoid or bennittite
cones are embedded among
from the vascular cambium to the leaves (Tidwell, 1998,
p. 196). Fossil cones suggest that the cycadeoid (bennittite)
cones were predominately unisexual early in their evolution
later became bisexual (Willis & McElwain, 2002, p.
137). The fronds of cycadeoids (bennittites) were spirally
at the top of branches. Although the leaves of cycadeoids
look like cycads there are important differences in leaf
epidermal structure. Cycadeoid (bennittite) had eustele
stems with a large pith surrounded by a ring of vascular
tissue. The exterior of the stem was covered with diamond-shaped
leaf bases. One group of cycadeoids had a short barrel-like
trunk crowned with large fronds. A second group was more shrub
or tree-like. Perhaps the most well known cycadeoid genus is
Williamsonia. Williamsonia resembled
a shrub or tree and grew to a height of 3 meters. Williamsonia reached
its greatest diversity during the Jurassic.
The cones of cycadeoids are remenescent of flowers. Some
paleotologists have suggested that the cycadeoids may have
a close evolutionary relationship with angiosperms (flowering
plants), although current evidence makes this tie unlikely.
Both Cycadales and Cycadeoidales most likely evolved from
medullosan seed ferns (Willis & McElwain, 2002, pp. 136-137).
Science Olympiad Fossil Event
The 2015 Science Olympiad Fossil List does not include
P. and Davis, P. (2004). Fossil Plants. Smithsonian
T.N., Taylor E.L. & Krings, M. (2009). Paleobotany:
The Biology and Evolution of Fossil Plants [2nd
Ed]. New York: Academic Press.
W.D. (1998). Common Fossil Plants of Western North
America. [2nd Ed]. Washington: Smithsonian Institution
K.J. & McElwain, J.C. (2002). The Evolution
of Plants. New York: Oxford University Press.