Sphenozamites, chiefly from French localities, which are referred to
the Cycads because of their similarity to the pinnate fronds of
modern Cycadaceae. In the succeeding Triassic system Cycadean
plants become much more abundant, especially in the Keuper period;
from Rhaetic rocks a still greater number of types have been recorded,
among which may be mentioned Nilssonia (fig. 10),
Anomozamites, Pterophyllum, Otozamites, Cycadites (fig. 11). The species
of Nilssonia shown in fig. 10 (N. compta) is a characteristic member
of the Jurassic flora, practically identical with a form from Rhaetic
rocks described as Nilssonia polymorpha. The large frond of
Cycadites represented in fig. 11 (C. Saportae) is from the Wealden
strata of Sussex, and possibly identical with Cycadites tenuisectus
from Portugal. In addition to these genera there are others, such
as Ctenozamites, Ctenis, and Podozamites, the position of which is
less certain. Ctenozamites occurs chiefly in the Rhaetic coal-bearing
beds of Scania, and has been found also in the Liassic clays of
Dorsetshire and in the Inferior Oolite beds of Yorkshire, as well
as in Rhaetic strata in Persia and elsewhere; it is characterized by
its bipinnate fronds, and may be compared with the recent Australian
genus Bowenia—peculiar among living Cycads in having bipinnate
fronds. Ctenis has been incorrectly placed among the ferns by some
authors, on account of the occurrence of supposed sporangia on its
pinnae; but there is reason to believe that these so-called sporangia
are probably nothing more than prominent papillose cells of the
epidermis. Podozamites (fig. 12) is usually considered to be a Cycad,
but the broad pinnae (or leaves) and their arrangement on the axis
suggests a possible relationship with the southern coniferous genus
Agathis, represented by the Kauri pine and other recent species.
The considerable variation in the size of the pinnae of Podozamites,
as represented by species from the Jurassic rocks in the Arctic regions
and various European localities, recalls the variation in length and
breadth of the leaves of Agathis. With regard to the distinguishing
features and the distribution of the numerous Cycadean leaves
of Mesozoic age, the most striking fact is the abundance of
fronds, which there is good reason to refer to the Cycadales in
Upper Triassic, Rhaetic, Jurassic and Wealden rocks in India,
Australia, Japan, China and elsewhere in the southern hemisphere,
as well in North America, Greenland, and other Arctic lands and
throughout Europe. It is
noteworthy that Tertiary
plant-beds have yielded
hardly any specimens that
can be recognized as
Cycads.
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Map B.—M1-M5, D, G, Distribution of the Matonineae, Dipteridinae, Ginkgoales. | ||||||||||||||||||||||||||||||||||||
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| Fig. 10.—Nilssonia compta. Inferior Oolite, England. |
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Fig. 11.—Cycadites Saportae. Wealden, England. |
A more important question is, What knowledge have we of the reproductive organs and stems of these fossil Cycads? Cycadean stems have recently been found in great abundance in Jurassic and possibly higher strata in Wyoming, South Dakota and other parts of the United States. Cycadean stems have been found also in the uppermost Jurassic, Wealden and Lower Cretaceous rocks of England, India and other parts of the world. An example of an Indian Cycadean stem from Upper Gondwana rocks is represented in fig. 13; the surface of the trunk is covered with persistent bases (fig. 13, A) of the fronds known as Ptilophyllum cutchense, which are practically the same as the European species Williamsonia pecten (fig. 17). In a section of the stem (fig. 13, B) a large pith is seen to occupy the axial region, and this is surrounded by a zone of secondary wood, which appears to differ from the characteristic wood of modern Cycads (see Gymnosperms) in having a more compact structure. It is interesting to find that G. R. Wieland of
