Bennettitales: Classification and Features | Gymnosperms
Contents:
- Origin of Bennettitales
- The Sporophyte and the Flower of Bennettitales
- Characteristic Features of Bennettitales
- Classification of Bennettitales
- Comparison between Williamsoniaceae and Cycadeoideaceae
- Genera of Bennettitales
- Indian Bennettitales
- Affinity of Bennettitales
1. Origin of Bennettitales:
Bennettitales resembled modern Cycads but had a more diversified habit with tuberous or columnar branched trunks with persistent leaf-bases. There was a crown of pinnate leaves. The reproductive organs were borne terminally on short axillary branches arising from the trunk. The strobili were typically bisporangiate unlike those of living Cycads.
The most common fossils among the Cycadophytes are leaves and it is difficult to distinguish the leaves of Mesozoic Cycadales and Bennettitales from their external features. Their stomatal characters are quite distinctive.
The Bennettitales, or the Cycadeoideales, appeared first in all probability in the Triassic flourished very luxuriantly in the Middle or Late Jurassic, and then went on declining in the Cretaceous. It is believed that this order arose side by side with the living Cycadales from a pteridospermous stock. The fossil remains are distributed extensively throughout the United States, Mexico, different parts of Europe, as well as India.
2. The Sporophyte and The Flower of Bennettitales:
The plant body of W. sewardiana is a small, stout tree resembling a miniature living Cycas. The trunk is columnar, bearing a cluster of pinnate compound leaves at its top. Intermingled with the leaves are scales, which leave small scars on the stem. There are lateral shoots, which are constricted at their bases. This suggests that they broke away, and served as propagative shoots. The leaf-bases formed an armour around the stem.
Internally, the pith of the stem is enclosed by a xylem ring. Tracheids of scalariform type are noted, and multiseriate bordered pits are present. The xylem rays are variable from uni- to multi- seriate type. The protoxylem is endarch, and secretory sacs are present in the cortex as well as in the pith.
The Flower of Bennettitales:
Flowers are produced on short-branched lateral shoots, which project beyond the armour of leaf cushion. Each flower is unisexual and ovulate, having a receptacle bearing the stalked ovules intermingled with interseminal scales. The nucellus of the ovule is enclosed by an integument, which prolongs into a micropylar beak.
3. Characteristic Features of Bennettitales:
(a) Vegetative Organs:
1. Plants were un-branched trees, usually 1-2 metres in height, with compressed internodes and ramentum, bearing a crown of large pinnately compound leaves on the top.
2. The trunks were covered with an armour of persistent leaf-bases.
3. The outer walls of the epidermal al cells are sinuous.
4. Guard cells and subsidiary cells are syndetocheilic, i.e., are produced by the division of the same initial.
5. The anatomical structure of the stem shows a thick cortex, a broad pith and narrow vascular cylinder composed of collateral bundles with exarch xylem.
(b) Spore-Producing Organs:
1. Flowers unisexual or bisexual in strobili.
2. Microsporophylls frond-like forming a loose cone.
3. Megasporophylls are not leaf-like, but found in specialised cone-like structure
4. Classification of Bennettitales:
Bennettitales are divided into three families, viz.,
(1) Williamsoniaceae,
(2) Wielandiellaceae and
(3) Cycadeoideaceae.
(1) Family. 1. Williamsoniaceae:
Plants are all older and primitive in age, mostly Triassic and Jurassic. Flowers were borne on relatively slender stems or pedicels and fully exposed. Materials are preserved as compressions.
Genera:
Williamsonia (whole plants, male and female flowers), Ptilophyllum (fronds).
(2) Family. 2. Wielandiellaceae:
These plants are also primitive and older like Williamsoniaceae. The chief difference is that their trunks are not covered by leaf armours as in Williamsoniaceae. Branches are slender and dichotomously branched.
Genus:
Wielandiella, Williamsoniella (fronds).
(3) Family. 3. Cycadeoideaceae:
Plants are younger and more advanced, mostly Cretaceous. The trunk is short, ovoid or columnar with an armour of leaf-bases. Flowers are embedded within leaf- base layer.
Genus:
Cycadeoidea (= Bennettites).
5. Comparison between Williamsoniaceae and Cycadeoideaceae:
The valid name for Bennettitales is Cycadeoideales as explained by Arnold in 1947, but the former name is still frequently used by many workers. They were the characteristic flora of the Mesozoic era, and were distributed throughout both the hemispheres. They had many resemblances with Cycadofilicales and Cycads in their general habit.
6. Genera of Bennettitales:
The following genera of Bennettitales are found in India:
i. Williamsonia:
Several species of Williamsonia are represented in India. Of these W. sewardiana is the most completely worked out species, which is found in the Rajmahal Hills as reconstructed by Prof. Sahni.
Other Indian species of Williamsonia are W. indica, W. microps, W. blanfordi, etc. But, the vegetative parts of these species are not well preserved.
ii. Ptilophyllum:
It is a leaf genus belonging to Williamsonia and occurs in the Rajmahal and Jabalpur series of India.
The two widely distributed species are:
a. P. acutifolium and
b. P. c.f. cutchense.
The leaf is pinnate having the pinnae linear, straightly falcate with parallel or sub-parallel veins. The pinnae are characterised by their attachment to the upper surface of the rachis covering it almost completely. The base of the lamina is rounded.
iii. Bucklandia:
This is stem genus belonging to Williamsonia. It has only one species, viz., B. indica. It is regarded as the stem of Williamsonia.
iv. Otozamites:
It is also a leaf genus belonging to the Family Williamsoniaceae, and occurs in Upper Gondwana, Rajmahal and Jabalpur series. There are four species of the genus, viz., O. bengalensis, O. parallelus, O. pecten and O. bunbaryanus. The leaves resemble those of Ptilophyllum but differ in the points that the pinnae have acute base and also spreading venation.
v. Dictyozamites:
It is also a leaf genus belonging to the Family Williamsoniaceae and occur in the Upper Gondwana of India. It has two Indian species D. falcatus and D. indica. The leaf resembles Otozamites, but differs mainly in anastomosing veins.
7. Indian Bennettitales:
The group is well represented in the Rajmahal Hills and other Upper Gondwana localities in India. No member of Cycadeoidaceae is known from India. Williamsoniaceae is represented in India by several species of Williamsonia. W. sewardiana found in Rajmahal Hills is the most completely known species.
This, as reported by Sahni is a small tree with a columnar trunk, covered with scars of foliage and scale leaves and a crown of ;slender pinnate leaves. Female flowers only are known and they are borne on short lateral branches, which are covered with leaf-bases and scales.
The flower consists of a conical fertile portion enclosed by linear heavy bracts and scales. The ovules are borne on long stalks and are surrounded by club-shaped interseminal scales. The trunk of W. sewardiana is known as Bucklandia indica and its leaves Ptilophyllum acutifolium.
Other Indian Williamsonias are not well preserved and their vegetative parts are unknown, e.g., W. indica, W. microps, W, blanfordi. Microsporangiate Williamsonias are W. sahni, W. santalensis, while W. sewardiana are unisexual. W. santalensis might be unisexual or bisexual.
Bucklandia has a compact secondary wood, uniseriate meduallary rays and tracheids have multiseriate bordered pits.
Five genera based on foliage are known from India, viz.,
a. Ptilophyllum,
b. Pterophyllum,
c. Zamites,
d. Otozamites and
e. Dictyozamites.
Ptilophyllum have linear leaves with pinnae attached to the upper face of the rachis, covering it almost completely. Base of pinna is rounded. Veins are parallel.
8. Affinity of Bennettitales:
Bennettitales have some resemblances with Ferns, Cycads and Angiosperms.
(a) Affinities with Ferns:
1. Lateral branching.
2. Ramentum present.
3. Direct leaf trace.
4. Presence of synangia.
5. Scalariform thickening.
(b) Affinities with Cycads:
1. Fronds of cycadeoidea ingens similar to enfolding of Encephalartos, macrozamia and Bowenia, otherwise frond character is different.
2. In general habit, to some extent, and anatomy Cycadeoids resemble Cycads, but no girdling of leaf traces and absence ofmedullary rays.
3. Probable swimming sperms.
4. Similar seeds.
(c) Affinities with Angiosperms:
Parkin’s theory is that, in both the groups flowers are alike, because:
1. Connection with sympetaly owing to common endarch siphonostele, etc.
2. Connection with Magnolias, because of floral analogies, but not true relations.
(i) Cycadeoidean bracts beneath stamen ring, homologous to the perianth.
(ii) Larger number of stamens on the staminate disc.
(iii) Cone-shaped ovuliferous receptacle bearing cluster of separate fruit-like structures.
(iv) Tracheids scalariform in all Cycadeoidaceae and most Williamsoniaceae, which is characteristic of Magnolia.
In certain respects the Cycadeoidales shows affinities with both the Pteridosperms and the Cycadales, particularly regarding the general habit, structure of wood, mesarch leaf trace, foliage and structure of the seed. Angiospermous affinities can also be considered by comparing their strobili with the flowers of Magnolia, but the similarity is only due to parallel evolution.
The relationship is drawn with the ferns in having the ramentum, synangia, and direct leaf traces. Hence, this order represents a combination of characters of Cycadofilicales, Cycadales, and Filicales along with some characters of its own.
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