The following points highlight the top five selected examples of Incertae Sedis. The examples are: 1. Vojnovskyales 2. Rhexoxylon 3. Nucellangium 4. Hydropteridangium 5. Czekanowskiales.
Incertae Sedis: Example # 1. Vojnovskyales:
Vojnovskya paradoxa, a gymnosperm of uncertain affinity, has been reported from the Lower Permian rocks of Palaeozoic era of the Pechora basin of the former USSR, and Nevberg (1965) placed it in a new order Vojnovskyales, which is still awaiting a proper placing in any recognized gymnospermous group.
Gaussia, another member of Vojnovskyales, has been reported from Pechora to Kuznetsk basins of former USSR and also from some other parts of the world including Transvaal, Argentina and North America. Sandrewia has been reported from some Permian rocks of North America.
Vojnovskya paradoxa (Fig. 16.1 A) was a large shrub or small tree. Its shoot had many leaf scars, and its leaves were large and flabelliform. Mamay (1976) described the reproductive system of this plant as the most fascinating morphological feature. According to him, the reproductive system consisted of many units of bisexual cone-like groups. These cones were quite compact and were of two types.
They were arranged spirally on a thick central axis. Each cone was attached to a vegetative branch by a slender peduncle. The cone attained a length of about 2.5-3 cm. In the male cones, the microsporophyll’s were present only on their apical part. They were long and slender bodies. At the terminal part of each microsporophyll were present two pairs of microsporangia.
Maheshwari and Meyer (1975), however, did not report any microsporangia in Vojnovskya. The megasporophylls had flat seeds. Each seed attained a length of about 1 cm. The megasporophylls were bilaterally symmetrical and each had a notched apex.
The flattened, pistil-like heads or fructifications of Vojnovskyales have been described as Gaussia. Each fructification had a single basal ovule. It was spathulate at the apex. A long and linear style-like portion was also present in each fructification.
The pollination liquid was filled in the expanded and funnel-like terminal part of the style. Several trichomes were present around the funnel-like style. A ligulate process was also present outside the funnel.
Sandrewia, another Vojnovskyales, has been reported from Lower Permian rocks of North America. Nephropsis-like leaves were present on its shoots. Bifid micropyles were present on the ovules. The ovules were pendulous and two in number on an indented disc.
Great phylogenetic significance is being attached with Vojnovskyales because these are the only Palaeozoic plants with bisexual flowers. Because of the disposition of the megasporophylls and microsporophyll’s and also because of the resemblance of the megasporophylls of Vojnovskaya with those of Liriodendron, Maekawa (1962) proposed Vojnovskaya as possible ancestor of angiosperms.
Some other workers did not agree with Maekawa and proposed that Vojnovskaya was possibly related either with Cordaitales or Bennettitales. Krassilov and Burago (1981) opined that Vojnovskayales descended probably from some lagenostomalean pteridosperms. Mamay (1976) stated that Vojnovskaya is probably a representative of “some bizarre, short-lived group of Late Palaeozoic gymnosperms”.
Incertae Sedis: Example # 2. Rhexoxylon:
Rhexoxylon is a fossil gymnospermous stem with uncertain affinities. Four species of this stem- genus have been reported from different parts of the world. These include Rhexoxylon africanum and R. tetrapteroides from Triassic of South Africa and R. priestleyi from South Victoria Land, Antarctica. Archangelsky (1968) reported R. piatnitzskyi from Triassic of Argentina.
The trunk of Rhexoxylon tetrapteroids (Fig. 16. IB) attained a length of over 3 metre and a diameter of 5-15 cm. It had a large parenchymatous pith with secretory ducts and sclerotic nests. The vascular strands were irregularly arranged. A ring of small steles or strands surrounding the pith constituted the perimedullary system.
The centripetal and centrifugal segments were present in each small stele. The perimedullary steles were surrounded by a ring of radial strands. The secondary wood and a little amount of primary wood were present in the ring of radial strands. Uniseriate medullary rays were present. Circular bordered pits were present on the radial walls of the tracheids.
Regarding its phylogenetic interpretations, Rhexoxylon was first considered as a medullosan stem. The structure of its wood, however, indicates that it was a coniferous plant Stewart (1983) emphasized on the similarities between the vascular organization and leaf-trace ongin of Rhexoxylon pianitzkyi and Pentoxylon, and opined that it had some relation with Pentoxylales.
Anatomical details of the wood of Rhexoxylon tetrapteroides and some modern angiospermic lianes (e.g. Tetrapteris) also provide an indication of some affinities between the two. Though nothing with certainity can be said at the moment, but on the basis of the available details it may be mentioned that Rhexoxylon is a gymnospermous plant still awaiting a proper placing in any well-defined group.
Incertae Sedis: Example # 3. Nucellangium:
Nucellangium, another perplexing genus awaiting a proper placing in the well-defined gymnospermous groups, has been reported by Andrews (1949) from the Upper Carboniferous coal balls near Des Moines (Iowa, USA).
Two types of seeds or reproductive bodies of this genus have been reported i.e. normal and proliferated (Fig. 16.1 C,D). The normal seeds were broadly ovate with a circular scar at one end. Their distal end was blunt and tapering.
An epidermal layer of radially elongated thick-walled cells surrounded the normal seeds. Inner to the epidermis was a layer of thick-walled cells which possessed many oval-shaped pits. The proliferated seeds were larger than the normal seeds. The parenchymatous tissue in these seeds developed into many fingerlike projections towards the centre.
Vascular strands were present in the peripheral part of the parenchymatous tissue. Segal (1968) observed micropyle and nucellus in these bodies and, therefore, these reproductive bodies have been considered as the complex mega-sporangia or true seeds.
Segal (1968) opined that these seed genera belong to Cardiocarpales. On the basis of several ontogenetic studies, Stidd and Cosentino (1976) proposed that Nucellangium was a member of Cordaitales.
Incertae Sedis: Example # 4. Hydropteridangium:
Hydropteridangium marsiloides is a fossil genus of still uncertain affinity. It has been described from the Rhaetic-Liassic rocks of East Greenland by Harris (1931). It has been reported in the form of its microsporangiate organ which possessed a main axis producing several lateral branches in all planes (Fig 16.1E).
The lateral branches also divided in all planes and had swollen capsules at their ultimate tips. Each capsule was a bivalved body, and on the inner surface of each valve were present seven microsporangia (Fig. 16.1F). Each microsporangium was filled with many microspores. The microspores were winged structures.
Formerly, the fossil specimens of Hydropteridangium were considered to belong to Hydropteridinae because they resembled with Marsilea. But Andrews (1961) opined that Hydropteridangium and some allied genera sprang from a common stock with Cycadales and belong to a unique group of gymnosperms.
Incertae Sedis: Example # 5. Czekanowskiales (Czekanowskia):
Czekanowskia, a Jurassic genus, was considered a member of Ginkgoales until 1951. But later on some palaeobotanists have recommended that a new group (Czekanowskiales) should be-created to accommodate Czekanowskia and some related fossil genera. The matter of its ultimate placing in some known gymnospermous groups is still uncertain.
Czekanowskia had leaves on short shoots. The leaves divided dichotomously three or four times into linear segments. Harris (1951) reported that some unique type of reproductive structures were associated with these leaves. Such reproductive structures were unknown in any group of the plant kingdom and were named as Leptostrobus (Fig. 16.1G).
Each reproductive unit consisted of a loose cone with bivalved capsules, resembling the two halves of a cockle-shell Each half had three to five seeds. Czekanowskia leaves along with Leptostrobus reproductive structures were reported in close association in the rocks of Western Siberia, Yorkshire and Greenland
Solenites and Sphenarion are also the foliage genera resembling Czekanowskia and probably belonging to Czekanowskiales. Ixostrobus is thought to be the strobilus of Czekanowskia. I whitbiensis attained a length of about 10 cm and possessed irregular whorls of six to eight coriaceous sporangiophores.
Krassilov (1977) opined that members of Czekanowskiales should be treated as Cretaceous and Jurassic pro-angiosperms but Stewart (1983) proposed that this fossil group should be treated as belonging to Mesozoic gymnosperms. According to Taylor (1981), the macerated megaspore wall of Czekanowskia suggests that its ovules were gymnospermous.