Fossil Genera and Progymnospermopsida

The following points highlight the five important fossil genera which have strengthen the concept of Progymnospermopsida. The fossil genera are: 1. Archaeopteris 2. Aneurophyton 3. Rellimia 4. Tetraxylopteris 5. Protopitys.

Progymnospermopsida: Fossil Genera # 1. Archaeopteris:

The frond-genus Archaeopteris of Pityales is known by about fifteen species, all of which had large fern-like fronds (Fig. 3.1 A). Beck (1960) has demonstrated the organic connection between Archaeopteris and the stem genus Callixylon. Both these genera have been reported from the rocks of the Upper Devonian period of North America, Russia and several other parts of the globe.

The trunk-like fossils of Callixylon attained a length of 8.4 m and a diameter of 1.5 m, and their most prominent structure was the abundance of pycnoxylic secondary wood. The fronds (Archaeopteris) were bi-pinnately branched, and all the pinnae developed in one plane. Some of the fronds attained a length of over a metre. A prominent midrib was absent.

Each pinnule had a single vascular bundle, which soon became dichotomously branched. The fertile fronds had sporangia. The sporangia replaced the pinnules of some of the lower pinnae in the fertile fronds. According to Phillips et.al. (1972) the sporangia in Archaeopteris halliana occurred over the adaxial surface of thrice-dichotomized appendages.

Most of the species of Archaeopteris appeared to be homosporous. A. latifolia, however, was heterosporous and possessed both microspores and megaspores on the same plant. It possessed two kinds of sporangia (Fig. 3.1C). The length of both the kinds of sporangia was almost the same (i.e. about 2 mm) but they differed from each other in their diameter.

The mega sporangia had a diameter of about 0.5 mm. and each possessed about sixteen spores of the size of about 300-350 μ. The narrower microsporangia attained a diameter of only 0.3mm and each bore about one hundred or more spores of the size of the about 30-35μ. Because of the heterosporous nature, Archaeopteris latifolia must have been a pteridophyte and not a seed plant.

Bonamo (1975) demonstrated heterospory in four species of Archaeopteris viz. A. halliana, A. jacksonii, A. latifolia and A.macilenta. In view of such conflicting reports of homospory and heterospory, some species of Archaeopteris may be considered as Progymnosperms while others may be given a full gymnospermous status.

Bonamo (1975) stated that “at present, this must remain only speculation” till some more details are not unearthed.

Scheckler (1978) placed Archaeopteris under Archaeopteridales which shows vegetative and reproductive features of Progymnospermopsida. He demonstrated that genera such as Actinopodium, Actinoxylon, Eddya, Siderella and Svalbardia are usually placed under Archaeopteridales but all these are simply the developmental stages of Archaeopteris. Archaeopteris is, therefore, the only genus of Archaeopteridales.

Progymnospermopsida: Fossil Genera # 2. Aneurophyton:

Krausel and Weyland (1923) collected Aneurophytori of order Aneurophytales from the Upper Devonian rocks of Germany. Goldring (1924) also collected these specimens from the rocks of the same age from New York and named them as Eospermatopteris. As per the rule of priority, the valid name now is Aneurophyton.

The trunks of these plants attained a height of about 13m or more and a diameter of about lm. They had a bulbous base. A loose crown of long fronds was present at the top of the trunk. Each frond attained a length of about 3m. Since the fronds branched in one plane, they appeared like that of ferns. The ultimate branchlets appeared to be dividing dichotomously.

At the tip of certain ultimate branchlets were present ovoid sporangia. Each sporangium was about 6mm in length and had many spores inside. Krausel and Weyland (1926) described the anatomical details of some specimens reported from Germany. A triangular solid protostele with mesarch protoxylems was present in the centre of the trunk.

The pycnoxylic secondary wood was present. The medullary rays were small and uniseriate. Bordered pits were present on the radial as well as tangential walls of the tracheids of the secondary wood. The fronds of Aneurophyton have been interpreted differently by different palaeobotanists.

Arnold (1947) regarded the rachis of the fronds as “stem” and the ultimate Y-shaped pinnules as the “leaves”. Majority of the palaeobotanists, however, call them as ‘fronds’ because the branches in them developed mainly in one plane.

Progymnospermopsida: Fossil Genera # 3. Rellimia:

Rellimia, a member of Aneurophytales, was earlier known as Protopteridium and Melleria. Leclercq and Bonamo (1971) re-described them from Givetian sediments of Goe in Belgium and named as Rellimia. In several of its characters (such as, probably spirally arranged three dimensional branching system, triangular or three-lobed protostele, compact pycnoxylic wood and dichotomizing leaves) Rellimia resembled Aneurophyton.

The fertile organs of Aneurophyton were, however, much smaller (0.5-1 cm) than those of Rellimia (3-4 cm). On the basis of the available morphological details. Rellimia appears to be a progyrrmospermous genus. But in the light of the available meagre anatomical details, nothing can be said with certainty.

All the branches, leaves and fertile organs in Rellimia were spirally arranged. In Protopteridium hostinense, P.pinnatum and P. thomsonii the fertile organs were spirally arranged and they all possessed almost similar complex organisation. Due to this, these three species of Protopteridium were united into a single taxon, Rellimia thomsonii, by Leclercq and Bonamo (1973).

In R. thomsonii the primary xylem was surrounded by compact secondary xylem. Numerous medullary rays were present. Reticulate and scalanform pittings were present on the walls of the tracheids. The sporangia were oval and pointed. Numerous round, trilete pseudosaccate spores were present in each sporangium.

Progymnospermopsida: Fossil Genera # 4. Tetraxylopteris:

Beck (1957) described Tetraxylopteris from the Upper Devonian deposits of New York. It also belongs to order Aneurophytales. The fronds were borne spirally on the stems and were branched three-dimensionally in an opposite decussate manner. The fronds of Tetraxylopteris were more stem-like than those of Aneurophyton.

Some fronds had sporangia on the tips of their ultimate branchlets. The size of all the spores in an sporangium was the same. The sporangia probably lacked annulus. A mesarch, four-lobed, solid protostele was centrally located. It was surrounded by a well-defined zone of pycnoxylic secondary wood. Bordered pits were present on the radial as well as tangential walls of the tracheids of secondary wood.

The fertile branching system, sporangia and spores of Tetraxylopteris schmidtii have been described by Bonamo and Banks (1967). Upcurving sporangial complexes were present on the nodes of the fertile branching system. These complexes were arranged in opposite decussate manner and decreased in size distally.

The mam stalk of each sporangial complex divided thrice to produce four branches, of which each divided again three times. The ultimate subdivisions were arranged alternately and pinnately. The sporangia were oblong or oval and tapered towards the tip. The spores inside the sporangium were spherical, trilete and pseudosaccate. Different types of ornamentations were present on the spores.

The characters, such as the presence of:

(i) Three-dimensional branching system,

(ii) Complex fertile organs, and

(iii) Spherical, trilete and pseudosaccate spores, are common in both Rellimia and Tetraxylopteris.

According to Beck (1976), however, the fertile branching system branched only once in Rellimia and twice in Tetraxylopteris. In Rellimia, the branching was helical, while it was decussate in Tetraxylopteris. Rellimia had a trilobed protostele, while Tetraxylopteris had a four-lobed protostele.

Progymnospermopsida: Fossil Genera # 5. Protopitys:

Protopitys is the only known genus of Protopitales. This stem genus occurred in the Lower Carboniferous period of Palaeozoic era, and is known to the palaeobotanists for about 150 years or so. P.buchiana was a large plant with its trunk attaining a diameter of about 45 cm.

The centrally located elliptical pith was surrounded by a regular but narrow zone of metaxylem. Densely-arranged tracheids and wood rays constituted the secondary wood, which was, therefore, pycnoxylic.

The rays were usually only one cell high and only one or two cells in width. Since the leaf traces originated “alternately from the opposite ends of the ellipse”, the plants of Protopitys must have resembled with Traveller’s Tree of Madagascar (Ravenala madagascariensis). The leaf was probably quite large and frond-like but much is not known about the kind of Protopitys leaf.

Due to several of its unique characters, Protopitys is still treated under Progymnospermopsida. Presence of medullated protostele shows its close relations with ferns (pteridophytes). But some palaeobotanists suggest its affinity with conifers because of the peculiar structure of its secondary wood.

Arrangement of its leaf trace suggests resemblance of Protopitys with Cycadopsida. Because of the discovery of the heterospory in P. scotica, some botanists (Walton, 1957) opined that Protopitys belongs to pteridophyta.

The spores of different dimensions (some with a diameter of 82μ, others 98μ and a few also of 147μ) have been reported in the elongated sporangia of P. scotica. In complexity and form, the fertile organs of Protopitys resemble Rellimia and Tetraxylopteris of order Aneurophytales.