Cliff Ferns

Historically, the higher classification of ferns has tended to be a bit wobbly. Compared to flowering plants, ferns often offer fewer readily observable features that may offer clues to relationships. As a result, the position of many fern taxa has long been uncertain. One such group is the cliff ferns of the genus Woodsia.

Woodsia scopulina, copyright Jim Morefield.

Cliff ferns, as their name suggests, are commonly found growing on rocks. There are a few dozen species, mostly found in cooler regions of the Northern Hemisphere. A single species, Woodsia montevidensis, extends into South America and southern Africa (Rothfels et al. 2012). They have short creeping rhizomes with a covering of scales and leaves bearing a mixture of scales and hairs. The most distinctive feature of the cliff ferns can only be seen on fertile fronds: the sori (spore packets) are covered by an indusium that is attached to the leaf basally relative to the sori. These indusia are commonly composed of an array of scales or filamentous sections, in contrast to the solid indusia of other ferns.

Underside of pinnule of Woodsia plummerae, showing the filamentous indusia, from here.

Historically, Woodsia has been placed in a family Woodsia with a number of superficially similar fern genera such as the bladder ferns of the genus Cystopteris. However, molecular phylogenetic analyses have disputed the monophyly of such a group. Rothfels et al. (2012) divided the 'woodsioid' ferns between no less than six different families with Woodsiaceae in the strict sense limited to the cliff ferns alone. Though some authors have divided the cliff ferns between multiple genera, an analysis of the group by Shao et al. (2015) found it difficult to reliably distinguish such subgroups and recommended recognition of only a single genus. They did, however, recognise three major clades within Woodsia identified by molecular phylogenetic analysis as distinct subgenera. The type subgenus Woodsia is distinctive among ferns in possessing articulated stems; species of this subgenus are widespread in the Palaearctic region. The subgenus Physematium is mostly found in the Americas and is characterised by bicolored scales on the rhizome. The third subgenus, Cheilanthopsis, is found in eastern Asia with the centre of diversity in the Himalayan region. The rhizome scales are concolorous, and the indusia are solid and globose rather than being composed of individual segments. In some cases in this subgenus, the sori are covered by 'false indusia', indusium-like structures that are formed from inrolled leaf margins rather than being independent membranes.

REFERENCES

Rothfels, C. J., M. A. Sundue, L.-Y. Kuo, A. Larsson, M. Kato, E. Schuettpelz & K. M. Pryer. 2012. A revised family-level classification for eupolypod II ferns (Polypodiidae: Polypodiales). Taxon 61 (3): 515–533.

Shao, Y., R. Wei, X. Zhang & Q. Xiang. 2015. Molecular phylogeny of the cliff ferns (Woodsiaceae: Polypodiales) with a proposed infrageneric classification. PLoS One 10 (9): e0136318.

Matoniaceae: Ferns with a Heritage

Ferns are one of those groups of organisms, like sharks and cockroaches, that are not really as ancient as most people imagine. For all that ferns are indelibly associated in the public conscience with antediluvian imagery of steamy coal swamps and great lumbering reptiles, the dominant fern groups that can be seen today did not arise until the Cretaceous and diversified as part of a flora that would have been largely modern in appearance (Schneider et al. 2004). Nevertheless, there are some fern lineages around today that might be said to have a genuine claim to a more venerable pedigree. One such group is the Matoniaceae.

Matonia pectinata, copyright Ahmad Fuad Morad.

In the modern flora, the Matoniaceae are a small family, including only three or four species in two genera, Matonia and Phanerosorus, found in south-east Asia (Lindsay et al. 2003). The two genera are distinct in appearance and habits. Matonia is found on more or less exposed montane summits and ridges and has pedate fronds with pectinate pinnae radiating from an erect central stipe that may grow well over a metre in height. Phanerosorus is found on vertical limestone walls and has pendulous, branching fronds whose pinnae are simple or more weakly pectinate (Kato & Setoguchi 1999). Both genera have the fronds arising from a long, hairy, creeping rhizome. Lateral veins in the pinnules show one or more bifurcations and in Matonia these branching forks may anastomose with each other to form a reticulate vein pattern. The genera also share features of the reproductive anatomy such as massive, deciduous sporangia.

Phanerosorus major, copyright Wally Suarez.

The fossil record of Matoniaceae indicates that they were far more widespread in the past; indeed, Matonia was illustrated from preserved compression fossils before it was described as a living genus (Klavins et al. 2004). Leaf fossils of Matoniaceae go back to the Late Triassic, and the Middle Triassic stem taxon Soloropteris rupex has been more tentatively assigned to the family (van Konijnenburg-van Cittert 1993). Fossil forms are more similar to Matonia in overall appearance and this is presumed to be the plesiomorphic morphology for the family. A certain resemblance exists between Phanerosorus and younger fronds of Matonia and it seems likely that the former genus evolved from Matonia-like forms by a process of paedomorphosis (Kato & Setoguchi 1998). The family was most widespread during the Jurassic and Early Cretaceous but became extinct in temperate regions of the Northern Hemisphere during the Late Cretaceous. It persisted longer in the Southern Hemisphere, with the stem taxon Heweria kempii known from the Early Tertiary of Australia, but at some point following that it became restricted to its modern localised range.

REFERENCES

Kato, M., & H. Setoguchi. 1999. An rbcL-based phylogeny and heteroblastic leaf morphology of Matoniaceae. Systematic Botany 23 (4): 391–400.

Klavins, S. D., T. N. Taylor & E. L. Taylor. 2004. Matoniaceous ferns (Gleicheniales) from the Middle Triassic of Antactica. Journal of Paleontology 78 (1): 211-217.

Konijnenburg-van Cittert, J. H. A. van. 1993. A review of the Matoniaceae based on in situ spores. Review of Palaeobotany and Palynology 78: 235–267.

Lindsay, S., S. Suddee, D. J. Middleton & R. Pooma. 2003. Matoniaceae (Pteridophyta)—a new family record for Thailand. Thai Forestry Bulletin 31: 47–52.

Schneider, H., E. Schuettpelz, K. M. Pryer, R. Cranfill, S. Magallón & R. Lupia. 2004. Ferns diversified in the shadow of angiosperms. Nature 428: 553–557.

The Dicranophyllales: An Early Branch of the Conifers?

Reconstruction of Dicranophyllum hallei, from here.

Popular works on the fossil record tend to give us a very uniform picture of the Carboniferous period. A watery swamp can be seen covering the landscape, from which large amphibians emerge onto sodden banks. Giant insects hover in the air. The vegetation is dominated by scaly-trunked lepidodendrons and enormous horsetails. The entire scene is primoeval, presenting us with the representatives of a generation of life long gone, whose like we shall never see again. But of course, not all of the Carboniferous world was given over to coal swamps. While the lepidodendrons and horsetails were indeed around, there were also the early representatives of more familiar plant lineages, though some of them may have been a bit difficult to recognise as such.

The Dicranophyllales may have been one such lineage. Though they survived for a long time, throughout the Carboniferous and Permian, and have been found in many parts of the world, they are generally uncommon in fossil deposits. In life, they would have been small trees or bushes, sparsely and irregularly branched (many reconstructions show them hardly branching at all). The branches bore long, needle-like leaves, not dissimilar to pine needles, in a helical arrangement. The longest of these leaves were over 20 cm in length. A single vein ran down the midline of the leaf, but because this was deeply imbedded it is often not visible in fossils. More prominent, and one of the characteristic features of the group, was a pair of deep grooves running the length of the leaf, one on each side close to the margin, containing the stomata (the openings through which planty leafs exchange gases with the surrounding atmosphere). The leaves were commonly branched towards the tips, at least once and sometimes more. The needle-like leaves, protected stomata, and uncommon preservation all suggest that the Dicranophyllales were mostly plants of drier environments (Wagner 2005). In many species, the leaves left a regular-shaped scar when they fell off, giving the trunk and branches an overall scaly appearance.

Reconstruction of a branch of Polyspermophyllum sergii, from Archangelsky & Cúneo (1990). Note the coiled fertile trusses at the ends of some leaves.

The majority of fossils of Dicranophyllales are of vegetative material (branches and leaves) only, and as a result they have mostly been assigned to the single genus Dicranophyllum, possessing the characters described above. Other genera of Dicranophyllales known from the Upper Permian of Russia include Mostotchkia, which differed in that the leaves were generally not branched, and Slivkovia, which had small scale-like leaves appressed to the branch surface in addition to the long needle-like leaves. Slivkovia and the Lower Permian Entsovia also differed from other Dicranophyllales in having a higher number of stomatiferous furrows on each leaf (Meyen & Smoller 1986). Reproductive structures are definitely recognised for only two species, the European Dicranophyllum gallicum, and Polyspermophyllum sergii from the early Permian of Argentina (Archangelsky & Cúneo 1990). Though Polyspermophyllum resembles Dicranophyllum vegetatively, it is distinct reproductively. In both species, the reproductive organs are broadly similar in appearance to the leaves, and occupy positions in the growth trajectory that would otherwise be occupied by leaves. Seeds are borne separately from each other on the female organs, which have been dubbed polysperms. In Dicranophyllum gallicum, the polysperms end in a bifurcation similar to that of a normal leaf, and the seeds are borne attached to the side. Unfortunately, the compressed fossils do not allow us to determine whether they were arranged helically or pinnately. The male organs were similar in organisation to the polysperms (Wagner 2005). In Polyspermophyllum, the polysperms are divided into multiple branches, and the seeds are borne in trusses at the ends of the branches.

Reconstruction of a section of Dicranophyllum gallicum bearing polysperms, from Seward (1919).

The affinities of the Dicranophyllales have been subject to debate. Some authors, such as Archangelsky & Cúneo (1990), have recognised two families in the Dicranophyllales: the Dicranophyllaceae containing all the taxa referred to above, and a second family including the Permian genus Trichopitys. Trichopitys is vegetatively similar to Dicranophyllales, but its leaves are arranged pinnately rather than helically, and its reproductive organs are borne axillary to the leaves rather than replacing the leaves in the growth sequence. As a result, other authors such as Meyen & Smoller (1986) have regarded the similarities between the two families as convergent. It has also been suggested that the Dicranophyllales might be early members of the lineage including the modern maidenhair tree Ginkgo biloba: under this model, the fan-shaped leaves of the ginkgo may be derived from branched leaves like those of Dicranophyllales by fusion of adjoining branches. However, Meyen & Smoller (1986) pointed out that the structure of Dicranophyllales leaves is less like those of a ginkgo that it is like those of early members of the conifer lineage. Some of the Cordaitanthales, a Palaeozoic group of plants related to the conifers, had furrows on their leaves similar to those found in Dicranophyllales. The leaves of Dicranophyllales also bear resemblances to those of early members of the conifers proper. And this is where the question of seed arrangement on the polysperms of Dicranophyllum becomes interesting: if they were helically arranged, then it becomes possible to the Dicranophyllum polysperm as a distant fore-runner of the modern pine cone.

REFERENCES

Archangelsky, S., & R. Cúneo. 1990. Polyspermophyllum, a new Permian gymnosperm from Argentina, with considerations about the Dicranophyllales. Review of Palaeobotany and Palynology 63: 117-135.

Meyen, S. V., & H. G. Smoller. 1986. The genus Mostotchkia Chachlov (Upper Palaeozoic of Angaraland) and its bearing on the characteristics of the order Dicranophyllales (Pinopsida). Review of Palaeobotany and Palynology 47: 205-223.

Seward, A. C. 1919. Fossil Plants: A text-book for students of botany and geology vol. 4. Ginkgoales, Coniferales, Gnetales. Cambridge University Press.

Wagner, R. H. 2005. Dicranophyllum glabrum (Dawson) Stopes, an unusual element of lower Westphalian floras in Atlantic Canada. Revista Española de Paleontología 20 (1): 7-13.