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.