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.

The Urbaum

Reconstruction of Archaeopteris, from Beck (1962).

It appears that it's been over a month now since I last posted anything at this site. I'm not going to go back and check, but I think this may be the longest hiatus that Catalogue of Organisms has been through since I first launched it nearly eight years ago. I have my excuses all prepared: it's been a busy period, what with trips back home to New Zealand, general job-hunting type stuff, and construction work around the house*. Nevertheless, I have had subjects lined up to present here all that time (nothing to do with construction, I promise you), and so I've found myself looking up material on Archaeopteris.

*An enterprise absolutely guaranteed to transform you into mind-breakingly tedious company for everyone else.

Archaeopteris, I hasten to explain, is nothing to do with Archaeopteryx, though certain parallels could be drawn (albeit with a long bow). Archaeopteryx, of course, is the Jurassic fossil genus that has become renowned as the Urvogel, the original bird. Archaeopteris is a much older fossil, coming from the Late Devonian. And if Archaeopteryx is to be known as the Urvogel, then Archaeopteris can claim to be the Urbaum, the original tree. It was not the earliest arborescent plant: the slightly earlier cladoxylopsid (a distant relative of modern ferns) Wattieza reached a height of at least eight metres (Stein et al. 2007). But Wattieza, with a single central trunk bearing a crown of fronds, would have been more similar to a modern tree fern or palm. Archaeopteris, with substantial side branches arising from its trunk, would have been more similar to the classic image of a modern tree.

Section of Archaeopteris branch, from Beck (1962). The globular structures are sporangia.

When it was first described, from its foliage alone, Archaeopteris was also believed to be an early fern. It wasn't until the early 1960s that fossils were described associating the fern-like foliage to large conifer-like logs that had been described from the same period. The entire tree was estimated to reach heights of at least sixty feet (about 18 metres) (Beck 1962). Archaeopteris was not a fern, but a member of the lineage leading to modern seed plants. As well as its overall habit, Archaeopteris resembled a modern tree in the presence of secondary thickening: a layer of cambium (generative cells) around the outer part of the trunk produced new phloem (nutrient-conducting cells) outside itself and new xylem (water-conducting cells) on the inside, thus allowing the trunk of the tree to expand as it grew (compare that to a tree fern, which gets no broader as it gets taller). However, as well as its fern-like foliage, Archaeopteris still resembled a more primitive plant in one very important regard: rather than producing seeds like a modern tree, it still reproduced through spores. Modified fronds produced clusters of sporangia, with at least some Archaeopteris species showing signs of the production of distinct male and female spore types. Whether these spores produced independent gametophytes in the manner of modern ferns is unknown, and likely to remain so: not only would such gametophytes probably be small and unlikely to be preserved, but they would have few if any features to associate them with the lofty trees.

Archaeopteris also exhibited a few other noteworthy differences from a modern tree. Most recent trees are more or less monopodial: they have a central main shoot from which branches arise laterally as adventitious primordia. Archaeopteris' main mode of growth was pseudomonopodial: instead of lateral branches arising de novo, they developed from the uneven division of the central shoot, with one part continuing upwards and the other part turning outwards. Though the end result would have looked broadly similar, there are some different functional implications. Archaeopteris' growth form may have been more constrained than most modern trees. Because branches were produced in the same spiral as leaves, there could have been a certain fractal-ness to Archaeopteris' appearance, with each major branch being something of a miniature of the tree as a whole (albeit a somewhat lopsided one, as at least some species produced larger leaves on the upper side of branches than on the lower side). Also, a purely pseudomonopodial mode of growth would not allow for the replacement of lost branches or other appendages: Trivett (1993) compared this model of the growth of Archaeopteris to "an inflating balloon or an opening umbrella with its increasingly empty interior". At the same time, she presented evidence that Archaeopteris could have produced a certain degree of adventitious growth, though it may still have been less resilient to damage than recent analogues. There is some circumstantial evidence that Archaeopteris may have sometimes shed leaves or minor branches en masse, though whether this was a seasonal occurrence or a response to stress is unknown.

Despite being potentially more vulnerable to damage than a modern tree, Archaeopteris was undeniably successful. Various species of the genus were found pretty much around the world, and were the dominant large plant wherever they were found until their extinction around the beginning of the Carboniferous. Perhaps resilience was simply less of an issue for Archaeopteris than for modern trees. After all, it lived in a world where there would have probably still been no major herbivores, and the main causes of appendage loss would have been the weather or disease. Also, long-term resilience may have simply not been so important for a tree that probably produced spores by the millions every year. Who knows how many Archaeopteris sporelings or gametophytes there may have been at a time, simply waiting their opportunity to provide a replacement for a fallen senior?

REFERENCES

Beck, C. B. 1962. Reconstructions of Archaeopteris, and further consideration of its phylogenetic position. American Journal of Botany 49 (4): 373-382.

Stein, W. E., F. Mannolini, L. V. Hernick, E. Landing & C. M. Berry. 2007. Giant cladoxylopsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa. Nature 446: 904-907.

Trivett, M. L. 1993. An architectural analysis of Archaeopteris, a fossil tree with pseudomonopodial and opportunistic adventitious growth. Botanical Journal of the Linnean Society 111: 301-329.

The Origins of Flowers

Reconstruction of the bennettitalean Williamsonia, a potential stem-angiosperm. Image from Turbo Squid.

I'm going to break one of the supposed blogging rules - I'm going to feed a troll. In the comments thread to the bird evolution post I wrote recently, one commenter brought up the supposedly intractable evolutionary problem of the "sudden" appearance of flowering plants. I briefly responded to this comment at the time, but I thought the question is an interesting enough one to deserve further investigation. So here is my presentation on why the "sudden" appearance of flowers was not so sudden.

The origin of the angiosperms (flowering plants) has long been considered one of the great unsolved questions of biology, and I must confess to having occassionally slipped into the hyperbole myself. However, we actually have some much better ground to stand on than the hyperbole might suggest.

First off, we need to ask what exactly makes flowering plants so distinct? What do they have that no other plant has? I bet some of you are fighting the urge to reply with, "They have flowers. Duh." To which I have to reply - wrong! After all, you could debate to what extent the reproductive structures of many flowering plants can really be called 'flowers'. Many flowering plants lack the petals and/or sepals of more classic flowers. They may have bracts (coloured leaves) instead, like poinsettias or bougainvilleas, while many wind-pollinated angiosperms simply do without ornamentation entirely. And if we argue that petals are not necessary to count as a flower - if those plants that surround their reproductive structures with bracts also count as having flowers - then flowers are not actually unique to angiosperms (as I'll explain in a minute). No, the really significant feature of angiosperms is the carpel, the protective covering of two integuments that encloses the ovule of angiosperms. In other living seed plants, the gymnosperms, the ovules generally have only one integument and are produced exposed on the ends of short branches, often surrounded by a protective whorl of leaves or leaf-derived structures to form a structure called a strobilus (in many conifer groups, these protective leaves have become hard and woody to form the scales of a cone with an ovule at the base of each scale). Morphological and molecular phylogenetic analyses disagree significantly about the relationships between angiosperms and living gymnosperms (Friedman & Floyd, 2001). Morphological analyses place angiosperms nested within gymnosperms, forming a clade with the Gnetales, while molecular analyses place the angiosperms as sister to all living gymnosperms, not closely related to Gnetales.

While there is a significant divide between the carpel-enclosed ovules of angiosperms and the exposed ovules of gymnosperms in living taxa, this divide (unsurprisingly) actually dwindles when we consider fossil taxa. Debate still rages about which fossil taxa are the closest relatives of angiosperms, but two taxa that pop up on a regular basis are the Bennettitales and Caytonia. These taxa are often closely related to angiosperms and the Gnetales in morphological analyses (Doyle, 1998), while if morphological analyses are constrained to match the molecular trees the angiosperms form a clade with Bennettitales, Caytonia and glossopterids (Doyle, 2006). The Bennettitales and Caytonia both put in an appearance during the Triassic and survived until the end of the Cretaceous, while angiosperms are first known from the early Cretaceous (Doyle, 1998). Caytonia is generally described as a "seed fern", which were usually trees, but articulated fossils are fairly rare. It produced multiple single-integument ovules reflexed and contained within a protective structure called a cupule. It does not take a significant leap to imagine the reduction to a single ovule per cupule and the cupule developing into the outer integument of the angiosperm carpel.


(From Frohlich & Chase, 2007) Reproductive structures of fossil stem-angiosperm candidates. a, Glossopteris showing cupules borne on stalk above a leaf. b, Caytonia male (above) and female (below) reproductive units. c, Caytonia cupule. d, Corystosperm (Umkomasia) cupule containing one ovule. Cupule wall almost surrounds ovule, except for a slit facing the stalk. e, Bennettitales (Williamsoniella) bisexual reproductive unit; each oval pollen sac consists of several fused microsporangia. Ovules are borne among scales on the central stalk; in Vardekloftia each is enclosed by a cupule wall. Green, cupule wall; red, ovule; yellow, pollen organ.

Bennettitales were plants fairly similar in appearance to modern cycads that lacked any such carpel-like arrangement and had ovules born along scales in the strobilus. What Bennettitales did have, however, were flowers (of a sort). The leaves of the strobilus were expanded into flower-like bracts that were quite large (and possibly quite colourful) in a number of taxa. Certain features of the bennettitalean bracts suggest that they had a role in attracting insect pollinators, just as modern flowers do today (Gottsberger, 1988). The largest bennettitalean "flowers" were found in Cycadeoidea, which had the bracts recurved to enclose a central chamber containing the reproductive organs. This is of great significance because similar arrangements are found in modern beetle-pollinated flowers, which are believed to be among the more basal flower forms. Also significant is the presence in bennettitalean fossils of the chemical oleanane, derived from a secondary metabolite that is only produced by angiosperms among living taxa, further supporting their relationship (Taylor et al., 2006).

The earliest major pollinators of flowers were probably beetles and flies (Kevan & Baker, 1983). Beetles in particular are the major pollinators of members of basal angiosperm orders such as Magnoliales and Nymphaeales. The two insect groups most commonly associated with pollination in most peoples minds, butterflies and bees, were unlikely to have been significant players in the origin of flowers for the simple reason that neither had come into existence yet - Lepidoptera as a whole only started making an appearance during the Cretaceous, while bees were not to appear until the Tertiary. As already noted, many of the basal angiosperm groups show adaptations towards beetle pollination (this is why magnolias, for instance, produce such a powerful perfume and white flowers - nocturnal beetles use smell more in finding food, while white stands out more at night than colour would). Many beetle-pollinated flowers have some sort of enclosed chamber, or close during the day, providing their pollinators with a safe haven from predators as well as providing food in the form of nectar or pollen (it is quite alright if the pollinator eats some of the pollen so long as the flower produces far more than the pollinator can eat - indeed, if the pollinator is actually going for the pollen then it will almost certainly be rooting around in it and getting covered with it), and this may have been the approach Cyacadeoidea was going for. On the other hand, another basal angiosperm family, the Winteraceae, have open and unspecialised flowers that attract a wide range of pollinators such as beetles, moths, flies and thrips.


Arabidopsis with induced mutation causing leaves to be partially converted into petals. Photo from University of California, San Diego.

Insect-attracting strobili such as found in Bennettitales could have quite easily given rise to the first flowers. Developmental genetics has confirmed the theory put forward many years previously that petals and sepals represent modified leaves, and by affecting the expression of the genes involved it has proved possible to make leaves grow instead of petals, and petals grow instead of leaves (Goto et al., 2001). So while we have still not entirely solved what Darwin so overquotedly referred to as the "abominable mystery", the answer has drawn tantalisingly close.

REFERENCES

Doyle, J. A. 1998. Molecules, morphology, fossils, and the relationship of angiosperms and Gnetales. Molecular Phylogenetics and Evolution 9 (3): 448-462.

Doyle, J. A. 2006. Seed ferns and the origin of angiosperms. Journal of the Torrey Botanical Society 133 (1): 169-209.

Friedman, W. E., & S. K. Floyd. 2001. Perspective: The origin of flowering plants and their reproductive biology - a tale of two phylogenies. Evolution 55(2): 217-231.

Frohlich, M. W., & M. W. Chase. 2007. After a dozen years of progress the origin of angiosperms is still a great mystery. Nature 450: 1184-1189.

Goto, K., J. Kyozuka & J. L. Bowman. 2001. Turning floral organs into leaves, leaves into floral organs. Current Opinion in Genetics and Development 11 (4): 449-456.

Gottsberger, G. 1988. The reproductive biology of primitive angiosperms. Taxon 37 (3): 630-643.

Kevan, P. G., & H. G. Baker. 1983. Insects as flower visitors and pollinators. Annual Review of Entomology 28: 407-453.

Taylor, D. W., H. Li, J. Dahl, F. J. Fago, D. Zinniker & J. M. Moldowan. 2003. Biogeochemical evidence for the presence of the angiosperm molecular fossil oleanane in Paleozoic and Mesozoic non-angiospermous fossils. Paleobiology 32: 179-190.

Of Serpentine Soils

Cape Reinga projects from the north-western end of the Aopouri Peninsula at the very top end of New Zealand. A lone lighthouse stands at the summit of the cape (image at left from Wikimedia), and a venerable pohutukawa tree hanging over the cliffs is pointed out as the very tree from which, in Maori tradition, the spirits of the dead clambered down to the ocean on their way back to Hawaiiki, the mysterious land that was the ancestral point of origin of the Maori to which they returned after their death*. Cape Reinga is a popular tourist spot as the northernmost point in New Zealand. It isn't. If you look carefully at the map at the top of this post (from the Far North District Council), you'll note that at the north-eastern corner of the country, there's a rounded prominence sticking further north. This is the North Cape, and that rounded prominence is the Surville Cliffs.

*There is an unfortunate tendency to refer to 'Maori tradition' as a single unit, when prior to European settlement the different Maori tribes each had their own collection of traditions, agreeing in some points and differing in others. While the concept of a return to Hawaiiki was, I believe, universal among Maori, I haven't been able to find out if this tradition was associated particularly with Cape Reinga for all Maori, or if tribes in other parts of the country identified their own departure points. Apparently more than one Christian missionary, including the memorable William Colenso, tried to have the Cape Reinga pohutukawa chopped down, but these attempts were always rebuffed by local Maori.

So why aren't the tourists all headed for the true northern tip of the country? The Surville Cliffs are part of a conservation reserve (the North Cape Scientific Reserve) that remains closed to the general public. The local Department of Conservation attempts to ruthlessly exclude and/or eradicate introduced taxa from the area, and the primary focus of this protection is a small area of about 120 hectares on the Surville Cliffs and the adjacent plateau that is home to a whole range of plant species found nowhere else on the planet, including Hebe brevifolia (Cheeseman) de Lange 1997, Carex ophiolitica de Lange & Heenan 1997 and Uncinia perplexa Heenan & de Lange, 2001.

Pittosporum serpentinum, an endangered species endemic to the Surville Cliffs area. Seedlings of this species have never yet been observed (photo by Gillian Crowcroft, from New Zealand Plant Conservation Network).

The range of plant species growing in any location is strongly dependent on the soil type. The effect of a change in soil type can be dramatic - sometimes you can practically see a line where one soil abruptly gives way to another. In the case of the Surville Cliffs, the presence of a distinct soil type not found elsewhere in New Zealand is to blame for the unique flora.

The soil at the Surville is serpentine, derived from the exposure of the Tangihua or Northland Ophiolite. Ophiolite forms when part of the sea-floor crust becomes uplifted and integrated into the continental crust. Major ophiolite belts are found in the Alps and the Himalayas where pieces of the oceanic floor between two colliding continental masses have been ripped up and wedged between the fusing continents. In the case of the Tangihua Ophiolite, the rocks that eventually became the ophiolite probably formed in the South Fiji Basin to the north-east of New Zealand (Whattam et al., 2004). They would have then become emplaced onto the New Zealand continental mass with the formation of a subduction zone along the north-east of New Zealand, probably due to the collision of the underwater Hikurangi Plateau with the New Zealand continental shelf further south.



Sketch map showing the disposition of tectonic elements adjacent to Northland. A, Immediately prior to the emplacement of the Northland Ophiolite. B, Immediately after emplacement and as subduction began. SFB, South Fiji Basin; VMFZ, Vening Meinesz Fracture Zone; HP, Hikurangi Plateau. From Whattam et al., 2004.

Ophiolite is very ultramafic rock, meaning it is high in heavy metals such as nickel, iron and magnesium. Soils formed from such rocks are toxic to the majority of plants, which is why they become the preserve of ultramafic specialists. In contrast, most ultramafic specialists do not do well when grown away from their toxic homes (de Lange, 1997; Heenan & de Lange, 2001), which is why the Surville Cliffs flora is so restricted in distribution. The greatest threat to the Surville Cliffs flora is probably invasion by introduced taxa such as Hakea and Cortaderia (pampas grass), though eradication programmes are currently underway to try and reduce the risk from these invaders. Some members of the Surville Cliffs fauna, such as Hebe brevifolia, are present in large numbers and are probably not under immediate threat despite their highly restricted distribution. Others, such as Uncinia perplexa, appear to have always existed in very low numbers, and are seriously endangered.

REFERENCES

de Lange, P. J. 1997. Hebe brevifolia (Scrophulariaceae) - an ultramafic endemic of the Surville Cliffs, North Cape, New Zealand. New Zealand Journal of Botany 35: 1-8.

de Lange, P. J., & P. B. Heenan. 1997. Carex ophiolithica (Cyperaceae): a new ultramafic endemic from the Surville Cliffs, North Cape, New Zealand. New Zealand Journal of Botany 35: 429-436.

Heenan, P. B., & P. J. de Lange. 2001. A new, dodecaploid species of Uncinia (Cyperaceae) from ultramafic rocks, Surville Cliffs, Northland, New Zealand. New Zealand Journal of Botany 39: 373-380.

Whattam, S. A., J. G. Malpas, J. R. Ali, I. E. M. Smith & C.-H. Lo. 2004. Origin of the Northland Ophiolite, northern New Zealand: discussion of new data and reassessment of the model. New Zealand Journal of Geology and Geophysics 47: 383-389.