Anatomy, Ontogeny & Ecology of Carboniferous Lepidodendrids

Anatomy, Ontogeny & Ecology of Carboniferous Lepidodendrids

Postby Jennifer_P » Thu Jan 24, 2008 2:25 am

Well, in case you're wondering where I've been hiding lately, this is it: the wonderful world of the lepidodendrids! I just turned this report in at 11:59 tonight, one minute ahead of the deadline (I HOPE) and now I might as well squander my free time here on the GoW forum.

Um, don't bother reading this report unless you're a botanist, or you like looking up every other word in a sentence (I know I sure didn't enjoy it...). It's meant to be more of a personal information cache on the Carboniferous for me than a report for you all--but no worries, I'll share a lot of this data later in a less technical format so that you don't have to keep an extra browser window open just to look up all the botany jargon. :roll:

Anatomy, Ontogeny, and Ecology of Carboniferous Lepidodendrids

Introduction
Lepidodendrids are extinct, arborescent lycopods which thrived during the Carboniferous period, a time of swamps and moisture. This environment was ideal for the well-adapted lepidodendrids, and as a result their genera rose to a level of prominence and dominance which they have not known since. While these arborescent lycopods later suffered extinction when the climate shifted and their habitat shrunk, their common fossilized remnants still have much to say about their anatomy, ontogeny, and ecology.

The Carboniferous
The Carboniferous period lasted from about 360 to 286 million years ago (the exact dates are disagreed upon). Due to the extensive amount of time this encompasses, the period is split into two subperiods (the Mississippian and the Pennsylvanian) at the 320 mya mark, with the Mississippian comprising the Lower Carboniferous and the Pennsylvanian the Upper (Levin, 1996). Lepidodendrids, comprising mainly Lepidodendron, Lepidophloios, Diaphorodendron, Paralycopodites (also known as Anabathra), and Sigillaria, appeared around the beginning of the Mississippian and increased until the Late Pennsylvanian, when their numbers dropped significantly and they were driven to near extinction in North America. The lepidodendrids lived in tropical swamplands where soft inorganic (clastic) or organic (peat) substrates were found; in addition, distributions of individual lepidodendrid species appear to have been dependent upon edaphic and soil drainage factors as well as disturbance frequency (Phillips and DiMichele, 1992).

Anatomical Structure
A lepidodendrid is composed of five basic parts: Stigmaria (the rooting system), trunk, branches, cones, and microphylls (simple leaves). While these parts often serve the same functions as those of modern angiosperms and gymnosperms, in many respects their functions are unique, with no equivalent among extant tree species.

According to Rothwell and Pryor (1991), the form genus Stigmaria describes the rooting system of members of the families Lepidodendraceae and Sigillariaceae. However, while Stigmaria served much the same purpose as do the roots of modern plants, it is not a true root; rather, it is a modified shoot adapted for underground existence.

The internal tissues of Stigmaria include a central hollow pith region with a perimedullary pith cylinder surrounding it. This hollow center is part of a structure similar to a dictyostele, in which separated vascular bundles containing central xylem encompassed by phloem surrounded by an endodermis are arrayed in a ring formation. The stelar structure consists of tracheids of xylem arranged radially around a central axis. Xylem maturation is endarch, with the protoxylem being produced centrally and the metaxylem abaxially (Rothwell and Pryor, 1991). Around this, a periderm enclosed Stigmaria to serve a supportive and protective role. Unlike the trunk periderm, this periderm was of a more pliable character, which suited the lesser support requirements of Stigmaria (Cichan, 1985). At its apex, Stigmaria possessed what appears to be a mass of parenchymatous tissue in place of the rootcap which is characteristic of most root systems today (Phillips and DiMichele, 1992). Stigmaria graded into the trunk tissues at the basal region of the pole.

In terms of overall morphological structure, Stigmaria is generally blunt-tipped, thick, dichotomously branching and probably lacks secondary xylem growth (except towards the basal end). A stelar groove paralleling the stele runs down down its length on the outer surface, and spirally arranged rootlets branch off of the main dichotomous appendages. Unlike the roots of most plants, Stigmaria retains a fairly constant width over its length, a growth formation which has been explained either as the result of a unifacial, determinate vascular cambium which develops new xylem cells adaxially or as the result of a primary thickening meristem. The latter option is the more likely alternative, as histological evidence of the postulated cambium is lacking and the morphological data collected by Rothwell and Pryor (1991) describing the constant diameter of Stigmaria point towards a primary thickening meristem (PTM) as the likely agent. If a vascular cambium were responsible for stigmarian expansion, then additional tissues would have been added basipetally and the stigmarian axis would inevitably be expanded towards the proximal end—which is not the case. Nonetheless, some secondary thickening of the stigmarian system did occur—for otherwise the entire basal girth of Stigmaria would have been required to be produced when Stigmaria began growing, a feat which would obviously be impossible for a young sporophyte. One possible scenario is that secondary thickening by means of a cambium was responsible for the more proximal girth increase, while towards the distal end of Stigmaria a PTM would have maintained the constant girth. Between the proximal and distal ends, secondary thickening could be gradually replaced by the activity of the PTM, resulting in a uniform girth down the length of Stigmaria (Rothwell and Pryor, 1991).

Interestingly, the hollow pith region present in Stigmaria apparently contained sufficient air so that the rootlets would have been able to achieve buoyancy. Connected with this concept is the fact that it may have been possible for Stigmaria to produce its own food by photosynthesizing underwater (Phillips and DiMichele, 1992). The reasons behind this speculation are twofold: firstly, it has been postulated that since lepidodendrids were unable to produce secondary phloem (food transport tissue) due to the fact that their vascular cambium was unifacial, they might therefore have produced photosynthates locally. In other words, the microphylls would supply the area near their own locations, the pole would feed itself with its leaf scars and interareas, and Stigmaria would feed itself by means of rootlet photosynthesis, with photosynthate sharing between the areas via secondary phloem being less important than in extant tree species. The second reason that it might have been possible for Stigmaria to photosynthesize is that its rootlets were in a location suitable for it—assuming, of course, that the surrounding water was sufficiently clear. The uppermost of Stigmaria’s thin, relatively short spirally arranged rootlets would break out of the mud below and reenter the water column, where it might have been possible for light to penetrate through to them and would have allowed them to photosynthesize if they possessed that capability. It is also possible that Stigmaria may have collected CO2 accompanied or unaccompanied by photosynthesis, either from the swamp water or by respiration (Phillips and DiMichele, 1992).

The trunk of a lepidodendrid was probably also photosynthetic. The trunk was covered with leaf scars produced by the abscission of the microphylls, a feature which earned the Lepidodendron the nickname “scale tree” (Wikipedia, 2007) due to the repetitive diamond shape of its leaf scars. Members of the family Lepidodendraceae exhibit spirally arranged leaf scars, while members of Sigillariaceae have leaf scars arranged in vertical rows (Hieb, 2006). The leaf scars and their spacing could be variable within a species depending upon environmental and other factors (Wnuk, 1989). The microphylls themselves (leaves distuinguished by paucity of phloem and supplied by a single vascular bundle (Phillips and DiMichele, 1992)) varied significantly in size depending on species, and could range in dimensions from centimeters in length to three-quarters of a meter long (Andrews and Murdy, 1958). Like the microphylls, the scars and their interareas possessed stomata and cuticle, which would seem to indicate that they were capable of photosynthesis. If both the stigmarian system and the pole were capable of photosynthesis, then the open canopy produced by the lepidodendrids’ nonshadowing methods of branching and leafing would make sense in context with the necessity of exposing their lower reaches to sunlight (Phillips and DiMichele, 1992).

As mentioned earlier, the trunk possessed a unifacial vascular cambium which produced secondary xylem. In order to continue the production of secondary xylem while dealing with the increase in girth that this xylary production created, it appears that lepidodendrids increased the size of their fusiform initials (lateral meristematic cells) to compensate (Cichan, 1985) for their increase in trunk circumference. The number of initials that they possessed also changed; in the case of Paralycopodites, there were initially many fusiform initials, but then as the tree's circumference increased the number of fusiform initials decreased. Eventually the decrease decelerated until a more or less fixed amount of initials was achieved.
The trunk's vascular cambium produced secondary xylem adaxially but no secondary phloem abaxially. It should not be assumed that this secondary xylem was extensive throughout the tree; in fact, the amount produced was rather small in comparison to the amount that extant tree species produce (Phillips and DiMichele, 1992). The fact that the limited xylary tissue (which occurred primarily in the form of wide tracheids with thin walls) was an excellent water conductor helped make up for the paucity (Cichan, 1985) of secondary xylary.

In comparison to the smaller and less significant secondary body of the tree, the primary body constituted much of a lepidodendrid’s eventual size, which leads to the conclusion that a correspondingly large primary meristem was present (Andrews and Murdy, 1958), possibly working in concert with a primary thickening meristem (Phillips and DiMichele, 1992). For this reason the lepidodendrids would change from short, thick poles into tall, slightly thicker poles, rather than growing from slim, short trunks into thick, tall trunks as dicot angiosperm tree species do (Phillips and DiMichele, 1992). In this respect, lepidodendrids are reminiscent of monocot palm trees, which maintain a cylinder of constant diameter with an only slightly tapering apex (Campbell, 1987).

Lepidodendrids relied on periderm rather than xylem for their main source of support, and this periderm was developed both in their stigmarian system and in their pole. In modern tree species, the fact that the xylem serves both supportive and conductive roles leads to a conflict in which a plant is forced to use xylem for one purpose at the expense of the other. By contrast, the lepidodendrids were able to avoid this problem by using their periderm for support instead of their xylem, which ties into the fact that they had so little xylem and it was so efficient. They no longer needed large masses of necessarily poorly conducting xylary tissue for their support system, and instead were able to get by with smaller amounts of efficiently conducting xylary tissue with support via their periderm. In the pole, the periderm developed beneath the leaf cushions from a bifacial cambium and was probably alive for most of the lepidodendrid’s lifespan. The periderm development also outlasted the secondary xylem development, with the periderm extending further up the pole and even into some of the branches (Phillips and DiMichele, 1992).

The periderm was divided into an inner and an outer region, with the discreteness of the transition between the two regions dependent upon the meristem of the particular genus of lepidodendrid. In all of the lepidodendrids encompassed in this report except Diaphorodendron, the transition was gradual with the entire periderm exhibiting a dense, easily-preserved aspect; in Diaphorodendron, however, a sharp division is observable between the inner and outer periderm, with the outer zone being dense and easily preserved and the inner zone changing gradually into the cortical parenchyma. The outer, stronger and more resistant periderm provided most of the support for the pole, in addition to serving as a sort of bark if the need should arise (Phillips and DiMichele, 1992). The periderm cells may have been sclerified (Cichan, 1985).

Adapical diminuition occurred in the trunk and the branches for both pole-like trees such as Sigillaria and Paralycopodites or for trees with ramified lateral branching such as Diaphorodendron and again, Paralycopodites, or for trees with terminal, dichotomously branching crowns such as Lepidodendron and Lepidophloios. This shrinking of the quantities of xylary tissues, phloem, periderm, etc. and the thinning and shrinking of leaf blades and their corresponding leaf scars is associated with the maturing of the tree and the end of the determinate growth which is characteristic of lepidodendrids as regards their Stigmaria, branching, and pole length (Phillips and DiMichele, 1992).

Branching in the lepidodendrids existed mostly for the purpose of holding cones (with a possible exception being one of the Diaphorodendron species). Accordingly, the branches of the lepidodendrids were generally small and short. All branching appears to have been dichotomous, with both isotomous and anisotomous types permitted. In some species, notably those in the genus Lepidodendron, the branches appeared only at the end of the tree’s determinate lifespan as a branching crown bearing cones; otherwise, the Lepidodendron lived as an unbranched pole for most of its growth period. Depending on species, cones could be borne laterally on the branches or terminally. The branches bearing cones were generally abscissed once their ability to support cones was at an end (Phillips and DiMichele, 1992).

Branches were not the only things to die when cone bearing was completed. Some lepidodendrids also exhibited monocarpy, in which the trees would die after producing cones. Other lepidodendrids practiced polycarpy, in which they were able to produce cones multiple times over their lifespans (Phillips and DiMichele, 1992).

Ontogeny
As Phillips and DiMichele (1992) have noted, lepidodendrid growth was determinate, with the trees attaining a certain set size within a maximum lifespan of 10 – 15 years and then dying. Lepidodendrids practiced both heterosporous and homosporous reproduction, depending upon the particular species, and so these giant trees began their life cycles as humble haploid spores released from the cones high above the forest floor. The spores would settle to produce the gametophyte generation, and eggs produced by the archegonium and sperm produced by the antheridium would combine to produce the diploid sporophyte which would eventually develop into a tree-sized lepidodendrid.

There has been some speculation as to the early development of the sporophyte as relates to the coordinated growth of the stigmarian system and the pole (Phillips and DiMichele, 1992), which developed in a bipolar manner (DiMichele and Bateman, 1996). To prevent the pole from simply tipping over as it increased in height, it is natural to assume that Stigmaria must have developed sufficiently to anchor the thick pole in the upright position, and that as a result coordination between the pole and Stigmaria must have been particularly close during the early growth. The extent of the stigmarian system during early pole formation would have depended heavily on whether or not Stigmaria was photosynthetic. If it was, then it would be not only natural but also possible to first develop an extensive stigmarian support system and then to grow the thick central pole upwards, for Stigmaria could support itself both physically and photosynthetically during the early development phase without need of photosynthetic help from the pole (Phillips and DiMichele, 1992).

After the pole had been developed, lepidodendrids practiced different branching habits, if they practiced branching at all. As noted previously, Lepidodendron spent most of its life as an unbranched pole; the microphylls grew directly out of the trunk, and as the pole increased in height the lower microphylls were abscissed, leaving the tree with a tuft of grass-like leaves at the apex and leaf scars down the rest of the trunk. Upon reaching a predetermined height, an anisotomous dichotomous crown bearing laterally emplaced cones developed. Lepidodendron was monocarpic, and after the spores had been released and the determinate growth was at an end, the tree died (Phillips and DiMichele, 1992).

By contrast, Diaphorodendron (once included in Lepidodendron genus, but since segregated) had both polycarpic and monocarpic forms and produced lateral branches which were either abscissed as the tree developed or possibly retained, depending upon the particular species. One of the polycarpic species utilized a long term but low key reproductive strategy, producing few reproductive materials but doing so over a long period of time. Another polycarpic species made an investment into its wood and branching systems, suggesting a lifespan longer than the ordinary for lepidodendrids. The monocarpic species resembled Lepidodendron in its growth habits (Phillips and DiMichele, 1992).

Sigillaria, for the most part, did not practice branching. A few scarce branches sometimes decorated the apex of this pole-like tree near the end of its determinate growth, and sometimes the apex would fork into two thick dichotomous tips. The pedunculate, polycarpic cones were borne in rings on the trunk or on the lateral branches (DiMichele and Bateman, 1996). The cones may have fallen from the tree and the reproductive contents would then have been spread by mechanical forces (Phillips and DiMichele, 1992).

Paralycopodites, one of the more primitive of the arborescent lepidodendrids (DiMichele and Bateman, 1996), had a pole-shaped trunk with short branches which grew opposite one another near the apex of the tree. Older branches were abscissed once their terminally borne cones had served their purpose. Like Sigillaria, the apex of Paralycopodites would split into a thick, dichotomous fork, a development which occurred towards the end of Paralycopodites’ life cycle. Cone production was polycarpic and occurred on a continual basis in great quantities (Phillips and DiMichele, 1992).

Lepidophloios, like Lepidodendron in many respects, probably lived a monocarpic lifestyle. At the end of its pole lifecycle, it produced an isotomously branching crown upon which pedunculate lateral cones hung (Phillips and DiMichele, 1992). This genus is distinguished by its leaf scars, which are wider than they are tall (Andrews and Murdy, 1958).

Ecology
Since the lepidodendrids were the largest and most prominent tree species in the Carboniferous forest, it may seem natural to assume that they represented the climax species. However, since they were not built to shade over the lower plants, they were in fact seral, and could be replaced by other plants. Each lepidodendrid genus has a preferred substrate type, some preferring one which was clastic and inorganic, others preferring one that is peat-rich and organic, and still others being ecotonal and preferring a mixture of the two types. In addition, the lepidodendrids had preferred levels of environmental stability. The swamp forests were subject to disruption events including fires, high winds, or marine flooding which left opportunities for colonizing lepidodendrids. Other lepidodendrids fared better under more stable, established conditions (Phillips and DiMichele, 1992).

Lepidodendron apparently preferred to grow in a clastic, nutrient-rich substrate rather than in nutrient-poor peat, although it utilized both environments. It was well adapted for life in standing water, a niche which it was able to dominate quite successfully. Lepidodendron could also grow in peat, provided that a river was present to supply the nutrients that it required (Phillips and DiMichele, 1992).

Diaphorodendron was not as particular about where it grew. Its mono- and polycarpic forms could and did occupy both peat and clastic swamp environments. More important for them was the level of disturbances in their environment. As mentioned earlier, the polycarpic form was long-lived, and so these kinds preferred areas which would permit them to live out their lifespan—i.e., areas seldom disturbed. Oddly, the monocarpic form preferred to settle in areas which had been disturbed, then nevertheless have lived out a long life (Phillips and DiMichele, 1992).

Sigillaria preferred a clastic substrate, although it could live in a peaty one as well. They apparently did well in the relatively drier areas of the wetlands, and preferred areas with good ground cover, or around stream channels (Phillips and DiMichele, 1992).

Paralycopodites was an ecotonal species adapted for “life on the edge.” This particular edge was the division between clastic and organic substrates, and such an environment would be especially suitable for this lepidodendrid if had been recently disturbed. Its prolific production of cones made it an excellent colonizer. It apparently did not share Lepidodendron’s preference for standing water (Phillips and DiMichele, 1992).

Lepidophloios occupied the peat swamps, and like the Lepidodendra, their counterparts in the clastic swamps, they could grow in standing water environments. They required a habitat that would remain undisturbed throughout their lifespan, effectively excluding them from the more disrupted areas of the coal swamps (Phillips and DiMichele, 1992).

The lepidodendrids were successful in dominating the wetland areas of the Carboniferous with a variety of anatomies adapted for lifestyles suited to their edaphic preferences. These adaptions were the key which brought about the rise to prominence of the arborescent lycopods.


References

Levin, H., 1996, The Earth Through Time: USA, Saunders College Publishing, 606 p.

Rothwell, G.W., Pryor, J.S., 1991, Developmental dynamics of arborescent lycophytes -- Apical and lateral growth in Stigmaria ficoides: American Journal of Botany, v. 78, p. 1740-1745

Wikipedia, 2007. Lepidodendron. http://en.wikipedia.org/wiki/Lepidodendron

Phillips, T.L., DiMichele, W.A., 1992, Comparative ecology and life-history biology of arborescent lycopsids in Late Carboniferous swamps of Euramerica: Annals of the Missouri Botanical Garden, v. 79, p. 560 - 588

Hieb, M., 2006. Fossil Plants of the Middle Pennsylvanian Period. http://www.geocraft.com/WVFossils/Lycopods.html

Campbell, N.A., 1987, Biology: U.S.A., Benjamin/Cummings Publishing Company, 1101 p.

Cichan, M.A., 1985, Vascular cambium and wood development in Carboniferous plants I. Lepidodendrales: American Journal of Botany, v. 72, p. 1163-1176

Wnuk, C., 1989, Ontogeny and paleoecology of the Middle Pennsylvanian arborescent lycopod Bothrodendron punctatum, Bothrodendraceae (Western Middle Anthracite Field, Shamokin Quadrangle, Pennsylvania): American Journal of Botany, v. 76, p. 966-980

DiMichele, W.A., Bateman, R.M., 1996, The rhizomorphic lycopsids: A case-study in paleobotanical classification: Systematic Botany, v. 21, p. 535-552

Andrews, H.N., Murdy, W.H., 1958, Lepidophloios--and ontogeny in arborescent lycopods: American Journal of Botany, v. 45, p. 552-560
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