Early to Middle Devonian – Plants and Animals

Australia: The Land Where Time Began

Early to Middle Devonian – Plants and Animals

Plants evolved to be more complex forms during the early parts of the Devonian, at least 3 new major groups arising at this time such as the zosterophylls and trimerophytes, both of which are now extinct, while other became more diverse, such as lycopods and club mosses, both of which are extant. Branching stems with small structures that were leaflike, rather than the simple stalks of earlier forms, and some grew to 2 m tall. Pertica, a trimerophyte, was one of the tallest. Lycopods were restricted to wet locations, though some evidence has been found that suggests that among them some had specialised to grow in different habitats. Clack¹ suggests they may have grown densely enough to stabilise stream banks, which would limit flooding (DiMichele & Hook, 1992). Evidence has been recovered from some sites where such plants could have been present in coastal habitats, in locations of either brackish water or those that were flooded periodically by marine incursions (Hotton et al.,. 2001).

Water containing silica has preserved flora and fauna spectacularly well by mineralising them at a locality in Scotland, the Rhynie Chert Formation, which has provided an immense amount of information about life in the Early Devonian. This site seems to have been a wetland in a hydrothermal basin, and several types of habitat, such as flood plains, lakes and marshlike areas are represented. Comparable information about the Middle Devonian has been provided at site in Gilboa, New York State, which is slightly younger.

It is known from these and a few other localities from the Middle Devonian that there were a number of arthropods living among these marginal plants. Among these millipede-like arthropods, centipedes, scorpions, pseudoscorpions, mites, and trigonotarbids that were large and predatory (Rolfe, 1980). Triops, the tadpole shrimp, and Artemia, the brine shrimp, close relatives of modern crustaceans, have been found in the Rhynie Shale, though these were purely aquatic. Gilboa, from the Canadian Early Devonian, has produced the first fossil spiders that are known to have produced silk. Also found in the Rhynie deposits are springtails, which are wingless hexapods, and recently work has shown that remains of true insects, possibly including insects with wings, though that suggestion is still speculative, were also present (Engel & Grimaldi, 2004). The fossil record is generally poor throughout the Devonian. According to Clack¹ most arthropods were predators that appear to have fed on each other, though some plant fossils have been found that have leaves and stems that are damaged, which is considered to be indirect evidence of a limited amount of herbivory among the arthropods. It has also been suggested the damage on the plants may have occurred after the fossils plant parts had become part of the detritus. Clack¹ suggests that true herbivory may not have arisen until the Early Carboniferous.

The climate was warm and seasonally dry, in at least some parts of the world, during the early and middle parts of the Devonian, and it was at this time that the land first came to be dominated by large plants. Bushy, shrublike forms and small treelike statures, which included primitive fernlike types, primitive ancestral types of the conifers and horsetails of the present, were achieved by several groups of plants in the Middle Devonian. Treelike morphologies were attained by lycopsids and progymnosperms, such as Archaeopteris, the latter growing up to 18 m tall, for the first time. It is suggested that a significant canopy layer would have been formed by these trees which would have provided shade and shelter for the early terrestrial animals. The island of Santiago (San Salvador) in the Galapagos Islands of the present the habitat was dominated by ferns and lycopsids, miniature representatives of the sort of landscape that would have existed in the Middle Devonian.

Around the world the appearance of land surfaces underwent unprecedented changes in the Middle and Late Devonian. There was an expansion in size, both height and the average diameter of the axis, taxonomic and morphological diversity, and habitats and area of ground cover that was occupied by land plants. The entire global system was profoundly affected by these changes (Algeo et al., 1995; Algeo et al., 2001; Algeo & Schleckler, 1998). The rate of removal rate from the atmosphere of CO₂ was increased by the increase of plant productivity, which led to a lowering of the temperatures globally and ultimately to the glaciation of the end-Devonian.

Significant changes in the flora become apparent at the boundary between the Frasnian and Famennian stages in the middle of the Late Devonian. Plants were diversifying and adapting to different types of landscape by this time, some to wetter, waterlogged places, such as Archaeopteris, while others were adapting to drier uplands that were better drained. Archaeopteris became a dominant form, where it remained until the opening of the Carboniferous, contributing to a great increase in density of plants at this time (Algeo et al., 2001). It arose in the Givetian, dominating the land surface until about the middle of the Famennian, after which it began a slow decline, and finally became extinct in the Carboniferous. It reached heights of 30 m at its peak of dominance, with trunk diameters of 1.5 m, producing many species throughout the interval and into the Early Carboniferous. Many different habitats were formed by these, the best known of these being extensive forests of trees with woody trunks that were up to 1 m in diameter, especially in wetter places such as floodplains and stream valleys.

Ferns such as Rachophyton were common in peaty swamps, while huge treelike forms, lepidodendroids, were evolved by lycopods, where they dominated the wetland forests so densely that their remains formed the first coal deposits. The dominance by the lycopods of these swamp forests continued until almost the end of the Carboniferous. These plant types which reproduced by spores and were restricted to wetlands were essentially relict species by the end of the Devonian. New forms, the seed ferns or pteridosperms, that reproduced by producing true seeds with a tough coat were evolving in the drier regions, which allowed them to cope with periodic environmental disturbances and to reproduce in conditions that were seasonally dry. They reached dominance in the Carboniferous. The initiation of the first forests that had a structure analogous to forests of the present occurred in the Famennian.

Tree lycopods such as Lepidodendropsis and Cyclostigma were among other plants that contributed to the expansion of the vegetation. Also making a substantial contribution to the vegetation of the wetland habitats were cladoxylalean ferns such as Pseudosporochnus and the zygopterid fern Rachophyton. According to Clack¹ Rachophyton appears to have been capable of tolerating a wide range of conditions that allowed it to be a pioneer plant species in the formation of marshy conditions (Greb et al., 2006). Complex root systems were produced by all these plants, though in the progymnosperms they were particularly deep and massive, disturbing the surface of the land and creating deep soils for the first time. They also shed leaves and branches (DiMichele and Hook, 1992), which created litter that provided a habitat for detritovores, bacteria and fungi.

Indications of seasonal stress, in the form of growth rings, have been found in some specimens of Archaeopteris that date to the late Frasnian (DiMichele & Hook, 1992). Trees of the present, form growth rings when growth slows during times of climatic stress (possibly annual, essentially winter) alternates with growth that is more rapid, as in spring and summer. Some of the earliest plants known to have had abscission layers at the bases of their leaves are Pseudosporochnus, from the Eifelian of Belgium and Wattiezia from the Givetian in New York State, the leaves being shed as these treelike forms grew (Berry & Fairon-Demaret, 2002). As the climate deteriorated they effectively became deciduous, that Clack¹ suggests was presumably another response to the changing climate.

It is suggested that the evolution of air breathing among vertebrates may have been influenced by the plant activity increase along the margins of water and on land nearby, with several factors possibly contributing to this. Deep organic soil formed as the plants grew larger as their roots grew correspondingly larger and the activity of their roots increased. A 100-fold increase in the average diameter of the stems of fossil plants has been shown (Algeo et al., 1995) to have occurred during the Devonian which was contemporaneously with the declining levels of atmospheric oxygen over the same time interval. The plants were allowed to expand their range further inland away from the edge of the water by the evolution of seeds that were resistant, and to increase the ground cover and plant material biomass. An increase in leaf litter and plant matter that was decaying falling into shallow water where the vertebrates lived resulted from all these developments. The appearance of the land surface and the production of soils were changed by such developments, which also influenced erosion from the land surfaces, and slowing the runoff rates as well as changing the character and content of the runoff. Acidic products being produced by the decomposition of plants and solutes from the soil leached into the water. This led to the lakes and rivers receiving quantities of carbon-based compounds and soluble nutrients that were increasing, and this encouraged the growth of algae (Algeo et al., 2001), which was important for vertebrates that lived in the water, as they used much of the oxygen that was dissolved in the water. When this was combined with high temperatures it would have resulted in periods when oxygen concentration in the water was low. It has been suggested by Algeo et al. that this resulted in all the water systems becoming anoxic, including marine environments, and they have shown that the events are linked to black shale horizons that recur throughout the Middle and Late Devonian (Algeo & Scheckler, 1998; Algeo et al., 1995, 2001). In what they describe as the “Devonian plant hypothesis” (Algeo et al., 2002, 2003), they suggest that this was related directly to the increase in terrestrial plant cover that occurred during this same time. According to their suggestion “elevated nutrient-borne fluxes may have promoted eutrophication of semirestricted epicontinental seas” and could have “stimulated algal blooms,” and it was this high marine productivity that resulted in the black shales. Anoxia that was intense on a continental scale is represented by these black shales. The fate of the contemporary stromatoporoid and tabulate coral reefs, which declined rapidly through the mid-late Frasnian, has also been linked by them to the widespread anoxia (Algeo et al., 2001: 227, 233).

Clack¹ suggests the presence of fallen leaves appears to have also had other effects, with fire becoming a routine aspect of floral ecology from the end of the Late Devonian, possibly exacerbated by fallen leaves and branches causing a fire hazard in drier regions (DiMichele & Hook, 1992). It has been shown (Scott & Glasspool, 2006) that the fossil charcoal record accords with the more recent atmospheric composition models that have been produced (Berner, 2006). There is a minimum of atmospheric oxygen level that is required to burn plant material. There is a small, though significant, record of fossil charcoal, but no evidence of charcoal has been found dating to the Middle or early parts of the Late Devonian, which was a time, according to Berner’s model, when atmospheric oxygen levels were less than the minimum required to ignite plant material. In the late part of the Late Devonian fossil charcoal is again found (Rowe & Jones, 2001). And from the Early Carboniferous it becomes abundant. Clack¹ suggests Berner’s model of atmospheric composition is corroborated, though indirectly, by this finding.

There were also other ways in which the form and growth of plants were affected by changing atmospheric conditions. Few, if any plants that had broad, planar leaves, such as are common among plants of the present, have been found that date to before the Late Devonian. Clack¹ says this is surprising, in spite of the length of time between the origin of terrestrial plants and the Late Devonian. The protracted decrease in the percentage concentration of carbon dioxide in the atmosphere at the time when the plant stem efficiency to transport water upwards had increased has, however, recently explained the phenomenon (Beerling et al., 2001). Clack¹ suggests this must have been another factor that influenced the amount of plant debris that was accumulating in water bodies which lowered its oxygen concentration.

Plants underwent renewed radiations into many more habits and habitats, as they recovered from the extinctions of the Famennian, as this was the time when the entire range of modern ecological types first became established. The landscape would have looked similar to that of modern rainforests, with ground cover plants, vines, scramblers, shrubs, and forest trees that had arisen, though the constituent plants were vastly different (DiMichele & Hook, 1992).

Terrestrial arthropods from the period between the Frasnian and the middle Carboniferous are not well known. Comparing what is known of forms from the Middle Devonian with those from the Late Carboniferous has been used to infer their evolution, and this very large gap in the fossil record of invertebrates is unfortunate as the radiation of terrestrial vertebrates has undoubtedly been influenced by the radiation of terrestrial invertebrates. It has been suggested (Ward et al., 2006) that the absence of the invertebrates at this time was a real phenomenon, as both the invertebrates and tetrapods were still confined to water during this period in which the levels of atmospheric oxygen was low.

Changes in the marine faunas were largely preceded by these changes that took place in the terrestrial environment. In the Frasnian-Famennian extinction event (McGhee, 1989) many species became extinct and were replaced by others.

Over the history of the Earth this was one of the major extinction events, the most catastrophic of which occurred at the close of the Permian. It has been suggested that the Frasnian-Famennian extinction may have resulted from the movements of the continents that led to the glaciation of Gondwana, which changed the world’s climate, lowering the overall temperature, though as Clack¹ suggests, contributions may have come from changes to the atmospheric composition and anoxia in the waters. At this time corals were particularly badly affected.

A time of major biotic crisis, from the Givetian to the Famennian, is exemplified by the Frasnian-Famennian extinction event, though this particular extinction event is only 1 of several crises that are evident among marine organisms over that time period. Anoxic marine events initiated in the Givetian are represented by a series of black shale horizons in the geological record, the last of these extinctions, the Hangenberg Event, occurred at the end of the Famennian.

The Hangenberg Event occurred at the Devonian-Carboniferous boundary, in the form of a thin black layer of shale at the stratigraphic boundary (Caplan & Bustin, 1999) appears to have been caused by a rise in water level, water stratification disruption, increased organic content, and it has been presumed there was also a decrease in the oxygen content of the waters. In East Greenland a layer of black shale has been found at the Devonian-Carboniferous boundary (Marshall et al., 1999). This black shale is one of a series of black shale layers to be found throughout the Late Devonian that have been interpreted as being associated with increased runoff of organic plant material (Algeo et al., 1995). The deposits in East Greenland are generally regarded as being freshwater sediment, though most of the localities where there are records of the Hangenberg Event are of marine origin, suggesting that the Hangenberg Event may have been much more widespread and general than realised previously, and, as Clack¹ says, it certainly correlates with a turnover of vertebrate faunal components that was unprecedented, and this has been documented in detail by a recent study (Sallan & Coates, 2010).

Sources & Further reading