The Late Devonian
The Late Devonian Epoch of the Devonian Period: 385 to 359 million years ago
Archaeopteris forests generated shelter, shade, and nutrients (in the form of small animals and invertebrates) that provided an ideal environment for the early tetrapodomorphs, thus leading to the
vertebrate conquest of land and the evolution of land animals (including, eventually, us).
Late Devonian stratigraphy has been intensively studied; and there seems to be considerable agreement on basics, particularly for the Euramerican world. The disagreement there is on what it means. The attached scheme is taken from Joachimski et al. 2004) and Sandberg et al. (2002). We have not tracked down each of the conodonts for which the conodont zones are named. However, they are almost all species of Palmatolepis. The exceptions are the Mesotaxis falsiovalis Zone (earliest Frasnian) and the Siphonodella praesulcata Zone (latest Famennian). Some variations in the conodont nomenclature are also found. Thus, the postera Zone is sometimes referred to as the styriacus Zone. The "costatus Zone" is also frequently mentioned. This corresponds (probably) with the upper half of the expansa Zone. Kaiser et al. 2004).
This arrangement has been correlated with strata from northern Europe and the Western United States. However, it is not clear whether it can be successfully applied to Gondwana lands. Critical points, such as the Frasnian - Famennian boundary and the base of the Carboniferous, have proven very hard to identify in (for example) Eastern Australia, Antarctica, and Africa. Kaiser et al. (2004). So, the Late Devonian may not be quite so neatly laid out as the figure might suggest.
Note also that the chronological portion of the chart is no longer consistent with ICS dating. You can get a reasonable approximation of the current ICS dates in the Famennian simply by adding 1 My to the ages on the chart. Things may not be so simple for the Frasnian.
The precision and world-wide correlation of this arrangement are of some interest because Sandberg and others have aggressively argued for a strong extraterrestrial influence over events in this epoch, as well as for an Ice Age, or actually several ice ages, covering all of the Famennian. This dramatic reading of the Late Devonian is almost entirely based on a close reading of strata and faunal lists from northern Baltica and western Laurentia, without so much as a dropstone or tektite (at a relevant time) to back it up.
Whatever value their hypotheses may have, these workers have given us a very close and detailed look at the sequence of events in two, well- separated regions of Euramerica, and that is something very much worth knowing. Briefly summarized, the story goes like this.
The Frasnian was a time of rising seas. This is normally a good thing for life. However, as in parts of the Mesozoic, sometimes the sea rises so rapidly that the reefs are unable to keep up. When this happens, near-shore communities may "drown" for various reasons: because benthic autotrophs find themselves below the level where
they can receive enough sunlight to survive, or because of unsuitable substrate, sedimentation, or grade. With the productive base of the food chain gone, the entire community may collapse. Such an event may have occurred in the rhenana Zone, or perhaps the jamiae (at the beginning of transgression cycle IId), as evidenced by the invasion of the normally pelagic conodont, Palmatolepis semichatovae, into near-shore waters and perhaps the appearance of anoxic bottom muds.
In the succeeding linguiformis Zone, the sea fell, recovered, and then fell precipitously and for an extended period. This is the time of the Frasnian-Famennian extinction or Kellwasser Event. The Kellwasser was devastating to marine life on a specific or generic level, but had little effect on higher taxa. In short, it was less a mass extinction than a mass turnover. The Kellwasser Event appears to be spread over occur a significant length of time, terminating in a black "extinction" layer followed, in some locations, by evidence of severe disturbances which seem to indicate storm or tsunami effects.
Particularly in view of the lack of more global sampling, it is hard to put a definite interpretation on this this data, and we will not attempt to do so. The Kellwasser Event was followed by generally falling sea levels in the Famennian, punctuated by reversals, particularly towards the end of the Age. Sandberg et al. (2002) ascribe this pattern to polar glaciation interrupted by interglacials. Again, absent more direct evidence of a Late Devonian ice age, we are reluctant to go so far.
ATW050113. Minor revisions ATW050612. Text public domain. No rights reserved.
Late Devonian Life
Extinction of coral reefs
About the middle of the Late Devonian epoch, towards the end of the Frasnian epoch, worldwide environmental changes, including anoxia (lack of oxygen in the oceans, perhaps the result of algal blooms) and a sudden drop in sea level (drying up the shallow seas where life thrived, and the replacement of a mild maritime climate with a harsh continental one ), caused one of the greatest mass extinctions in the history of life. The victims included many important marine organisms, and especially reef biota. The previously flourishing reefs were decimated. The long association between algae, sponges, stromatoporoids, and corals that began in Ordovician times and had continued for some 130 million years without significant disturbance came to a sudden end.
Plants: Lycopsids included some of the stems were of diameter and their bark superficially resembled that of Carboniferous tree-lycopsids, implying that some, such Protolepidodendropsis, were a few metres high, and by the end of the period the group included substantial forms such as Cyclostigma.
Links between the evolution of land plants, weathering processes, and marine anoxic events
"The Devonian Period was characterized by major changes in both the terrestrial biosphere e g the evolution of trees and seed plants and the appearance of multi-storied forests, and in the marine biosphere an extended biotic crisis that decimated tropical marine benthos, especially the stromatoporoid tabulate coral reef community. The connections between these terrestrial and marine events are poorly understood but a key may lie in the role of soils as a geochemical interface between the lithosphere and atmosphere/hydrosphere, and the role of land plants in mediating weathering processes at this interface. The effectiveness of terrestrial floras in weathering was significantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were (1) arborescence tree stature), which increased maximum depths of root penetration and rhizoturbation [see diagram below], and (2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats and allowed colonization of drier upland and primary successional areas. These developments resulted in a transient intensification of pedogenesis (soil formation) and to large increases in the thickness and areal extent of soils. Enhanced chemical weathering may have led to increased riverine nutrient fluxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottom water anoxia, and high sedimentary organic carbon fluxes. Long-term effects included drawdown of atmospheric pCO2 and global cooling leading to a brief Late Devonian glaciation, which set the stage for icehouse conditions during the Permo-Carboniferous. This model provides a framework for understanding links between early land plant evolution and coeval marine anoxic and biotic events, but further testing of Devonian terrestrial-marine teleconnections is needed."
Thomas J. Algeo and Stephen E. Scheckler, "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events", Phil. Trans. R. Soc. Lond. B (1998) 353 113-130
earlier<-------------------------->later
increasing terrestrial plant root depth penetration with time during the Devonian, leading to increasing soil depth and weathering. rhy=rhyniophytes such as Aglaophyton or Horneophyton; tri=trimerophytes such as Psilophyton; lyc-he = early herbaceous lycopsids such as Asteroxylon or Drepanophycus; lyc.tr=early tree lycopsids such as Lepidosigillaria or Cyclostigma; prog-ao=aneurophyte Progymnosperms such as Tetraxylopteris; prog-arc=Archaeopteris progymnosperms; gym=early gymnosperms such as Elkinsia or Moresnetia; and Zyg=zygopierid ferns such as Rhacophyton. Scale bar, 1 meter.
The Late Devonian period marked the time when the first tetrapods animals evolved from their sarcopterygian ancestors. There were at least two successive and distinct waves of tetrapodomorph evolution, the Frasnian Elpistostegalians, which seem to have been exterminated by the Frasnian-Famennian extinction event, and the Famennian tetrapods (such as Acanthostega and Ichthyostega) which were to become the ancestors of the Carboniferous labyrinthodonts. These tetrapods were almost entirely aquatic, although they were clearly able to crawl up on the mud and move about out of water. The first tetrapod skeletons are known from the Late Devonian of Greenland, but other traces of Devonian fossil evidence, such as some tetrapod footprints and fragmentary remains from Australia, hint at a much greater distribution of Devonian tetrapods than is currently revealed in the fossil record.
Murderous Trees
Developing a Sequence Stratigraphic Framework for the Late Devonian Chattanooga Shale of the southeastern US - "The Late Devonian Chattanooga Shale of Tennessee and Kentucky is in most areas a thin black shale deposit of less than 10 meters thickness. It is a distal equivalent to the almost
3000m thick Catskill sequence, and encompasses most of the Frasnian and Famennian, approximately 14 million years of earth history."
Devonian times - going upstream
"Brachiopod faunal extinction and recovery during the Frasnian-Famennian biotic crisis" Number 04 (April) 1999, Volume 47 of Przegl?d Geologiczny (Geological Review) Polish Geological Institute
The Late Devonian Mass Extinction (The Critical Moments and Perspectives in Paleobiology and Earth History Series) by George R. McGhee, Jr. - a book about the Frasnian-Famennian extinction