End-Permian collapse and survival
Prior to the mass extinction, the palynological assemblages of Greenland reflect gymnosperm woodlands (see figure). Ferns and other spore plants played a subordinate role in the understory. Located in a northern subtropical region, the forests consisted of the long-leaved Cordaites (extinct gymnosperms), seedferns and conifers. An indication for already ongoing destabilization of the ecosystem during the Late Permian is the presence of fungal remains. These are thought to be a result of excessive heterotrophic activity, caused by the dieback of trees. A sudden decline of pollen of the dominant conifers and seedferns, coinciding with the collapse of marine ecosystems, gives firm evidence for the loss of woodland (a). This is followed by a rapid increase in spores of more stress-tolerant community members: ferns, quillworts, clubmosses (b). Large amounts of the spores are found in unseparated tetrads, which might be an indication of environmental stress. Following this 'spore peak', communities start to restructure (c). Herbaceous quillworts, clubmosses, and seedferns colonized resource-poor wet habitats. In more stable environments the residual 'endangered species' regain dominance, but they had to share their dominance with several newcomers. These newly appearing gymnosperms were cycads and gnetaleans, which were not present in the original communities. Renewed increase of spores (d) heralds a second blow to the reign of woodland elements (e). This time the conifers and Cordaites really go extinct, and only a fraction of the original gymnosperms survive. After a period of 200,000 years, a low-diversity, stable open shrubland with seedferns and quillworts (f) is the final stage of vegetational change during the initial survival phase. Transitions from all kind of woodland types to open shrubland can be recognized worldwide in other Permian-Triassic palynological records. This picture confirms the well-documented drastic loss of provinciality at the end of the Permian.
During the collapse and initial part of the survival phase, the landscape changes from closed woodland, via vegetation mainly composed of opportunistic herbs, to a stable open shrubland. The palynological record from Greenland revealed two unanticipated ecological patterns. First, the crisis resulted in an increase in diversity, instead of the expected decrease. This was related to the proliferation of the herbaceous quillworts, clubmosses, and seedferns, while the late Permian dominants were in decline, but still present. In addition, several initially absent gymnosperms invaded the communities. Second, in Greenland the time-lag between initiation of terrestrial ecosystem collapse and selective extinction among characteristic late Permian plants takes about 200,000 years. Such a delayed extinction resembles predictions of population dynamics models. Computer modeling of plant communities' response during progressive habitat destruction shows a delayed extinction of species, the so-called 'extinction-debt'.
Looy C.V., Twitchett R.J., Dilcher D.L., Van Konijnenburg-Van Cittert J.H.A. and Visscher H., 2001. Life in the end-Permian dead zone. Proceedings National Academy of Science, USA 98: 7879-7883. PDF
|End-Permian Isoetalean spore tetrads|
|End-Permian fungal remains|