Geoscientist Online

 

Leyla Seyfullah (University of Göttingen), with financial help from the Society’s Garwood Fund, has just returned from rescuing precious fossil plant remains from a building site in suburban Chemnitz.

Geoscientist 21.02 March 2011

If you should ever try to imagine a late Palaeozoic ecosystem on land, what would it look like? Is it the “typical” Carboniferous coal-swamp setting, a humid subtropical mire-forest with huge odd looking plants (and possibly giant insects)? Our knowledge of coal-swamp flora and fauna from this time is quite extensive; but as a palaeobotanist I would like to know what other types of ecosystem had developed by this time, and, as the coal-swamps of Euramerica declined into the early Permian, what other plants might have been growing? One possible insight into a hitherto unknown Late Palaeozoic terrestrial ecosystem comes from the fossil lagerstätte of Chemnitz.

The area around Chemnitz, Saxony, East Germany is justly famous for its Petrified Forest (Versteinerter Wald). The first finds occurred in the first three decades of the 18th Century. In 1752 a huge silicified trunk was uncovered with its roots still attached. Great numbers of large and often very colourful plant fossils from this Petrified Forest were unearthed when residential areas were constructed at Hilbersdorf and Sonnenberg around Chemnitz at the end of the 19th Century. The fossil stems, branches of different orders (main or side branches, offshoots) and roots were collected as prettily-coloured objects in their own right; but far more interesting is the fact that the original three-dimensional shape of these fossils has been preserved – a very unusual thing in plant fossils.

Within these often complicated structures we see exceptional preservation of the plant organs, as noted by the German palaeobotanist Johann Traugott Sterzel (1841-1914), who re-started the study of this material after initial work by the German geologist Bernhard von Cotta (1808-1879) in 1832. The amazing detail of the individual cells of the plant tissues of each organ, along with their external morphology, is why Chemnitz became the type region for many silicified fossil plant taxa, and forms the basis for our understanding of Permian plants with anatomical preservation.

Other Permian localities in Europe more commonly yield flattened impression/compression materials, instead of this three-dimensional anatomical assemblage. Nowadays, however, many historically important former quarry sites are no longer worked as the city of Chemnitz (population c. 254,000, twinned with Manchester, among other places) has swallowed them up. For this reason the remains of this classic deposit were deemed “inaccessible”. Hilbersdorf and Sonnenberg are both suburbs of Chemnitz now. However, a new opportunity recently presented itself and Chemnitz has once again provided us with the only collectible Early Permian silicified plant assemblage.

REUNIFICATION

Straight after the reunification of Germany there was a rush to develop the city’s of out-of-town shopping facilities, leaving the heart of the city somewhat neglected. However, since 1999, there has been a period of investment and reconstruction, with new building developments in the city centre. This development meant that formerly inaccessible localities might be reopened and, potentially, yield new finds.

So Dr Ronnie Rößler, director of the Museum für Naturkunde in Chemnitz, decided to lead the city’s application to have the Petrified Forest recognised as a World Heritage Site by UNESCO. To help convince UNESCO he, with help from the city planners, organised an “emergency excavation” on a site scheduled for redevelopment in the residential suburb of Hilbersdorf. This site was deemed sufficiently local to be a good choice for an attempted “rescue” dig for any Petrified Forest before reconstruction either destroyed it or rendered it inaccessible. An area of 18m × 24m was set aside, just a few metres from the road. This was eventually excavated to a depth of 5.5m.

I had been lucky enough to meet Ronnie at a conference where he had presented beautifully-preserved silicified specimens with amazing anatomical detail. We got talking about the material, and I was invited to have a look at the materials housed in the Chemnitz museum once I had finished my PhD. Ronnie is interested in the evolution of sporing plants, such as tree ferns and the giant extinct horsetails. He works on these plants as preserved in volcanogenic sediments across the world, using the incredible anatomical detail to determine how the plants worked in terms of the mechanics of their tissues. He has worked mostly on Permian plants from Chemnitz and South America that have been preserved in a similar way. My interest lies in the evolution of plants that have seeds rather than spores (like today’s conifers and their ancestors) and in understanding how these plants were organised anatomically. So, after finishing my PhD, I contacted Ronnie and found out about this unique opportunity to access the historical locality and its potential finds.

APPLICATION

Being rather excited about this opportunity, but unfunded at the time, I wrote a desperate application to the Geological Society for fieldwork assistance. I asked the Society to help me get to Chemnitz, so I could get into the excavation to be able to observe the in situ materials and so get to understand the depositional environment; to be able actually to collect, myself, from this historic area and then work on the seed-plants being discovered there. The Society decided to award me the Garwood Fund for the Petrified Forest rescue attempt. This paid for 20 days’ field work to uncover the petrified fossil forest, as one of a team led by Ronny and Ralph Kretzschmar (Chemnitz Museum für Naturkunde) with volunteers and students from different universities, mainly from the Freiberg University of Mining and Technology. The fieldwork took place in June 2010.

The Chemnitz Petrified Forest originates from rhyolitic explosive volcanism that occurred around 290 million years ago. A recently obtained SHRIMP age put this episode at 290.6 (+/- 1.8) Ma. At this time, a series of volcanic eruptions northeast of Chemnitz resulted in the formation of a pyroclastic sequence now referred to as the Zeisigwald Tuff Horizon, which is part of the Leukersdorf Formation - approximately 800m of sedimentary and volcanic deposits. The whole succession consists of three depositional cycles, which start at their erosive base with alluvial coarse clastics and develop, fining upward into fine-grained palustrine or lacustrine cycle tops.

The stratigraphic position of the Leukersdorf Formation corresponds to the Upper Asselian/Lower Sakmarian, indicated from the palustrine Rottluff Coal microflora situated within the lower part of the Leukersdorf Formation. The Zeisigwald Tuff Horizon resulted from a multi-storey caldera eruption - a depositional sequence of several devastating glowing-cloud eruptions and repeated pyroclastic flow deposits. One giant lateral surge associated with the hot, dense and unsorted flows is thought to have been the key process in creating the Chemnitz Petrified Forest. Descending ashes buried and conserved a unique ecosystem, and this rapidity and intensity have resulted in some of the most complete and perfectly preserved fossil plant assemblages known, providing insights in to both the well-preserved Permian floral elements and the permineralisation process itself. A modern analogue to this mode of preservation is found in the eruption of Mount St Helens (1980), when many trees were found snapped off, stripped of their branches and bark, and dropped in alignment with the blast direction, whereas other trees remained upright and were less damaged.

HIGH DENSITY

So far we have uncovered 48 in situ plants from this new locality measuring 24m x18m x 5.5m, giving a high density; although additional trunks and branches were clearly not in situ and had been brought in as part of air-fall tuffs or pyroclastic flows, which were shown to have flowed westward. The in situ plants are rooted in the underlying sedimentary layer and protrude through into the pyroclastic sediments that have a basal fine-grained, horizontally-bedded layer, followed by a thin, strongly lithified ash with obvious accrectionary lapilli, containing rare three-dimensional fossil moulds, and finally, a massive blocky tuff layer. This uppermost flow deposit of the Zeisigwald Tuff Horizon is purple-red in colour and massively blocky. It is also where the best anatomically-preserved fossil plant materials are found trapped.

The plants themselves consist of plants from several different groups, there are sporing plants: ferns, Arthropitys (tree-like horsetails) and Psaronius (tree ferns) preserved in situ, along with various seed plants: Cordaites (extinct conifer ancestor), conifers and pteridosperms (seed plant ancestors). The medullosan pteridosperms show a large amount of anatomical variation, which we had thought was just down to ontogenetic (developmental, age related) variation within the same species; but it is clear from examining neighbouring in situ trunks that differing subspecies/species grew side by side, so further work to fully delimit and describe each species is now needed. It is interesting that some of the trees appear de-barked on one side, mirroring the surge direction Also the floral understorey appears to be impoverished, so there few smaller shrubby and no herbaceous plants present in the assemblage. We suspect this to be a taphonomic effect – an effect of preservation – with the hot surge destroying more delicate plants like ferns, whereas the wood and outer bark of the tree-forms could withstand the heat and inertial energy of the surge so close to the original volcano. The team believes the original volcano to have been located just over one kilometre away.

As the large blocks from the (partly silicified) tuff were removed, attention then focused on the detrital layer preserved in the first ash-fall deposit, as this proved to be unusual. This is the level at the base of the site. Normally, this layer would contain fallen and shed materials (seeds, leaves, branches, animals) in various states of decay, on the original soil level - making this layer a useful indicator of the general ecology. The detrital layer can preserve organs and species not seen in the overlying layers of a surge event. It appears that the majority of this detrital layer had been “boiled” or scoured away by this extremely devastating flow, due to the proximity of the source volcano.

However, there were some traces of the detrital layer (about 1000 compression/impression specimens), preserved as a fine mud with fragile plant fragments, several tetrapod remains (three reptiles, two amphibians and isolated bones) and different arthropod remains (an Arthropleura leg, two diplopodes (millipedes), one aphantomartid arachnid) found predominantly around the bases of the in situ tree-trunks. The detrital layer contains a higher frequency of leafy remains and a greater diversity of species than that of the overlying tuff, including delicate pteridosperm reproductive organs, although these fossils are not permineralised, they either appear either as infillings of the slightly compressed three-dimensional moulds with either iron or manganese oxides, or preserved as calcium phosphate traces on the mud layer.

EXCITING

For the team, the scientifically most valuable find was found lying horizontally, embedded in the tuff. It is the more than 10m multi-branched terminal portion of a woody horsetail-tree (Arthropitys bistriata). It has been named German "Fossil of the Year 2010", the first time a fossil plant has won this accolade, as it shows for the first time that these giant sporing plants from the Permian could be branched, and challenges the assumption that the Permian forms were never branched.

For me, however, the most exciting find was something that was originally thought to have been a rooting system of one of the large seed plants (which in itself would have settled many questions about how the stems were really rooted). This, in fact, turned out to be the broken-off crown of a medullosan pteridosperm tree, with the branches still attached, preserved upside-down! Painstaking excavation with fine chisels and paint brushes revealed that the very long, flattened branches of the inverted tree-crown had pinnules of the fern-like foliage attached, and although not fertile, this specimen represents the first known example of how such large branches (with foliage) were actually borne by the medullosan pteridosperm trees on comparatively slender trunks.

This glimpse into the world of the Sakmarian is extremely valuable; there is a notable decrease in the fossil plant record (when compared with the Carboniferous) as the terrestrial Permian record is often represented by hiatuses or unfossiliferous strata. This rare parautochthonous flora also bears evidence of a drying environment that would have had little preservation potential, had it not been for this lucky volcanism. Some intriguing questions remain, particularly around the taphonomy. What processes led to many of these petrifactions being preserved by purple calcium fluoride, rather than being silicified, for example?

For me, the most unexpected part of the fieldwork was the interest that local people took in our excavation, and the plants we uncovered. The site itself was less than two metres from a road and was overlooked by multi-storey apartment blocks. For safety reasons, the site was walled off; but numerous visits were made by many different groups, (6000 visitors from Germany during a few weekends when the excavation was opened to the public, as well as 80 scientists from more than 20 countries who also came to see the site). Many volunteers laboured long and hard in the hot sun, breaking up the tuff and clearing it by hand, vigilant for any interesting structures, while others (including me) sat with small chisels and brushes cleaning the delicate structures before noting their measurements and directions, then photographing and removing the fossils.

We also digitally mapped each fossil so that a complete picture of the finds in the site has been accurately recorded in 3D. The combined effort of so many genuinely interested people was great to be a part of (even for a short time). Due to the unexpected interest in recovering the Petrified Forest, the city of Chemnitz has granted a new site for excavation, in the Sonnenberg area of the city, next year.

Meanwhile, we are getting on with preparing the specimens and comparing them to existing collections in Berlin and publishing the results – all the time looking forward eagerly to grubbing around in suburbia once again, in 2011.