Preserving plants in pyrite

Pyrite permineralisations are an important source of information on plant anatomy including water-conducting cells in the earliest land plants and woody tissues in Eocene twigs and roots. Detailed analysis of the fabric of pyrite in relation to the quality of preservation in the Devonian and Eocene fossils has allowed elucidation of the processes involved and tentative explanations for the differences in amounts of organic material present in the fossils. A model system using Apium petioles (celery) and the FeS-H₂S reaction to produce pyrite was designed to simulate pyritisation in the laboratory. The destruction of microcrystalline pyrite precipitated within cellulose cell walls, cements between cells, and on the inner walls of the cells is consistent with observations of Devonian plant fossils, but we have yet to produce a fully pyritised celery fossil to confound future palaeobotanists.

Mesofossils from the Late Cretaceous of Antarctica

Painting the broad picture: a plea for a multidisciplinary approach in community reconstruction

• a broader picture of the flora emerges (the extra plant parts yield supplementary information)

• the accuracy of the identifications can be corroborated/called into question and alternatives suggested

• differences in representation can be used in the spatial reconstruction of the flora

If possible, other data on the site of deposition and its environs (e.g. sedimentology, palynofacies, phytoliths, diatoms, insect- and vertebrate-remains) should be integrated into such multidisciplinary studies.

It is clear that no one individual can undertake all this work. So how can we accomplish this objective? The solution lies in multidisciplinary teams. However, this is easier said than done. The members of the team must be compatible and be willing to let the common goal take precedence over their individual careers.

Unusual preservation of silicified wood by quartzine from the Jurassic Purbeck Formation

Plant taphonomy in the fluvio-lacustrine basin of Uña-Las Hoyas (Upper Barremian, Southwestern Iberian Ranges, Spain)

Mid Cretaceous fossil forests of Alexander Island, Antarctica

The fossil plants and trees are found preserved in sequences of fluvial sandstones, siltstones and palaeosols. Sandstones show features such as in situ fossil tree trunks and leaves, some current bedding and rip-up clasts of the palaeosol below. Thick siltstone units are horizontally laminated and contain an abundance of well preserved plant fossils. This sedimentary sequence represents flood plain and channel bar deposits of a braided river which evolved into a meandering river system. The sandstones were formed during flood events which deposited vast amounts of sand over the riverbanks, covering the soils and vegetation. Signs of deformation in underlying sediments suggest that deposition of the sands was rapid and therefore flood events were catastrophic. The siltstones were formed from suspension fallout in standing water pools. The palaeosols are immature but have an abundance of rootlets and plant material within them indicating that they supported vegetation. The palaeosols represent a period of emergence along a riverbank with enough time for colonisation of plants and trees. Sequences of palaeosols, sandstones and siltstones suggest that when water subsided, new soils formed and plant colonisation began again.

The fossil plant assemblages suggest that a thick canopy of araucarian and Elatocladus conifers with an understorey of ferns and the small shrub Taeniopteris, dominated the vegetation within these Cretaceous fossil forests. Other minor components of the vegetation included liverworts, angiosperms and Ginkgo.

Terrestrial plant remains in late Cenozoic shallow marine deposits of the Po Plain (Italy)

The second case study is represented by the succession of Oriolo (nearly 20 m thick) in the SE Po plain, near Faenza, which should be chronostratigraphically located at the Matuyama/Brunhes boundary. Here a rich collection of reddish leaf impressions (and a few fruits and seeds) was gathered by Dr. M. Sami. The leaf assemblages originate from a few silty layers with chaotically disposed leaf sheets. Several samples contain mainly whole leaf specimens, suggesting that fragmentation during transport has not been dramatic. Most specimens show distinctive leaf architectural features, which permit to assign them to about three dozens of taxonomic groups, most of them still living in Italy.

Obviously, the leaf and «carpoid» assemblages of both study sites experienced a consistent transport that could have biased the composition. However the palaeoenvironmental setting of both sites was characterised by inclined slopes very close to the coastline, with a rather narrow coastal plain in between, so that plant remains did not need to be transported more than a few thousend metres. Such assemblages may be interpreted as masses of continental plant brought by rivers into the shallow-sea during distinct flood events. Individual «carpoids» obviously floated in the sea for several weeks (months?) before burial, because they were colonised by Teredo. On the other hand, due to the presence of delicate leaves, it seems improbable that their floating time in the sea would be as long; yet very rare specimens encrusted by balanids and bryozoans were found in other Pliocene sites of the western Po Plain. In order to reconstruct the area and the vegetation types which were sampled by the transport agents, the best tool seems to be an ecological analysis based on nearest living relatives, especially for the Pleistocene of Oriolo. In this site several living taxa distinctly indicate a riparian forest; other forms indicate a floodplain forest and a slope forest, likely associated with clearings yielding shrubby vegetation. Even for the Cervo River leaf flora there are some indications of such a broad vegetation sampling. The carpological assemblages invariably indicate a main provenance from well-drained terrestrial environments, but is difficult to say from how many vegetation types because most species are extinct. Less common taxa prove that also freshwater wetlands (Proserpinaca) and marine seagrass communities (Cymodocea) had been sampled. Of course, a sound actuopalaeontological analysis of analogous taphocoenoses will be the key to get more information from this rich and diverse kinds of fossil assemblages.

Taphonomy of amber and its inclusions

Amber is a natural fossilized resin exuded by a large diversity of trees, especially some groups of conifers and angiosperms. Today these large resin producers are mainly distributed in the tropical and temperate forests and savannah, and they are largely used by the wood industry, chemistry and pharmacy. Resins are a complex mixture of terpenoid compounds, the more common ones are oxygenated terpenes, such as acids, alcohols, and esters, secreted from plant parenchyma cells. Terpenes may be volatiles at normal environmental temperatures but in particular cases they are not. The latter are the most significant for us since they transform into amber after milions of years of diagenesis, while volatiles are gradually lost in that time.

After resin secretion a number of processes including oxidation and polymerization occur; resin becomes harder in a short time to form a product known as copal, which may reach ages of up to a few thousand years. Copal has different physical and chemical features than amber, which needs some millions of years to be formed.

When does amber taphonomy begin? It is obvious that in the case of arthropod or other animal inclusions, the taphonomic processes begin when they are trapped by the sticky resin, but this is more difficult to decide in the case of their "matrix" (the amber). The boundary between biostratinomy and diagenesis is also different for insects and the amber itself. Whereas foramber inclusions, diagenesis begins when they are totally embedded in resin, this boundary is not as straightforward for the amber. Does it begin with the polymerization of terpenes or rather when the resin is buried in sediments ?, what about the resin secreted by roots ?

These questions may be answered after reviewing the formation of resins in trees. Resins may form in internal crack fillings, under the bark, in resin pockets in the wood, in pockets between the bark, as fillings from tree wounds, as external stalactite shapes, as stalactites, as external drops and swellings. Not only resins are usually related with the bark or wood but also with roots, leaves, etc. It is necessary to say that one species of tree may produce diverse types of resins, with different chemical compositions and obviously different rates of decomposition. For example Agathis australis (Araucariaceae) produces at least 5 different types of resin. All these features determine the future amber formation, and the preservation of inclusions.

In this workshop a number of crucial topics for the taphonomic history of amber may be discussed:

a) Resin production and their producers. What kind of trees produce today large quantities of resins ? Differences between their recent distribution and their distribution in the fossil record should be taken into account.

b) The drying of resin. During this event a large quantity of animal and plant remains may stick in resin. Are the organism remains found embedded in amber selected in a certain extent? This may condition the palaeoenvironmental and palaeoecological reconstructions made from amber remains.

c) The transport of resin until its final depository. Are the amber deposits mainly autochthonous or allochthonous ? Which sedimentary environments are more favourable for the preservation of resin?.

d) Diagenetic processes that affect resin and their biological content. Is the original composition of inclusions preserved in ambers? Why do we study the palaeobiological content by transmitted light microscopy (through the amber)? and why do we not extract it?