natural process of polymerisation takes place over several years, generally at high temperature and pressure. Just like the formation of all fossils, the process of converting resin into amber is one which is fraught with improbabilities. The original resin has to be resistant to mechanical and biological decay for quite long periods of time, which many plant resins are not, so that there is time for the polymerisation to take place. This will render the resin more resistant to decay or destruction, but does not instantly produce the finished product. These conditions are similar to those thought to be needed for creation of coal, so it is hardly surprising that amber can be found in coal seams.
Initial polymerisation at high temperature and pressure turns the resins into copal. This is a term which originally only applied to resins from South America, but then became a general term for the halfway house between resin and amber. Copal can be used to make a very high quality varnish when mixed with suitable solvents. During the eighteenth and nineteenth centuries, large quantities of copal were consumed specifically to be used as varnish, as it could be applied to any subject that needed a high gloss clear varnish, from carriages to paintings. To complete the polymerisation and to turn the intermediate copal into amber, the pressure and temperature have to be continued. If the pressure is for some reason reduced, but the temperature maintained, the amber, or nascent amber, will break down into its constituent chemicals. In the final stages of polymerisation to make amber, the solvent terpenes are driven off leaving the tree resin as a complex polymer of great resilience.
As one would expect of a product that originates from trees at a time of massive forestation, the distribution of amber is worldwide but heterogeneous in species origin. The majority of amber is generally regarded as being cretaceous or of a more recent in age, which at 142 million years ago, or less, corresponds with the proliferation of flowering plants. Since not all trees produce free resin, it is not so surprising that amber seems to be associated with specific botanical families, of which there are still extant living examples. This is even though the plant families of interest are both ancient and not necessarily flowering. The three family groups that seem to have produced most amber are:
Araucariaceae, these include the monkey puzzle trees and the kauri trees of New Zealand. They are large evergreen trees which are now almost exclusively found in the wild in the southern hemisphere, but when they were one of the dominant tree species, they were worldwide in distribution. In parts of Turkey, fossilised wood from members of the Araucariaceae is carved and used in jewellery.
Fabaceae, although most of these legumes are herbs and edible crops, there are some large trees in the family. There is a single tree species in east Africa from which copal is used as incense. They have a widely distributed fossil record, as flowers and pollen as well as leaves.
Sciadopityaceae, there is only a single species left in this family, the Japanese Umbrella Pine. Although there are no close living relatives, this was a widespread clade with a fossil record extending back more than 200 million years.
It should be emphasised that these are not the trees which originated amber, they are not ‘living fossils’, they are the current species of the lineage that produced most of the amber we know today. With amber being strictly plant in origin, it should not be a surprise that it is a frequent inclusion in some forms of coal, which were laid down from plant material at more or less the same period as amber was being formed.
Although we all have an idea of the colour of amber, having given its name to the shade of orange which we describe as amber, this is only the commonest of the colours associated with it. For example, there is a form of amber which comes from the Dominican Republic that is quite different. In this form, Dominican amber is predominantly blue. The colour is thought to originate from inclusion in the amber of a molecule called perylene. This is a polycyclic aromatic hydrocarbon with the empirical equation of C20H12. Perylene is basically two naphthalene molecules joined by two carbon/carbon bonds. The molecule itself is not blue, but fluoresces shades of blue when illuminated with ultraviolet light, depending upon the wavelength of the ultraviolet radiation. As it is sensitive to a wide range of wavelengths and, of course, ultra violet light is a normal component of daylight, under natural conditions the colour will always appear to be the same. Consequently, the amber will look blue in daylight, but less so, if at all, in artificial light. The unusual inclusion of perylene into Dominican amber implies either a different, possibly unique, species of origin or a considerably modified method of creation.
It is not just by the inclusion of animal material in amber that it is possible to preserve organisms in a near life‐like form without the mineralisation normally associated with fossilisation. Along with the inclusion of animal material in amber, there are also conditions in which large‐scale remains can be preserved for quite long periods of time. One of these which has yielded some quite startling finds is effectively pickling, in some cases with associated freezing. Although, as we shall see, this latter process can be good enough on its own to render stunning levels of preservation of details after death.
The process of pickling involves an organism rapidly finding its way after death into anoxic conditions, as would be expected in a peat bog where the oxygen has been depleted by large‐scale organic decay, usually of plant material. This in itself would cause preservation, although it would depend on long‐term stability of anaerobic conditions to preserve organisms intact. In the composite system of preservation, if the remains move to the next step, which is freezing, then the entire animal may be kept in very good condition for as long as the climate permits it. This can been seen very clearly in mammoths removed from permafrost where very little decay has taken place over the millennia of entombment in deep frozen condition. This process of preservation by partial chemical treatment followed by freezing could take place almost anywhere that long‐term permafrost can be found.
An extreme example of permafrost preservation was demonstrated at the 47th annual dinner of the Explorers Club held at the Roosevelt Hotel, New York, in 1951. Among the various courses was 250 000‐year‐old mammoth. This was only a taster as supplies were understandably limited, but we do know that it was provided by the Reverend Bernard Hubbard from an animal found at Wooly Cove on Akutan Island. Akutan is one of the Aleutian Islands in Alaska, where glacial permafrost is widespread. A more recent example of permafrost preservation came with the discovery in 2007 of a frozen mammoth calf. This animal, now called Lyuba, is thought to be the best preserved mammoth mummy ever found. Lyuba is a member of the species Mammuthus primigenius and died about 41 800 years ago. It is considered most likely that she suffocated in mud during a river crossing and the lactic acid produced by bacteria in her stomach partially pickled her. So well preserved is she that there is identifiable milk in her stomach. It was not certain at the time of the discovery that the calf would be preserved at all, as while the original discoverers went to report the find to their local museum, the corpse was lifted and removed for display outside a shop. It was retrieved, with a little damage due to local dogs, and has been widely exhibited since. The permanent display is at the Shimanovsky Museum and Exhibition Centre in Salekhard, Russia. Salekhard is reputedly the only town where the Arctic Circle actually runs through the town.
Completely submerging an organism in what is in effect a preservative is another way in which plant remains or animal corpses can survive for very long periods. One of the ways this can happen is found in tar pits. These are rare sites, but can be quite extensive, for example, Pitch Lake in Trinidad is the largest natural deposit of asphalt in the world, covering about 44 ha. Other such deposits include Binagadi asphalt lake in urban Baka, Azerbaijan, and the second largest lake, Lake Guanoco in Venezuela.
Probably the most investigated and well known of all the asphalt deposits are the Tar Pits of La Brea in Los Angeles. This is the most well studied of the rare tar pits, and it is also unique in having been an asphalt mine during the nineteenth and early‐twentieth century. The position of the tar pits, in what is now an extensive suburban area of Los Angeles, has added to the development of interest in this particular site. The current pits are mostly man‐made, a leftover from asphalt mining
