Inclusions in Baltic amber

The most significant and the largest known amber deposits in the world are distributed on the south–eastern part of Baltic Sea. These deposits are linked to the development of the modern plant world in the Late Mesozoic and Early Cenozoic, when the angiosperms and gymnosperms (specifically the conifers) diverged.

Amber deposits can only form, if the resins are flushed out of the forest floor by rivers during or shortly after their formation and transported into oceans or large lakes, where they are protected from weathering processes after deposition.

Many existing scientific problems about Eocenian period could be resolved, if it were possible to date individual amber specimens directly. At present, this cannot be achieved using either physical or chemical methods. Using carbon-14 dating, the only radioactive element in amber, is only adequate for up to 60,000 years. Only studies of amber inclusions trapped long time ago could help in solving many questions dealing with climatic, ecologic, geographic and other conditions of that period.

As is the case with the majority of all fossil resin deposits, Baltic amber is today no longer found at the site where the resins were originally produced and deposited in the forest floor.

The oldest deposits known today indicate that the amber forest must have already existed in the Lower Eocene (about 50 million years ago).

The complex of animals and plants found in Baltic amber is associated with distribution area of resin-producing trees and with conditions which prevailed in amber forest. The conditions were not so simple there, because groups of insects entrapped in amber now are found in forests, swamps, meadows, mountains, lakes and rivers. This means that so called amber forest was complex of different habitats.

The formation of amber in northern and central Europe probably already began in the Early Eocene and may have ceased towards the end of the Mid Eocene. Amber originated from a single “amber forest” that had a stable subtropical-tropical climate. It existed for a period of roughly ten million years and covered an enormous area of prehistoric northern Europe.

The insects are the most abundant among Baltic amber inclusions. They make 86-92% of all inclusions, arachnids make 7.5-13%, the other groups only 0.1-1.7%. Plants make only 0.4% of all inclusions.

The fauna preserved for us in Baltic amber is almost exclusively restricted to the arthropods, with insects and arachnids making up a majority at nearly 99%. Inclusions of representatives of other phyla (worms, mollusks, vertebrates) are extremely rare and only verified on the basis of a few, isolated finds in some cases. The numerical distribution clearly illustrates the different preservation potential of individual animal groups, which is influenced by multiple factors. One significant selection factor is size. Most of the animals enclosed in Baltic amber are small, with a body length of between one and five millimeters. Strong, large animals were usually able to free themselves from the resin and are therefore found only rarely, if at all. Exceptions to this rule are exuviae and animals that got caught in the resin after they were already dead. A few inclusions of fairly large specimens show that the animals stuck to the resin became prey to others before being completely entombed by the resin. This can frequently be observed in caddisflies and termites, whose abdomen is completely eaten away and filled with the resin of the subsequent flow. In some instances, only traces of relatively large animals have been left behind in the form of individual wings and extremities, or molting remains. The preservation potential of the individual animal groups is primarily determined by their habitats in the amber forest. Animals living on the resin-producing trees themselves or in the immediate vicinity were by far the most frequent victims of the resin traps. The numerous animal communities in the different ecological niches of the “amber tree”, from the root zone all the way up to the treetops, have been preserved in great diversity and detail. In addition to real tree-dwellers, we also know of a considerable number of “visitors”, who visited the amber tree for various reasons (e.g. hunting, reproduction, rest) and ended up getting captured in the resin traps.

In special circumstances, the chance of coming into contact with resin can also be greatly influenced by an organism’s specific habits, such as the swarming flight of certain insect groups (Diptera). The insects living inside wood or in underground tunnels had no chance of becoming caught. Naturally, the preservation potential was also much higher for organisms moving about on the trunks of the resin-producing trees (ants, spiders) or inhabiting the crevices and cracks in the bark (certain Microlepidoptera). In contrast, the danger of nocturnal arthropods getting entrapped by the resin may have been much less, as the resin surfaces hardened faster at lower nighttime temperatures or the resin flow stopped completely. Finally, the seasonal factors may also have had an influence on the composition of animal inclusions.

Current observation reveals that resin secretion is also highest in spring and summer in subtropical climates, which could significantly limit the possibility of preserving typical fall or winter faunas. The extreme rareness of winter crane flies (Trichoceridae) in Baltic amber is offered as evidence.