Amber provides a unique window into past organisms and ecosystems

By Dane McDonald

The sight of fossilized amber, be it a photograph (Image 1) or the real object (not available!!), tends to captivate the viewer immediately. In spite of its beauty, amber provides unique insight into terrestrial forest paleoenvironments and also into micro- and macro-evolutionary processes [1].

Image 1. A Miocene, Nymphalid butterfly in Dominican amber [2]

Amber is the fossilized form of tree resin and has properties similar to amorphous (a solid that is not crystalline; Oxford dictionary of science, 2003) polymeric glass [1]. The fossilization of amber occurs due to polymerization (i.e. cross-linking of organic compounds with hydrogen bonds) of the tree resin [5], which consists of a complex mixture of terpenoid and/or phenolic compounds [1].

Fossil amber is derived from several different tree families of which the Pinaceae, Araucariaceae, and Leguminoseae are most common. [1,4]. Most of the ambers that subscribe to these families have a Miocene (25-14 mya) Dominican or Eocene (55-38 mya) Baltic origin [4]. There are also Saxon (Europe), Florrisant (Colorado, U.S.) and Lebanese ambers. The Baltic region contains approximately 80% of all amber deposits. Geological processes can explain the occurrence of this large deposit- an ancient river course (“Eridanus”) transported resin, still plastic and soft, into a delta [6]. This delta emptied out into an ancient sea basin called the ‘Tethys’. It is in this basin that the resin underwent much deposition and began its long metamorphosis into amber [6]. Most resin deposits would have been fossilized in a similar manner.

Due to the botanical, geographical, and age differences of amber deposits (especially Baltic versus Dominican), researchers have questioned whether comparisons of amber palaeoecosystem structures are possible [4]. With this question in mind, Penney and Langan (2006) analyzed the size distribution of 671 fossilized spider species preserved in Dominican and Baltic ambers to determine whether amber-forming resins trapped organisms in a uniform manner. They found that Baltic amber generally contained larger arboreal web-spinning spiders. This however, had nothing to do with the amber’s differing properties. They concluded that the greater structural complexity of the Baltic amber-producing trees compared to the Dominican amber producers, favoured larger aerial web-spinning spiders. Thus no resin-entrapment biases were evident. Furthermore it follows that Baltic and Dominican amber forests are directly comparable in this respect [4].

Besides the fact that there may be differences between fossil ambers (as shown above these are proving to be irrelevant in comparisons), they are collectively unique in the sense that they ‘capture’ fossil organisms, especially terrestrial arthropods, in a manner that reproduces their ancient ecological interactions (Image 2). This is shown by ‘syninclusions’ where interactions between two or more organisms are preserved in the same piece of amber. These fossil ambers preserve interactions such as mating, mate guarding, parasitism, commensalism, disease, egg laying, defecation, and maternal care (E.g. ants carrying larvae and pupae) [1]. Similar observations are seldom, if ever, seen in the non-amber fossil record due to differing taphonomic processes that control preservation in different mediums (E.g. carbonate rocks vs. amber) [1].

Image 2. A group of winged ants interacting in Dominican amber [1]

Taphonomy can be defined as the study of decaying organisms over time [7]. It enables a better understanding of biases present in the fossil record. Biases are evident when a fossil assemblage contains more of one type of fossil than another [7]. This can be observed in amber, which has an affinity for preserving insects, spiders, and other terrestrial arthropods [1]. As a result one can either infer that the organism was present in greater numbers, or that its remains are more resistant to decomposition [7].


The preferential fossilization of terrestrial arthropods, especially insects, is extremely important for a variety of palaeontological reasons. According to Reaka-Kudla et al (1997) this is the most dominant and important group (besides humans) that affect life on earth. Furthermore insects and their relatives live on all continents and occupy a variety of microhabitats (E.g. deep soil, tree-tops). It follows that arthropods, as a result of their pervasiveness, play an integral role in life within ecosystems (pollination, nutrient cycling, and population control) (Reaka-Kudla et al, 1997). These important roles are not just evident in recent time but would have been very important during our geologic past. It is important to note that a profusion of insects in amber could explain a benign environment across many habitats and vast geographical areas in the past (i.e. benign in a sense of plenty water and moderate climate) [8].

Although amber-preserved arthropods play a very important role in understanding the past, other amber-preserved organisms also have much to contribute. Bauer et al (2005) described an early Eocene gecko from Baltic amber that had important implications for understanding the evolution of adhesion in climbing geckos. A study by Schmidt et al (2004) identified four new amoebae taxa in Cenomanian (i.e. late Cretaceous, approximately 100 mya) amber from Schliersee (Southern Germany). They went further and explained that “…the presence of 100-mya-year old fossils with extant representatives suggests evolutionary stasis of these freshwater amoebae”.

Image 3. An early Eocene gecko in Baltic amber [9]

In addition to the excellent preservation afforded [2], fossilized amber has the potential to make a significant contribution to contemporary research areas such as global climate change [1]. In analyzing palaeontological evidence from the amber fossil record scientists will be able to infer the patterns of past climate changes. This should help in predicting “…the potential consequences of climate change resulting from similar processes, in addition to the influence of anthropogenic factors”[1].

There is also the possibility, albeit remote, for the extraction of viable ancient DNA [3]. The debate of whether geologically ancient DNA exists or not, remains highly contentious. This is because previous claims have not been verified by independent replication, which is an important criterion in authenticity [3]. A rigorous study by Austin et al (1997) in which they attempted to reproduce DNA sequences from amber- and copal-preserved bees and flies failed to produce any authentic ancient insect DNA. This lack of reproducibility suggested that DNA could not survive over millions of years even in amber, which seems to be the most promising fossil environments.

In conclusion it is relatively clear that fossil amber has made and will continue to make an important contribution to the reconstruction of past biodiversity.



References:

1] Penney, D (2006) Fossils in amber: unlocking the secrets of the past. Biologist, 53#5:247-251

2] Penalver E, Grimaldi DA (2006) New Data on Miocene Butterflies in Dominican
Amber (Lepidoptera: Riodinidae and Nymphalidae) with the Description of a New Nymphalid. American Museum of Natural History, #3519.

3] Austin JJ, Ross AJ, Fortey RA, Richard TH (1997) Problems of reproducibility–does geologically ancient DNA survive in amber-preserved insects? Proceedings of the Royal Society of London, 264:467-474

4] Penney D, Langan A (2006) Comparing amber fossil assemblages across the Cenozoic. Biology Letters, The Royal Society.

5] http://www.fossilmall.com/Stonerelic/amber/Fossil_Amber.htm

6] http://www.gplatt.demon.co.uk/baltic.htm

7] Wikipedia contributions: http://www.en.wikipedia.org/

8]http://jrscience.wcp.muohio.edu/fieldcourses03/PapersCostaRicaArticles/ComparativeAnalysisofInse.html

9] Bauer AM, Bohme W, Weitschat W (2005) An Early Eocene gecko from Baltic amber and its implications for the evolution of gecko adhesion. Journal of Zoology, London 265: 327-332

10] http://www.sciencedaily.com/releases/2004/11/04/041108021411.htm

11] Reaka-Kudla ML, Wilson DE, Wilson EO (1997) Biodiversity II: Understanding and Protecting Our Biological Resources. Joseph Hendry Press, 560 pp. ISBN: 0-309-520754