Fossils in amber (Part 1): Biodiversity

Dr David Penney and Dr David Green (UK)

It is almost two decades since the original blockbuster movie, Jurassic Park, brought the existence of fossil insects in amber (fossilised tree resin) into the limelight. Since then, numerous books and research papers have been published. Fossiliferous amber deposits are still being discovered, including, in recent years, the first major deposits in Africa, India and Australia. The market for fossils in amber experienced a boom in the 1990s, but it has since declined for various reasons, including fakery, copal (sub-fossil resin) being sold as genuine amber and the current economic conditions.

Nevertheless, there are many reputable sources for those wishing to develop their passion for amber – a substance that has fascinated people for millennia. It has been endowed with mystical, magical and medicinal properties, and used as an artistic medium and in jewellery. However, today, it is probably most famous for the fossil insect inclusions it preserves with life-like fidelity. It is these that are the focus of this article.

This is the second part of a series of articles on fossils in amber. The first is: Fossils in amber (Part 1): Preparation and study.

Important fossiliferous deposits

There are almost 200 known amber deposits around the world, some dating from as early as the mid-Carboniferous. Relatively few have produced abundant biological inclusions and those that do occur only in strata of Tertiary or Cretaceous age. Many of these ambers were produced by different tree families under somewhat different environmental conditions. However, due to the uniform trapping mechanism of tree resin, the different faunas they enclose can be considered to be broadly comparable in an ecological context. Nonetheless, the fossil assemblages from many of the deposits exhibit their own idiosyncrasies. Some of the more important deposits are shown in the table and are briefly discussed below.

Dominican Republic amber, from the Caribbean island of Hispaniola, probably exhibits the highest degree of preservation of inclusions of all known ambers. Despite the relatively late onset of study of this Miocene deposit in the 1960s, more than 1,000 fossil species have been described. Most of their closest relatives live in the neotropics today, but some do not. For example, the fossil termite, Mastotermes electrodominicus, has its closest affinities with extant Australian species.

Mexican amber, from the state of Chiapas in southern Mexico, is considered to be contemporaneous with Dominican amber and was produced by the same genus of tree. However, the quality of preservation of its inclusions is usually much poorer, possibly due to exposure to increased temperature and pressure, resulting from past volcanic activity in the region. The fossil fauna has received a lot less attention than Dominican amber, yet comparison of both faunas (one being continental, the other insular) has great potential to help us understand the origins of Caribbean biodiversity.

Bitterfeld amber, from Germany and sometimes referred to as Saxonian amber, has always been somewhat in the shadow of amber from the adjacent Baltic region. Indeed, its independent origin from the latter has been questioned on many occasions. The current consensus is that it is a different, younger (Miocene) deposit, despite the fact it shares many fossil species with Baltic amber.

Cape York amber, from Australia, was discovered in 2003 and is currently subject to intense studies. At least 25 families of terrestrial arthropods have been found, in addition to botanical inclusions, such as flowers and leaves. Fungi, air and water bubbles, and also small feathers and mammalian hair, are all preserved in this Tertiary amber, such that it represents a rare opportunity to conduct a quantitative investigation of a richly fossiliferous amber deposit before it is subjected to commercial development or selective collecting.

Baltic amber, from the Eocene of northern Europe, is by far the most famous and longest studied deposit, with more than 3,000 fossil species described from most arthropod orders. Unfortunately, many of the early descriptions date from the 1800s and are inadequate by modern standards. To compound the problem, many of the specimens on which the original species descriptions were based (type specimens) have been lost over the years. Despite the intense study of this deposit, there is still considerable debate regarding the identity of the tree providing the source of the amber.

Rovno amber, from the Ukraine, has only recently been investigated palaeontologically and demonstrated to be different from Baltic amber. Although the two ambers are chemically identical and have many shared fossil species, there are, importantly, also many unique to this amber. A remarkable 227 species of gall midges have been recorded from a single family (Diptera: Cecidomyiidae).

Indian amber, from Vastan, Gujarat is the only known deposit from the Indian subcontinent, but was only discovered this century. The investigation of its inclusions is still at a very early stage, but this has the potential to be informative from a palaeo/biogeographic point of view, because this lowermost Eocene amber was formed before the Indian–Asian tectonic plates collided, which occurred approximately 50Ma.

Oise amber was discovered in 1996 near Creil, Oise in France and is still under intensive investigation by scientists in Paris. The quality of preservation is excellent. It is extremely unusual for a Tertiary amber, in that it lacks jumping spiders (Salticidae) as fossil inclusions. These are the most diverse spider family on the planet today and are extremely common in other Tertiary ambers, but absent from Cretaceous deposits.

Canadian amber is of particular interest because it constitutes the last known diverse fossil arthropod assemblage before the end-Cretaceous extinction event. It comes from two main areas: Grassy Lake in Alberta and Cedar Lake in Manitoba, the latter being a secondary deposit of the former. Unfortunately, little research has focussed on this fauna, despite the large collections preserved in Canadian museums.

New Jersey amber is the most significant amber deposit in the United States. Discovered in the 1990s, this new Cretaceous outcrop has yielded thousands of inclusions, many of which are still being studied. Particularly notable discoveries to date include only the second Cretaceous tardigrade (a close relative of arthropods) to be found anywhere in the world and a high diversity of ant subfamilies. It also contains an unusually high abundance of scale insects (Coccoidea) and Lepidoptera.

Ethiopian amber, from the Cretaceous Debre Libanos Sandstone Unit, which contains bountiful fossil inclusions, was first reported in 2010. It provides the earliest African evidence of important ecological groups, such as insects and spiders, and is of major significance for understanding the biogeography and evolutionary history of the African biota.

Charentese amber, from southwest France, has only received attention for its inclusions this century. Most specimens are opaque, so specialist techniques using synchrotron radiation technology is used to identify and image the preserved inclusions. In addition to the usual arboreal taxa, this Cretaceous amber has a large proportion of litter and soil dwelling organisms, and is particularly unusual for the relatively large number of brackish and marine groups encountered as inclusions.

Burmese (Myanmar) amber was originally considered to be Tertiary in age, but subsequent examination of the insect inclusions demonstrated the presence of several families and subfamilies, suggesting that it is, in fact, from the Cretaceous. This was confirmed by additional studies on foraminifera, palynomorphs and also the presence of a Cretaceous ammonite in a sandstone bed above the amber layer. Burmese amber is found in both Cretaceous primary deposits and also as re-worked material in Tertiary rocks. In particular, it preserves a particularly diverse neuropterid (lacewings and allies) fauna.

Spanish amber, with inclusions, has only recently been discovered and has received intensive investigation over the past decade. The identity of the amber producing tree is still unclear. Thousands of arthropod inclusions have been recovered, many of which are of significant taxonomic and palaeoecological value, as they originate from early in the Cretaceous, when many insect groups were radiating to become pollinators of the first flowering plants.

Lebanese amber (along with deposits from Jordan) is the oldest Cretaceous deposit yielding a diverse array of biological inclusions. It is contemporaneous with the appearance of angiosperms and newly evolving ecosystems, and documents the diversification of modern arthropods and the disappearance of some archaic groups. Some of the oldest specific behaviours of numerous insect groups in the fossil record (for example, mating in scatopsid flies, mite parasitism on chironomid midges, and so on) are documented from this amber deposit.

Biodiversity of amber inclusions

The diversity of organisms preserved in amber is staggering. It ranges from minute micro-organisms, including bacteria and protozoa, to fungi, lichens, bryophytes and liverworts, nematodes, annelid worms, rotifers, molluscs, velvet worms or onychophorans, frogs, lizards, birds (feathers and egg shells) and, occasionally, even small mammals. Even disease-vector associations are preserved, as in the microscopic trypanosomid parasites found in situ in their sandfly host. Plant structures are also preserved, including leaves, leaflets, stipules, petioles, bracteoles, buds, sepals, petals, stamens, receptacles, pistils and pollen, including from the amber-producing trees and sometimes in association with pollinating insects.

The most common inclusions are arthropods, especially insects, but, as well as insects, there are a substantial range of other creatures that can be found, as can be seen from the box entitled Animals found in amber as inclusions.

The high diversity of inclusions in many amber deposits permits both quantitative and qualitative ecological investigations of palaeobiotas to assess the impact of important events throughout the geological history of the Earth. Such events include mass extinctions, predator-prey ‘co-evolution’ over time, and the origin and radiation of angiosperms and their pollinators.

The closest living relatives of many inclusions in amber no longer occur in the same region today and some demonstrate extremely disjunct distributions (for example, some fossils in Dominican amber have their closest living relatives in Australia, and some Baltic amber taxa have their closest relatives in South Africa and Madagascar). Therefore, fossils in amber are important for addressing questions in current and past biogeography, in addition to providing additional data for phylogenetic studies, although many scientists working on living organisms still overlook, or even ignore, the potential importance of these fossils.

Probably, one of the most remarkable features of amber as a preservation medium is its ability to fossilise direct evidence of species interactions, described by some as ‘frozen behaviours’. These include mating, mate guarding, egg-laying, brood care, predation, parasitism (endo- and ecto-), relationships between animals that are neither harmful nor beneficial (that is, commensalism), swarming, dispersal (for example, the transport of one animal by another more mobile one in pseudoscorpions), camouflage and defensive behaviours.

In some specimens, it may be possible to determine the organisms’ previous meals, such as the camouflaged, possibly predatory insect larva illustrated that has disguised itself with remnants of prey carcasses in this case, an ant, a spider (family Oonopidae) and a pseudoscorpion. Alternatively, these may be random bits of debris collected by the organism to conceal itself. It follows that, when examining amber specimens, it is always important to look for the bigger picture. Most specimens have more information to divulge than merely the presence of single fossil inclusion.

Animals found in amber as inclusions

While the most common inclusions are arthropods, especially insects, the following are also found: arachnids (spiders – Araneae, scorpions – Scorpiones, pseudoscorpions – Pseudoscorpiones, mites and ticks – Acari, whip spiders – Amblypygi, schizomids – Schizomida and sun spiders – Solifugae), myriapods (centipedes – Chilopoda and millipedes – Diplopoda) and crustaceans (woodlice – Isopoda and even some marine forms such as amphipods). The commonest insects found tend to be flies (Diptera), beetles (Coleoptera) and small wasps (Hymenoptera). Ants (Hymenoptera: Formicidae) are particularly abundant in some Tertiary ambers, but are considerably less common in Cretaceous deposits. Silverfish (Zygentoma), bristletails (Archaeognatha), true bugs (Hemiptera), termites (Isoptera), caddis flies (Trichoptera) and bark lice (Psocodea) are also reasonably common. Lacewings (Neuroptera), crickets (Orthoptera), stoneflies (Plecoptera), mayflies (Ephemeroptera) and cockroaches (Blattodea) all have a reasonable number of species described from fossils in amber, but they are uncommon compared to the groups mentioned above. The following orders are rarely encountered and are highly prized by collectors: twisted-winged parasites (Strepsiptera), butterflies (Lepidoptera – micro-moths are more common), webspinners (Embiodea), zorapterans (Zoraptera), dragonflies and damselflies (Odonata), fleas (Siphonaptera), scorpion flies (Mecoptera) and alderflies and so on (Megaloptera). This pattern tends to hold true for most of the different ambers discussed in this article, and many of these orders have not been recorded from smaller or less well-studied deposits.

Distinguishing amber from copal and fakes

Given the prices that museums and wealthy private collectors (or even tourists on holiday) are prepared to pay for rare amber inclusions, such as small vertebrates or large arthropods, it is not surprising that forgeries are commonplace. These are relatively easy to make and, in some cases, can be difficult to detect. They can also have serious financial consequences for the buyer and important scientific ramifications, if their systematic descriptions or biogeographical data enter the scientific literature.

The increasing awareness of the existence of fakes lessens confidence in buyers. This is especially so for specimens offered by small, local dealers, to the extent that some people only buy from large, reputable institutions (such as museums), who guarantee authenticity and provenance, although often at much higher prices. Therefore, fake amber specimens have economic consequences for sellers as well as buyers. Even buying amber directly from miners or collectors at the source does not guarantee authenticity, although it is less likely the specimen will be a fake.

The techniques for faking amber fossils are varied. Recent organisms may be embedded in copal, synthetic resins or in a hollowed-out piece of authentic amber and sealed with a natural or synthetic resin. Methods available for detecting forgeries include:

• Fluorescence under ultra-violet light (plastics do not fluoresce, but amber does).
• Floatation in concentrated salt solution (amber and copal float, most — but not all — plastics sink).
• Hot-needle test (amber and copal give off a resinous odour when pierced with a red hot needle, plastics smell of burning plastic).
• Surface scratch test (amber generates a powder, plastic forms a coiled thread).
• Ethanol, ether or xylene drop reaction test (the surface of amber reacts with none of these, but copal becomes tacky, while some plastics react only with ether).
• Infrared spectroscopy, pyrolysis-gas chromatography, pyrolysis-gas chromatography/mass spectroscopy (chemical signatures distinguish real amber from fake).

Identifying authentic amber can be relatively easy, although many collectors are unwilling to submit their specimens to tests that may damage them, even slightly.

There is a second part to my discussion of insects, see

Preparation and study of fossils in amber

Careful examination of the matrix and the inclusion can provide insights into whether or not a piece is likely to be genuine. The presence or absence of impurities and imperfections is a good indicator, but not wholly reliable. Some authentic pieces may be devoid of any material other than the inclusion (which might lead one to suspect a fake), whereas some forgers sprinkle particulate debris in their synthetic resin to make it look more convincing.

Symmetrically preserved organisms (that is, legs and/or wings splayed out equally on both sides of the body) should be immediately treated as suspicious. Amber often preserves the death throes of the entombed arthropods as they struggled to escape the sticky exudates, for example, in the form of wing movements, disarticulation of body parts, detachment of legs (sometimes also with fossilised blood droplets or haemolymph preserved), so the legs and wings usually end up jumbled and folded. In faked inclusions, it is often possible to detect a thin layer of air between the organism and the matrix, because polyester does not have the wetting properties of natural resin.

Sophisticated forgeries have included the Bement Frog in the American Museum of Natural History, New York and the Piltdown Fly in the Natural History Museum, London, both of which involved inserting dead, extant organisms into pieces of authentic amber.

About the authors

David Penney is a Visiting Scientist in the Faculty of Life Sciences (Preziosi Lab) at the University of Manchester, who specialises in research on amber palaeobiology and spiders. David Green is a Visiting Scientist in the School of Earth, Atmospheric and Environmental Sciences at the University of Manchester and has a keen interest in fossils in amber and photomicroscopy. They are currently working on several projects together.

References

Grimaldi, D.A., Shedrinsky, A., Ross, A.J. & Baer, N.S. 1994. Forgeries of fossils in ‘amber’: history, identification and case studies. Curator, 37: 251–274.

Penney, D. (ed.) 2010. Biodiversity of fossils in amber from the major world deposits. Siri Scientific Press, Manchester. (Available directly from the publisher at http://www.siriscientificpress.co.uk.)

Penney, D. & Green, D.I. 2011. Fossils in amber: snapshots of prehistoric forest life. Siri Scientific Press, Manchester. (Available directly from the publisher at http://www.siriscientificpress.co.uk.)

Poinar, G.O. Jr. 1992. Life in amber. Stanford University Press.

For more articles in past issues of Deposits on amber, see:

Deposits, Issue 19: Amber deposits of the Dominican Republic’s northern cordillera by George Burden.

Deposits, Issue 18: Rare amber inclusion of harvestman donated to Natural History Museum by Terence Collingwood.

Deposits, Issue 11: The return of Burmite Amber by David Lamb.

Deposits, Issue 10: Amber – frozen moments in Time by Gary Platt.