Biodiversity of fossils in amber

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.

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.

Botanical origins and ages of amber deposits mentioned in the text.

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.

A predatory fly, Empis sp. (Empididae), with its cecidomyiid prey caught on the wing, in Baltic amber. (From the collection of D Penney.)

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 50mya.

Spider: Mastigusa bitterfeldensis (Dictynidae) in Baltic amber, showing the extremely modified pedipalps characteristic of the genus. (From the collection of D Penney.)

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.

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