Sub-fossils in copal: An under-valued resource
Dr David Penney and Dr David I Green (UK)
Copal (derived from the Spanish copalli meaning incense), the precursor of amber, is subfossilised tree resin not old or polymerised enough to be classed as amber. Given that the transformation of resin into copal and then into amber is dependent on factors such as temperature and pressure, there is no set age at which one turns into the other and the nomenclature (with respect to age) of these different transitional stages is still being debated.
Some authors have proposed an arbitrary age of 2Ma to demarcate the transition from copal to amber, whereas others have suggested classifying anything that can be carbon dated as copal and anything too old for radiocarbon dating as amber. The debate continues and it seems that the age at which copal becomes amber will remain controversial for the foreseeable future. Furthermore, reaching a consensus terminology has been hampered by both amber researchers and dealers complicating the issue with terms such as sub-fossil resin, young amber, copal amber and so on.
Nonetheless, copal preserves insects and other arthropods in the same way as amber and, given the younger age, the inclusions are often preserved with stunning, life-like fidelity. Remarkably, and in contrast to amber, very little research has focused on inclusions in copal because of its young age relative to amber. Such specimens are not deemed old enough to be of any significance by many palaeontologists and, for similar reasons, few analytical studies have been undertaken.
A bibliographic search on Science Direct using the terms “fossil AND amber” (in all fields, for all years) yielded 2,015 results, whereas “fossil AND copal” yielded only 106. Colombian copal is most probably Pliocene/sub-Recent, with some samples ranging from as young as ten-years-old up to approximately 1,700-years-old, although ages dating back as far as Tertiary have been proposed in the literature, albeit without any supporting evidence.
In some samples from Madagascar, evidence of enhanced 14Carbon content (due to artificial 14Carbon from thermonuclear weapon tests) indicates those particular samples are very young indeed. Other sources of fossiliferous copal include the Kauri gum of New Zealand, Mizunami in Japan and deposits from Cotui in the Dominican Republic. However, inclusions in copal (even very young samples) can be informative at many different levels and the aim of this article, which is the third in a series of articles in this magazine on fossil arthropods preserved in resins, is to highlight the palaeontological potential of this resource and to discuss some possible future research directions.
Distinguishing copal from amber
Fossil inclusions in copal are readily available in the marketplace, both online and through retail outlets. In many cases, they are sold as copal, but sometimes they are passed off as amber and sometimes the terms ‘copal amber’ or ‘young amber’ are used. Occasionally, fossils described in the scientific literature as being preserved in Baltic or Dominican ambers, have subsequently been demonstrated to be much younger, sub-Recent fossils preserved in Madagascan copal.
In some instances, the copal had been treated by heating it under pressure in an autoclave to make it harder and change the colour, so as to resemble amber more closely. There are undoubtedly still early examples of nineteenth century descriptions currently accepted as being in Baltic amber that actually refer to specimens in copal, although in many such cases the specimens have been lost.
Copal is softer than amber, but will still take on a high polish. However, because the volatiles are still evaporating from the sub-fossil resin, the surface may become highly crazed after only a few years (this process takes much longer with amber). Nonetheless, in some specimens of Zanzibar (East African) copal, the polished surface appears to have remained in relatively good condition even after 180 years of exposure to air. Copal is usually lighter in colour and may even appear colourless, compared to the yellow–orange colour usually seen in amber specimens.
More often than not, copal specimens tend to be larger and also to contain a wide range of inclusions, usually belonging to extant (living) species. Sometimes, fossils in amber show signs of compression and transformation as a result of exposure to high temperature or pressure. We have not come across this phenomenon in copal specimens. With some experience, it is usually possible to tell whether or not a specimen is amber or copal from its appearance and the general feel of it.
However, if in doubt, a relatively simple test is to apply a small drop of alcohol or other solvent on the surface of the specimen. If the specimen is amber, this will have no effect, but if it is copal it should dissolve the surface slightly, leaving it tacky to the touch. Another simple comparison is the surface scratch test. Copal scratches readily with a pin generating a rough powder, whereas amber is much harder and takes considerably more effort to scratch.
Understanding the bias of amber preservation
Reconstructing fossil tropical ecosystems relies on data from inclusions in amber. Most pieces of excavated amber tend to be less than 10g in weight, usually with a single or only a few syninclusions (fossils preserved together in the same piece) and, therefore, an indication of the community structure preserved at any one time is limited. This can be overcome to some degree by studying different pieces with shared syninclusions (that is, a gathering of different inclusions in one amber sample), but this cannot guarantee that particular species co-existed at the same time and in the same location. In contrast, specimens of Colombian copal up to 1kg are not uncommon and can be packed with inclusions (several hundred), all of which perished within minutes of one another, usually no more than a few centimetres apart.
If left unearthed and discovered 20Ma from now, this copal (as amber) would provide future investigators with a window on our current biodiversity crisis, often referred to as the sixth mass extinction. Indeed, some of the few species described from Colombian copal are already extinct, for example, a species of orchid bee described in 2007 and even a butterfly that has not yet been formally described. Of course, it may also be the case that such identifications refer to undiscovered extant taxa, but, with highly apparent organisms such as bees and butterflies, it would seem unlikely.
Such a situation did occur recently with a fossil spider found in Madagascan copal, which was initially thought to represent a new extinct species. However, a subsequent search of a museum collection identified a recently collected specimen of the same species preserved in alcohol. This research was published based on an excellent study using X-ray computed tomography to compare the fossil and Recent material. The bark louse (Psocodea) illustrated in Fig. 3 represents a new undescribed species of Epipsocus (family Epipsocidae), but it would be highly premature to suggest that this was an extinct species given the diverse and largely unknown extant Colombian psocid fauna.
For some of the more cryptozoic groups, such as scorpions, both fossil and extant species have been described. Some insect families are known as ‘fossils’ only from Colombian copal, for example, the bark louse (Psocodea) family Ptiloneuridae. To future researchers, these fossils would provide an insight into the biodiversity of the neotropical forests we are striving to conserve at present. Such ecosystems have notoriously poor potential for fossilisation in the usual sense, due to their hot, humid nature and vast armies of decomposers and recyclers.
Therefore, as with the long extinct subtropical Cretaceous forests of Lebanon, New Jersey and so on, and the Tertiary of the Baltic region, it can be expected that millions of years from now, little will remain of our current neotropical forests other than arthropods entombed in amber (assuming humans are extinct and existing biodiversity data from museum collections and the like have vanished). Future investigators would be faced with similar problems that amber researchers face today, such as not understanding the biases of preservation or what the implications of these data may be for any conservation efforts directed at their extant forests.
The community structure of fossils in Colombian copal can serve as a proxy for understanding the bias of preservation in amber. This is because entrapment in Colombian copal forming resin can be expected to be analogous to entrapment in Dominican amber forming resin – both occurred in neotropical forest ecosystems (with similar faunas) and both were produced by the same tree genus, Hymenaea.
Furthermore, spiders in Dominican amber have been demonstrated as having South American origins, or at least affinities, and a study of the ant assemblage preserved in Colombian copal showed greater similarity to that preserved in Dominican amber than to the extant Colombian fauna (although the latter may have been an artefact of sampling methodology). Therefore, both fossil faunas (Colombian copal and Dominican amber) can be considered comparable.
Biodiversity and ecology
Copal preserves a range of arthropods similar to that seen in amber and they can be studied using similar techniques (see Issue 26, Biodiversity of fossils in amber, and Issue 27, Preparation and study of fossils in amber).
However, in contrast to amber, most of the inclusions in copal belong to extant species. Preliminary examination of a 3kg sample of Colombian copal identified the following arthropod orders: Araneae (spiders), Acari (mites), Diptera (true flies), Coleoptera (beetles) , Hemiptera (true bugs), Hymenoptera (ants, bees and wasps), Mantodea (mantids), Blattodea (including Isoptera) (cockroaches and termites), Thysanoptera (thrips), Psocodea (bark lice), Lepidoptera (butterflies and moths), Orthoptera (crickets and so on), Trichoptera (caddisflies), Dermaptera (earwigs), Ephemeroptera (mayflies) and Archaeognatha (bristletails). As with amber, scorpions and pseudoscorpions are rare, but these have been described.
It can be expected that most arthropod orders will be recorded eventually. However, given the often larger pieces of copal (compared with amber) that are excavated, it is not uncommon to encounter interesting instances of replicated co-occurrence that are unlikely to have occurred by chance and which are unlikely to be found elsewhere in the fossil record. For example, the beetle Termitodius sp. (Scarabaeidae: Aphodiinae) with Coptotermes (Rhinotermitidae) worker termites. Extant species of Termitodius are termitophilous (termite associated) and are easily recognised by their carinate (keeled) dorsal surface.
The subfossil illustrated in Fig. 13 appears very similar to, and may well belong to, T. coronatus, the larvae of which live in the sides of termite nests. Such observations would be considerably less likely in Dominican amber because of the smaller specimen sizes, nor are they likely to be evident from Recent specimens in natural history museum collections that are usually sorted systematically. Therefore, the study of fossils in copal also has the potential to shed light on the current ecology of tropical ecosystems, in addition to providing palaeobiological data and new geographic records.
Previous claims of arthropod DNA extraction from amber appear to have been in error, with the DNA sequences that were obtained probably deriving from modern, non-insect DNA of fungal and/or vertebrate origin that had contaminated the samples. However, the problems of extracting DNA from amber specimens many millions of years old cannot necessarily be extrapolated to copal. Theoretical and empirical data indicate that ancient DNA fragments greater than 50 base pairs long are present in well-preserved materials up to at least 100,000 years in age.
Given their excellent preservation and relatively young geological nature, in addition to advances in next generation DNA sequencing techniques, it is not unreasonable to expect successful extraction of DNA from copal. If this is indeed possible, it will provide data on intra and interspecific genetic diversity to pre-date any current information based on museum collections. Genetic information from copal specimens would bridge the gap between contemporary/museum biodiversity and the fossil record, and would directly inform phylogenetic and evolutionary genetic research. Perhaps, most importantly, copal could yield molecular data on how genetic variation within species (a less familiar but important component of biodiversity) has changed over the scale of a few thousand years, if not longer.
Copal specimens, with genetically related individuals, are not uncommon, especially with regard to the traditional social insect orders (although these are common in amber also), but there are also examples of less common, communal (and therefore individuals that are genetically related) orders such as spiders. Indeed, the specimen illustrated in Figs. 14 and 15 represents the first ever (sub)fossil record of presumably communal spiders. Hence, there are excellent opportunities for potential research in the new discipline of molecular palaeobiology.
Rather than seeing copal as a confusing and frustrating amber imitator, we hope that this article has highlighted the potential value of this sub-fossil resin, not only in terms of the beauty and diversity of its inclusions, but also in terms its scientific value and potential.
About the authors
David Penney is an honorary lecturer in the Faculty of Life Sciences (Preziosi Lab) at The University of Manchester and a fellow of the Royal Entomological Society. He specialises in research on amber palaeobiology, and fossil insects and spiders. David Green is an honorary researcher at the National Museums Wales and has a keen interest in fossils in amber and photomicroscopy.
The authors are currently working on several projects together. For this article, we are grateful to David Grimaldi (American Museum of Natural History, New York) for his Colombian copal radiocarbon dating data; to Edward Mockford (Illinois State University) for Psocodea identification; to Hans Henderickx (University of Antwerp, Belgium) for the image of the scorpion in Madagascan copal; to Simon Barry Titchener and Barry George Titchener (Lyme Regis Fossil Shop, UK) for the loan of the Colombian copal specimen containing the communal spiders; and Sue Shawcross (University of Manchester, UK) for the loan of the specimen containing the evaniid wasp and the orb-web spider.
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