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 2myrs 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.