A review of amber and copal occurrences in Africa and their paleontological signi ﬁ cance

– The paleontological interest for fossil plant resins (amber and copal) has greatly increased in the last decades, as ﬁ eld studies have resulted in the discovery of various new deposits worldwide. Yet, amber-rich deposits remain particularly scarce on continents from former Gondwana. Here we review the known occurrences of copal and amber from Africa, with a state-of-the-art regarding the age dating, the putative plant sources, the fossil content, as well as the paleoenvironmental settings. The ﬁ rst African ambers known to yield arthropods and other organismal inclusions, found recently from the early Cretaceous of Congo and the Miocene of Ethiopia, are brie ﬂ y overviewed. ’ Ethiopie, sont brièvement présentés.


Introduction
Fossilized resins represent important sources of paleontological data for reconstructing ancient terrestrial ecosystems. A single fossiliferous deposit can provide abundant, various, and often exquisitely preserved organic inclusions (mostly arthropods, plants, protists, and fungi; more rarely microalgae, molluscs, and vertebrate remains). Fresh liquid resins solidify and progressively transform into fossil resins during burial through the chemical process of polymerization, during which their volatile compounds are lost (Anderson and Crelling, 1995;Clifford and Hatcher, 1995;Seyfullah et al., 2018). Amber is a fully polymerized, cross-linked resin that can at most softened using organic solvents. A resin that is incompletely cross-linked and not fully polymerized can dissolve in various solvents and is sometime called copal, although the use of this term is ambiguous (Anderson and Crelling, 1995;Langenheim, 2003;Vávra, 2009;Lambert et al., 2012;Penney, 2016;Delclòs et al., 2020). The time needed for full polymerization greatly varies depending on the resin composition and the burial conditions, so that there is no temporal feature to characterize copal and amber. Commonly, however, copals are considered as young, semifossilized resins ranging from Recent up to the Pliocene age (5.3 Ma) (Poinar, 1992;Grimaldi, 1996;Delclòs et al., 2020). But there are older resins that are only partially polymerized and can readily dissolve, or that remain sticky and aromatic when cut or polished, and should therefore be copals. Nonetheless, these are generally classified as ambers because of their antiquity (e. g., Eocene Cambay "amber" from India, or Miocene "amber" from Indonesia; see Rust et al., 2010;Mazur et al., 2014;Narudeesombat et al., 2014). In the present review of all known fossil resins from Africa, "copal" and "amber" are used in this temporal sense rather than chemical sense, i.e. regardless of their level of polymerization.
The vast majority of amber deposits known today occurs in the Northern Hemisphere, particularly from the former Laurasian continent. The paucity of Gondwanan amber deposits may results from a long lack of prospection for this geological material in the Southern Hemisphere, combined with typically limited outcrops due to the dense plant covering in intertropical zones. Amber deposits of Gondwanan origin comprise Indomalayan occurrences in India (early Eocene; Rust et al., 2010) and possibly Myanmar (early Cenomanian; at the time probably an island in the Tethys Ocean but originating from Austral Gondwana, see Rasnitsyn and Öhm-Kühnle, 2018, Poinar, 2019or Westerweel et al., 2019; Australasian occurrences in Australia (Triassic to Neogene; Hand et al., 2010;Sonibare et al., 2014;Stilwell et al., 2020) and New Zealand (Cretaceous, Oligocene and Miocene; Schmidt et al., 2018;Mays et al., 2019;Stilwell et al., 2020); South American occurrences in Brazil (early Cretaceous; Pereira et al., 2011), Ecuador (Aptian-Albian; Cadena et al., 2018), Venezuela (Miocene, Pérez et al., 2016), and Peru (Miocene; Antoine et al., 2006); and rare African occurrences that are detailed and reviewed hereafter along with the known copal deposits.

Copal
Copal had once a major export value for its industrial use in varnishes and lacquers. A rich literature exists on African copals, their botany and their trade that developed during colonialism in the nineteenth and twentieth centuries. Abundant copal resources have been exploited mostly from two regions of the continent: Zanzibar and the German East Africa, that became a British territory after WWI, and are now corresponding to the modern Tanzania (Kirk, 1871;Schlüter and von Gnielinski, 1987;Sunseri, 2007); and the Democratic Republic of the Congo (DRC, formerly Congo Free State, then Belgian Congo) (Pynaert, 1924;De Wildeman, 1927;Vervloet, 1932;Hellinckx, 1935;Anonymous, 1942;Billing, 1944). Copal is more generally known from several countries in East Africa and West Africa (Fig. 1). All African copals derive from legume trees in the family Fabaceae, mostly Hymenaea or Guibourtia in the Caesalpinioideae group, but also Colophospermum, Copaifera, Daniellia or Tessmannia in the Detarioideae group (Howes, 1949;Langenheim, 2003;McCoy et al., 2017;Delclòs et al., 2020). Page 2 of 11 V. Bouju and V. Perrichot: BSGF 2020, 191, 17 2.1 East African copals Pleistocene or even younger copal has been reported from various countries along the African East coast: Kenya, Tanzania, Mozambique, Madagascar (Fig. 1). Copal was traded into Indian Ocean networks, including India, by Swahili communities as soon as in the eighth century AD, but a rise in the trade came with the arrival of European and American ships in Zanzibar between the sixteenth and nineteenth centuries (Hedley Barry, 1932;Howes, 1949;Sunseri, 2007;Delclòs et al., 2020). Most of the copal traded on the Zanzibar market, however, originated from the coastal forests of mainland Tanzania. This brings confusion as to whether the bioinclusions described from the so-called Zanzibar copal are actually from there or from Tanzania (Schlüter and von Gnielinski, 1987;Stroiński, 2007). Here we use Tanzanian copal in its broad sense including deposits from coastal hinterlands of Tanzania as well as the Zanzibar archipelago. The trade of East African copal, particularly that of Madagascar, has been recently reviewed by Delclòs et al. (2020).
According to botanical observations in the above references, the legume tree Guibourtia (with species formerly assigned to the genus Copaifera) is considered to have produced most of the Congo and other West and Central African copals. But some Congo copal is also derived from Tessmannia, and copal from Angola is thought to originate from Colophospermum. Copaifera or Daniellia are also considered as important producers of copals from Benin, Ghana and Togo (Léonard, 1950;Langenheim, 2003). Resins of all these genera and Hymenaea are relatively similar in their molecular composition and are thus difficult to distinguish chemically (Lambert et al., , 2009).

Amber
Amber has been comparatively less frequently encountered than copal in Africa (Fig. 1). The rare occurrences until now have mostly consisted in small deposits yielding only few amber pieces apparently devoid of organic inclusions. Recently, however, the first two fossiliferous African ambers have been discovered, in Congo and Ethiopia (Schmidt et al., 2010a(Schmidt et al., , 2010bPerrichot et al., 2016Perrichot et al., , 2018. The study of these deposits and their fossil content is still in a nascent stage, so that only a preliminary review is given herein.

Lesotho
The oldest fossilized resin known from Africa dates back to the Upper Triassic, in the Molteno Formation, and consists of few millimeter-sized amber drops found fossilized within gymnosperm cones (Ansorge, 2007). The seed-producing gymnosperm was evidently the plant source of the resin, but its precise affinity remains unknown and a chemical characterization of such a tiny amber sample remains challenging. The tiny size and preservation within seeds has also prevented the finding of any organismal inclusions within this material.

South Africa
Numerous small (max. 7 mm) pieces of yellowish to red amber have been found in the Middle-Upper Valanginian (Lower Cretaceous) Kirkwood Formation (Gomez et al., 2002a(Gomez et al., , 2002b. The amber-bearing rock also yielded abundant remains of Brachyphyllum, a conifer genus of ill-defined position within the Araucariaceae or Cheirolepidiaceae, and possibly the resin-producer. This amber is devoid of inclusion. Paleobotanical and taphonomical studies suggest that the resin was produced under a warm to hot, semi-arid climate, and was washed from its primary environment into a fluvial deposit during a flooding event (Gomez et al., 2002a).

Congo
The only fossiliferous Cretaceous amber from the continent has been discovered in 2011 near Doumanga, in the Mayombe belt, 70 km NE of Pointe-Noire. Amber and Page 4 of 11 V. Bouju and V. Perrichot: BSGF 2020, 191, 17 plant debris, including lignite and fusinite, have accumulated in lags within the Vembo Shales, a member of the Chéla Formation that was dated as middle Aptian based on the palynomorphs and ostracods from the amber-bearing shales Perrichot et al., 2016). An excavation by one of us (VP) in 2013 yielded about 5000 amber pieces, all dark, red to orange in color (Figs. 2B and 2C), commonly pebble-shaped, and up to 12 cm in diameter. Analysis of small, translucent pieces in light microscopy revealed the first arthropods, while synchrotron imaging has been used to radiograph a first portion of dark, larger pieces. The studied sample appears moderately fossiliferous, with a dozen families identified in 9 arthropod orders (Tab. 1; Figs. 3A and 3B). Plant debris (stellate hairs, wood fibers, spores) or fungal/bacterial remains also occur in many pieces.
The molecular composition of Congolese amber indicates a Group A in 13 C-NMR (sensu Lambert and Poinar, 2002) and Class Ib in THM-GC-MS (sensu Anderson, 1994), indicative of resins from the conifer families Araucariaceae, Cupressaceae, Podocarpaceae, and Cheirolepidiaceae. The wood remains associated in the amber-bearing rock belong to Brachyoxylon and Agathoxylon, and the palynomorphs comprise Araucariacites and dominant Classopollis, suggesting an Araucariaceae or Cheirolepidiaceae as the plant source. Amber and plant debris were washed into a saline, lacustrine environment in a small, inland, rift basin connected to the ocean, and accumulated in flood related gravity driven deposits, as indicated by sedimentological and taphonomical studies. In addition with paleobotanical data, it also suggests a hot paleoclimate with contrasted, humid and dry seasons .

Niger
Small, isolated pieces of reddish amber have been collected in 2016 at Takalmaoua, Department of Dakoro, in Maradi Region. The amber was found in a well, at a depth of about 50 m, within rocks of Cenomanian/Senonian age (A. Moumouni, personal communication). A chemical characterization of this amber by thermochemolysis gaschromatography-mass spectrometry (THM-GC-MS) has indicated a Class Ib amber (sensu Anderson, 1994), thus originating from a conifer (Perrichot, unpublished data). The few available pieces did not provide any inclusion.

Nigeria
A Lutetian or Lower Bartonian (Eocene) amber, named Amekit, has been found in the Ameki Formation near Umuahia, in southeastern Nigeria (Wilson, 1925;Arua, 1979;  Page 5 of 11 V. Bouju and V. Perrichot: BSGF 2020, 191, 17 Savkevitch and Arua, 1990). Amber chunks occur within three successive sedimentary facies of this formation, and are generally associated with lignite and various marine organisms, suggesting they were washed into a marginal lagoon environment from a nearby mangrove-swamp area (Arua, 1979(Arua, , 1986). An angiosperm source has been suggested for Amekit based on the absence of gymnosperms among the fossil plants from the amber-bearing strata (Rao and Kumaran, 1988). But the molecular characterization of the amber by infrared spectroscopy and gas chromatography-mass spectrometry (Py-and THM-GC-MS) indicates a Class Ib amber typical of conifers, possibly the family Cupressaceae (Sonibare et al., 2012). No organismal inclusions has ever been mentioned from this amber despite large pieces of yellow or dark red, translucent amber (Arua, 1979). However, there has been virtually no material available for screening by paleoentomologists since the formal description of this amber four decades ago. It is possible that an older amber also exists in southern Nigeria, as mentioned by Le Gall et al. (2010). Indeed, some pieces were found associated with mosasaur teeth and ammonites in the Upper Cretaceous (Maastrichtian) strata that outcrop between Enugu and Port Harcourt, thus in the surroundings of the Ameki Formation (Ph. Courville, personal communication). Whether this corresponds to a true Cretaceous, primary deposition, or a redeposition after the erosion of above layers would require further investigation. But fieldwork in Nigeria is currently hampered by the geopolitical situation.

Ethiopia
The richest sources of amber in Africa are outcrops located in the gorges of the Wenchit, Jemma, and Mugher rivers incising the northwestern Ethiopian plateau in the North Shewa Zone of the Amhara Region. At least 20 kilograms of amber are extracted annually by villagers, apparently with all excavations in a same siltstone that was erroneously attributed to a Cretaceous unit, the Debre Libanos Sandstone (Schmidt et al., 2010a;Kiefert, 2015). The Cretaceous age of the rock was suggested by local geologists based on field observations and correspondence with the geological map of Belay et al. (2009); it was also estimated based on the analysis of the chemical features of the amber and the sporomorphs of the associated sediment (Schmidt et al., 2010a). However, two of the sporomorphs that had been identified as Cretaceous taxa were mistaken, and other sporomorphs that had been left undetermined have eventually been identified as long ranging Cenozoic taxa . Recent investigation of additional amber material has provided arthropod and plant inclusions that belong almost exclusively to extant genera (Tab. 1), therefore modern lineages which definitely rule out a Cretaceous age. Additionally, the rocks associated with this new material have yielded palynomorph assemblages that are indicative of an Early Miocene age . According to geological studies, the only sedimentary rocks known in this area are either Upper Cretaceous, referred to as Debre Libanos Sandstone or Upper Sandstone; or Upper Page 6 of 11 V. Bouju and V. Perrichot: BSGF 2020, 191, 17 Miocene, referred to as "Tertiary sediments" (Belay et al., 2009).
Data are yet insufficient to explain this incongruence with the Early Miocene age suggested by our paleontological results.
Ethiopian amber is hard, translucent, with colors ranging from orange to yellow greenish, light green, or dark green (Figs. 2D-2F), and pieces are commonly 5 cm or more in size. Analysis of nine amber samples in THM-GC-MS has consistently recovered a Class Ic amber, regardless of the color, and comparison with the spectra obtained from Dominican and Mexican ambers suggests the angiosperm family Fabaceae as the plant-source (unpublished pers. data).
The finding of fossil leaves and stamens of Hymenaea in several amber pieces similarly suggests this legume tree as the resin producer (J. Szwedo, personal communication).
The first study of Ethiopian amber had already recovered a diversity of arthropod and fungal inclusions (Schmidt et al., 2010a(Schmidt et al., , 2010b. Newly investigated material confirms the richness of Ethiopian amber inclusions and undoubtedly shows taxonomic affinities with Cenozoic biota (Tab. 1; Figs. 3C and 3D). Remarkably, this amber yields a fairly high proportion of plant inclusions, among which frequent remains of liverworts (Marchantiophyta) and angiosperms (Fabales), more rarely Page 7 of 11 V. Bouju and V. Perrichot: BSGF 2020, 191, 17 leafy mosses (Hypnales) and lichens (Lecanorales). Most plant and insect taxa found as inclusions are reported for the first time from the African fossil record (e.g., Frullaniaceae, Lejeuneaceae, Pylaisiadelphaceae, Formicidae: Dorylinae, Pseudomyrmecinae, Mycetophylidae). And many taxa are the first fossil occurrence of extant genera, thus bridging a gap in the evolutionary history of the corresponding lineages. A particular focus has been made on ants, revealing at least 13 genera, 10 of which are still extant. Several of these contemporary genera have relatively recent crown-group origins, for instance with the following age estimates obtained from molecular phylogenetic analyses ("Mean" values, in Ma): Hypoponera (30-37) for Ponerinae (Schmidt, 2013); Ravavy (27) and Technomyrmex (23-33) for Dolichoderinae (Ward et al., 2010);Cataulacus (18.3-20.4) and Trichomyrmex (20-22.7) for Myrmicinae (Blaimer et al., 2018). These occurrences support a Miocene maximal age dating for Ethiopian amber.
Finally, the palynomorph assemblages are suggestive of a lowland tropical rainforest, without any marine input. The presence of abundant liverworts assignable to Lejeuneaceae similarly indicates a humid tropical forest. Ethiopian amber was thus derived from Hymenaea trees likely growing in evergreen rainforests, rather than in dry environments like some recent Hymenaea species (Langenheim et al., 1973).

Conclusion
Several evolutionary trends can be seen from the few African deposits of fossil resin (Fig. 4). Ancient, Cretaceous to Eocene resins were all derived from conifers (class Ib type resins) while younger, Miocene to Recent resins derive from angiosperms in the Fabaceae family (class Ic type resins). Also, Araucariaceae or Cheirolepidiaceae were apparently the dominant amber-producers during the Early and mid-Cretaceous (South-Africa, Valanginian; Congo, Aptian), possibly replaced by Cupressaceae in the Eocene (Nigeria). This is similar to the global pattern, observed from amber deposits worldwide, of a shift to dominance of resin-production by Cupressaceae or Pinaceae in the Late Cretaceous, and by (sub) tropical angiosperms in the mid-Cenozoic, with the exception of the kauri copal produced by Agathis (Langenheim, 2003;Nohra et al., 2015;Seyfullah et al., 2018).
This change in the resin-producing trees was accompanied by a change in climate and environment. For the Cretaceous, sedimentary and fossil data in South Africa and Congo indicate a warm, subtropical climate with at least temporary arid conditions. From the Eocene onward, ambers from Nigeria and Ethiopia, and copals from eastern and western-central Africa have derived from lowland, tropical evergreen rainforests with (Nigeria) or without (Ethiopia) marine influence. Further study of fossil inclusions of fungi, plants, and arthropods from Congolese and Ethiopian ambers will thus likely further substantiate such contrasted habitats.