New candidates for the Baltic amber source plant

habitats, non-flooded areas also existed which were covered by mixed mesophytic conifer-angiosperm forests (Sadowski et al. 2017a [5]).

4.1.4 New candidates for the Baltic amber source plant

Although a coniferous origin of Baltic amber was proven (chapter 1.6.2), the source plant is still a mystery. Regarding the newly described conifer taxa from Baltic amber presented in the previous chapters, new candidates should be further investigated for assessing the botanical origin of Baltic amber. Mostly, the Baltic amber source plant has been suggested as being pinaceous, such as Pinus succinifera which has been discussed as Baltic amber tree for almost two centuries. Besides numerous palaeobotanical studies (e.g. Schubert 1961, Dolezych et al. 2011) and chemical analyses of Baltic amber and extant resins (e.g. Kosmowska-Ceranowicz 2015, Wolfe et al. 2009), it is still unclear if Pinaceae resin is really suitable for the formation of amber or not. Several types of diterpene acids (e.g. abietic and pimaric acids), which are abundant in pinaceous resins do not polymerize and thus, are less likely to persist in the rock record and to form large amber deposits (Langenheim 2003, Ragazzi and Schmidt 2011). However, there is conflicting evidence from several studies discussing Pinaceae, especially Pinus, as Baltic amber source tree.

For instance, Mosini and Samperi (1985) discovered correlations between Baltic amber and resin of extant Pinus halepensis, after they had artificially aged resin samples of four pine species by heating them at 110°C for 30 to 60 days maximum.

A gas chromatography–mass spectrometry (GC/MS) analysis of the ‘aged’ resins and Baltic amber revealed similarities, especially in resin acids which were transformed during the aging process (Mosini and Samperi 1985).

A further study linking the amber to a pinaceous origin was published by Dolezych et al. (2011) who analysed in-situ amber of a wood inclusion from Baltic amber. The wood itself was assigned to Pinus (subgenus Strobus, section Parraya and/or Strobus), and by applying IR analyses the in-situ amber was identified as gedano-succinite (Dolezych et al. 2011). The latter is a ‘transitional type’ between succinite and gedanite, combining chemical properties of both amber types, such as specific peaks in their IR spectra and the amount of succinic acid (Stout et al. 1995, Vávra 2015). Stout et al. (1995) interpreted the similarities between gedanite, gedanite-succinite and succinite as indicator for a common botanical source and suggested that the structural differences between the named amber varieties are caused by diagenetic processes.

Another amber type with pinaceous affinities was suggested by Yamamoto et al. (2006) who identified Pinus or Picea as source tree for Bitterfeld succinite, indicating that Pinaceae taxa can be source trees of large amber deposits. This result has previously been supported by Wolfe et al. (2016) who applied along FTIR and isotope analyses, time of flight-secondary ion mass spectrometry (ToF-SIMS) to study Baltic and Bitterfeld amber. Structural and chemical characteristics of both ambers show similarities to resin properties of extant Pinaceae and Sciadopityaceae, but a definite taxonomic assignment to a source plant was still impossible. Although

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both ambers are “broadly contemporaneous”, they are not equivalent to each other, meaning that succinite of the Bitterfeld and Baltic area derived from different botanical sources and localities (Wolfe et al. 2016). There is no doubt that numerous Pinus species existed in the Baltic amber flora (see chapter 4.1.1); however, based on the state of knowledge, the morphological-anatomical evidence, as well as structural and chemical indications are still too contradictory to consider or exclude Pinus species as Baltic amber source tree.

Among Pinaceae, Pseudolarix should again be considered as putative Baltic amber source tree, as it was already done by Anderson and LePage (1995) who discovered several conifer taxa on Axel Heiberg Island of the Canadian Arctic Archipelago (Anderson and LePage 1995). Middle Eocene sediments of the Buchanan Lake Formation preserved a coniferous swamp forest with in-situ amber, meaning that the amber was associated with identifiable plant fossils, which allowed the linking of the amber directly to its source plant (Anderson and LePage 1995).

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) analyses were conducted to study the chemical properties of the ambers as well (Anderson and LePage 1995), and in combination with fossil evidence, revealed that the ambers originated from Metasequoia, Pinus and Pseudolarix. Interestingly, the Pseudolarix amber not only derived from polylabdanoid resins, but also contained succinic acid.

Both features are two key characteristics of Class Ia resins, including Baltic amber (Tab. 1). Furthermore, Wolfe et al. (2009) found that resin of extant Pseudolarix amabilis exhibits a subdued ‘Baltic shoulder’ in its IR spectra, suggesting affinities of Baltic amber to Pseudolarix. But differences in the labdane configuration of both ambers and the absence of the ‘Baltic shoulder’ in the IR spectra of Pseudolarix amber, as well as lacking fossil evidence from Baltic amber raised doubts about Pseudolarix being a Baltic amber source tree (Anderson and LePage 1995, Langenheim 2003, Wolfe et al. 2009). Despite this, the recently described first record of Pseudolarix needle inclusions from Baltic amber by Sadowski et al. (2017a [5]) shows that Pseudolarix is not yet ruled out as a source tree of Baltic amber. Despite the differences between both ambers, the chemical similarities between the ambers and extant Pseudolarix amabilis supports the idea of Anderson and LePage (1995) that both amber source trees were not alike but at least shared a common ancestor.

A further pinaceous origin of Baltic amber was suggested by V. Katinas (Stroganov 1987) who considered the Atlas cedar Cedrus atlantica as Baltic amber source tree. However, besides a newspaper article by Stroganov (1987) no further details about Katina’s studies are available. Regarding the latest update of conifers from Baltic amber (Sadowski et al. 2017a [5]), inclusions with affinities to cedars have not been discovered yet, questioning whether Cedrus was a constituent of the Baltic amber forest at all.

Another conifer which should be considered as putative amber tree is Cupressospermum saxonicum of the extinct Geinitziaceae. Fossils of this ancient conifer were discovered in open cast mines of the Bitterfeld amber deposit (Upper Oligocene, Saxony, Germany), but also in the Lusatian Miocene of Saxony and Brandenburg (Barthel and Hetzer 1982, Kunzmann and Schneider 2013). In-situ

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resin in wood remains, twigs and cone scales of C. saxonicum indicated excessive resin release in stands which were inundated by brackish waters (Barthel and Hetzer 1982, Sadowski et al. 2017a [5]). However, in inundated stands of C. saxonicum, which were non-tidal influenced, resin release was observed to be present, but not as pronounced as under brackish-water influence (pers. comm. Dr. Wilfried Schneider, Hoyerswerda). IR spectra of this in-situ resin were similar to Bitterfeld amber and thus, C. saxonicum was suggested as a source tree of Bitterfeld amber (Barthel and Hetzer 1982). However, Barthel and Hetzer (1982) did not clarify which type of Bitterfeld amber was used in their IR analyses for comparing it to the resin of C.

saxonicum. Despite this, in a further publication about Bitterfeld amber by Krumbiegel and Kosmowska-Ceranowicz (2007), the authors were more precise and stated that Barthel and Hetzer (1982) had identified the amber type gedanite from fossil cone scales of C. saxonicum. But it remained unclear how Krumbiegel and Kosmowska-Ceranowicz (2007) knew that it was gedanite, since this was not mentioned by Barthel and Hetzer (1982). Gedanite is an amber variety which was first described from the Baltic amber deposit (Stout et al. 1995). Although gedanite was suggested to be related to C. saxonicum (Krumbiegel and Kosmowska-Ceranowicz 2007), its source plant is still not verified, and is further confused since Krumbiegel and Kosmowska-Ceranowicz (2007) also mention that IR spectra of gedanite were similar to extant resin of Agathis australis (Araucariaceae).

In contrast to Barthel and Hetzer (1982), Yamamoto et al. (2006) detected strong differences when comparing the chemical composition of Cupressospermum resin to Bitterfeld succinite (the main amber variety of the Bitterfeld deposit). But Yamamoto et al. (2006) discovered similarities of Cupressospermum saxonicum resin to stantienite, another rare form of amber, which also occurs in the Blue Earth layer (Vávra 2015). Despite of the conflicting chemical evidence, C. saxonicum was a resinous conifer and has been recently reported from Baltic amber (Sadowski et al.

2017a [5]), too. Although the chemical composition of C. saxonicum resin is different to Baltic amber, C. saxonicum still needs to be considered when discussing possible source plants of further amber varieties, besides succinite from the Baltic amber deposit.

Wolfe et al. (2009) used FTIR to compare extant resins of the suggested source conifers of Baltic amber to the amber itself (Pinus contorta, Metasequoia glyptostroboides, Pseudolarix amabilis, Agathis australis, and Sciadopitys verticillata). Moreover, the authors conducted FTIR for further amber types which had a similar age to Baltic amber and whose botanical affinities were also proven by palaeobotanical evidence, including Pseudolarix ambers from the Canadian Arctic (see above). Although Anderson and LePage (1995) highlighted the strong similarities of Pseudolarix amber from the Canadian Arctic and that of Baltic amber, Wolfe et al. (2009) underlined differences between both ambers, mainly the absence of the ‘Baltic shoulder’ in the absorption spectrum of the Pseudolarix amber.

Following Wolfe et al. (2009), Baltic amber showed most similarities to the spectrum of Sciadopitys verticillata, including the ‘Baltic shoulder’. Hence, Wolfe et al. (2009) proposed Sciadopityaceae as source plant of Baltic amber, although extant S.

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verticillata resin is devoid of succinic acid but contains verticillol, a typical compound found in cladodes of S. verticillata but which is missing in Baltic amber.

The authors argued that diagenetic transformations of the amber explained these inconsistences; however, they could not present unambiguous fossil evidence proving the presence of Sciadopitys in the Baltic amber flora.

Just recently, Sadowski et al. (2016a [3]) restudied a needle-shaped inclusion from Baltic amber, which was presented by Wolfe et al. (2009) showing sciadopitoid affinities. In their study, Sadowski et al. (2016a [3]) revealed that the putative sciadopitoid inclusion lacked specific features of cladodes of Sciadopitys (e.g.

papillous groove on the underside, ‘double leaf tip’) and rather showed characteristics of an angiosperm leaf, especially in the morphology of the stomata complexes. However, Sadowski et al. (2016a [3]) found two Baltic amber inclusions of cladodes which possessed the unique features of Sciadopitys and thus clearly proved the presence of this taxon in the Baltic amber flora (Sadowski et al. 2016a [3]). Thus, there are chemical and structural indications, as well as palaeobotanical evidence for a potential sciadopitoid provenance of Baltic amber.

Besides Pinaceae and Sciadopityaceae, other conifer families also exhibit resin properties which facilitate amber formation. For instance, Cupressaceae resin possesses labdane-type acids which polymerize more easily and thus, are more likely to form amber (Langenheim 2003, Ragazzi and Schmidt 2011). As discussed in the previous chapters, Sadowski et al. (2017a [5]) proved the presence of the cupressaecous taxa Calocedrus, Quasisequoia couttsiae and Taxodium in the Baltic amber flora. Also, there are numerous Cupressaceae inclusions from Baltic amber, especially twig fragments and pollen cones (Figs 1-4) which, however, could not be assigned to specific taxa yet (see chapter 4.1.2 for details). Despite their abundant occurrence in Baltic amber, Wolfe et al. (2016) eliminated Cupressaceae as potential source of Baltic amber, based on recent chemical and structural analyses of extant resins. However, among 133 extant Cupressaceae species (Farjon 2005), the authors only analysed resin from 11 cupressaceous taxa. Considering the high diversity of extant and fossil Cupressaceae, especially in the Baltic amber flora, resins of more Cupressaceae genera should be examined, including the verified conifer taxa from Baltic amber, to test their affinities to the chemistry of Baltic amber.

In conclusion, despite using a wide range of techniques and new fossil data from Baltic amber, no consensus about the botanical origin of Baltic amber was found so far (see Tab. 8 as overview). For resolving the origin of Baltic amber, more data about chemical and structural properties of extant and fossil resins across all conifer taxa are needed. Another challenge which needs more attention is the unknown effect of diagenetic processes on amber and how they change its properties (Anderson et al. 1992). Furthermore, palaeobotanical evidence from Baltic amber should be included more often in those studies. Wood inclusions with in-situ amber are an especially promising tool to infer the Baltic amber source plant. Based on different types of amber from the Baltic region, as well as the high coniferous diversity, it also should be considered that there might be more than one amber source plant.

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Tab. 8: Arguments for (in blue) and against (in red) different suggested source trees of Baltic succinite. Indistinct arguments were left blank. Table is adapted from Langenheim (2003) and extended with subsequent results, as indicated in the references. NA indicates information that was not available.

Suggested source Agathis Pseudolarix Pinus Sciadopitys

Family Araucariaceae Pinaceae Sciadopityaceae

Chemical and structural resin and amber properties

Molecules

Baltic shoulder absent present absent absent present

Succinic acid absent present present absent absent

Positive wavenumber ratio (FTIR)

absent absent absent present

FTIR spectra dissimilar dissimilar dissimilar correlating

Potential of

Baltic amber flora fossils absent fossils present fossils present fossils present Reports of in-situ

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