Abundance and formation of inclusions from Baltic amber

In Baltic amber, as it is the case for many other amber deposits, animal inclusions are more abundant than plant inclusions. Especially Arthropoda are very well represented, as they comprise 80% of all animal inclusions from Baltic amber.

Following the most current numbers by Weitschat and Wichard (2010), 539 families of Arthropoda were hitherto described from Balitc amber. Diptera represent the by far most abundant group of arthropods with 800 species being described, followed up

Tab. 1: Classification system for ambers, taken from Anderson et al. (1992; and citations therein), Anderson and Botto (1993), Anderson (1994, 2006), and Anderson and Crelling (1995), Yamamoto et al. (2006), Bray and Anderson (2009), Rust et al. (2010), Vavra (2009; and citations therein), Ross et al. (2010) and Poulin and Helwig (2012).

Class Characteristics Selected examples Botanical affinity

Class I

based on polymers of labdanoid diterpenes, including especially labdatriene carboxylic acids, alcohols and hydrocarbons

Class Ia

based on polymers and copolymers of labdanoid diterpenes (regular configuration), including

based on polymers and copolymers of labdanoid diterpenes (regular configuration), including/not limited to communic acid, communol and biformene;

devoid of succinic acid

based on polymers and copolymers of labdanoid diterpenes (enantio configuration), including/not limited to of ozic acid, ozol and enantio bioformenes;

devoid of succinic acid

based on polymers and copolymers of labdanoid diterpenes with enantio configuration; incorporation

Class III basic structural feature is Polystyrene

siegburgite: Siegburg and Bitterfeld

Hammelidaceae (Liquidambar) some New Jersey ambers

Class IV basic structural feature is sesquiterpenoid, based on

Cedrane (IX) skeleton, non-polymeric ionite: Pliocene of California unknown

Class V

non-polymeric diterpenoid carboxylic acid, especially based on the abietane, pimarane and iso-pimarane carbon skeletons

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by Araneae (587 species) and Hymenoptera (448 species; Weitschat and Wichard 2010). In contrast, there are only approximately 130 species of plants (conifers and angiosperms) that were described from Baltic amber so far [based on a species list by Czeczott (1961)]. This low species number of plants is strongly connected to the low percentage of botanical inclusions in unselected samples of Baltic amber, ranging from 0.6 % to 24.9 % (Hoffeins and Hoffeins 2003, Sontag 2003), depending whether stellate plant hairs were counted individually or not (Sontag 2003).

Although these estimations vary, it is clear that plant inclusions from Baltic amber are very scarce. Reasons for this rareness may be collection bias (Szwedo and Sontag 2009), but could also be related to the taphonomy of plant inclusions which, however, has not been studied yet.

For animal inclusions, certain factors which bias trapping in resin have been discussed already (Martı́nez-Delclòs et al. 2004, Solórzano Kraemer et al. 2015) and some of them could be considered to be of similar importance for the formation of plant inclusions. Depending on the resin viscosity and stickiness, surface tension might be too high to allow the trapping of insects into the resin. In this case, the size of the insect is a crucial factor as well: too high surface tension inhibits the intrusion of very small insect into the resin, while larger insects may penetrate the resin.

However, due to their larger size, they can escape more easily (Martı́nez-Delclòs et al. 2004); a similar situation could hold true for plant fragments: high surface tension of resin flows might prevent plant fragments getting stuck on the resin (pers. comm.

M. M. Solórzano Kraemer, Frankfurt).

As it is the case for animals, the size of a plant fragment likely biases the trapping as well: depending on resin properties, small plant remains are probably more easily retained by resin than larger ones. The location of the source tree is another crucial factor: animals which occur close to the source tree and within its immediate environment are more likely to be captured than animals outside of this area (Martínez-Delclòs et al. 2004). However, animals which occur in habitats other than that of the resin bearing plant also may be captured in amber, since they can actively move around; anyhow, they are more scarce in amber than those animals which live in close proximity to the amber source plant (Martı́nez-Delclòs et al.

2004). For plant inclusions, a similar situation is possible: plants located close to the source trees or those which are even epiphytic on the resin secreting plant are more likely to be abundant in amber than other plant taxa with different ecologies. For instance, inclusions of bark overgrown with the leafy liverwort Frullania were recently reported from Burmese amber, indicating that the liverworts were likely epiphytic and removed from the bark by a resin flow (Heinrichs et al. 2012).

In contrast to animals, plants cannot ‘actively roam around’ and get stuck to fresh resin flows, thus plants become passively stuck to the resin outpourings. It is more likely that fresh resin drops covered plant remains coincidently, while falling on the forest floor (pers. comm. M. M. Solórzano Kraemer, Frankfurt). So-called

‘litter amber’, which was reported from French Cretaceous amber deposits, could be an indicator for that since it contained taxa which were specific for soil biotas indicating its proximity to the forest floor (Perrichot 2004). Observations from extant

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habitats with resin-releasing trees, such as Araucarian forests of New Caledonia, support this idea, showing that large fresh resin flows on the forest floor covered litter and entombed plant fragments (Girard et al. 2009). Thus, inclusions of plants more likely represent local floras, originating from the same environment. However, it is also possible that plant fragments were transported passively by wind into fresh resin flows, which however, depends on the vegetation structure, since very dense forests may inhibit wind transport (Martı́nez-Delclòs et al. 2004).

In contrast to plants, animal behaviour can strongly influence the probability whether they get caught by resin or not. For instance, insects maybe attracted by volatile terpenoids and then accidentally be trapped on the sticky resin (Martı ́nez-Delclòs et al. 2004). Also swarming insects are more prone than others to become inclusions (Martı́nez-Delclòs et al. 2004). These factors favour the entrapment of animals in resin, and as a result, they are not only more abundant than plant inclusions, but also often represent heterogeneous taphocoenoses when occurring as syninclusions (Seredszus 2003). For instance, Seredszus (2003) studied inclusions of chironomid midges and their syninclusions from Baltic amber, reporting that terrestrial and aquatic taxa often co-occurred. Seredszus (2003) argued that the swarming behaviour of insects, but also the close proximity of habitat types likely was the reason for heterogeneous taphocoenoses from Baltic amber.

In summary, plant inclusions are rarer than animal inclusions, but have a great potential to portray the immediate environment they derive from. In comparison, animal inclusions might represent an assemblage of different habitats.

However, these are hypotheses that still need verification by actualistic experiments, studying and comparing certain habitat conditions and how they may influence or even bias the trapping of plants and animals in resin. Biased preservation of certain organisms in amber should always be considered when reconstructing palaeohabitats on the basis of amber inclusions. Based on the recent knowledge of the taphonomy of plant inclusions, it is challenging to estimate these biases and which group of plants might be underrepresented in comparison to others.

After being embedded into resin, several processes facilitate the formation of amber inclusions. Dehydration of the organism, comparable to mummification, is crucial to inhibit the degradation of the tissue (Henwood 1992). In some cases, it is suggested that volatile compounds of the resin diffused through cell walls and replaced the cellular water, resulting in the high-quality preservation of internal tissues (Grimaldi et al. 1994, Stankiewicz et al. 1998). Antimicrobial compounds of the resin inhibit the degradation of the inclusions by fungi and bacteria, protecting the entombed organism from decay (Martı́nez-Delclòs et al. 2004). However, the preservation also depends on the amber type and the diagenetic processes discussed above (chapter 1.2).

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