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. 2007 Oct 16;104(42):16581-5.
doi: 10.1073/pnas.0707949104. Epub 2007 Oct 10.

Aquatic organisms as amber inclusions and examples from a modern swamp forest

Alexander R Schmidt  1 , David L Dilcher
Affiliations

Aquatic organisms as amber inclusions and examples from a modern swamp forest

Alexander R Schmidt et al. Proc Natl Acad Sci U S A. .

Abstract

To find aquatic organisms in tree resin may seem to be highly unlikely, but the fossil record provides numerous amber-preserved limnetic arthropods (e.g., water beetles, water striders, and crustaceans) and microorganisms (e.g., bacteria, algae, ciliates, testate amoebae, and rotifers). Here we explain the frequently discussed process of embedding aquatic organisms in tree resin based on field studies in a Florida swamp forest. Different aquatic arthropods and all major groups of limnetic microorganisms were found embedded in resin that had contact with swamp water. The taphonomy of aquatic organisms differs from that of terrestrial plants and animals that get stuck on resin surfaces and are enclosed by successive resin outflows. Large and highly motile arthropods are predestined for embedding. The number of microbial inclusions is increased when tiny drops of water with aquatic organisms become enclosed in resin while it is flowing in an aquatic environment. Bacteria and fungi may grow inside the resin as long as it has not solidified and therefore become secondarily accumulated. In contact with air, even resin that had initially been flowing into water may solidify and potentially form amber.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Resin in Dilcher's swamp forest east of Gainesville (Florida). (A) Pinus elliottii and other trees standing in the water. (B) Resinous exudate spreading at the water surface. (C) Small elongate resin pieces of up to 4 cm in length hanging at the water surface attached to the tree trunk. (D) Solidified resin piece at the water surface with trapped insects and anthers (arrows). (E) Subaquatic resin with enclosed detritus flowing downward on the tree trunk. (F) Subaquatic pillow-like resin piece of ≈5 cm flowing on the swamp floor. (G) Extensive subaquatic resin flow of ≈20-cm length on the swamp floor. (H) Pillow-like resin piece of ≈2.5 cm width in lateral view with trapped water beetle (arrow). (Scale bars: 1 cm.)
Fig. 2.
Fig. 2.
Aquatic organisms in tree resin. (A) Tiny water drop in resin containing testate amoebae, green algae, and desmids. (B) Assemblage of green algae, diatoms, and desmids at the place of a former tiny drop of water in the resin. (C) Testate amoeba of the genus Arcella surrounded by green algae at the place of a former tiny drop of water in the resin. (D) Desmid genus Xanthidium well preserved in resin. (E) Euglenoid cells of the genus Trachelomonas. Note that flagella are not visible in the resin. (F) Ciliate. (G) Rotifer. (H) Nematode. (I) Mite. (J) Ostracod. (K) Water beetle entirely trapped in subaquatic resin. (L) Water beetle slightly exposed at the resin surface. (M) Fungal hyphae growing in liquid resin. [Scale bars: 5 mm (K and L), 200 μm (A, B, and H–J), and 30 μm (C–G and M).]
Fig. 3.
Fig. 3.
Comparison of testate amoebae and filiform algae enclosed in modern resin (A, C, and D) and in 100-million-year-old Cretaceous amber from Schliersee, Germany (B and E). (A) Shell of Centropyxis aculeata (Ehrenberg) Stein surrounded by a tiny drop of water in modern resin. (B) Fossil Centropyxis delicatula Penard surrounded by a circular structure that probably indicates a former water drop. (C) Well preserved filament of the genus Desmidium in modern resin. (D) Filament of the genus Spirogyra with well preserved (arrows) and badly preserved cells in modern resin. (E) Filaments of the Cretaceous conjugatophyte Palaeozygnema spiralis Dörfelt with well preserved (arrows) and badly preserved cells. (Scale bars: 30 μm.)
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