Microbe‐like inclusions in tree resins and implications for the fossil record of protists in amber

Abstract

During the past two decades, a plethora of fossil micro-organisms have been described from various Triassic to Miocene ambers. However, in addition to entrapped microbes, ambers commonly contain microscopic inclusions that sometimes resemble amoebae, ciliates, microfungi, and unicellular algae in size and shape, but do not provide further diagnostic features thereof. For a better assessment of the actual fossil record of unicellular eukaryotes in amber, we studied equivalent inclusions in modern resin of the Araucariaceae; this conifer family comprises important amber-producers in Earth history. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), we investigated the chemical nature of the inclusion matter and the resin matrix. Whereas the matrix, as expected, showed a more hydrocarbon/aromatic-dominated composition, the inclusions contain abundant salt ions and polar organics. However, the absence of signals characteristic for cellular biomass, namely distinctive proteinaceous amino acids and lipid moieties, indicates that the inclusions do not contain microbial cellular matter but salts and hydrophilic organic substances that probably derived from the plant itself. Rather than representing protists or their remains, these microbe-like inclusions, for which we propose the term 'pseudoinclusions', consist of compounds that are immiscible with the terpenoid resin matrix and were probably secreted in small amounts together with the actual resin by the plant tissue. Consequently, reports of protists from amber that are only based on the similarity of the overall shape and size to extant taxa, but do not provide relevant features at light-microscopical and ultrastructural level, cannot be accepted as unambiguous fossil evidence for these particular groups.

Amber from the Triassic to Paleogene of Australia and New Zealand as exceptional preservation of poorly known terrestrial ecosystems

“…Early to mid-Paleocene (~66-62 Ma) amber has been recovered from the Department of Primary Industries (DPI) Core Lab in small amounts (N = 6) in 2013-2015 from drill cores in the Gippsland Basin of southern Victoria, Australia, in the petroleum well Barracouta-1 (spudded in December 1964; originally named the Gippsland Shelf-1 well) at 1967.9 m depth. This record is represented by translucent to semi-opaque orange and red amber devoid of inclusions to date, apart from particulate debris and pseudoinclusions 29 . This Paleocene amber from the lower Lygistepollenites balmeii spore-pollen Zone was deposited in sediments of the mid-Cretaceous to Eocene Latrobe Group, which mostly comprises fluvial, floodplain and coastal plain siliciclastics interspersed with thin coal beds 30 .…”

“…The Tuna-1 amber is represented by a dominance of mostly transparent with rare semi-opaque orange and rare red amber that ranges from lenticular, angular, wedge-shaped, and irregularly shaped pieces. Possible filamentous inclusions have been noted in amber pieces obtained at 2668.1 m depth; otherwise, only minor amounts of pyrite, pseudoinclusions sensu Thiel et al 29 and particulate debris have been noted in the amber. The lowermost levels containing amber at 2673.1 m is characterized by siltstone with dispersed organic matter, which is overlain by sandy siltstone with coalified wood fragments, massive siltstones with coal laminations and organic matter, and capped by partially sideratized siltstone with poorly developed laminations and minor bioturbation.…”

Morphological stasis in the first myxomycete from the Mesozoic, and the likely role of cryptobiosis

“…1I in Thiel et al . 36 ). The recent report of a structure in Burmese amber, interpreted as a dictyostelioid slime mold 37 is likewise unconvincing.…”