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World’s Oldest Slime Mold Found in Amber

The fossilized fruiting bodies of a myxomycete from the extant genus Stemonitis preserved in Kachin amber date back some 100 million years (mid-Cretaceous period) and represent the oldest record of myxomycetes known to date.

Overview of the Kachin amber specimen showing the close proximity of the myxomycete sporocarps (arrowhead) and the hind leg of an agamid lizard. Scale bar - 1 mm. Image credit: Rikkinen et al, doi: 10.1038/s41598-019-55622-9.

Overview of the Kachin amber specimen showing the close proximity of the myxomycete sporocarps (arrowhead) and the hind leg of an agamid lizard. Scale bar – 1 mm. Image credit: Rikkinen et al, doi: 10.1038/s41598-019-55622-9.

Myxomycetes or ‘slime molds’ are a monophyletic lineage of eukaryotes that produce intriguing, morphologically complex fruiting bodies,” said University of Helsinki’s Professor Jouko Rikkinen and colleagues.

“They are phylogenetically an ancient lineage within the Amoebozoa, but like most amoeboid microorganisms, direct evidence of their evolutionary history is extremely scarce.”

The piece of amber containing the fossil myxomycete came from the outcrop near Tanai, a village in Kachin State, northern Myanmar.

The specimen consists of a group of six stalked fruiting bodies (sporocarps) entrapped in resin while young, with almost entire spore mass.

“The fragile fruiting bodies were most likely torn from the tree bark by a lizard, which was also caught in the sticky tree resin, and finally embedded in it together with the reptile,” Professor Rikkinen said.

“The lizard detached the fruiting bodies at a relatively early stage when the spores had not yet been released, which now reveals valuable information about the evolutionary history of these fascinating organisms.”

Fossil Stemonitis in Kachin amber: (a) general habitus of sporocarps; (b) surface of sporotheca, showing details of capillitium; (c) base of sporotheca, showing stalk continuing as a columella into the sporotheca; (d) detached spores. Scale bars - 200 µm in (a), 20 µm in (b), 10 µm in (c), and 5 µm in (d). Image credit: Rikkinen et al, doi: 10.1038/s41598-019-55622-9.

Fossil Stemonitis in Kachin amber: (a) general habitus of sporocarps; (b) surface of sporotheca, showing details of capillitium; (c) base of sporotheca, showing stalk continuing as a columella into the sporotheca; (d) detached spores. Scale bars – 200 µm in (a), 20 µm in (b), 10 µm in (c), and 5 µm in (d). Image credit: Rikkinen et al, doi: 10.1038/s41598-019-55622-9.

According to the team, the sporocarps of the ancient myxomycete are indistinguishable from Stemonitis, one of the two main lineages of the myxomycetes.

“The fossil provides unique insights into the longevity of the ecological adaptations of myxomycetes,” said University of Göttingen’s Professor Alexander Schmidt.

“We interpret this as evidence of strong environmental selection. It seems that slime molds that spread very small spores using the wind had an advantage,” Professor Rikkinen added.

“The ability of slime molds to develop long-lasting resting stages in their life cycle, which can last for years, probably contributes to the remarkable similarity of the fossil to its closest present-day relatives.”

The team’s paper was published in the journal Scientific Reports.

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J. Rikkinen et al. 2019. Morphological stasis in the first myxomycete from the Mesozoic, and the likely role of cryptobiosis. Sci Rep 9, 19730; doi: 10.1038/s41598-019-55622-9

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