At the 1933 Chicago World’s Fair, the Ripley’s Believe It or Not Pavilion contained an exhibit titled, “Hair Growing on Wood — Believe it or Not!”. You shouldn’t have believed it.

Although it certainly can look like a log sporting locks, what visitors were actually seeing was the slime mold Stemonitis, pictured at the top of this post.

Now another Stemonitis has gained some notoriety, according to a new article in the journal Scientific Reports.

In this piece of southeast Asian amber, trapped alongside a KFC-style leg o’ lizard, is a Stemonitis indistinguishable from species alive today. It is 100 million years old — 60 million years older than the previously oldest known example.

Arrow indicates slime mold. Lizard leg center. Scale bar 1 millimeter. Credit: Rikkinen et al. 2019

Scale bar 200 micrometers. Credit: Rikkinen et al. 2019 

Plasmodial slime molds, the group that Stemonitis belongs to, are giant amoebas. Although they exist in several other forms, their most famous manifestation is a slime-encased bag of cytoplasm that crawls around land terrorizing microbes (it eats them) and vacuuming up detritus (eats this too).

But that’s just their day job. When properly “recruited”, they also wow us with debatably intelligent behavior in solving mazes, running urban planning simulations, and acing memory tests.

But slime molds have another trick. When their food supply is exhausted, they metamorphose into a spectrum of spore-making forms called sporocarps or sporangia that could not look less like a crawling bag of slime. They are dry, stationary, and full of intricate hard structures. In short, something that moves like an animal becomes something that looks like a fungus over the span of just a few short hours. They are very odd creatures. 

Even with this sturdier form, slime mold fossils remain rare. Until now, the oldest unambiguous slime mold sporocarps were found in Baltic amber deposited in the Eocene, around 40 million years ago. Intriguingly, one of them is also a Stemonitis. But it didn't look likely any more would be found any time soon.

Luckily for us, sometime around 100 million years ago in a wet tropical forest, a slime mold and a lizard had a bad tree-related day. Six slime mold sporocarps two millimeters long and about 200 micrometers wide — the spore-making structures of Stemonitis — found themselves plonked in tree sap. It’s possible the lizard dislodged the slime mold during its fatal struggle to escape. The sap hardened into amber, which ultimately made its way into a miner’s hand in Myanmar, and then, crucially, into scientists’ in Finland, the United States, and Germany.

What is most striking about this specimen is that all the relevant anatomy and dimensions match the range of a species we today call Stemonitis virginiensis. The sporocarps contain spores ornamented with a network of ridges. The sporocarps contain a network of fibers called capillitium that serve to disperse the spores. They also contain stalks and columellas — internal stalks that continue up into the spore mass.

There was one other telling detail: stuck to the base of the sporocarps were grains of sand, hinting at a sticky plasmodium that crawled to the site before sporulating.

Sticklers would argue that it is impossible, after such an immense timespan, for this to be the same species. Romantics like myself would argue that not only is it possible, it’s kind of magical. However, because slime mold appearance is so variable due to environmental conditions, it is not currently possible to say for sure. Even if it could be considered the same species, the outward similarity does not preclude a great deal of invisible molecular change.

How much further back in time might slime molds have existed? A lot further is possible. Older species of slime molds could well be among us, but we would have little idea due to the slime mold fossil shortage. The group into which slime molds belong, the Amoebozoa, is an ancient lineage, and slime molds are an ancient lineage within it. Humid tropical forests, slime molds' preferred habitat, have existed on Earth since the mid-Paleozoic, about 380 million years ago.

And as I wrote about here last year, possible marine slime molds, which may or may not have been related to the existing lot, may have lived as far back as 2.1 billion years ago.

Why and how Stemonitis has persisted for such a long time are also intriguing questions. According to the Scientific Reports authors, the best clues are found in organisms very distantly related to slime molds: water bears and tadpole shrimp.

Water bears are tiny adorable eight-legged animals also called moss piglets that trundle about in lichens and moss. In an eerie parallel, the water bear Milnesium tardigradum is very similar to a ~90 million year old specimen from New Jersey amber.

Fossils of tadpole shrimp - predatory crustaceans with big head shields that make them look like tadpoles — from the Triassic are also very similar to an existing species, which means that that genus has persisted for at least 230 million years.

What unites these organisms with slime molds is a power called cryptobiosis. “Secret life” is the rough, tantalizing translation, but what it means in practice is that they can all shut down their metabolism and enter a state of indefinite hibernation. In it, they can survive conditions that would destroy other life.

Water bears have famously resisted extreme temperatures, pressures, radiation, dehydration, and the vacuum of space. Tadpole shrimp and can endure water near the boiling point for short periods of time and complete dessication for decades, an adaptation of supreme importance in the ephemeral pools in which they live. During rough times, slime molds may form “sclerotia”, dry, tough, ugly little chunks of plasmodium that hang out in soil or leaf litter waiting for salad days to come again. Slime mold spores may also remain viable for decades.

Even though slime molds, water bears, and tadpole shrimp are all very distantly related, they accomplish cryptobiosis the same way: by replacing the water in their cells with a sugar called trehalose, a disaccharide composed of two molecules of glucose.

Of course, the ability to shut down one’s body for extended periods cuts down on the number of generations per unit time; fewer individuals on which natural selection can act necessarily slows evolution’s pace. 

But ridding their cells of water also somehow makes them impervious to the vicissitudes of dehydration, radiation, heat, and, at least in the case of the water bears, space (a place where, admittedly, few of these organisms are likely to find themselves, and so a superpower of limited utility).

Hibernation also releases them from the the need to adapt to adverse conditions, because they can just check out whenever these come along. Stuffing your cells full of trehalose is a de facto “Get Out of Evolution Free” card.

Organisms such as these with such unwavering loyalty to their bodies are often called living fossils. Other examples include horseshoe crabs, coelacanths, and dawn redwoods. Humans — with our mighty, mighty brains and prodigious sense of importance — have been around for a half million years or so. Lowly Stemonitis has been lumbering about through four geological periods, the collision of an impactor that wiped out the planet's reigning vertebrates, countless ice ages, and millennia beyond comprehension. Whatever else you might say about them, slime molds consistently demonstrate they should not be underestimated.