“It’s an encouragement to really pull out all the stops and start looking at the kinds of Ordovician rocks that might have the next exciting organism.”
The enterprising fungus Tortotubus, which lived 440 million years ago,is the oldest land-dwelling organism ever found, according to new research published in the Botanical Journal of the Linnean Society.
These tiny microfossils, each of which is smaller than a human hair, date back to the late Ordovician period, when Earth's biodiversity was almost entirely limited to the oceans. They beat out the previous record-holder for oldest land-dweller, an undistinctive and still unnamed plant-like organism, by a sliver of only a few million years. The next oldest land-dweller after that is the Cooksonia plant, which emerged nearly 20 million years after the oldest Tortotubus fossils.
As an intrepid early explorer on land, Tortotubus left behind an amazing tale of terrestrial colonization etched into fine-grained rocks of Scotland and Sweden. Though traces of these fungi were first discovered in the 1980s, it has taken decades for scientists to accrue enough samples to build a solid picture of the organism's morphology and behavior.
"It's like having the individual stills from a movie," Martin Smith, an evolutionary biologist at Durham University and the author of the new study, told me over the phone. "Suddenly, there are enough of the stills that you can play the movie and you can see the developmental trajectory."
Indeed, by assessing hundreds of these microfossils, Smith has demonstrated that Tortotubus foraged by extending slim filaments through its earthy environment until it struck upon something to snack on. Over time, these filaments developed into sturdy cords that delivered food wherever it was needed, possibly to spore-bearing mushroom caps above ground.
"This sort of behavior has evolved three or four times, but only in the mushroom-forming fungi," Smith said. "We don't have any mushrooms preserved, but it's possible this was a mushroom-forming fungus."
Mushroom or not, the filament-based feeding strategy seems to have worked wonders for the fungus's ambitions, because once it shows up in the fossil record, it thrives for about 60 million years. It's an important stretch of evolutionary history, bridging the Ordovician, Silurian, and Devonian epochs, when life on land began to rapidly diversify.
Tortotubus was not only party to this biome's transition from a barren wasteland to a lush forest, it was an active participant in the process. "Once the fungi starts making stable soil, there's more nutrients and larger plants are able to grow, so there's more biomass for the fungi to feed on," Smith explained. "Once you get big enough plants, you get animals coming out to feed on those plants, and creating new organic dumps for the fungi to feed on as well."
"You've got an enormous evolutionary transition of this fossil plodding on throughout that whole period," he added.
As influential as Tortotubus was, it was not the first organism that ever struck out on land—just the first to be found enshrined in the fossil record. The fungus had to be feeding on something, after all, probably algae, lichens, or microbial organisms. But given the difficulty of studying the incredibly subtle remains of the fungus itself, you can imagine the sheer unlikeliness of tracking down any trace of its prey.
"It's a challenge," Smith said. "The reason that we see these fungi is that they've got solid cell walls that they put really tough fibres into, and so they've got some survivability. Things like algae just don't have that robustness and resistance. You need something really special to come along to actually preserve those."
Every so often, circumstances do conspire to produce such a treasure trove of information. For instance, the Rhynie chert deposits in Scotland have preserved an entire Devonian ecosystem, thanks to the petrification-friendly conditions of a hot spring environment. Smith is optimistic that there are similar finds out there, that may further illuminate the fascinating world in which Tortotubus lived, and the larger ecosystem in which it was situated.
"This is another line evidence that says there is going to be something interesting if we look in the late and middle Ordovician," Smith said.
"It's an encouragement to really pull out all the stops and start looking at the kinds of Ordovician rocks that might have the next exciting organism, and help us reconstruct even earlier ecosystems."