A drip tip can tell you a lot. The vegetal water slides make the thick and rounded leaves they’re connected to look as if they have melted a little, like the cascade of a burning candle held out at an angle. The tongue made of cell walls and cellulose has a specific purpose: Drip tips allow collected water to run off a leaf, tumbling in a small spurt onto whatever is below. The chute prevents water from building up on the leaf and hosting algae or collecting debris that would obscure the solar-sucking cells the parent plant relies on to make food.
Alone, a drip tip is just a curious piece of plant anatomy. It’s an adaptation that has evolved time and again over tens of millions of years; another simple, ingenious outcome of evolution. You can find the water-shedding wings not only among plants that mingle their branches to become rainforest canopies today, but also among the paleobotanical sampling of Earth’s oldest rainforests.
Dense forests of dark green leaves shining with precipitation might seem like they are incredibly ancient, holdovers from the time of the dinosaurs. But it should perhaps come as no surprise to find that Jurassic Park lied to us—feathery and scaly dinosaurs did not tromp through sweltering rainforests. And they would have absolutely hated the Hawai’i locales the movie’s location scouts chose to evoke the favorite jungles of Tyrannosaurus and Velociraptor.
Instead, it was ginkgoes, monkey puzzle trees, and other gymnosperms that were common during the heyday of the terrible lizards, growing in stands and woodlands that were kept relatively open by the constant trampling and munching of herbivorous reptiles. Plants could not grow close enough to knit shade-throwing canopies together with the likes of the 100-foot-long, 80-ton Patagotitan wandering around, swinging its elongated, muscular neck side to side to gobble branches with peg-like teeth, trampling saplings with its massive clawed feet, and knocking trees over with its muscular, tapering tail.
The world’s oldest known tropical rainforests are about 60 million years old, about 6 million years after the asteroid impact that erased nearly three quarters of Earth’s species in a cataclysm of fire and dark impact winter.
The asteroid impact of 66 million years ago did far more than leave birds as the only surviving dinosaurs. Plants suffered a mass extinction just like the reptiles. Entire forests were burned to ash, leaving little but what was held safe in the soil’s seed bank. As survivors of the disaster began to grow, angiosperms—plants more prone to flower and fruit—grew quicker than the conifers, ginkgoes, and other gymnosperms, their wild growth assisted by an infusion of iron into Earth’s soils from pulverized rock the asteroid’s collision sent all over the planet.
Within 1 million years after impact, the first beans were growing alongside ancient relatives of willows, pepper, palms, cacao, and more, making their own space in a world that was so stripped of its megafauna that even the largest land mammals would only ever reach a fraction of the size of their dinosaurian counterparts. Forests could grow close again.
***
Life’s enthusiastic resurgence from Earth’s fifth biological catastrophe is partly told through fossil leaves uncovered in a Colombian strip mine. More than 2,000 leaf fossils have been found at the spot, within a geological unit experts know as the Cerrejón Formation—the same formation that has yielded strange prehistoric crocodiles and the immense constrictor Titanoboa.
The reptiles are the celebrities, but their essential context is etched among the leaves. The fossil leaf collection includes 65 different shapes, representing an array of tropical plants whose close relatives still grow in equatorial forests today.
A leaf’s shape isn’t merely a matter of ancestry or the result of speciation after speciation through time. A leaf is the parent plant’s interface with the world, the surface that unfurls into the light, catches rain, flutters in the wind, and is arguably the busiest part of the entire plant. The shape of a leaf has been honed by climate and precipitation, yet also broadly delimits where a plant might successfully grow. No leaf is suited to all occasions.
Through time, leaf size is closely associated with a place’s moistness: the rain and streams and humidity making water droplets condense and fall. In an arid place, like deserts only briefly kissed by rain, small leaves smush the major veins close together and make it less likely that there will be some leak, burst, or other problem with the plumbing during droughts. In dripping forest, however, too much water is a problem. Leaves need to lose excess water through transpiration, a process made easier by a broader surface area. The greater number of broad-leafed species found in a locality, the more likely the habitat was regularly doused with water—the sort of place where drip tips would provide an advantage in keeping the leaves free of excess water, algae, and other detritus.
Naturally, there are temperate and cool rainforests, too. Hot and wet do not always pair together. But the outlines of the Cerrejón leaves affirm that Titanoboa would not have been a shivering snake. The ecosystem was warm, the leaves attest, because they are mostly smooth around their outer edges. Serrations are most often found in woody plants in cooler places marked by stronger sways between seasons. Botanists are not sure why. The reason could have to do with gas exchange, defense against plant predators, releasing excess pressure from the roots, or something else entirely. But the correlation remains: Hot and wet habitats see multi-layered forests of smooth leaves, and the ragged edges of a rose, oak, or cannabis spread out in cooler places.
Look at a few leaves and you can quickly get a sense of a place, whether you’re taking in living leaves moving in the wind over your head or gazing at a leaf imprinted into an ancient stone.
The combination of broad leaf surfaces with smooth edges speaks to a hot, wet local climate, part of a greenhouse world that favored cold-blooded animals like Titanoboa and nudged warm-running mammals to stay relatively small to avoid overheating. Insect damage among these leaves speaks to a hidden revival. Plant life was so thoroughly denuded after the asteroid impact that some ant species turned to farming fungus growing on the world’s decay to survive—the forerunners of today’s leafcutters and other agricultural ants.
Most insects that fed on plants, however, perished. Insect-created leaf damage across the pre- and post-asteroid boundary shows a precipitous drop. By 60 million years ago, however, surviving insects had evolved to snip into leaves, tunnel through them, and otherwise make the most of the green life, signaling that Earth’s ecosystems were no longer shaken communities of survivor species but had begun to build anew from the bottom up.
A single fossil leaf from the Cerrejón is a novel if you read it carefully enough.
***
A fossil leaf feels like it should be impossible. We watch the foliage of maples and oaks lilt toward the ground every year, offering shelter to the little leaves beneath as they slowly decay through days of bright and cold sunlight to dissolve back into the soil as if they were never there in the first place. The persistence of something so thin and fragile across millions upon millions of years is a minor miracle.
And that is to say nothing of what it takes to survive an afterlife marked by geological violence. To be folded into the fossil record in the first place, a leaf must be buried in enough sediment to block out and even suffocate any potential scavengers—nature’s plant press. The ash, silt, mud, sand, or muck then transforms, changing the leaf within it. Ordinary groundwater, suffused with unseen minerals, percolates through the changing grains and bathes the leaf time and time again, causing microscopic minerals to replace neighborhoods of walled cells, making a copy that both is the leaf and is not. The fossil is the shape of a life that once was. The fossil is an anatomical echo, requiring that we walk back through the steps of transformation to the original life.
Sediment becomes stone. Rock layers pile up, and older layers, the lakebottom mud of some 100,000 years before or the sand of a floodplain stream that coursed ! million years ago, are pushed lower. Some may never pop back up again. The world is full of fossils that may never be exposed and may even be shoved back toward the mantle, melted to form new rock.
But the Earth’s near-constant motion nudges and thrusts old rocks back toward the surface. They do not come out crisp and clean. They crack and tilt as mountains form.
The moment the primordial rocks meet the air again, they begin their next transformation. The acidity of rainwater, the scour of the wind, the persistent pounding of the sun’s rays, the relentless quest of roots to find anchor points, and the mechanical scraping of all the world’s busy little creatures begin busting apart and grinding down the stone. Chaotically and yet somehow effectively, they remove the interfering layers above the fossil slabs. It is only at this fleeting interface, with the leaf at the surface or very near it, that the fossil has a breeze’s chance of being found. Against the depth of its history, a fossil leaf is such an astoundingly lucky find that it’s almost incomprehensible.
Leaves are finding their way into the fossil record even now. What story they write in the stones about our time on Earth, we’ll never know. But leaves and other paleobotanical clues will be there. I can’t help but wonder what they might reflect about humanity, what shade they might unintentionally reveal to the paleontologists of the far future, watching plants track the changes we’ve scoured into our planet.
The thin leaves are such fleeting things, delicate enough to tear like tissue, and yet some will last far, far longer than these very pages. The world they belong to, our world, can be opened with just a few questions.
This story first appeared in Atmos Volume 11: Micro/Macro with the title, “Leafing Through Time.”
Biome
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