From dinosaur to chicken: Isoetes and its ancestors

Today, the closest living relative of Tyrannosaurus rex is arguably a chicken.

All right, that isn't strictly true: birds are certainly the closest living relatives of dinosaurs (and are in fact dinosaurs themselves, by any reasonable measure), but chickens are no closer to T. rex than are swans, ostriches, or toucans. They're also no more distantly related than any of those, though -- and isn't that remarkable? Goofy, awkward, halfway-flightless domesticated featherbags have as much claim to the tyrannosaur's legacy as anything else alive.

Evolution does this sort of thing all the time, altering species in directions quite different from any human notion of "progress." The plant kingdom has hundreds of examples, and none quite as spectacular as the story of Lepidodendron and Isoetes.

Artist's reconstruction of Lepidodendron.
This one was as tall as a 9-story building.

An extinct genus of giant lycopsids (early ferns) with thick scale-patterned bark and leafy, forking branches, Lepidodendron -- the scale trees -- once dominated great rainforests that stretched around the world. Despite having no true wood to hold them up, they grew up to 30 meters tall with trunks a meter thick. For sixty million years, they flourished worldwide. Their rainforests produced so much plant matter that instead of decaying or fossilizing normally, it compacted into vast beds of coal. Huge scale-tree fossils -- massive trunks, spiky-leaved branches, sporing cones longer than a human hand -- have captured botanists' imagination for as long as miners have been chipping them free of the bedrock. It's no exaggeration to call these vanished giants the Tyrannosaurus of the plant world.

300 million years ago, though, the scale trees went extinct. The Carboniferous ended in a planetary cold snap that brought glaciers creeping down from the poles; practically overnight, the rainforests collapsed. Giant lycopsids all but vanished, and tree ferns took their place in the cold new forests.

A clump of Isoetes tegetiformans with a U.S. penny for scale. 
Not much to look at unless you know what to look for.

Today, Lepidodendron's closest living relatives are the little grass-like quillworts of the genus Isoetes. They're tiny compared to their ancestors: a quillwort 20 cm tall is quite unusual, and the largest species alive only grow to one meter. Their rolled-up, rush-like leaves don't look much like scale-tree branches, and they have nothing at all like bark. They keep their spores tucked close inside the base of the leaves; there is no separate sporing cone. Most of them are aquatic, growing in calm freshwater. The North American species are particularly good at surviving in cold, infertile lakes where little else will grow.

These pointy little proto-ferns look about as much like the great Lepidodendron as a chicken resembles Tyrannosaurus. Just like the chicken, though, there are similarities: the structure of their veins, their different male and female spores, and the history traced in their genes. It's all there, if you can find the right way to see. Inside every quillwort is the shadow of an ancient rainforest.


Image sources:

• Heimans, Eli. "Stigmaria [Lepidodendron] reconstruction." Retrieved 20 Feb 2016 from Wikimedia Commons: <https://en.wikipedia.org/wiki/File:Stigmaria_Heimans.jpg>
• US Fish and Wildlife Service. "Isoetes tegetiformans, on granite outcrop in eastern half of Georgia Piedmont." Retrieved 20 Feb 2016 from Wikimedia Commons: <https://commons.wikimedia.org/wiki/File:Isoetes_tegetiformans.jpg>

The delicate, translucent leaves of filmy ferns (family Hymenophyllaceae) are only one cell thick -- but some of them grow to over half a meter long!

It's a leaf! It's a root! It's a... wait, what?

Tmesipteris elongata at Otira, New
Zealand. Looks pretty normal, right?

At a basic, developmental level, plants have only three types of tissue: leaves, roots, and stems. Everything else is modified from these. The trunk of a tree is just a modified stem. A potato's tuber is a modified stem, too. The grasping tendrils of ivies and peas are modified leaves. Each part of a flower -- each petal, sepal, stamen, and carpel -- is a modified leaf. I could go on for quite a while. Those three basic tissues have been modified into the whole dizzying variety of plant parts, and that is an enormously diverse variety indeed.

Sometimes, though, diversity isn't exactly what is needed, and plants simplify instead. Tmesipteris is a genus of unusual, "primitive" ferns, found only in a few scattered places in the South Pacific. To a casual eye, it looks as though it has all the usual parts of a fern -- roots, stems, leaves, and even the sort of spore capsules you'd expect on such an ancient lineage. It takes a much closer look, by which I mean microscopes and developmental analysis, to realize that something is odd there.

Those broad green structures along its stem, the ones that do most of its photosynthesizing? They're not leaves. The thready bits at its base, the ones that attach it to the bark where it lives? They're not roots.

Every single part of a Tmesipteris plant is, if you look closely enough, a modified stem.

At some point in the evolutionary past, this plant's ancestors found that leaves, roots, the whole shebang, were all -- for whatever reason -- unnecessary. As I pointed out before, evolution ditches unnecessary things pretty quickly. Why bother with leaves when you can just grow flat bits of stem, and they'll photosynthesize just fine? Why bother with roots when you can just grow thready stem-anchors, and they'll keep you nailed down just as well as you need? Other plants have found plenty of reasons to bother, but for Tmesipteris, there just wasn't a need. It's gotten along very well this way for something like 400 million years, thankyouverymuch, and it has no reason to change now.


Image source: Liefting, Alan [Public domain]. Tmesipteris elongata. Retrieved February 12, 2012, from Wikimedia Commons: <http://commons.wikimedia.org/wiki/File:Tmesipteris_elongata.jpg>

Cryptobiosis: a convenient "death"

Water is life. Nothing on earth can live in its total absence, and few organisms can go without it for very long. A human can survive only a week or so without drinking. A camel, with its water- and food-storing hump, can go much longer, but will still die of thirst when its stores run out. Some cacti can survive for a year on the water stored in their stems, replenishing it only in the desert's short rainy season.

There's more than one way to survive dryness, though. Camels and cacti use about the same amount of water all the time; when there's none to be had outside, they fall back on their internal stores. A spikemoss species called Selaginella lepidophylla has found another way.

When it's well-hydrated, S. lepidophylla is green and perky, with tiny leaves in scale-like patterns that conjure images of steamy Jurassic jungles. When there's no water around, though, it does what most ferns would do: it dehydrates and shrivels, its branches curling into a sad little desiccated clump. You'd think it truly dead, right until the rains return and it promptly earns its common name -- the resurrection fern.

That lifeless ball of brittle twigs sucks up the water with desperate greed, and then it unfurls. The brown leaves soften and go green again. In a few short hours, the resurrection fern comes right back to life. It will live like this, soaking up the water and the sun, for as long as the water lasts; then, just as it did before, it will dry out and "die." It can do this over and over, and it can survive in dry state for months or years on end.

The video below is a time-lapse of a store-bought resurrection fern rehydrating, over the course of about five hours. Watch and enjoy.

The phenomenon -- an organism tamping down its metabolism to wait for more hospitable conditions -- is called cryptobiosis. The most famous cryptobiotic organisms are all microscopic: bacteria in cryptobiotic cysts can hibernate for centuries in unlikely places like permafrost and bedrock, and encysted tardigrades (water bears) have been exposed to excessively lethal doses of radiation, and even to the vacuum of space, with no ill effects.

Compared to that, Selaginella lepidophylla may not seem impressive, but keep in mind that it's many orders of magnitude bigger than those champions. It's resurrection on a familiar scale, and it happens over and over again. Beat that, Lazarus.