Monday, February 22, 2016

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. Their spores, they keep 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.


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