A graptolite sampler, from an ancient Encyclopedia Britannica

Occasionally, life looks like it isn’t. In the eastern forests of North America and in a thin strip along the Pacific Northwest (but sadly not in Colorado), hidden in plain sight on tree trunks you can find the gracefully named elven script lichen, Graphis scripta. With a little imagination, the lichen looks like secret writing, not like an eruptive fungal-algal symbiont that specializes in cohabiting in tree bark.

In the 18th century, Linnaeus, the father of taxonomy, faced a similar biological dilemma. He found patterns in rock he suspected were chemical or geological formations that looked as if they had been alive, but actually had not. He called them “graptolites” (= rock writing), and gave the name to a variety of things may or may not have been alive. Over time, however, the term was co-opted by paleontologists for a group of strange fossils that very much had been alive. The only problem was, no one knew what the living parts of these things actually looked like — or how they might be related to anything alive today.

At left, you can see a variety of these creatures’s tube-like houses, called coenecia (se-nee-see-a), which are found abundantly through marine rocks from the Ordovician, Silurian, and part of the Devonian. Some were branched and tree like (dendritic, see13, 18, and 27, left) and probably bottom-dwellers, and others took on a variety of other bizarre forms that scientists interpret as the products of a planktonic form.

Many tended to have characteristic “hacksaw” shapes (see 4a, 7, 15, and 19, left) either in tuning forks, or coiled up in spirals like watch springs, as if something had poked out of the teeth lining these tubes. But no one knew what. The few cases where some actual ex-animal had fossilized were apparently more like ex-animal smudges than ex-animal fossils.

The pterobranch Rhabdopleura, in a lovely study in blue and gold. Note the tentaculate feeding/breathing prongs, aka lophophores.

Meanwhile, in another part of the science universe, scientists were describing and identifying members of a group called pterobranchs (= winged or feathered gills). Stretching little more than a centimeter long and living in proteinaceous* banded tubes cooperatively secreted by their shield-shaped probosci, they humbly go about their business stretching their ciliated tentaculate arms (which may remind you of bryozoans’lophophores, which they merely resemble convergently because that’s what they probably are (see comments)) into the water currents to catch prey and exchange gases. Inside their proboscis is a true lined body cavity, or coelom (seel-um). They sometimes live on their own, but usually grow in colonies connected by stems, or stolons**, in that colony of fused tubes called the coenecium. Some species have a pair of gill slits, just like fish (For a nice look at the general structure, see here, here, or, yes, the plush version here).

And this may very well reflect pterobranchs’ position in the shrub of life. Pterobranchs, it turns out, are hemichordates, in the same group as the acorn worms (enteropneusts) I described here last year. They are animals that evolved from animals just on the verge of becoming chordates, or nerve-corded animals like ourselves. They have a tripartite body plan of proboscis, collar (whence the tentaculate arms spring), and trunk. Like echinoderms (seastars, etc.) and chordates (us, etc.) they have a complete digestive tract whose mouth forms from the second indentation in the hollow ball of cells formed after a fertilized egg starts dividing (= deuterostome). Like echinoderms (but unlike chordates), they have no body segmentation and a special kind of larva called a dipleurula (chordates have no larvae). Signficantly, a hollow neural tube grows in some species early in development.

Most suggestively, pterobranchs and the fossil graptolites seemed suspicously similar, although little more than two dozen species of pterobranch live today, and for millions of years in the Paleozoic (from the Cambrian to the middle of the Devonian), graptolites were the dominant zooplankton in the world’s oceans. Tantalizingly, the microstructure of graptolite fossil tubes is very similar to the microstructure of pterobranchs, a detail discovered when electron microscopes first peered inside tubes of both animals and ex-animals in the 1970s. But no preserved fossil animal could confirm this.

Looks like a graptolite. Quacks like a graptolite. Could be a graptolite. Wouldn’t it be great if we had some soft tissues preserved in a graptolite fossil! Well, now we do.

Galeaplumosus, which was probably a two-armed model. The right arm is broken off, but two tentacles are still visible on it. "You don't look a day over 500 million years. You and Rhabdopleura could be sisters!" From Hou et al., Current Biology. Click for link.

In a March paper in Current Biology, scientists report the discovery of a tentaculate graptolite 525 million years old from the lower (early) Cambrian.

Finally, in all its glory, an animal poking out of a conical graptolite tube. And what an animal!For pterobranchs, they are, at shy of two inches (four centimeters), Yao-Ming-class. Which is fitting, because the fossil was found in China and dubbed Galeaplumosus abilus, from galea (helmet) and plumosus (feathered), and ab (away from) and nubilus (cloudy). Yunnan, where the fossil was found, means “south of the clouds”.

The fossil provides the clincher on graptolites’ true identities: a banded (probably secreted) cooperatively-made tube with contractile stalk and tentaculate feeding arms projecting from the opening is the M.O. of extant pterobranchs.

Looking carefully at the fossil, scientists were even able to discern possible cilia (silly-uh — little hairs that beat back and forth to draw in particles of food) on one tentacle, and a possible contractile stalk inside the shell. What scientists have, apparently, is the earliest, largest hemichordate animal (zooid) ever found, alive or dead, and it seems to show that their way of feeding and building a house have changed virtually not-at-all in 525 million years. Take that, sharks***.

The authors of the paper hypothesized that the rarity of specimens like this is probably a result of most graptolites’ planktonic lifestyle: on the long trip to the big sleep, most graptolites/pterobranchs probably decayed before they hit bottom, while the shell has proved decay-resistant in modern tests. That this animal was preserved, they suggest, means it was likely a bottom-dweller.

And that would fit with what we know about graptolite natural history. Scientists long suspected that the first graptolites, which tended to be tree-shaped (dendritic) and evolved in the Cambrian, were likely sessile bottom dwellers. Only later, in the next era, the Ordovician, did a floating planktonic form also emerge, the Earth’s first large zooplankton — and by far the dominant plankton of the early Paleozoic oceans. With their collaborative approach to constructing a floating colony, they were a bit like floating bee hives or wasp nests, if the wasps were all attached at the abodomen by stems, secreted their own cells (instead of building them from chewed up wood or mud), and never left the nest. Like the vast floating chains of colonial salps in today’s oceans (though much smaller), they must have been strange indeed.

The graptolite Pendeograptus fruticosus from the Lower Ordovician (477-474 mya) near Bendigo, Victoria, Australia. This style is referred to as the "tuning fork".

These planktonic co-ops evolved so abundantly and so quickly that they are commonly used as “index fossils” by the geology and petroleum geology sets to date rocks relative to each other with fine detail. In their heyday, thousands of species filled the oceans, common, widespread, quickly evolving and easily identifiable: a rock dater’s dream (errr . . . yeah. Ammonites have also been used this way.) In the Silurian, for instance, 40 different graptolite zones have been described, with an average duration of .7 million years — incredibly fine detail for geologic time, where dating anything to within a few tens of millions of years is usually considered spectacular.

Sadly, the planktonic graptolites went extinct in the middle Devonian, about 380-400 million years ago. Thus, the first (Galeaplumosus et al.) and last forms we find in the fossil record (from the mid-Cretaceous, near the end of the age of non-avian dinosaurs) are bottom-dwelling dendritic forms — as are the handful of species alive today, the humble survivors of a formerly world-dominating group****. But let us take the sunnier view. This post could have been titled “Graptolites Are Not Extinct!”.

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To learn more about graptolites and pterobranchs, see here, and a nice page here by the British and Irish Graptolite Group (BIG-G : ) ).

*made of a collagen-esque material, a family of animal proteins that help keep your muscles attached to your bones and your skin perky.

**A term for horizontal connections between organisms. Stolon, incidentally, is also the term botanists use for stems (*not* roots) that run along or jut below the ground from plant to plant (aka runners). If you’ve grown strawberries you have experienced this phenomenon.

***420-million year-old posers

**** much like brachiopods

Hou XG, Aldridge RJ, Siveter DJ, Siveter DJ, Williams M, Zalasiewicz J, & Ma XY (2011). An Early Cambrian Hemichordate Zooid. Current biology : CB PMID: 21439828

Not going to win any beauty contests: a typical lab-grade acorn worm, an enteropneust. Creative Commons Necrophorus

Behold the lowly acorn worm, ocean filter feeder. Not so impressive, is it? Well, neither would you be were you stuffed for years in a jar filled with formalin. This was the creature portrayed in the video posted last time, as correctly guessed by Mr. Kevin Zelnio of Deep Sea News. As you may have noticed, it has an odd mouth positioned behind a fleshy proboscis attached by a trunk to the center of its mouth.  This configuration is the source of the acorn worm’s name. The proboscis fitted into its collar appeared to someone with Naming Powers at some point to look like an acorn. I think acorn is the modest interpretation . Other descriptors come to mind, although another worm already called dibs on that name. Perhaps you can see the acorn better in this figure where the proboscis is not so long:

An Acorn Worm Revealed, from the 1911 Encyclopedia Britannica.

The proboscis seems to function as trowel, fly paper, and conveyor belt. It digs, it’s coated in sticky mucus, and it is covered with little beating hairs, or cilia, that move food particles back toward the mouth. Inside the mouth are cilia that draw water in as well.

So what’s the big deal about this little worm (also called an enteropneust)? Inside that unprepossessing pallid little body lies a hollow nerve cord and two rows of slits (gills) used for feeding and breathing. By virtue of those gill slits, we know our friend the acorn worm is a descendant of the immediate ancestors of chordates (organisms with a stiffened, fortified nerve cord) and vertebrates (organisms with bony vertebrae defending the nerve cord), and is thus representative of one of the more important evolutionary links in the chain that led to sharks, duck-billed platypuses, and George Clooney (this was obviously a very successful lineage). Very primitive vertebrates may have looked something like them — and at one point, even embryonic you briefly sported a pair of gill-like apparati.

Acorn worms were traditionally classed in a group called the hemichordates, which along with the echinoderms (sea stars, basket stars, sea cucumbers, etc.) are  the vertebrates’ closest living relatives. As Colin Tudge notes in “The Variety of Life”, this is quite a remarkable thing.

In short, the chordates, which — in the vertebrates — include the most powerful, most swift, most versatile, and most intelligent creatures that have ever lived, emerged from amongst the ranks of animals that in large part are either sessile or seem to have a hankering to be so and some of which (notably the echinoderms) are without brains, not simply in the loosely perjorative sense but quite literally. Chordates, in short, have strange bedfellows. Nature it seems, likes to startle.

Together, these three groups — the acorn worms, echinoderms, and vertebrates (and possibly one or two other straggler groups) — make up the deuterostomes, a term you may have heard before. The characteristics that reliably define the group are not flashy features found in adults, but minute and highly conserved features of embryonic development (which are thus protected somewhat form the fiercest forces of natural selection) like anal pore formation, embryonic cell cleavage patterns, and larval types, so we will save that story for another day.

As per usual, the biologists are still fighting over what the exact relationships are between these groups. This diagram is as good as any for exploring it.

Most books describe acorn worms as creeping around or living in muddy or sandy U-shaped burrows on the sea floor. They are always depicted as frumpy and vaguely suggestive little creatures like the one pictured at top. When I taught introductory biology in grad school, our teaching specimens came from the same lot, and I remember dutifully fishing one of these not-so-impressive little creatures out of a jar to show my students.

And so the ugly larva grew up to be a beeeeee-uuutiful enteropneust. Photo courtesy of David Shale.

So — and here’s the point of this post — imagine my surprise when last week I opened up a slide show on wired.com of an expedition to either side of the Mid-Atlantic Ridge (run, do not walk,  to see) of new finds by British researchers in the Census of Marine Life aboard the RSS James Cook that was practically a debutante ball of brightly colored deep sea acorn worms. Worms with the frill and fuss of bustled, petticoated ladies. Worms that were observed swimming. Acorn worms, that, in short, look nothing like acorn worms. Homely burrowers my arse! Look at the diversity of color and form! On seeing these pictures, it is less hard to imagine the common ancestors of these organisms and us evolving into fish. But as always, these pictures beg the question: What else is awaiting discovery out there . . . ?