But before we take a closer look, let’s take a step back . . .

Hallucigenia at the Smithsonian, Washington, DC. Creative Commons jylcat.

This is Hallucigenia. It is an odd creature, one who has been reconstructed both right side up and upside down (presuming only one of the directions was correct in real life, which given the general weirdness of early life, may not be a safe assumption), and which of the two the current reconstructions is correct may still be anyone’s guess. It has been interpreted variously as walking on pointy tentacles with flexible feeding arms on its back, or walking on flexible feet with pointy spines on its back (the currently favored interpretation, pictured above).

It was such an odd duck that the famous paleontologist Simon Conway Morris, whose study of bizarre Cambrian animals in the Burgess Shale Stephen Jay Gould wrote about in the book Wonderful Life,  named it after a hallucination. Morris also happened to originally reconstruct the animal in the currently non-accepted fashion, meaning perhaps that with creatures so strange, even great paleontologists can be fooled (we saw that that was the case for Anomalocaris too).

Hallucigenia was probably a lobopod, a type of creature with hollow, lobe-like legs that are extensions of the body wall. The fossil beds are full of strange incarnations of lobopods, some of which you should see here, here, and here to get a sense of the variation and general weirdness in the group.

But the creature debuted this week in nature may take the cake. (Alas — I lack copyright permission to use the images, so check out Ed Yong’s photos of the fossils here.) One cannot help but compare Diania to a first-grader’s pipe-cleaner art project. It is shockingly, wonderfully, delightfully weird. Can you imagine this thing shambling along the bottom of the Cambrian* oceans? What was it eating? How did it . . . er. . . make little walking cacti?

Aside from the delight that their sheer beauty and knowledge of their existence inspires, these organisms are important for another reason. They may have been among the ancestors of the arthropods, the tremendously successful joint-legged creatures that include insects, millipeds, and crustaceans.For, true to name, they appear to have hardened, jointed legs. But unlike arthropods, their bodies are not. The authors of the paper in Nature that revealed this creature to the world suggest that that may mean jointed legs came before jointed bodies, meaning the name “arthropod” (jointed foot) for the group was well chosen indeed.

Scientists were especially excited by the find because they had lacked any intermediate forms between the apparently soft-bodied lobopod fossils and the stiff and jointy arthropods. This looks like evidence of life in that transition.

What would Diania have looked like in real life, and how would those spindly legs have moved? Living lobopods — the modern day relatives of Diania — might give us a clue. Today, there are probably only two surviving members of the lobopod group — the velvet worms,

Creative Commons teague_o. Click for link and license.

and the water bears, or tardigrades (remember?). Tardigrades are the foil-hatted survivalists of the animal world, withstanding pure alcohol, the vacuum of space, temperatures from -272 to 149 degrees C and ionizing radiation with remarkable aplomb. It’ll be them, cockroaches, and twinkies in the fallout of the apocalypse. Yet belying that ability is their adorable plush appearance, as they trundle along on their cute little claw-footed stumpy legs, as you can see in this video.

They can live practically anywhere — from moss to the deep sea to hot springs.

Velvet worms, on the other hand, are skilled predators with an incredible way to snare their prey. As usual, David Attenborough puts it best in this video:

This is a little animal called a water bear, or tardigrade. On the mosses, lichens, forest litter, ponds, beaches, snowbanks (and even hot springs) of the world, this little guy plods along, oblivious to the larger world. At just 100 micrometers (.1 mm) to 1.5 mm long, they are cute on paws. Did you notice the little fingers?

Hug me! The caterpillar from Alice in Wonderland meets Heimlich from A Bug's Life? Creative Commons Rpgch

Discovered in the 18th century, these little guys were named water bears for their trundling, bear-like gait — that is, if can you imagine a bear with *four* pairs of legs and a penchant for shriveling up in winter rather than curling up in a cave. Tardigrade, in fact, just means “slow walker”.

Water bears exist at a strange junction between the world of the large and the world of the small. They are multicellular organisms with intestines, brains, eyes, fingers, and a chitinous cuticle that they shed, but in many ways they behave like protists, which are also microorganisms but not animals at all. Some tardigrades don’t defecate until they moult. Others don’t mate until that happens. The fertilized eggs stay behind in the moulted skin and incubate there, or sometimes adhere to a nearby surface. They are also eutelic (you-tell-ik), which just means that every water bear grows exactly the same number of cells, and once that number is reached, they can grow larger only by growing those cells. This isn’t uncommon for microbial life. Their mouths are armed with stylets with which they pierce and suck the delicious contents of plant cells, algae, and small invertebrates.

Creative Commons Rpgch

As for their bizarre survivalism streak, withstanding the vacuum and scorching solar radiation of space seems to be a byproduct of their ability to survive dry spells (they can go for a decade without water), just as it is for the bdelloid rotifers, whom I’ve also covered here. They can reversibly enter a state of suspended animation called cryptobiosis, in which their metabolism screeches to a halt and their water content plunges to a hundredth of normal. This helps protect their DNA, and a sugar called trehalose helps protect their membranes. For further information, see here. In 1997, they were launched into low-earth orbit and survived the vacuum of space for 10 days. Yes, Tardigrades . . . in . . . Space! Several went on to lay and hatch eggs normally. Interestingly, there is even a sci-fi sounding word for their state of suspended animation: when so ensconced, they are called a “tun”.

Taxonomically, water bears are most closely related to arthropods, or all the crustaceans and insects of the world, and onychophorans, the velvet worms. Biologists would say they are one of the bilaterian crown groups, or one of the earliest lineages to split into their own group after animals developed mirror-image symmetry. Other early-diverging animals either had no symmetry (sponges) or were radially symmetrical (jellyfish et al.) You can check out their neighborhood of the life family tree here (look for tardigrada). You may notice they’re also in a group called “ecdysozoa”, which is just a code word for “all the organisms that moult exoskeletons”, which actually does seem to be a true, historical, one-time evolutionary inovation (i.e., synapomorphy in bio-speak), and thus make it a taxonomically valid group.

Final cool factoid: few tardigrades have fossilized, but of those that have, one was named Beorn leggi, which will be delightful to those of you who have read The Hobbit. And in case you were wondering if someone actually had the chutzpah to do it, yes, yes someone did. It was screaming to be done. Behold the plush tardigrade.

It’s a whip scorpion, in the order Thelyphonida, although this one has sadly somehow lost its long thin tail, or “whip” (called technically, like those of protists and sperm, a flagellum — but they are *not* evolutionarily-related structures). This one seems to be very well fed, though thankfully not on Jen. I’m taking a short arachnology class at the Denver Museum of Nature and Science right now, and this was one of our subjects. According to my classmate, these animals, also commonly called vinegarroons because of the defensive acetic acid (vinegar) glands they possess near their tails, are the nerds of the arachnid world: “They just kind of bumble along, smelling like a salad.” Raptoral pedipalps (big scary pincers) aside, the one I held did seem to be a sweet, gentle creature. I’ve now held a whip scorpion! Yay!

I haven’t talked about arthropods at this blog much yet, and a paper published in Nature a few weeks ago together with my play date with Stumpy, above,  provide the perfect opportunity to correct that. This post is called “The Creepy-Crawly Branch of the Family Tree”, but it could equally well be called the Floaty-Swimmy Branch, or the Bloody-Sucky Branch or the Borey-Eggs-Iny-that-Hatchy-and-Devour-the-Insides-of-your-Hosty Branch. There are arthropods that do all these things. So let’s have a look at the broad shape of the tree as revealed by this new analysis of the evolutionary relationships among members of Arthropoda:

Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences Jerome C. Regier, Jeffrey W. Shultz, Andreas Zwick, April Hussey, Bernard Ball, Regina Wetzer, Joel W. Martin & Clifford W. Cunningham Nature 463, 1079-1083(25 February 2010) doi:10.1038/nature08742

Now there are a lot of scary words on this diagram, it’s true. But take heart! Look how many drawings of awesome creatures there are! And it’s way better than the alternative that most biologists have to deal with, which I also had to learn to read in school. Before I get to what’s new and cool about this tree, let’s talk a little bit about what trees like this are, and then about the main groups you see on it.

This tree is called a phylogeny, or phylogram (you also hear cladogram). It is a hypothesis of evolutionary history. That doesn’t mean scientists are hypothesizing that these creatures evolved. That’s a foregone conclusion. The hypothesis is what the specific relationships are between the different groups. In other words, the question is, “How is everyone related?”, and this tree is one possible answer. In some trees, as appears to be the case here, the branch lengths are proportional to the evolutionary distance between the different groups. That is, the longer the branches, the more evolvin’ that’s been going on. Evolution, in this case, is measured in DNA nucleotide changes. DNA, as you’ll recall, is made of many base pairs called nucleotides. There are four kinds. When one changes to another, that is called a point mutation. The more of these changes that build up, the greater the evolutionary distance between groups.

For this tree, scientists studied 62 genes in 75 arthropod species. They sequenced them all and compared the changes. They put all the data into a special computer program designed to figure out which sequences are most similar to which other sequences in the five-jillion possible combinations of relationships embodied by 62 genes in 75 species. Then they cranked the computers up to 11 and probably waited a few days (or maybe even weeks! I have heard stories of scientists locking computers in closets during this time) for them to churn out the solution to this hyper-space chess problem. The lone tree you see above is the result.

So what do we see? At the top is Hexapoda, which as you may guess are insects and friends — the six-legged among us. Below them you see an interesting group called Xenocarida. More on them later. Below that group are the Vericrustacea and Oligostraca, which are both, as far as I can tell, basically crustaceans. In both groups you see some old friends: the copepods (some freshwater species of which carry Guinea Worm larvae, a topic I covered in January), the ostracods (who we looked at in a post on deep-sea bioluminscent organisms last year), and the Decapoda, which has a high taxonomic tastiness index: it includes lobsters, crayfish, crabs, and shrimp.

Next are the myriapods: centipedes and millipedes. Below that are the chelicerates, or organisms with special mouthparts called chelicerae — sea spiders (pynogonids), horseshoe crabs, scorpions, ticks, mites, tarantulas, spiders, and Stumpy. And rounding out the base of the tree are the outgroups — the groups we use to “root” the tree, or give it a direction. They are usually the most closely related organisms not in the group of interest, here arthropods. In this case, they are the ridiculously cutely-named water bears or moss piglets — the tardigrades — and velvet worms, the onychophorans. Velvet worms are half of the subject of a crazy-*** theory that somehow got published last year hypothesizing that metamorphosing insects like butterflies were the result of an unholy chimerical union between velvet worms and a larva-less insect.

Also looming large in the arthropods but not on the tree simply for reasons of chronological discrimination (and also because, being extinct, we have no DNA to sample) are the the trilobites. According to my copy of Colin Tudge’s Variety of Life, they branched off somewhere between the Tardigrades and Chelicerates.

OK, so now that you’ve waded through all of that, what were the surprises in this new tree? Scientists also used to think millipedes and centipedes were closely related to insects. They’re both land arthropods, after all. My two college biology texts (published 1995 and 1996) show this relationship, though Tudge(2000) is agnostic on whether millipedes and centipedes or crustaceans are more closely related to Insects. Now it appears certain that, since all crustaceans are aquatic, insects and centipedes/millipedes represent a seperate evolutionary invasion of land by arthropods, much as seals and whales represent two seperate re-invasions of the sea by mammals.

This study also supports the hypothesis that insects evolved from a crustacean, which is why we can’t use the term “Crustacea” any more — the group as traditonally defined doesn’t include the insects, but this tree shows that it should (since the principles of modern evolution-based taxonomy require proper groups to include an ancestor and ALL of its descendants). The term “Reptiles” poses the same dilemma, because it should technically include  birds. So some scientists have stopped using that term as a taxonomic classification, too. Little-r reptiles is OK, though, as informal name for the group.

Finally, it appears hexapods’ (insects’) closest relatives are an obscure underwater-cave-dwelling group newly dubbed the Xenocarida. Carl Zimmer goes into that in admirable detail here.

But the take-home message of this tree for you is simple: look, admire, and marvel at the variety and abundance. In fact, I give you a homework assignment, should you choose to accept it: pick a group on that tree that looks interesting that you’ve never heard of before. Look it up. Find out what it is, what it does for a living, and where it directs its mail. You’ll be glad you did, I promise.