Meet Diania the walking cactus, an early cousin of life's great winners

By Ed Yong | February 23, 2011 1:00 pm

Around 520 million years ago, a walking cactus roamed the Earth. Its body had nine segments, each bearing a pair of armour-plated legs, covered in thorns. It was an animal, but one that looked more like the concoction of a bad fantasy artist. Jianni Liu from Northwest University in Xi’an discovered this bundle of spines and named it Diania cactiformis – the “walking cactus from Yunnan”. And she thinks that it sits at the roots of the most successful group of animals on the planet.

If Liu is right, Diania is one of the earliest relatives of the arthropods – the group that includes insects, spiders, crabs, and more. These species all share a segmented body, a hard external skeleton and jointed legs. They are life’s winners, the most diverse of all animal groups.

To understand what made them so special, we need to look at where they came from. “Delving around the roots of arthropods might help us understand drivers of their current huge biodiversity,” says Michael Benton from the University of Bristol. For a start, how did they evolve from soft-bodied worm-like creatures into the armoured, legged animals we know today? Diania might help to provide some answers. If Liu is right, it was an animal that was “close to the point of becoming a true arthropod”.

Liu found several specimens of Diania in a famous fossil site at Chengjiang, China. The fossils have spent half a billion years underground, but three of them are still beautifully preserved.

Its trunk was a six-centimetre-long cylinder consisting of an odd, wrinkled head, followed by nine segments (it’s facing left in the reconstruction). Each segment was topped with a hard disc and separated from the next by spiny rings. Sprouting from the trunk were ten large armoured ‘legs’, covered in thorns and ending in two spines. “We think that the spines could have protected Diania against predators,” says Liu. “[And] although we don’t know for sure what Diania ate, it’s possible that the spines also helped it to grab and hold onto prey.”

Each leg consisted of 16 hardened segments, and Liu thinks that the gaps between each of these probably acted as primitive joints. The ones at the front stuck out at a forward angle and Diania might have used them for grasping and feeding. Those at the back stuck out to the side and they were probably classical legs, used for walking.

Diania is a lobopodian, a group of animals that look like “worms with legs”. They may have given rise to the arthropods themselves, as well as other fascinating animals like the glue-spitting velvet worms and the toughest animals alive – the tardigrades.

Liu compared Diania to other fossil arthropods and lobopodians to create a family tree that links the groups together. Based on her tree, she thinks that Diania is an early cousin of the arthropods, which branched away at the very bottom of their dynasty. If she’s right, it suggests that these animals evolved armoured legs before armoured bodies.

But that’s not certain. Even though the Diania specimens are well-preserved, they are still frustratingly enigmatic. Their heads are hard to make out and it’s difficult to say for sure if the legs really were armoured. “We accept that are some uncertainties about the exact position of Diania,” says Liu.

“The basic problem is that there are a whole series of interesting Cambrian fossils which show some (but not all) of the features seen in modern arthropods. Diania, for example, has jointed legs, which is a very important arthropod character (and we would argue the most important character). Other fossils living about 500 million years ago show things like heads, eyes or gills; features we don’t see in Diania. We don’t yet know in which order all these different body parts evolved, and we very much hope that future fossil discoveries will help to clarify Diania‘s position.”

This uncertainty isn’t uncommon. “The Chengjiang locality is famous and well-established of course for its remarkable fossils, but many have been debated,” says Benton. Take the aptly named Hallucigenia, another lobopodian from the Cambrian period, which was discovered at the Chengjiang site. When it was discovered, it was so bizarre that no one knew which way was up or which end was the front. Simon Conway Morris thought that it walked on long spines and fed with raised tentacles. Now we know that Morris got the animal upside-down. It walked on the stilt-like, claw-tipped legs (formerly the tentacles) and has long spikes sticking out of its back (formerly the legs).

Diania continues Hallucigenia’s tradition of tantalising confusion. As Benton says, “These are remarkable fossils of another outrageous Cambrian organism, but as ever it appears to raise as many questions as it solves!”

Reference: Liu, Steiner, Dunlop, Keupp, Shu, Ou, Han & Zhang. 2011. An armoured Cambrian lobopodian from China with arthropod-like appendages. Nature http://dx.doi.org/10.1038/nature09704

More on lobopodians and odd Cambrian animals:

CATEGORIZED UNDER: Animals, Invertebrates, Palaeontology

Comments (21)

  1. It looks like one of the sculptures Catherine O’Hara’s character in Beetle Juice designed. Truly terrifying.

  2. Cool. The fossil is neat, although I would not want to run across one of these in my kitchen. ;) Horseshoe crabs are also an interesting glimpse into the past. Most of us struggle with even the basic tree of life puzzles; this must be a huge challenge. Entomologists have their work cut out for them.

  3. Sven DiMIlo

    Wow…very cool animal.

    Take the aptly named Hallucigenia, another lobopodian from the Cambrian period, which was discovered at the Chengjiang site.

    Of course it was first ‘discovered’ in the Burgess Shale of Canada. A putative second species was found much later at the Chinese site.

  4. Robert S-R

    This would be an amazing period in Earth’s history to visit. We could breathe the air that was around back then, right?

  5. Torbjörn Larsson, OM

    Earth oxygenation is a fluid [sic!] subject right now, seems to this layman. A recent description of early fossil assemblages:

    “The deep-water Avalon biota (about 579 to 565 million years old) is often regarded as the earliest-known fossil assemblage with macroscopic and morphologically complex life forms1. It has been proposed that the rise of the Avalon biota was triggered by the oxygenation of mid-Ediacaran deep oceans2. Here we report a diverse assemblage of morphologically differentiated benthic macrofossils that were preserved largely in situ as carbonaceous compressions in black shales of the Ediacaran Lantian Formation (southern Anhui Province, South China). The Lantian biota, probably older than and taxonomically distinct from the Avalon biota, suggests that morphological diversification of macroscopic eukaryotes may have occurred in the early Ediacaran Period, perhaps shortly after the Marinoan glaciation, and that the redox history of Ediacaran oceans was more complex than previously thought.”

    The Lantian biota was 635 – 577 Ma, and had oxygen enough for multicellulars close to the ocean surface, at times. Avalon was 579 – 565 Ma, perhaps 40 My later, and had oxygen enough for multicellulars at depth. And this is another 50 My.

    So yeah, maybe modern animals could have breathed the air at 520 Ma. But maybe one shouldn’t bet on it.

  6. Josh K

    I came across your blog a couple of days ago and its absolutely brilliant. It takes me an age to read a blog post because I end up following all the links to more cool stuff – thanks and keep on writing!

  7. Gunnar

    Thank you for bringing this awesome phylogeny to my attention.

  8. Brian Too

    Do you suppose the head end was on the left or the right of the illustration?

  9. @Brian Too – It’s facing left.

  10. Head looks pretty simple, do they have any idea if it had primitive eyes or mandibles or anything?

    Also, for the first time in my life, I am grateful for cockroaches. Cause at least we have cockroaches, and not…those…*shudder*.

  11. Ben

    “They are life’s winners, the most diverse of all animal groups”

    How about bacteria? Is life always about animals? ;-)

  12. chris y

    followed by nine segments (it’s facing left in the reconstruction).

    Not wishing to quibble, but I count 10 post-cranial segments in the reconstruction (excluding the pygidium), unless the first set of appendages are regarded as cranial. But they seem to attach to the thing the same way as all the rest. Less clear in the photo of the specimen.

  13. @Ben – The only thing worse than zoocentrism is microbiologists constantly whining about bacteria not being represented. And the only thing worse than that are people who complain about not mentioning Archaea when you only talk about bacteria ;-)

    @Chris Y – I specifically followed this up. The front set of appendages are cranial. They don’t count as a “segment”.

  14. Jason

    I’m actually one of the authors of this study and I’m deligted to see it generating comment and interest. In response to a couple of the points: the head end is to the left with a tube like proboscis and no, there is no evidence for mouthparts which presumably developed later in arthropod evolution. The exact count of legs at the front end is slightly complicated because the legs all lie jumbled on top of each other, but we’re pretty sure it’s 10 as per the reconstruction. I wouldn’t say it really has a true head. Heads, like mouthparts, came later on in the evolutionary tree and this is why we think we have something very close to the point where these so-called ‘lobopod’ animals turned into true arthropods

    JD

  15. @Jason – I can’t tell you how much I appreciate it when scientists answer reader questions in blog comments. I only wish everyone would do this. Thanks for taking the time, and indeed for the awesome study.

  16. Sven DiMIlo

    Heads, like mouthparts, came later on in the evolutionary tree

    How would you defend that statement in light of the cladogram presented?
    Tardigrades, onychophorans, Opabinia, Anomalocaris, etc. all had perfectly good heads, no?
    And why don’t the Great Appendages of Anomalocaris count as “jointed”?

  17. Michael Fisher

    Questions from a layman:

    So Diania cactiformis is believed to be a close relative to the ancestor of modern arthropods (but not descended from that common ancestor)
    I understand that modern aquatic arthropods breathe using gills borne on appendages or body segments
    Did the early aquatic arthropods that evolved after Diania cactiformis breathe using the same ‘technology’ ?

    Did this animal have gills ?
    If so, where are they in the reconstruction illustration ?
    More generally did all multicellular aquatic animals from this era use gills or are there other ways of extracting oxygen from water ? For example ~ would simple diffusion work on a creature of this thickness ?

    Why is the sclerotin on the limbs shaped into spikes ? Assuming there were prey/predators with eyes in that era perhaps to imitate aquatic vegetation ? Increased armour ?

  18. Sean L

    Wow! What a beautiful specimen. The Chengjiang-group beds are just endlessly surprising.
    For those who don’t follow this stuff much, the important thing here is the clear external segmentation. In [living] arthropods this is largely controlled by expression of the [italics]engrailed[] gene; in lobopods, which share a similar internal organization but lack pronounced segmentation of the ectoderm and re-enforced cuticle, the expression of the very ancient gene is quite different.
    I question whether the production of scleroderms is a direct physiological or phylogenic precursor to, say, the formation of mineralized chitin plates; the processes appear – to this inexpert observer – to be rather distinct. This leads me question the identification as a stem arthropod rather than, perhaps, a near-basal lobopod – with extensive scleral plating being the primitive condition. Or maybe I should just hunt down a copy of the paper.

  19. The question of where exactly it sits on the evolutionary tree is undoubtedly interesting and almost certainly going to be controversial. From reading the paper (and especially the Supplementary info), it really seems to me like the authors have produced the best tree they can with the info they have, but even they aren’t entirely convinced about where Diania sits on it. Just something about the tone of the paper conveys uncertainty to me, which, I guess, can only be encouraged.

  20. Sean L

    Ah, found it!
    If anyone should be interested, the link below is to a recent (2008) paper concerning another ‘armoured’ lobopod from the same lagerstatte, with some discussion of possible relationships.

    http://www.bioone.org/doi/pdf/10.4202/app.2008.0209

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