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

By: David

David — Wed, 02 Mar 2011 05:35:48 +0000

I’d eat it.

By: Sean L

Sean L — Mon, 28 Feb 2011 10:07:20 +0000

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

By: Ed Yong

Ed Yong — Sat, 26 Feb 2011 02:25:37 +0000

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.

By: Sean L

Sean L — Sat, 26 Feb 2011 02:18:22 +0000

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.

By: Michael Fisher

Michael Fisher — Fri, 25 Feb 2011 11:35:17 +0000

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 ?

By: Sven DiMIlo

Sven DiMIlo — Thu, 24 Feb 2011 23:26:01 +0000

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"?

By: Ed Yong

Ed Yong — Thu, 24 Feb 2011 17:29:03 +0000

@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.

By: Jason

Jason — Thu, 24 Feb 2011 12:15:54 +0000

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

By: Ed Yong

Ed Yong — Thu, 24 Feb 2011 10:56:44 +0000

@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”.

By: chris y

chris y — Thu, 24 Feb 2011 10:39:33 +0000

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.