Aspidella on the move?

This is Aspidella:

(Peterson et al. [2003] via Palaeos)

The Internet tells me this is also Aspidella:

(Amy Campbell)

And so is this:

(Menon et al., 2013)

(How on earth did all of those things end up with the same name???)

Aspidella, you see, is one of those problematic Ediacaran fossils that may or may not belong to a single kind of organism, which may or may not be an animal. It’s an impression of something soft with a rather variable assortment of surface features, and hence it’s pretty hard to tell what made it, although the wide holdfast of some bottom-dwelling, filter-feeding animal is a popular opinion. This nice Charniodiscus specimen (Tina Negus via Wikipedia) explains why:

Seeing how fossils like these are one of our precious few sources of evidence on the early history of animals, any additional evidence to help us figure out what they were is awesome. It’s especially cool to find evidence of behaviour, because “behaviour” is something that only certain groups of organisms exhibit, and some of the candidates for Ediacaran thingies like this (e.g. fungi, lichens, microbial mats) specifically don’t.

In a short paper in Geology, Menon et al. (2013) argue that they have found such evidence in some Aspidella specimens from the mid-Ediacaran Fermeuse Formation of Newfoundland. There are two kinds of features they report on. First, there are shallow, short trails that look like whatever made the impressions slid or hopped along a soft sediment surface in short movements. Some of the trails show faint impressions of the radiating ridges some conventional Aspidella specimens possess (like the one below, taken from the paper):

They are fairly rare, the best bet for finding them being slabs of rock practically carpeted with Aspidellas. A couple of things indicate that they weren’t just made by some random current or mudslide sweeping hapless Aspidella creatures along. For one thing, even in a whole pile of Aspidella imprints, you’ll find only a few such trails. (Although that could be because most of the living creatures would have been firmly rooted to the sediment!) For another, neighbouring trails point in all kinds of random directions, so if it was a current, it must have been the most chaotic one in earth history.

The other kind of evidence is what looks like the “evolution” of vertical burrows, layers of sediment dipping downwards like there used to be something sitting on them that gradually relocated further up as more sand and mud accumulated around it. Of course, an animal sitting in the mud isn’t the only thing that can produce similar features, so the authors considered a few alternatives.

They didn’t find any signs of water or gas bubbles escaping. They also didn’t think the features looked like sediment slumping into a hole, which they actually experimented with by piling sand and mud on top of dissolving liquid capsules (laundry capsules?? :o). The dips produced by falling sediment get conspicuously shallower towards the top, which the fossil dips don’t seem to do, plus the latter also have round structures like small Aspidella on top. Personally, I find the photos of the fossil dips really hard to compare with the picture of the experimental dips, though. Here’s perhaps the best specimen they show alongside one of their experiments:

Yeah… I can kind of see where you’re coming from, but…

So the idea is that an animal lived with its rear end buried in the sediment and its feeding structures up in the water column. As the water brought in more sediment (the Fermeuse Formation is thought to be marine in origin), the unknown creature moved upwards to avoid complete burial. Eventually, it would die, leaving behind a stack of little dips indicating its previous seats, topped by a good old-fashioned Aspidella impression.

Interestingly, only small Aspidella are associated with these vertical traces. Did young and old Aspidella creatures live in different ways, or do larger specimens simply belong to a different organism?

The authors specifically think the Aspidella animal was cnidarian-like because other possible candidates such as sponges and giant moving protists haven’t been observed to move vertically through sediment. Only well-muscled creatures like sea anemones (and bilaterians, but there’s absolutely no reason to think this thing was a bilaterian) are known to do that.

Which is really pretty exciting – more Ediacarans directly associated with traces of movement! I maybe should have mentioned that the paper keeps going on about Retallack (2013), mainly to say that it was Wrong, but I thought it was interesting enough in its own right. The fact that it discusses signs of animal-like behaviour in a kind of fossil that’s also common in the Australian rocks reinterpreted by Retallack as terrestrial is kind of beside the point.

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References:

Menon LR et al. (2013) Evidence of Cnidaria-like behavior in ca. 560 Ma Ediacaran Aspidella. Geology advance online publication 06/06/2013, doi: 10.1130/G34424.1

Peterson KJ et al. (2003) A fungal analog for Newfoundland Ediacaran fossils? Integrative and Comparative Biology 43:127-136

Retallack GJ (2013) Ediacaran life on land. Nature 493:89–92

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Oh, look, an argument!

It seems like forever since I posted about the very old putative bilaterian burrows Pecoits et al. (2012) reported in Science. I read the paper, thought about the implications, wrote the post and then filed the whole thing away in the giant messy cabinet at the back of my mind.

But a big claim like the one Pecoits et al. made – burrows from bilateran animals that appear before the first Ediacaran fossils! – is unlikely to go unchallenged by the scientific community. Now the argument has broken out. Gaucher et al. (2013) wrote a comment in Science criticising the reasoning that put such an old date on the formation where the burrows were found. Pecoits et al. (2013) responded. The plot is thickening!

The main bone of contention seems to be whether the huge body of granite that gave the actual radiometric date of 585 million years lies below the burrow-bearing formation (in which case it must be older than the fossils) or cuts through it (in which case it’s younger). The other question is whether the fossils and the rocks they’re found in actually belong to another nearby formation that is thought to be Permian in age. Burrows in Permian rocks would be no surprise at all . By that time reptiles and the ancestors of mammals walked the earth, insects of all kinds flew over it, and armadas of worms had been boring through soft sediments for hundreds of millions of years. Burrows that far into the Precambrian, on the other hand…

The argument is all very geological, and as I repeatedly said, I’m not much of a geologist. Looking at the figures wouldn’t help me decide who to believe at all. I’m rather amused by some of the snark that gets into the text, though. I have this feeling that Pecoits et al. are annoyed. Watch this, for example:

In this case, Gaucher et al. (1) take no notice of the outcrop-scale relationships and instead prefer to show five photographs from just one hand sample that they assigned to fossil site C to discredit the intrusive nature of the granite [figure 1, B to F, in (1)]. We do not want to speculate on the origin of this sample, but we see no evidence that it comes from fossil site C; it is not the ferruginized basal sandstone we previously documented [figure S3C in (2)].

Oh, yeah. “We do not want to speculate,” but we think something’s fishy with your evidence, only we’re too polite to say it in so many words!

Tee-hee. Academia’s version of an online flame war.

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References:

Gaucher C et al. (2013) Comment on “Bilaterian burrows and grazing behavior at >585 million years ago”. Science 339:906

Pecoits E et al. (2012) Bilaterian burrows and grazing behavior at >585 million years ago. Science 336:1693-1696

Pecoits E et al. (2013) Response to comment on “Bilaterian burrows and grazing behavior at >585 million years ago”. Science 339:906

Is Ediacara really stranded?

Heh, when I wrote a confused post about a paper by Greg Retallack that argues that classic Ediacaran fossils like Dickinsonia come from a terrestrial rather than an underwater environment, I said there’s sure to be responses. And I completely managed to miss the responses in the very same issue of Nature, apparently published online on the same day. *shameface* (I don’t think I got the commentary piece by RSS???)

One of them was actually quite nice to Retallack. L. Paul Knauth’s name doesn’t ring a bell, I suspect he’s the “geologist” out of the “palaeontologist and a geologist” the intro mentions. Of Retallack’s analysis itself, all he has to say is that Precambrian sediments can be very difficult to interpret, and one will need genuine expertise in fossilised soils ‘n’ stuff to evaluate Retallack’s claims. However, Knauth rejoices over the mere fact that there are unorthodox opinions like Retallack’s out in the open. In which he is certainly right – science wouldn’t go anywhere without disagreements.

The other commenter, Shuhai Xiao, is not so kind. (Him I’ve actually heard of; he’s published some seriously interesting stuff about Ediacaran fossils.) His commentary is kind of a polite way of saying “what a load of nonsense”. Like Knauth, he considers the evidence for the terrestrial origin of these rocks ambiguous, but he also emphasises features of the rocks that fairly unambiguously point to a marine environment. Funnily enough, he brings up geology that isn’t totally impenetrable to me as evidence, like a neat photo of Dickinsonia specimens on a slab of rock covered in nice symmetrical-looking ripples (the kind that forms under quiet waves). There’s also the fact that I forgot about when I wrote the other post: Dickinsonia itself is sometimes associated with crawling traces. Whatever that thing was and wherever it lived, it ain’t no lichen.

That’s reassuring in terms of not standing my worldview on its head, but I really wish Xiao had been less vague about some of his points. For instance, “the isotope signatures of carbonate nodules in the Ediacara Member can be accounted for by post-depositional alterations that do not involve pedogenic processes,” he says, with no further explanation and no citations. I’m thus far on Xiao’s side, but that doesn’t turn the above into a good argument…

Oh well. Let the debate rage on 🙂

(As of yet, no citations of Retallack’s paper on Google Scholar. We’ll definitely check back later. If I remember…)

Ediacaran Underground

As you may have guessed from my blog title, I’m fascinated by the early history of animals. Part of the problem with this early history is that its fossil record is extremely sketchy, extremely difficult to interpret, or both. One of the things that marks the diversification of animals late in the Ediacaran (the last period of the Precambrian) and even more during the Cambrian explosion, is the appearance of burrows and other trace fossils. Marks on the seafloor don’t have to be made by what we usually think of as “complex” animals, though. Burrowing sea anemones are well known, and even single-celled creatures can plough a track in the sediment. That makes things rather complicated when you are looking for the first signs of complex bilaterian animals in the rocks.

Pecoits et al. (2012) are convinced that the trace fossils they found in Uruguay are such a sign, perhaps indeed the oldest. The gently meandering little burrows they report in Science are shaped much like the burrows made by some modern molluscs and annelid worms, possessing features that are hard to explain with a rolling protist or a simple animal. Among other things, they have minute indentations in their walls that may indicate the places where leg-like body parts pushed against the sediment as the animal pulled itself along.

The mysterious burrowers’ path was relatively simple, meandering in broad sine waves that may preserve the unknown creature’s search for food. Burrows often cross, suggesting that their makers made no effort to avoid each other. However, they sometimes disappear and reappear a few millimetres later, as if the animal made a detour upwards or downwards. The few abrupt turns, combined with the width of the burrows, indicate that the creature who left these traces was small, less than a centimetre long.

These burrows – if they are indeed that – are somewhere between 585-600 million years old, at least 30 million years older than the oldest uncontested bilaterians. My question, though, is are they? Bilaterian burrows, I mean. It’s a good thing I double-checked about trace-making protists, since it turns out that the protists in question leave tracks that have supposedly bilaterian characteristics, like consisting of two ruts on either side of a central ridge (Matz et al., 2008). This is one of the similarities the new paper draws between modern bilaterian burrows and their Precambrian precursors! (The weirdest thing is they cite Matz et al. but kind of dismiss the eerily bilaterian character of protist traces…)

Pecoits et al. (2012) do take some time to argue against a protist or non-bilaterian origin of the burrows. There are the little indentations that may have come from something like a worm’s parapodia but are perhaps more difficult to reconcile with a simple rolling ball of cytoplasm. There are also the disappearances and reappearances that indicate the creature moving up and down levels. The authors also argue that the burrows are definitely burrows – i.e. actually under the sediment surface -, though I’m not sure this follows from their evidence.

Then again, they’re the experts here, I’m just a geologically challenged biologist looking at pictures of grooves in rocks… The disappearance-reappearance thing does seem to imply that whatever left these fossils could burrow. (Unless it suddenly hopped off the bottom and landed nearby? And why can’t protists burrow anyway? We didn’t even know they made fake animal traces until a few years ago… I think I’m beginning to sound silly…)

As far as arguments for a bilaterian trace-maker go, a lot is made of the infillings in the burrows, and of the parts that look like the roof collapsed after the animal moved on, but honestly I can’t see what they are talking about in the photos, so I’ll stay on the safe side and reserve judgement there 🙂

And here I meander off track (terrible pun fully intended)

Aaaaanyway, let’s assume they are right, and there’s a reasonably complex worm behind these burrows.

That would be awesome.

However, it poses some questions. In fact, it poses the same questions my friend Kimberella does. I’ve been ruminating about this since I wound up explaining what we know of the bilaterian ancestor to some random guy on the internetz. Let me pour out the contents of my brain here, and let’s hope they make sense >_>

Kimberella. This creature is at the very least a bilaterian, but probably not too far off from molluscs. Its amazing fossil record includes incontrovertible evidence that it could move around and graze on whatever it ate (microbes, probably) by scraping it off the seafloor. It was covered in a knobby armour – fairly flexible and probably not made of a single peace, but definitely a shell of sorts. That’s one of the things that suggest ties to molluscs. Either way, something that we can identify as a member of a subgroup of bilaterians must postdate the bilaterian common ancestor by a fair bit – all those lineages needed time to split and evolve their recognisable body plans.

Palaeontologists and developmental biologists (Budd and Jensen, 2000; Erwin and Davidson, 2002) made a good case in arguing that the last common ancestor of bilaterians must have been small and simple. We know this creature must have predated Kimberella by some time – and this is where we run into problems. One of the strongest arguments for a small and simple bilaterian ancestor is the paucity of Precambrian trace fossils. Large and complex bilaterians make a big mess of seafloor sediment. They dig into it, they walk over it, they churn it up and eat it and spit it out. Nonetheless, the Precambrian is full of virtually undisturbed microbial mats, an unexploited bonanza for any bilaterian with the means to graze. So the conclusion is that large, complex bilaterians must have been rare or non-existent. But, Kimberella? Burrows of a large-and-complex bilaterian* that predate Kimberella by 30 million years?

Something odd is going on here.

I suppose the question could be put as: if these creatures were around in the late Precambrian, why weren’t there more of them? (And where are the bodies?) Why didn’t they swarm out and eat all the microbial mats that made the preservation of many Ediacaran fossils possible (Narbonne, 2005)? Kimberella was doing its best to graze them off the face of the earth, yet this same type of preservation is common even in the formations everyone’s favourite proto-mollusc comes from.

Too little oxygen? But Kimberella is large, relatively compact and partly covered in armour. It’s the sort of creature that we’d expect to have a specialised respiratory system even in an oxygen-rich modern sea, so at least one animal clearly solved this problem…

I suppose it all comes back to what caused the Cambrian explosion, which is a tough question. (Marshall [2006] is a pretty nice review if memory serves.) If we figure out why molluscs and worms and other bilaterians didn’t take over the oceans long before the Cambrian, we’ll have figured out why they did so in the Cambrian.

I’m not sure that did make sense in the end, but I’m glad I could get it off my chest 😀

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*A centimetre may not sound very large, but a pretty big percentage of the animal kingdom comes nowhere near it in size.

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References:

Budd GE & Jensen S (2000) A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews of the Cambridge Philosophical Society 75:253-295

Erwin DH & Davidson EH (2002) The last common bilaterian ancestor. Development 129:3021-3032

Marshall CR (2006) Explaining the Cambrian “explosion” of animals. Annual Review of Earth and Planetary Sciences 34:355-384

Matz MV et al. (2008) Giant deep-sea protist produces bilaterian-like traces. Current Biology 18:1849-1854

Narbonne GM (2005) The Ediacara biota: Neoproterozoic origin of animals and their ecosystems. Annual Review of  Earth and Planetary Sciences  33:421-442

Pecoits E et al. (2012) Bilaterian burrows and grazing behavior at >585 million years ago. Science 336:1693-1696