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.

***

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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