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