Burrowers: knowing front from back

In sedimentary rocks below the base of the Cambrian there is not only a dearth of body fossils, but signs of creatures burrowing and stirring up the sediment are most uncommon. A burrower needs several criteria to be fulfilled: a supply of oxygen; sufficient food; a body able to penetrate and an ability to move back and forth, but forth would probably do fine, provided the animal could turn corners. The amount of oxygen in bottom waters would have influenced its availability beneath the seabed. Whatever the conditions, dead organic matter falls and is buried by sediment before it is oxidised away, even nowadays.  There is little sign that there was any marked change between the oxygenation of the planet just before and after the start of the Cambrian Period, so the main control over burrowing is that of animal morphology.

Many modern burrowing animals are pretty flaccid but moving sediment aside and upwards demands some muscle power. Most important, the creature needs a means of navigation, albeit of a rudimentary kind, and since what goes in beneath the surface – food – must go out – excreta – there must be a front- and a back end. That ‘fore-and-aft’ symmetry is the essential feature of bilaterian animals. Only a limited range of animal taxa don’t have that built-in. Sponges are the most obvious example, having no discernible symmetry of any kind. Radially symmetrical animals such as jellyfish and coral polyps only have a top and a bottom. An absence of inbuilt horizontal directionality stops non-bilaterians from burrowing in any shape or form. But, so what?

The vast majority of animals have some kind of bilateral symmetry; even echinoderms have it from their 5-fold symmetry that is also the simplest kind of radiality. By the start of the Cambrian, not only had bilaterians split off from the less symmetrical but almost all the phyla living today, and several that became extinct in the last 542 Ma, have representatives in the Cambrian fossil record. The only logical conclusion is that emergence of bilaterians and their fundamental diversification took place in the Precambrian: they are absent  from earlier strata only because they had no hard parts. Comparing the DNA of living representatives of the main bilaterian phyla and with that of non-bilaterians can help date the times of genetic and morphological separation, but only crudely. This ‘molecular clock’ approach points to some time between 900 and 650 Ma ago for the last common ancestor of bilaterians.

Uruguayan fossil burrows from late Neoproterozoic (Credit: Pecoits, E. et al. 2012)

Getting a handle on the minimum time for the split depends either on finding fossils or unequivocal signs of bilaterian activity. The oldest unequivocally bilaterian fossils occur in rocks about 550 Ma old, which doesn’t take us much further back than the base of the Cambrian. But there are trace fossils that are significantly more ancient (Pecoits, E. et al. 2012. Bilaterian burrows and grazing behaviour at >585 million years ago. Science, v. 336, p. 1693-1696). They are tiny burrows in fine-grained sediments from Uruguay, so tiny that there is a chance that they may be traces of grazing bacterial films on the seabed rather than beneath it. The decider is the mechanics of trace fossil formation. Surface tracks only a millimetre or so across would only penetrate the biofilm, so on lithification they would simply disappear. Burrows on the other hand penetrate the sediment itself to get at food items. Even if this was a biofilm, the track would be in sediment above the film, so compaction would preserve it. The Uruguayan exam-[les are exquisite horizontal burrows, and they push back the minimum age for the origin of the bilaterians to at least 40 Ma older than the start of the Cambrian. In fact 585 Ma is a minimum age for the sediments as it is the U-Pb age of zircons in a granite that intrudes and metamorphoses them.

An equally significant observation is that the burrows only appear towards the end of a glacial episode – probably the last of the Neoproterozoic ‘Snowball Earth’ events – as marked by tillites below the burrowed shales and occasional ‘dropstones’ in them. Could it be that the climatic and other stresses of a global glaciation triggered the fundamental division among the Animalia?

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