In the last video I presented an overview of the various aspects of the Cambrian Explosion and we reached a more modern definition of this phenomenon: the dramatic change in metazoan animals that took place during the first 21 million years of the Cambrian Period. A great number of factors were involved in either starting or fuelling the Cambrian Explosion and I would now like to go through the most important of them in a more systematic manner. For practical purposes I will divide them into abiotic and biotic factors. Abiotic factors have to do with the physical environment, whereas biotic factors relate to the characteristics of the animals and plants themselves, as well as their interactions. The first abiotic factor that must be included is the phenomenon of global glaciation, which was discussed in much greater detail in an earlier presentation. At least two major global glaciations took place during the Cryogenian period: the Sturtian Glaciation 716 million years ago and the Marinoan Glaciation 635 million years ago. Those events gave rise to the concept of Snowball Earth. It should be noted that there are significant indications that these glaciations were repeatedly interrupted by periods of open water, leading to the alternative concept of Slushball Earth. The important thing is that each glaciation was followed by a huge release of carbon dioxide within the oceans, resulting in the deposition of massive amounts of carbonates. These layers are called cap dolomites and the one that followed the Marinoan Glaciation is the largest single planet-wide marker bed that exists. It is up to 11 m thick and can be recognized in Canada, Scotland, Namibia, India, China and Australia. The only other global marker bed that even comes close to this is the one that resulted from the asteroid impact that ended the Cretaceous Era and probably led to the extinction of non-avian dinosaurs. That marker bed is usually only a few centimeters thick. A final global glaciation occurred during the Ediacaran, the Gaskiers Glaciation 580 million years ago – so about 40 million years before the onset of the Cambrian. The oldest Ediacaran faunal assemblages followed shortly after that, about 575 million years ago. These glaciations were significant because the stagnant conditions that prevailed in the oceans during them were followed by periods of greatly improved ocean circulation and oxygenation. The concentrations of hydrogen sulphide and methane in the stagnant oceans were very high and these substances were flushed out when the glaciation ended. And this probably reduced toxicity and presumably favoured evolution. The next factor is continental drift. During the Ediacaran most of the world's land masses were concentrated in the supercontinent Pannotia. It began to break up during the beginning of the Cambrian, accompanied by considerable tectonic activity. As areas of open water arose around the new continents, upwelling along the continental shelves increased significantly. This enriched the water column with nutrients, especially phosphates, and we know this because considerable layers of phosphates were deposited during the Early Cambrian. We also know that life was concentrated in relatively shallow waters close to the continents and as the continents broke up, the total area of shallow ocean habitat increased significantly. This gave evolution a much larger stage on which to play out. I have mentioned phosphates a couple of times now. In my opinion, they may have been the X-factor that triggered the Cambrian Explosion. We have evidence that primary producers, the phytoplankton, in the Ediacaran Period had such high carbon to phosphate ratios that they were of limited nutritional value. This constrained the evolution of metazoan feeding strategies. The increased phosphate that became available during the Early Cambrian, along with rising levels of oxygen, stimulated the production of more nutritional phytoplankton and supported the radiation of the metazoans. In particular, higher phosphate levels allowed both metabolic rates and growth rates to rise, due to the enhanced production of phosphate-rich ribosomal RNA. On the other hand, excessive amounts of phosphate in the diet can be detrimental. It appears that the deposition of phosphate, as a mineral called apatite, by animals in various mineralized structures such as shells, may initially have been an adaptation to deal with this potential toxicity. Interestingly, the presence of high levels of phosphate in the oceans was lucky for us as well, because it favoured the fossilization of smaller animals than was typically the case during the Ediacaran, where most of the fossils that we have are very large. The last abiotic factor that I will mention is oxygenation. The concentration of oxygen in the atmosphere increased steadily from the beginning of the Ediacaran Period. Oxygen concentrations in the ocean are balanced with atmospheric levels through surface interaction. An atmospheric oxygen concentration of 1% of present-day levels is regarded as having been an important threshold in the development of the biological environment for two reasons. Firstly, it was at this level that an efficient ozone layer could develop in the upper atmosphere, and this allowed plankton to flourish closer to the surface of the ocean than previously. Since they were closer to the sunlight, this lead to a huge increase in the available biomass of phytoplankton. Secondly, it is believed that metazoans, and especially bilaterians, could not attain large body sizes until oxygen levels had increased. This is partly because collagen synthesis is not effective at lower levels. Collagen is probably the most important molecule in structures that bind cells and tissues together, and they provide bodies with both rigidity and flexibility. It is thus possible that low concentrations of oxygen had placed an upper limit on the body volume of metazoans during the Ediacaran. Another threshold occurs at an atmospheric oxygen concentration of about 7% of present-day levels. It was at that level that the formation of both mineral and organic skeletal elements like shells became effective. Furthermore, predation on other animals is metabolically expensive, and it only became possible on a large scale after oxygen levels had risen. Thus, there is no evidence for the existence of carnivory at all until the very end of the Ediacaran. To sum up, the most important abiotic factors that helped to trigger the Cambrian Explosion were the end of global glaciations, continental drift, increased levels of phosphate in the ocean and increased levels of oxygen in the atmosphere and oceans. We will now turn to biological factors. But as I go through them individually, please keep in mind that they all took place more or less at the same time and that they influenced and amplified one another through a series of complex and continuing feedback mechanisms. The first biological factor that I will talk about is skeletonization. Skeletons are understood here in a broad sense to include all hardened structures that protect an animal, and they also provide support for muscles and other soft tissues. So, in addition to mineralized skeletons of calcite, aragonite or phosphate, they include such structures as silicate tests and spicules, agglutinated tubes and organic shields that are typically cuticular. Although the rise of skeletons may not have been the first biological transformation in chronological terms, it is usually regarded as being the most important one. The famous palaeontologist Martin Brasier went so far as to declare that the Cambrian Explosion was "primarily the evolution of easily fossilized mineral skeletons". As we talked about earlier, the biomineralization was facilitated by significant changes in ocean chemistry. Another extremely important factor in the Cambrian Explosion is predation. The rise of skeletonization is believed to have been – at least in part – a response to the simultaneous rise in predation that is clearly observed to have happened during the Early Cambrian. Predators appear to have been absent or very rare during the Ediacaran. Boreholes that have been found in some specimens of a Late Ediacaran tube-dwelling organism called Cloudina, but that is the only known exception. On the other hand, there is plenty of evidence for the widespread occurrence of predation during the Cambrian. Direct evidence of this occurs in the form of bite marks on trilobites and the presence of animals called hyoliths in the gut of the priapulid Ottoia. Indirect evidence is provided by the increased occurrence of skeletons and by the feeding structures of the presumed predators themselves, for example various kinds of jaws or grasping appendages. The steadily increasing occurrence of burrows within the sediment, which we will talk about later, was probably partly a protective response to the presence of predators as well. The oldest known predator of the Cambrian is Protoherzina, which is known only for its sickle-shaped spines that are assumed to have been part of a feeding apparatus. Protoherzina appeared as early as 539 million years ago, a mere 2 million years before the start of the Cambrian. We now move on to plankton. As we discussed earlier, continental drift, rises in phosphate levels in the oceans and increased levels of oxygen in the atmosphere and ocean created the opportunity for an adaptive radiation of the phytoplankton, in particular the calcareous acritarchs. The availability of phytoplankton facilitated the rise of grazing zooplankton. And it appears that this was one of the decisive innovations of the Early Cambrian - the rise of zooplankton. We know of the existence of zooplankton because we have found sieving appendages belonging to an arthropod from the Early Cambrian. The presence of zooplankton had two effects of great importance for the development of Cambrian ecosystems. First of all, they became prey to planktonic and nektonic predators. Most importantly, however, as an integral part of their feeding activities, the zooplankton produced coprolites or faecal pellets. These coprolites consisted of packages of only partially digested phytoplankton and they had a diameter 10 to 100 times greater than that of the individual phytoplanktonic organisms. These packages sank more rapidly to the sea bottom, (think of it as a conveyor belt) creating a huge influx of new food to the bottom. This new resource induced increases in the complexity of the benthos, the bottom-living animals, and in particular the infaunal organisms that worked some of these coprolites into the sediment by their burrowing activities. The increased phytoplankton and zooplankton also stimulated the development of benthic filter feeders. Together with the new infaunal organisms they comprised the prey for new forms of benthic predators. So the benthos changed dramatically during the Cambrian Explosion. Trace fossils from the Ediacaran show that hardly any of its animals burrowed beneath the surface of the sediment – almost all of the burrows were horizontal. The substrate was typically covered by a microbial mat and the water-substrate interface was very sharp, with relatively little water being present in the hard-packed upper sediment. But from the beginning of the Cambrian, trace fossils demonstrate that burrows gradually penetrated deeper and deeper into the sediment. And by the Middle Cambrian this layer of bioturbation was typically 15 cm deep. The transformation of the upper sediment was so stark that it is referred to as the Cambrian Substrate Revolution. Increased bioturbation meant that the water content of the upper sediment also rose, this reduced the sharpness of the water-substrate interface. An interesting aspect of this is that the burrows became more and more complex, and this indicates that animals that produced these burrows were becoming behaviourly and neurologically more sophisticated - more complex. The Cambrian Explosion saw the rise of an infauna – animals that lived within rather than on top of the sediment. And this meant that the benthic environment was transformed from an almost 2-dimensional to a 3-dimensional habitat. The first burrowers were probably filter feeders and surface deposit feeders. However, by reworking the sediment they created the conditions necessary for the appearance of subsurface deposit feeders as well. In the next video I will talk about the Cambrian ecosystem.
