[Note: yellow rocks above are Chengjiang ones]. It is a veritable freakshow. What will now follow is a simple showcase of the more famous fossils.
Anomalocaris is the poster child of the Cambrian Radiation, a large and probably vicious predator with two large eyes, an enormous toothy appendage and a characteristic pineapply mouth,
Originally, it was described as two animals, the appendage being one animal and the pineapple mouth another. It wasn’t until the Whittington et al. reinvestigations that it was discovered to be one single animal.
Anomalocaris is the original, but many more relatives have been found with very different morphologies. Only three have body fossils: Anomalocaris (top left), Hurdia (top right) and Laggania (bottom left). The rest are known from isolated appendages.
As a sidenote – going into the stem-group arthropod fossil record and systematics is impossible in this space – this appendage is called the great appendage, but it evolved in other taxa independently, although they are erroneously lumped together as “great appendage arthropods“.
Hallucigenia, as the name suggests, is a very strange animal. Originally, it was described upside down, but then was flipped over. It is a stem-group velvet worm (Onychophora) with spikes on its back, presumably for defence.
Sanctacaris is another strange arthropod, thought to be on the stem line to the Chelicerata (spiders, scorpions et al.)
Opabinia is the classic freak of the Cambrian Radiation – when its reconstruction was first unveiled at a scientific conference, the auditorium reportedly burst out laughing. And it’s not hard to see why, with its five stalked eyes and that ridiculous proboscis ending in a grasping appendage.
To move away from the arthropods, we also have Wiwaxia. It’s an animal whose body is entirely covered by sclerites, and it also possesses those elongated spines. There is no fossil available yet to shows us what lay beneath all the armour. It is assumed to be either a stem-group mollusc or annelid.
And for a brand new reconstruction, here’s a reinterpretation of Nectocaris pteryx as a cephalopod.
It also has a varied history, first described an an anomalocarid, then as an arrow worm, before ending up as a cephalopod. This is a testament to the difficulty of getting the interpretations of these ancient fossils right.
So basically, when you see a picture like this, the only thing that is fictional are the colours. No matter how strange the organisms in a Cambrian reconstruction look like, they are most likely to be real.
So now that we know what was around, it’s time to dig into why this is such a significant event in the history of life. What makes the Cambrian Radiation so special?
It was the first appearance of bilaterian animals in the fossil record. The Bilateria is the node numbered 573 above – all animals excluding Cnidaria (jellyfish) and Porifera (sponges). Before the Cambrian, what was present was the stem-group of all these phyla. But it was only in the Cambrian that the crown groups started appearing.
For a more graphical perspective, here is another picture, to put a face to all the perhaps unknown names.
Source: Barton, N. H., Briggs, D. E. G., Eisen, J. A., Goldstein, D. B. & Patel, N. H. 2007. Evolution. CSHL Press. [See http://evolution-textbook.org/]
In the talk, these arrow-fests were brought gradually by clicking, but here you can take your time and examine them carefully.
The Cambrian Radiation is the result of a new innovation in developmental biology: triploblasty, i.e. a third germ layer. In sponges, there is only one germ layer, the endoderm, which builds the digestive system and epithelial linings (and lungs, in those animals who have those). Cnidarians go one step further and have an ectoderm, out of which arises the nervous system.
Bilaterians all have three germ layers with the addition of the mesoderm. This allows some very significant changes in morphology to happen, as the skeleton, the coelom and the circulatory system arise from it.
In abstract terms, what these lead to is an expansion of the possible morphospace. In practical terms, there is more support available for differing morphologies: with muscles come new ways of moving, circulatory systems allow larger body sizes, coeloms provide structural support.
There were two major changes in the nature of the fossil record in the Cambrian. The first is the appearance of massive biomineralisation – the building of shells and carapaces.
This not only plays into the support-for-morphologies aspect (and other aspects, as we will see), but is also a fundamental change for the fossil record, as it is the beginning of ‘regular’ fossilisation.
Obviously, biomineralisation started out small. There are isolated examples from the Ediacaran fauna, but mineralisation first became popular with the rise of the small shellies in the Tommotian stage of the Early Cambrian.
For some background, biomineralisation is not simply a process by which a lump of hard material is thrown on an organism’s back – although it may have started that way. It’s a tightly-regulated process. Even in snails and bivalves, the shelled molluscs, the shells do not consist solely of minerals, but the minerals are integrated into a complex cocktail of proteins. In arthropods such as shrimp, biomineralisation is also present in the head shield, where trace amounts of carbonates are embedded in the chitin matrix. Vertebrate bones are the same: they are not clumps of calcium phosphate, they are living tissue.
Anyway, the appearance of biomineralisation can easily be linked to the geological changes of the time. With the break-up of Rodinia, more nutrients were leeched into the oceans and they became saturated. Combined with the warming of the climate, crystallisation was favoured. This was incorporated and controlled by the organisms because biomineralisation is very useful for shells and for weapons, as we will see.
The other major change is in the trace fossil record. The picture shows Trichophycus pedum, a trace fossil whose earliest presence at 535.5 Ma is the accepted boundary between the Proterozoic and the Cambrian periods.
Note that it has now been found at earlier times, but the generalisations in the next page are still valid.
Trichophycus is a deep and very sculpted trace, something typical of a burrowing animal. Traces like these are only found from the Cambrian onwards – Ediacaran trace fossils are mere scratches on the surface.
Such an advance in a trace fossil can only be brought about by advances in morphology – this is the effect of triploblasty and the development of muscles and active movement.
In turn, this leads to what is referred to as the “Cambrian substrate revolution”. Animals could now invade a new, up to now unexplored territory: the sediment. They could anchor themselves or dig tunnels. In essence, the increase in trace fossil complexity heralds the start of regular marine ecology, and it can all be traced back to the development of triploblasty.
The above picture summarises what we see as the Cambrian Radiation: a drastic radiation of animals, concurrent with an increase in biomineralisation and the complexity of trace fossils.
So now we can begin exploring some of the more general aspects of the Cambrian Radiation and write a basic synthesis. Written in red are key rules of thumb that should always be kept in mind.
We already went through the Ediacarans, but just as a reminder: it was not a “garden”, as it is sometimes called in popular articles. It contained animals.
What triggered the radiation of the animals was the environmental changes of the time – the rise in temperatures allowed a general increase in biological activity (heat = energy, after all).
The deglaciations caused flooding and a great increase in the space available for animals to live in. At the start, these were colonised by the Ediacaran faunas, which formed what is called a permissive ecology, i.e. one in which even inefficient organisms like the Ediacarans could survive.
However, their reign was ended due to the rise of oxygen. Oxygen was the key to taking advantage of triploblasty: it allowed circulatory systems to develop and body size to increase, by allowing the biosynthesis of many types of tissues and fibers, most notably collagen. These, combined together, allowed the triploblasts to rise to dominance.
Another aspect to keep in mind is the evo-devo (evolution of development/evolutionary developmental biology) one. It is well-known that many, if not most, genes, especially developmental ones, are conserved across all animal taxa. The early evolution of animals proceeded largely by cooption of existing genetic pathways, and a few select duplications.
This, combined with the environmental chaos, led to a whole load of experimentation, producing such weird freaks of nature as the vetulicolians, pictured above.
With natural selection, the successful “body plans” (I use that term loosely for the sake of pedagogy!) were kept and development was “canalised”, i.e. focused along narrower and narrower paths. In abstract terms, the few fitness peaks on the morphospace were gradually being discovered and climbed.
This plays into a larger issue in palaeontology and evolutionary research, namely how genetic changes result in morphological ones. This is another issue which would require a specific lecture (series) to deal with properly, so it is summarised in red: you cannot simply correlate genetic change to morphological change.
This may seem like an irrelevant quip to make for this topic, but whether the Cambrian Radiation is a real event is actually a controversial subject. Molecular phylogeneticists employ a molecular clock in order to measure divergence dates. This molecular clock is not at all as foolproof as is made out to be and is faulty more often than it is correct (for reasons that would require their own talk to explain!).
Earlier molecular clock studies had outrageous dates for the origin of animals (cf. above example). Nowadays, they are closer to the fossil record, but still have an (expected) gap, mostly placing the origin of animals between 700 and 600 Ma.
This has led to talk of the Cambrian Radiation being merely an illusion, brought about by a gappy fossil record. This is complete poppycock: you cannot wish for a better fossil record than that in the Cambrian. It is the molecular clocks that are wonky, not the fossil record.
Very basically, the molecular clock finds out when genetic intermixing stopped – i.e. the point of speciation. However, because genetic changes are not directly measurable in morphology, this is impossible to see in the fossil record.
Note that I am oversimplifying the matter to the point of perhaps being false. If wanted, contact me for more detailed info on the difference between molecules and morphology, and molecular clocks and the fossil record.
So by now we have our diverse animals that can move around in whole new ways. The driver of the Cambrian Radiation was how the animals employed their new abilities both in dealing with their environment and among themselves.
The Cambrian substrate revolution is the main novelty in the former, but it should not be forgotten that active swimming and movement was also a novelty.
However, the real star of the show is the latter: how animals started influencing each other. I mentioned the preservation of organismal interactions in the Burgess Shale. These can all be led back to a single selective pressure: predation.
Biomineralisation was selected for because hard shells offer protection. In response, predators took advantage of the same effect to make weapons. In response to those, animals lower on the food chain reinforced their shells. And so on. This pattern is called a coevolutionary arms race and it is most evident in the Cambrian Radiation.
Predation explains why we have animals such as Opabinia with five eyes, or anomalocarids with huge eyes: they were predators and needed to see their prey. The main use of shells is for defence, and they needed to be in top shape at all times, as evidenced by this particular picture with the shell damaged by predation. Even the Cambrian substrate revolution played into the predation aspect: burrows are great hiding spots.
In a nutshell, environmental changes triggered the radiation, developmental biology and geochemistry allowed animals to diversify, and the radiation was made all the more spectacular by all-new types of ecological interactions, most importantly predation.
To move on from the Cambrian Radiation, we can look at the next striking Konservat Lagerstätte: the Orsten. Initially, it referred to a single locality from the Late Cambrian of Sweden, but it is now known as a special taphonomic window (i.e. a place that preserved specific things) from all over the world.
It preserves small animals in small nodules with extremely fine detail and in 3D, as seen above. The majority of these are arthropodan larvae.
For more info: Maas, A., Braun, A., Dong, X.-P., Donoghue, P. C. J., Müller, K. J., Olempska, E., Repetski, J. E., Siveter, D. J., Stein, M., Waloszek, D. 2006. The ‘Orsten’—More than a Cambrian Konservat-Lagerstätte yielding exceptional preservation. Palaeoworld 15, 266-282.
For pictures and updates: http://www.core-orsten-research.de/
As an example of how spectacular these fossils are, here’s Cambropycnogon, the first instar larva of a pycnogonid (sea spider).
Even more informative, we have larvae of stem-group crustaceans, which provides us with a lot of info and data on the systematics and evolutionary history of the crustaceans that is otherwise unobtainable.
So by the end of the Cambrian, this is what the sea-floor looked like: it was by and large arthropod-dominated, with trilobites being the most diverse group, some competing with the hyoliths (those conical animals with the handlebars). A couple of epibenthic bivalves were also common.
Picture source: Harper, D. A. T. 2006. The Ordovician biodiversification: Setting an agenda for marine life. Palaeo3 232, 148-166.
The Cambrian had a unique type of fauna, with low species diversity. With the beginning of the Ordovician, though, this changed as one of the largest radiations in the history of life on Earth took place.
It should be clarified that the Cambrian Radiation did not lead to such a huge increase in diversity, but was a diversification of “higher levels of systematics” (of phyla and form). However, it was in the Ordovician that the lower levels (orders, classes) known from the fossil record and today originated, as seen in the diagram above.
This radiation is explained by more environmental changes. In the Ordovician, the oceans rose more and huge epicontinental seas developed, leading to more colonisation space for the opportunistic organisms to take advantage of quickly.
These included the brachiopods, who managed to diversify into many forms (now extinct).
Picture source: Zhan, R.-B., Jin, J. & Rong, J.-Y. 2006. β-diversity fluctuations in Early–Mid Ordovician brachiopod communities of South China. Geological Journal 41, 271-288.
Another group that radiated in the Ordovician were the trilobites, reaching high taxonomic, morphological and ecological diversity.
The echinoderms reached their highest diversity in the Ordovician, with most classes (including the ones still living today) originating in this radiation.
For more on echinoderm palaeodiversity: Smith, A. B. 2005. The pre-radial history of echinoderms. Geological Journal 40, 255-280.
What all these radiations led to was a drastic shift in the way marine benthic ecosystems are structured. The Cambrian seafloor was crawling with very mobile species; in the Ordovician, these were replaced by many mostly-sessile organisms such as sea lillies (crinoids) and bivalves. Brachiopods were at their peak, and molluscs were beginning to rise to dominance.
However, it is from this point that we start getting the effect of the Cambrian Radiation on the fossil record. The animals that radiated in the Ordovician all have hard parts and are preserved in any case.
Soft part preservation depends on anoxic, undisturbed deposition. With the advent of burrowing in the Cambrian Radiation, such depositional enbironments were hard to come by: any carcass that gets buried will just be fed on by an organism living in the sediment.
What this means is that this radiation of shelly- and skeletonised organisms, while it did definitely take place, may also have been accompanied by diversifications in non-shelly taxa, such as most arthropods (besides trilobites).
This highlights the importance of Konservat Lagerstätten, only one of which exists for the Ordovician: the Fezouata Formation in Morocco. What we find there, besides the shelly taxa, are remains of many soft-bodied arthropods, many of which can be linked back as relatives of the Cambrian freaks. For example, above you see Furca, a relative of Marrella, the most abundant arthropod of the Burgess Shale and most abundant arthropod of the Cambrian.
As a summary, here is what happened in the Ordovician Radiation. It is a classic case of shifting ecological dominance. The cause was the appearance of new and widespread shallow oceans, allowing theorthids (cephalopods, the straight-coned ancestors of the ammonites) and snails to radiate and push the previously dominant trilobite and early crinoid fauna into deeper waters. The same happened in the Late Ordovician, when bivalves took over and pushed those groups into mid-deep water.
However, this was rather short-lived, as at the end of the Ordovician, the second-largest extinction event of all time happened: the end-Ordovician mass extinction.
However, this was not as drastic as it may seem. Sure, biodiversity took a big hit, but the new ecological structure that was established in the Ordovician Radiation re-emerged unscathed, just with other taxa filling those ecological guilds.
The cause of it is simple. These new oceans that had formed were exceptional, in that they were the result of melted glaciers. As soon as a cold climate came back into play, these oceans disappeared quite suddenly, and the shallow-water organisms had no time to move or adapt to the deeper waters to survive.
To continue with our Konservat Lagertätten theme, we move on to the Silurian, where again, there is only one of note: the Hereforshire locality.
Here, organisms were buried in volcanic ash and preserved in 3D (like in Pompeii). However, there is one unique aspect: these organisms cannot be prepared out of their nodules. Instead, many very thin slices are made (on the several µm-scale), and all these slides are scanned and digitally stacked together to form a 3D model of the organism inside. An example of a slice can be seen on the left, middle.
Through the years, the technique has been improved, and we can now make such detailed pictures as the coloured ones above, showing 2 stem-group arthropods (left), a brachiopod with pedicle (middle) and a pycnogonid (right; modern analogue for comparison below).
Again, examination of the specimens in this locality provides data we would otherwise not have on diversity for many organisms, and in enough detail to make proper morphological analyses possible.
As a small case study for the importance of Konservat Lagerstätten, consider the anomalocarids. Select examples are pictured above. These are all from the Cambrian.
This is where Schinderhannes bartelsi comes in. It is an anomalocaridid from the Lower Devonian Hunsrück Slates, another Konservat Lagerstätte, and the only [at the time, see note] non-Cambrian anomalocaridid known. [Note added in press: just this week, an Ordovician anomalocaridid was published, from the Fezouata Formation, see above. Interestingly, I had predicted this during my talks, meaning that I have divine powers and thus you should send me money. Uhh, where was I? Right, reality…]
Without its finding, it would logically be assumed that the anomalocaridids died out at the end of the Cambrian. This is the true importance of the Konservat Lagerstätten.
And to round off the talk, some more about these Hunsrück Slates, since I’ve done some work in them myself (in the same team that described Schinderhannes, although I joined later than that). The locality is 405 Ma, from western Germany. The fossils come from three quarries, Simmern, Gemünden and Bundenbach; see the map above, prepared by Gabriele Kühl (Uni Bonn) based on Rothe (2005) and Mittmeyer (1980).
Mittmeyer HG. 1980. Zur Geologie des Hunsrückschiefers. In: Stürmer W, Schaarschmidt J & Mittmeyer HG (eds.). Versteinertes Leben im Röntgenlicht. Kleine Senckenberg-Reihe 11, 34-39.
As you can see, the quarry is now left alone and collectors can just walk in and pick up slabs of slate. However, be warned that the fossils are very, very rare, especially the spectacular ones. The picture above is by Alexandra Bergmann (Uni Bonn).
On the deposition, the Hunsrück Slates are the result of turbidites (underwater mud flows). They swept through from shallow waters to deeper waters, carrying organisms and habitats with them, before depositing them in an anoxic basin. The anoxia was caused by bacteria who fed on the carcasses. Importantly, these bacteria caused the muscles to be replaced by pyrite – the ocean waters were saturated with iron and sulphur. This is what causes the soft-past preservation: pyrite is a hardy mineral.
The Hunsrück Slate fossils are pyritised. Pyrite is a radiodense material, meaning X-Rays cannot pass through them. This means we can X-Ray them, and in fact, the first fossils to have been studied by X-Ray were Hunsrück Slate ones, just a couple of years after the discovery of X-Rays.
In recent years, the advent of computer tomography has made it possible to X-Ray them in 3D, as shown above. This can potentially provide more information, or make it easier to interpret enigmatic structures.
Basically, what I want to say is that palaeontologists are not hole diggers and stamp collectors, and we also need and use technology to get the most out of our materials.