This talk will take us through the origin and initial diversifications of animal life.
One theme that will be very prominent throughout is that of Konservat Lagerstätten, or sites of exceptional fossil preservation. Whereas 99% of the fossil record consists of bones, shells, teeth and other hard parts, these localities preserve soft parts, such as muscle and tissue. We will see just how important this is in general for palaeontology, but also in the study of this particular topic of the origin of animals.
We left off the previous talk with oxygen levels at ~10% PAL. Another rash oxidation event happened at ~700 Ma which led to the atmosphere reaching a similar oxygen level to today. This is correlated with the break-up of the supercontinent Rodinia.
This break-up caused, or played into, several severe changes happening in the geosphere at the time. The Earth was coming off the grips of a Snowball Earth period. So advanced was the glaciation that it was initially thought that the Earth was frozen right up to the Equator; now it is thought to be more of a “slushball Earth”. With the increasing temperatures and melting glaciers, the sea levels rose.
Accompanying this is the presence of drastic geochemical anomalies; the diagram above shows carbon isotope anomalies, but the same patterns can be seen with other geochemical elements, including sulphur.
As of the last glaciation, we have the earliest fossil record of animals. A pertinent question is whether these anomalies were the cause of the origin of animals, or whether the origin of animals caused the geochemical anomalies, or to which degree they influenced each other.
Diagram Source: Marshall, C. R. 2006. Explaining the Cambrian “Explosion” of Animals. Annual Review of Earth and Planetary Sciences 34, 355-384.
As I mentioned, Konservat Lagertätten are important when discussing this topic, and here you can see why: there are a lot of them in this period of time. We will see why later. The ones in bold are the ones we will look at specifically.
Bitter Springs preserves cells in 3D, and I include it here as it is 800 Ma, but contains no animals.
Dengying contains the earliest true Ediacarans (see later).
Uratanna is the original Ediacaran Fauna formation, where they were first discovered.
It is thought that their sister group is the choanoflagellates, a group of flagellated unicellular eukaryotes. As the picture above shows, they can clump together to form multicellular colonies, and it is imagined that this is how the first animals (characterised by multicellularity) could have joined together from their unicellular precursors.
So now we can start looking at the first confirmed animals: the Ediacaran Fauna.
The diagram above shows their temporal distribution. The earliest fossils are small, from 635 Ma localities. These were interrupted by a last glacial period, after which the Ediacaran faunas radiated.
Diagram Source: Xiao,S. & Laflamme, M. 2009. On the eve of animal radiation: phylogeny, ecology and evolution of the Ediacara biota. Trends in Ecology & Evolution 24, 31-40.
Picture Source: Hofmann, H. J., Narbonne, G. M. & Aitken, J. D. 1990. <a href=”http://dx.doi.org/10.1130/0091-7613(1990)0182.3.CO;2″>Ediacaran remains from intertillite beds in northwestern Canada. Geology 18, 1199-1202.
Charniodiscus, Charnia and Rangea are referred to as problematics, i.e. we don’t know what they are. They bear a superficial resemblance to sea pens, but they have nothing to do with them systematically, or with any other taxon known. As such, we place them as separate phyla in their own right.
Tribrachidium is similarly problematic, however some authors place it among the medusoids (jellyfish).
The others are at the focal point of a lot of research, as they are thought to be stem-group representatives of the phyla that radiated in the Cambrian Radiation, i.e. of the animals besides sponges and cnidarians. Kimberella is thought to be a stem-group mollusc, Parvancorina a stem-group arthropod, Dickinsonia a stem-group mollusc or annelid, and Spriggina a stem-group annelid. These are all very tentative, though, and a full discussion would require a talk of its own.
In the past, a popular hypothesis pushed by Adolf Seilacher gained prominence: the Vendobiont hypothesis. Adolf Seilacher is a pioneer and revolutionary of trace-fossil palaeontology, and he postulated that the Ediacarans were something like colonies of unicellular organisms forming layers separated by air, like air-mattress organisms. This is now discounted even by Seilacher, as there is significant evidence that the Ediacaran faunas were animals, not just aggregated unicellular organisms.
Another hypothesis states that some of the fossils are actually algal or lichen trace fossils. This may sound outlandish, but experiments in algal and lichen taphonomy (grow them in similar mud and see what trace they leave behind) shows that their death trace bears an uncanny resemblance to some of the Ediacaran fossils, particularly Dickinsonia and the “compressed jellyfish”.
However, it is undeniable that animals were present amid the Ediacaran biota. The proof of this comes courtesy of the first of the very significant Konservat Lagerstätten, the Doushantuo Formation, China. Fossils from Doushantuo are all microfossils: nothing above a certain size gets preserved. However, these microfossils are impeccable, preserved in 3D and preserving cellular detail. Among them, we have found these microfossils pictures above.
These are embryos, which I sorted by cleavage stage. The only organisms that produce embryos like this are animals.
However, there is no extant animal taxon that has this cleavage pattern, so these embryos are not sortable systematically.
2012 update: many of these have been shown not to be embryos, but clustered single-cells organisms!
Pictures Source: Xiao, S., Zhang, Y. & Knoll, A. H. 1998. Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite. Nature 391, 553-558.
From now we will look at the Cambrian Radiation of animals, starting with the three classic localities documenting it, before reviewing the causes.
The earliest of these localities is the Chengjiang outcrop of China. On the palaeogeographical map, it would be on the ocean floor on the outskirts of Gondwana, near where it says “South China”, whereas the Burgess Shale would be near Laurentia (where it says “Alaska”).
The fact that they are so wide apart, but are still so similar (as we will see) makes it reasonable to conclude that what is observed there are representative of the situations around the globe.
Map source: Scotese Palaeomap Project
Chengjiang and the Burgess Shale are very similar in every way, as seen in the diagram on the right with faunal compositions. 65% of the fossils there are arthropods – and almost all of them are non-mineralised, i.e. they are all soft parts.
Among them we can count all the main ecological guilds, from deposit feeders to carnivores.
Besides arthropods, we also find sponges, brachiopods and various “wormy” taxa. Also, the earliest chordate, Haikouella, is found in Chengjiang.
By far the most abundant Chengjiang fossils are these small, bivalved crustacean-like organisms called bradoriids. These can be imagined as swimming in swarms, like the krill of today, and probably served as the main food source for predators.
Picture Source: Hou, X., Williams, M., Siveter, D. J., Siveter, D. J., Aldridge, R. J. & Sansom, R. S. 2010. Soft-part anatomy of the Early Cambrian bivalved arthropods Kunyangella and Kunmingella: significance for the phylogenetic relationships of Bradoriida. Proceedings of the Royal Society B 277, 1835-1841.
The chronologically next Lagerstätte is the Sirius Passet of Northeast Greenland.
Sirius Passet is most well-known for its small shelly fauna. These include fragments of larger organisms, as well as whole organisms. As the name suggests though, they are all below a certain size and are essentially microscopic.
Kerygmachela is a stem-group arthropod.
Halkieria is thought to be a mollusc, but could also be an annelid. It has two shells (‘as’ = anterior shell, ‘ps’ = posterior shell), and the soft body is slug-like between them. The body is surrounded by sclerites (‘sc’).
One particularly common group of enigmatic microfossils are the mobergellans. They are little shells with 9 muscle scars on them; no such arrangement is known from other taxa, fossil or recent.
Some history on the place: it was discovered by Charles Doolittle Walcott, a Canadian geologist, in 1901. He was out looking for trilobites when he stumbled on peculiar fossils. He later got permission to dig out a quarry there and by 1916 had accumulated and described hundreds of specimens, and left thousands more in the archives of the Canadian Geological Department.
Through no fault of his own, the fossils were inadequately described – phylogenetic thought was not known or popular at the time. In the 1970s, Harry Whittington and his two students Derek Briggs and Simon Conway Morris started redescribing all these specimens in a brand new way: not as “weird animals”, but as stem-group representatives of modern phyla.
This is, of course, the correct approach (although some disagreed, notably Stephen J. Gould), and it was this reinvestigation, which is still ongoing with a new generation of students, that led to the discovery that there was indeed a Cambrian radiation of animals. Keep in mind that this was the first Cambrian Konservat Lagerstätte to be discovered, the rest came later.
One can imagine the Burgess Shale as being a somewhat deeper water environment, with all the organisms either in, on or swimming above the ocean floor. The habitat was right beneath a cliff. At some point, this cliff got dislodged, perhaps due to an earthquake, leading to an underwater mudslide which buried the entire habitat.
The main organism group we find there are arthropods, especially this one, Marrella.
Also among the significant organisms are the first cephalochordates (Pikaia) and early echinoderms (sea stars, brittlestars, sea urchins, sea cucumbers and sea lillies are the five classes still alive; there were 42 classes in their history, many of which had very strange morphologies, e.g. the helicoplacoid above).
Picture Source: Skovsted, C. B., Brock, G. A., Lindström, A., Peel, J. S., Paterson, J. R. & Fuller, M. K. 2007. Early Cambrian record of failed durophagy and shell repair in an epibenthic mollusc. Biology Letters 3, 314-317.
This is also obvious from the number of ecological guilds preserved, which span the entire possible spectrum.