The Cambrian Explosion: a broader view

The base of the Cambrian has long been defined as the level where abundant shelly fossils and most phyla first occur in the stratigraphic record. That increase in diversity led to the nickname ‘Cambrian Explosion’, despite the fact that sheer numbers and diversity of lesser taxa took a long time to rise to ‘revolutionary’ levels. Yet a great deal of animal evolution was going on during the preceding Proterozoic Era that was revealed once palaeobiological research blossomed in rocks of that age range. Today, the earliest occurrences, or at least hints, of quite a few phyla can be traced to the last 100 Ma of the Precambrian. Clearly, the Cambrian Explosion needs a fresh look now that so many data are in. Any palaeontologist would benefit from reading a Perspective article in the latest issue of Nature Ecology & Evolution (Wood, R. and 8 others 2019. Integrated records of environmental change and evolution challenge the Cambrian Explosion. Nature Ecology & Evolution, v. 3, online publication; DOI: 10.1038/s41559-019-0821-6)

Rachel Wood of Edinburgh University and co-authors working elsewhere in Britain, Canada, Japan and Finland sift the growing wealth of fossil and trace-fossil evidence that predate the start of the Cambrian. They also consider the geochemical events that stand out in the Ediacaran Period that succeeds the Snowball Earth events of the Cryogenian. Their account recognises that the geochemical changes – principally a series of carbon-isotope (δ¹³C) excursions – may have resulted from tectonic changes. The carbon-isotope data mark a series of short-lived penetrations of oxygen-rich conditions deep into the ocean water column and longer periods of oxygen-starved deep water. Such perturbations in oceanic redox conditions ‘speed-up’ thorough the late-Ediacaran into the Cambrian: a profound and protracted transition from the Neoproterozoic world to that of the Phanerozoic. Over the same time span there is a ‘progressive addition of biological novelty’ in the form and function of the evolving biota, so that  each successive assemblage builds on the earlier advances.

The fossil evidence suggests that the earliest Ediacaran fauna was metazoan but with no sign of bilaterian affinities (i.e. having ‘heads’ and ‘tails’). The rise of bilaterians of which most animal phyla are members occupied the later Ediacaran , with the first evidence of locomotion – and almost by definition animals with ‘fore’ and ‘aft’ – being around 560 Ma. Each discrete shift from more to less oxic conditions in the oceans seems to have knocked-back animal life, the reverse being accompanied by diversification of survivors. Oxygenation at the very start of the Cambrian marked the beginnings of a diversification clearly manifested by animals capable of biomineralisation and the secretion of hard parts with clear patterns. Such ‘shelly faunas’ are present in the latest Ediacaran sediments but with a multiplicity of seemingly arbitrary forms, although trace fossils suggest soft-bodied animals did have definite morphological pattern.

Diorama of the Lower Cambrian Qingjiang fauna (Credit: Fu et al. 2019; Fig 4)

Adding yet more information to early metazoan history is the recently discovered Cambrian Qingjiang lagerstätte of Hubei Province in southern China dated at 518 Ma; similar in its exquisite preservation to the Burgess (508 Ma) and Chengjiang (518 Ma) biotas (Fu, D. and 14 others 2019. The Qingjiang biota—A Burgess Shale-type fossil Lagerstätte from the early Cambrian of South China. Science, v. 363, p. 1338-1342; DOI: 10.1126/science.aau8800). The two previously discovered Cambrian lagerstättes are notable for their very diverse arthropod and sponge faunas. That at Qingjiang adds an abundance of cnidarians, jellyfish, sea anemones, corals and comb jellies, rare in the other two biotas, plus kinorhynchs or mud dragons – moulting invertebrates known only from Cambrian and modern sediments. The fossils at Qingjiang include only about 8% of the taxa of the same age found at Chengjiang, suggesting different environments

The idea of a sudden, discrete explosive event in the history of life, which coincided with the start of the Cambrian, now seems difficult to support. This should not damage the status of 541 Ma as the start of the Phanerozoic because stratigraphy basically gives form to the passage of time and has done since its emergence in the 19^th century, so keeping the names of the divisions is essential to continuity.

Related articles: Daley, A.C. 2019. A treasure trove of Cambrian fossils. Science, v. 363, p. 1284-1285; DOI: 10.1126/science.aaw8644. Switek, B. 2019. Fossil Treasure Trove of Ancient Animals Unearthed in China (Smithsonian.com)

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The great Cambrian unconformity

My first field trip from the Geology Department at the University of Birmingham in autumn 1964 was located within hooter distance of the giant British Leyland car plant at Longbridge. It involved a rubbish-filled linear quarry behind a row of shops on the main road through south Birmingham. Not very prepossessing but it clearly exposed a white quartzite, which we were told was a beach deposit laid down by a massive marine transgression at the start of the Cambrian. An hour later we were shown an equally grim exposure of weathered volcanic rocks in the Lickey Hills; they were a sort of purple brown, and said to be Precambrian in age. Not an excellent beginning to a career, but from time to time other Cambrian quartzites sitting unconformably on Precambrian rocks entered our field curriculum: in the West Midlands, Welsh Borders and much further afield in NW Scotland, as it transpired on what had been two separate continental masses of Avalonia and Laurentia. This had possibly been a global marine transgression.

In North America, then the Laurentian continent, what John Wesley Powell dubbed the Great Unconformity in the Grand Canyon has as its counterpart to the Lickey Quartzite the thrillingly named Tonto Group of the Lower Cambrian resting on the Vishnu Schists that are more than a billion years older. Part of the Sauk Sequence, the Tonto Group is, sadly, not accompanied by the Lone Ranger Group, but the Cambrian marine transgression crops out across the continent. In fact it was a phenomenon common to all the modern continents. Global sea level rose relative to the freeboard of the continents then existing. A recent study has established the timing for the Great Unconformity in the Grand Canyon by dating detrital zircons above and below the unconformity (Karlstrom, K, et al. 2018. Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons. Nature Geoscience, v. 11, p. 438-443; doi:10.1038/s41561-018-0131-7). Rather than starting at the outset of the Cambria at 542 Ma, the marine transgression was a protracted affair that began around 527 Ma with flooding reaching a maximum at the end of the Cambrian.

Extensive flooding of the continents at the end of the Cambrian (credit: Ron Blakey , Colorado Plateau Geosystems)

It seems most likely that the associated global rise in sea level relative to the continents was a response to the break-up of the Rodinia supercontinent by considerable sea-floor spreading. The young ocean floor, having yet to cool to an equilibrium temperature, would have had reduced density so that the average depth of the ocean basins decreased, thereby flooding the continents. The creation of vast shallow seas across the continents has been suggested to have been a major factor in the explosive evolution of Cambrian shelly faunas, partly by expanding the range of ecological niches and partly due to increased release of calcium ions to to seawater as a result of chemical weathering.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

The launch of modern life on Earth

To set against five brief episodes of mass extinction – some would count the present as being the beginning of a sixth – is one short period when animals with hard parts appeared for the first time roughly simultaneously across the Earth. Not only was the Cambrian Explosion sudden and pervasive but almost all phyla, the basic morphological divisions of multicellular life, adopted inner or outer skeletons that could survive as fossils. Such an all-pervading evolutionary step has never been repeated, although there have been many bursts in living diversity. Apart from the origin of life and the emergence of its sexual model, the eukaryotes, nothing could be more important in palaeobiology than the events across the Cambrian-Precambrian boundary.

One of the evolutionary experiments during the Cambrian, Opabinia regalis, from the Burgess Shale. (credit: Wikipedia)

This eminent event has been marked by most of the latest issue of the journal Gondwana Research (volume 25, Issue 3 for April 2014)in a 20-paper series called Beyond the Cambrian Explosion: from galaxy to genome (summarized  by Isozaki, Y., Degan, S.., aruyama,, S.. & Santosh, M. 2014. Beyond the Cambrian Explosion: from galaxy to genome.  Gondwana Research, v. 25, p. 881-883). Of course, these phenomenal events have been at issue since the 19^th century when the division of geological time began to be based on the appearance and vanishing of well preserved and easily distinguished fossils in the stratigraphic column. On this basis roughly the last ninth of the Earth’s history was split on palaeontological grounds into the 3 Eras, 11 Periods, and a great many of the briefer Epochs and Ages that constitute the Phanerozoic. Time that preceded the Cambrian explosion was for a long while somewhat murky mainly because of a lack of means of subdivision and the greater structural and metamorphic damage that had been done to the rocks that had accumulated over 4 billion years since the planet accreted. Detail emerged slowly by increasingly concerted study of the Precambrian, helped since the 1930s by the ability to assign numerical ages to rocks. Signs of life in sediments that had originally been termed the Azoic (Greek for ‘without life’) gradually turned up as far back as 3.5 Ga, but much attention focused on the 400 Ma immediately preceding the start of the Cambrian period once abundant trace fossils had been found in the Ediacaran Hills of South Australia that had been preceded by repeated worldwide glacial epochs. The Ediacaran and Cryogenian Periods (635-541 and 850-635 Ma respectively) of the Neoproterozoic figure prominently in 9 of the papers to investigate or review the ‘back story’ from which the crucial event in the history of life emerged. Six have a mainly Cambrian focus on newly discovered fossils, especially from a sedimentary sequence in southern China that preserves delicate fossils in great detail: the Chengjian Lagerstätte. Others cover geochemical evidence for changes in marine conditions from the Cryogenian to Cambrian and reviews of theories for what triggered the great faunal change.

Since the hard parts that allow fossils to linger are based on calcium-rich compounds, mainly carbonates and phosphates that bind the organic materials in bones and shells, it is important to check for some change in the Ca content of ocean water over the time covered by the discourse. In fact there are signs from Ca-isotopes in carbonates that this did change. A team of Japanese and Chinese geochemists drilled through an almost unbroken sequence of Ediacaran to Lower Cambrian sediments near the Three Gorges Dam across the Yangtse River and analysed for ⁴⁴Ca and ⁴²Ca (Sawaki, Y. et al. 2014. The anomalous Ca cycle in the Ediacaran ocean: Evidence from Ca isotopes preserved in carbonates in the Three Gorges area, South China. Gondwana Research, v. 25, p. 1070-1089) calibrated to time by U-Pb dating of volcanic ash layers in the sequence (Okada, Y. et al. 2014. New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges, Weng’an and Chengjiang areas, South China. Gondwana Research, v. 25, p. 1027-1044). They found that there were significant changes in the ratio between the two isotopes. The isotopic ratio underwent a rapid decrease, an equally abrupt increase then a decrease around the start of the Cambrian, which coincided with a major upward ‘spike’ and then a broad increase in the ⁸⁷Sr/⁸⁶Sr isotope ratio in the Lower Cambrian. The authors ascribe this to an increasing Ca ion concentration in sea water through the Ediacaran and a major perturbation just before the Cambrian Explosion, which happens to coincide with Sr-isotope evidence for a major influx of isotopically old material derived from erosion of the continental crust. As discussed in Origin of the arms race (May 2012) perhaps the appearance of animals’ hard parts did indeed result from initial secretions of calcium compounds outside cells to protect them from excess calcium’s toxic effects and were then commandeered for protective armour or offensive tools of predation.

Artists impression of a Snowball Earth event 640 Ma ago (credit: guano via Flickr)

Is there is a link between the Cambrian Explosion and the preceding Snowball Earth episodes of the Cryogenian with their associated roller coaster excursions in carbon isotopes? Xingliang Zhang and colleagues at Northwest University in Xian, China (Zhang, X. et al. 2014. Triggers for the Cambrian explosion: Hypotheses and problems.  Gondwana Research, v. 25, p. 896-909) propose that fluctuating Cryogenian environmental conditions conspiring with massive nutrient influxes to the oceans and boosts in oxygenation of sea water through the Ediacaran set the scene for early Cambrian biological events. The nutrient boost may have been through increased transfer o f water from mantle to the surface linked to the start of subduction of wet lithosphere and expulsion of fluids from it as a result of the geotherm cooling through a threshold around 600 Ma (Maruyama, S. et al. 2014. Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion. Gondwana Research, v. 25, p. 910-944). Alternatively the nutrient flux may have arisen by increased erosion as a result of plume-driven uplift (Santosh, M. et al. 2014. The Cambrian Explosion: Plume-driven birth of the second ecosystem on Earth. Gondwana Research, v. 25, p. 945-965).

A bolder approach, reflected in the title of the Special Issue, seeks an interstellar trigger (Kataoka, R. et al. 2014. The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution in the late Neoproterozoic and Cambrian periods. Gondwana Research, v. 25, p. 1153-1163). This looks to encounters between the Solar System and dust clouds or supernova remnants as it orbited the galactic centre: a view that surfaces occasionally in several other contexts. Such chance events may have been climatically and biologically catastrophic: a sort of nebular winter, far more pervasive than the once postulated nuclear winter of a 3^rd World War. That is perhaps going a little too far beyond the constraints of evidence, for there should be isotopic and other geochemical signs that such an event took place. It also raises yet the issue that life on Earth is and always has been unique in the galaxy and perhaps the known universe due to a concatenation of diverse chance events, without structure in time or order, which pushed living processes to outcomes whose probabilities of repetition are infinitesimally small.

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Plate tectonics and the Cambrian Explosion

A rough-and-ready way of assessing the rate at which silicic magmatic activity has varied through time is to separate out grains of zircon that have accumulated in sedimentary rocks of different ages. Zircon is readily datable using the U-Pb method, if you have access to mass spectrometry. While some of the zircons will date from much older continental crust that was exposed while the sediments originated, sometimes there are grains that formed only a few million years before the sediments accumulated. Those are likely to have crystallized from silica-rich volcanic rocks above subduction zones where ocean-floor has been driven beneath continental crust; i.e. at continental volcanic arcs. Such young zircons therefore help assess the tectonic conditions close to sedimentary basins. The potential of detrital zircon geochronology was first suggested to me by Dr M.V.N. Murthy of the Geological Survey of India in 1978, long before anyone could aspire to mass zircon dating. M.V.N. had by then amassed kilograms of zircon grains from every imaginable source in India, and may have been the first geologist to realise their potential. It has become a lot quicker and cheaper in the last two decades, thanks to methods of dating single zircon grains both precisely and accurately and M.V.N.’s prescient suggestion has been borne out globally.

A detrital zircon grain about 0.25 mm long. (Photo credit: Wikipedia)

Results for the late Precambrian to early Palaeozoic have recently been compiled (McKenzie, N.R. et al. 2014. Plate tectonic influences on Neoproterozoic-early Paleozoic climate and animal evolution. Geology, online publication doi:10.1130/G34962.1). One of the striking correlations is between the abundance of ‘young’ zircons relative to Cambrian sedimentary deposition and the pace of diversification of animal faunas during the Cambrian.  During the Cambrian Period there may have been far more continental-margin arc volcanism than in the preceding late Neoproterozoic or later in the early Palaeozoic. That would match with evidence for the Cambrian atmosphere having reached the greatest CO₂ concentration of Phanerozoic times and the fact that the Gondwana supercontinent (comprising the present southern continents plus India) was assembled at that time by collision of several Precambrian continental masses. Global temperatures must have been rising.

Earth at abround the start of the Cambrian showing the cratons that collided to form Gondwana (Photo credit: Wikipedia)

The rapid emergence of all the major animal groups by the middle Cambrian – the Cambrian Explosion – took place during and despite climatic warming. Environmental stress, perhaps increased calcium and bicarbonate ions in sea water as a result of acid conditions, may have forced animals to develop means of getting both ions out of their cells to form carbonate skeletons: the Cambrian Explosion really marks the first appearance of shelly faunas and a good chance of fossilisation. Yet at the peak of volcanically-induced warming faunal diversity, especially of reef-building animals, fell-off dramatically to create what some palaeobiologsts have termed the Cambrian ‘dead interval’. Marine life really took-off in a big way during the Ordovician while temperatures were falling globally; so much so that the close of the Ordovician was marked by the first major glaciation focused on Gondwana. The zircon record indicates that continental-arc volcanism also declined during the Ordovician, and maybe the Cambrian silicic volcanics were chemically weathered during that Period to remove carbon-dioxide from the atmosphere, along with renewed reef building to bury carbonate fossils.

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Land almost colonized during the Cambrian Explosion

One of the major shale-gas source strata in the eastern USA, the Middle Cambrian Conasauga Shale, formed in a shallow inland sea. Consequently the sedimentology of the lowest Palaeozoic Era of the region and the strange structures affecting it during deformation that formed the Appalachian Mountains have become a focus of intense tectonic and stratigraphic interest – economic potential generally helps fund academic research at a time when money for pure science is short. This has extended into the deepest part of the Cambrian lying unconformably just above the crystalline Precambrian basement. The Lower Cambrian of the Appalachians marks the earliest stage of rifting that flooded former dry land and comprises the multicoloured mudstones, siltstones and sandstones of the Rome Formation. Though only sparsely fossiliferous, the Rome formation contains archetypical trilobites of the genus Olenellus, typical of the Lower Cambrian and used to correlate sedimentary rocks of this age far and wide. They occur far across the North Atlantic in coeval rocks of the Northwest Highlands of Scotland, but not in those a mere couple of hundred kilometres to the south in Wales. This faunal disparity forms a major line of evidence that the olenelid fauna occupied one side of a once major ocean – Iapetus – another different bunch of early trilobites being characteristic of its opposite flank. The almost hemispherical extent of similar faunas was long regarded as an indication that they inhabited open ocean water. In fact, their wide distribution is as much due to juvenile arthropods being planktonic, while adults may have occupied all sorts of marine environments. It now turns out that Olenellus lived in very shallow water (Mángano, M.G. et al. 2014. Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion. Geology, online pre-publication doi:10.1130/G34980.1).

Gabriela Mángano of the University of Saskatchewan and colleagues from Argentina and the US found that the Rome Formation is full of sedimentary structures typical of modern intertidal zones. Tidal-flat strata are full of suncracks but are also criss-crossed by tracks made by substantial arthropods, only fossil olenellid trilobites being big enough to have made them while feeding , maybe on microbial mats formed on the mudflats or on worms that burrowed the muds. Clearly these animals were literally only a few steps away from colonising the land very shortly after abundant, sturdy animal life appeared in the Cambrian Explosion. Currently the dominant hypothesis for permanent entry of animals onto land is that the colonizers first adapted to fresh- or brackish water habitats. Yet, apparently, there was little to stop a direct invasion from the sea.

Origin of the arms race

Global paleogeographic reconstruction of the Earth in the early Cambrian period 540 million years ago. (credit:Ron Blakey, Northern Arizona University)

Palaeontologists generally agree on one broad aspect of animal evolution: the central role of predation versus defence in animal diversification to occupy different ecological niches. Indeed that co-relation has to an extent been responsible for the diversification of potentially habitable niches themselves. Armour and arms form a dialectic within the animal world, but one that only rose to dominate when hard materials became an integral part of animal morphology, allowing some to bite, gnaw or rasp and others to develop shelly or horny skeletons. The Kingdom Animalia within the domain of the eukaryotes – organisms based on cells that bear a nucleus – is united by one life style, that of feeding directly or indirectly on other living things. They are heterotrophs unable to generate energy and tissue through the fundamental harnessing of chemistry and physics to use the inorganic world directly, as do autotrophs.  One of the earliest discoveries about the history of animals was that fossils in the widely accepted meaning of the word appeared suddenly in the geological record, earlier rocks containing virtually no tangible signs of life: fossils explode in numbers from the start of the Cambrian Period at 542 Ma. Subsequently, geologists did discover imprints of clearly quite complicated organisms in rocks a few tens of million years older than the start of the Cambrian. But these were flaccid, bag like creatures that recent research has shown to rely on filtering microorganisms from water or directly absorbing organic matter through their skin.

An animal from the late Precambrian(Photo credit: Wikipedia)

Another feature of sediments of the oldest Cambrian is that in many parts of the world they rest with or=profound unconformity on deformed older rocks of Precambrian age. Throughout Britain the lowest Cambrian rocks are almost pure quartz sandstones that rest upon older more complex rocks ranging from only a few tens of million years older than 542 Ma to some of the oldest rocks in Europe, the Lewisian complex dating back 3 billion years. Many of the hills of North West Scotland have a gleaming white cap of Lower Cambrian quartzite above what has been termed the Great Unconformity where it occurs in Arizona’s Grand Canyon. Sedimentary sequences that continuously record the Precambrian to Cambrian transition and the biological explosion at the juncture are rare. But they show two curious features in sediments that immediately predate those bearing recognisable fossils: a complete lack of evidence for burrowing and millimetre-scale shell-like bodies made of calcium phosphate and carbonate, which are thought to have adorned the skins of otherwise unprotected animals.

Creatures from the Cambrian Period (credit: Wikipedia)

Calcium, while a very common element is one of the most dangerous to life. Traces are essential for the signalling that goes on in cell metabolism, and too little snuffs out those vital processes.  Yet too much – still a very low concentration in cell cytoplasm – results in the growth of minute mineral crystals within cells, also spelling cell death. This results from the limited solubility of calcium in water, compared with those of other common metals.  At an early stage in evolution cells developed means of restricting the admission of calcium ions and efficient means of expelling excess amounts of calcium. The ubiquitous occurrence of Ca-rich marine limestones throughout the geological record bears witness to two things: the abundance of calcium ions in seawater; a closer look reveals that a great many limestones, going back some 3.5 billion years show traces of biomineralisation that helped form the limey sediments. In the second case, the calcium carbonate in most Precambrian limestones was secreted by photosynthetic blue-green bacteria in minutely thing layers, probably in the form of a slimy film excreted to avoid calcium toxicity. Palaeontologists have long suspected that the earliest skeletal materials formed by animals evolved from the need to excrete biomineralised films by turning a metabolic necessity into functional and integral parts of their body plans: arms and armour. Yet limestones are not rare signs of the presence of a dissolved calcium threat, so why the sudden adoption of waste products in this way?

A fairly old hypothesis is that calcium in seawater must have risen above a threshold that posed toxic threat to all living things and excretion had to increase to maintain the balance, perhaps matched with increasing sizes of animals in the late Precambrian. Only recently has support been found for this suggested evolutionary trigger, initially from analysis of brines trapped in crystalline materials within sediments, such as salt (NaCl). But the very presence of such halite in a sediment is a universally accepted sign of evaporation increasing ionic concentrations in isolated seawater lagoons, whereas a general increase in marine calcium would be needed to present sufficient chemical stress that the whole of animal evolution would require a step-change for survival.  It turns out that support for the hypothesis stems from two isotopic systems most usually associated with dating the formation and weathering of continental  crust: those of strontium and neodymium. The global record of ratios of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd show unusually large changes in the run-up to the Cambrian Period, the first rising to the highest level recorded in geological history and the second reaching a historic nadir during the Cambrian. This anti-correlation signifies the greatest chemical weathering of older continental crust in the history of the Earth (Peters, S. & Gaines, R.R. 2012. Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion. Nature, v. 484, p. 363-366). Not only would this have poured dissolved ions, including those of calcium, into the oceans and raised their concentrations in seawater, but vast areas of the continents would have been eroded to form huge coastal plains, ripe for marine inundation. The last is exactly what the near-universal unconformity at the base of the Cambrian signifies. Presaging this long drawn-out grinding of continents to their gums had been a protracted bout of continental assembly to form the Rodinia supercontinent around 1000 Ma though collision and mountain building. Then Rodinia broke apart, its fragments being driven by plate tectonics to reassemble, along with vast chains of new crust formed in volcanic island arcs, by yet more orogenesis: tectonically high-energy times matched by the processes of denudation on land.

A nice example of planetary interconnectedness on the largest scale with the greatest conceivable consequences, for we vertebrates anyhow. This comes as a great comfort to me in the twilight of my career, since in 1999 I stuck out my neck with a similar concept in Stepping Stones only to meet a suitably stony silence.

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