During an interval of some 25 million years, in the early and mid Ordovician, there was an explosion in diversity in the history of life. It is now proved to be one of the really major events in Earth history.
Putilivo Quarry is a large hole in the ground some 70 km east of St. Petersburg. Here, the Middle Ordovician limestones have been quarried for many years principally as building stone but also for aggregate and cement. Many of the elegant buildings in the city of St. Petersburg were built of Ordovician limestone. I and my colleague Arne Thorshøj Nielsen had been working in the area with Andrej Dronov (then at St. Petersburg’s State University) and our students for several years mainly concentrating on the very fossiliferous Volkhov Stage on the Lynna River; the section was particularly rich in fossil brachiopods, a type of shell fish and trilobites, an ancient marine arthropod.
During late spring 2002, however, we camped together with our students and our Russian colleagues in the actual quarry. The weather was cold and frosty with snowfalls, but intermittently there were warm spells of sunshine. The food was basic; sometimes the breakfast kasha was spiced up with chicken bones from the previous evening meal, otherwise we had to rely on processed cheese, tomatoes and black bread. Occasionally small cans of pork were provided, signalled by a happy piglet on the label; this was probably fairly accurate since the can seemed to include the entire body parts of a pig compressed into the small container. Water was in short supply but our truck driver could always find some vodka to wash down our evening repast. The evenings were enjoyable spent around the camp fire under a starry sky punctuated by passing satellites and Russian folk songs from our colleagues.
James O’Donoghue, New Scientist, June 2008.
Sampling had focussed on the Volkhov Stage again, with our Russian colleagues carefully extracting rocks from each horizon while we removed all the fossils before moving onto the next level. In this way we began to construct a bed-by-bed diversity curve for this part of the Volkhov; as we suspected there were no huge shifts in diversity in this section. The quarry, however, has a number of different levels and we hoped to sample the next stage, the Kundan in a more distant part of the quarry. This part was guarded by giant rusty cranes, relicts of the Soviet era, towering menacingly above us. The Kundan limestones appeared harder and more dolomitized but it was already clear to me and my student Christian, that these rocks were very fossiliferous. Our expectations were extravagantly confirmed when Christian began to analyse and indentify the material in detail, back in Copenhagen. Our data showed that there was a jump in brachiopod diversity from just over 20 species at the top of the Volkhov to nearly 50 at the base of the Kundan. This was a near doubling of species diversity over a relatively short time interval. Had we hit precisely the start of the Great Ordovician Biodiversification Event? Well yes, but it had taken life nearly 4 billion years of Earth history to get this far. There was clearly a very long fuse.
The Ordovician System
The Ordovician System (488-444 Ma), named after the Ordovices tribe of Wales, was proposed by the English geologist Charles Lapworth (1842-1920) in 1879 to solve the controversy caused by the overlap between the upper Cambrian and lower Silurian, but the current Ordovician was not officially accepted as an international unit until the Internal Geological Congress in 1960.
How did it all start?
Charles Darwin knew from his basic training in geology and his friendship with contemporary geologists such as Charles Lyell, that the fossil record would provide the ultimate test of his theory of evolution. Unfortunately he was equally aware that the fossil record was markedly incomplete and lacked the sorts of evolving lineages and missing links that would prove the gradual evolution of life through deep time. More worryingly most of his lineages, indicated on his branching tree of life in ‘Origin’, apparently began at the base of the Cambrian with no tangible roots in the immense expanse of Precambrian time. This, ‘Darwin’s dilemma’ required some special pleading. Rather than revert to a creationist model, Darwin argued that life had its origin back in the Precambrian but the highly metamorphosed and tectonized strata had yet to yield any fossil material. His prediction was supported by the demonstration, in the late 1850s by the Canadian palaeontologist Sir William Logan, of Precambrian fossils from the rocks of the Laurentian craton; these were subsequently studied and named Eozoon canadense, the dawn creature, by the Canadian geologist Sir J. William Dawson. After lengthy, heated debates most geologists and palaeontologists conceded that Eoozon was in fact inorganic and of mineral rather than of biologic origin. But although the American palaeontologist Charles Walcott demonstrated that stromatolites, the cryptozoans of the 19th Century, did in fact occur in Precambrian strata, a fact also hotly contended, it was not until the 1960s that Precambrian palaeontology graduated to the high table.
During the 1960s clear evidence of mid Precambrian life was illustrated from the Gunflint Cherts from Ontario, containing a range of microbes, whereas the occurrence of stromatolites, the products of cyanobacteria, was now widely recognised in Neoproterozoic rocks. Most remarkable was the description of the soft-bodied complex organisms of the late Neoproterozoic Ediacara fauna containing evidence of some of the earliest animals and some 50 million years later the ‘small shelly fossils’ at the base of the Cambrian demonstrated that animals had already acquired hard skeletons. In more recent years the bacteria record has been extended back into the Archean, much more is known about the distribution and ecology of the Ediacara and small shelly biotas, whereas Carbon and other isotope data from the 3.8 billion year old Isua Complex of West Greenland suggest that not only life but photosynthesis was already a feature of early Earth. Darwin’s dilemma was solved but there was still a large gap between these microbial and soft-bodied communities of the Precambrian and the complex and diverse animal and plant communities of the modern continents and oceans (GEO 01/2009; «Livets utvikling i urtiden, fortsatt en gåte»).
The cast takes to the stage
During the late 1970s and early 1980s the American palaeontologist Jack Sepkoski statistically analyzed the ranges of virtually all the families of marine animals through time. The results were illuminating. The English palaeontologist John Phillips had carried out a similar study in 1860, based on of course less data, and had divided the Phanerozoic into the Palaeozoic, Mesozoic and Cenozoic eras based on their distinctive faunas and floras. Sepkoski’s divisions were slightly different. He recognized Cambrian, Paleozoic and Modern (Mesozoic+Cenozoic) evolutionary faunas, each with own distinctive groups of organisms and definable ecosystems. The Cambrian evolutionary fauna was dominated by the trilobites together with various primitive groups of brachiopod, echinoderm and mollusc. In general terms both the animals themselves and their community structures showed a great deal of variation suggesting there was not only flexibility in the growth and shapes of organisms but also in the way their communities were constructed. But one fact sets the Cambrian apart from most other geological systems: the large number of exceptionally-preserved faunas or Lagerstätten.
Much of our information on the ‘Cambrian Explosion’ is derived from three such faunas: Burgess Shale (Canada), Chengjiang (South China) and Sirius Passet (North Greenland). These faunas contain a selection of both soft-bodied animals and skeletal animals with some of their soft parts preserved. Some 13 separate phyla are represented in these faunas including some very weird animals such as Anomalocarus, Hallucigenia, Opabinia, Odontogriphus and Wiwaxia. The apparent, sudden appearance of this carnival of the animals suggested a real explosion of new body plans or perhaps the animals had just acquired skeletons and the optimum size to be detected in the fossil record. Stephen Jay Gould in his best-selling book, Wonderful Life: The Burgess Shale and the Nature of History, noted the sudden appearance of these very distinctive types of animal, suggesting that the Cambrian explosion was a time of great experimentation never again repeated. Modern statistical analyses have not confirmed Gould’s hypothesis. Rather the animals look different and odd because they are difficult to compare with our modern fauna; living arthropods are just as different from each other as those of the Cambrian.
Strangely, following the appearance of these animal body plans very little happened for the next 30 million years. But during an interval of some 25 million years, in the early and mid Ordovician, there was an explosion in diversity at the family, genus and species level unprecedented in the history of life. The diversification involved a new cast of players that together consolidated a suspension-feeding benthos that was to survive for the next 200 million years before the ecosystem was destroyed by the end Permian extinction event. Brachiopods, graptolites and trilobites together with bryozoans, corals and molluscs formed the basis of the Paleozoic evolutionary fauna. Within the benthos organisms began to burrow deeper into the sediment, build up complex tiering networks like Manhattan skyscrapers, develop reefs and reef communities and diversify into a whole new range of life modes; pelagic and planktonic organisms exploited a variety of new niches in the water column and predation became a more apparent way of life. Unfortunately there is no one unique explanation for this biodiversification, but there are a number of contenders.
Seeking a reason
There are five major extinction events in the more recent history of life. During the 1960s and 1970s there were many hundreds of explanations for such extinction events; for example perhaps the diet of the dinosaurs caused them to succumb to constipation although on the other hand it may well have been diarrhoea. Many of the explanations were rather vague and not really testable, depicting some palaeontologists as rather amateurish, stamp collectors. This was to change in the 1980s. First Luis and Walter Alvarez related an Iridium anomaly together with shocked quartz at the Cretaceous – Tertiary boundary coincident with the major end Cretaceous extinction event and generated a testable and viable hypothesis for this major extinction that removed the dinosaurs together with the vast majority of the ammonites and belemnites and many other groups of marine and terrestrial organisms. A few years later David Raup and Jack Sepkoski presented evidence for a 26 myr cyclicity of extinction events during the last 250 million years. This generated considerable interest amongst astronomers, astrophysicists and cosmologists. Although few would now support this analysis, palaeontology and more specifically extinction events had at last become important research areas for more serious scientists!
The Great Ordovician Biodiversification Event (GOBE) is a relatively new topic but there has already been a proliferation of reasons for this burst in diversity. It may take some time to test all these models and move towards a realistic explanation for this remarkable event.
The current explanations fall into three main categories. Those associated with 1) the palaeogeography of the period, 2) its climate and sea level changes and 3) biological or palaeoecological processes within the Ordovician biotas themselves.
Firstly the Ordovician was a period of significant continental dispersal with high rates of sea-floor spreading and the rapid movement of tectonic plates. The oceans were littered with many small terranes forming the parts of arc systems or merely the rift products from crustal extension. Continental separation generated a number of provinces amongst the shelf faunas whereas the arcs and microcontinents provided a range of Galapagos-type islands, such as Otta in the Norwegian Caledonides, and a source for new taxa.
Secondly most evidence suggests that for much of Ordovician the Earth enjoyed warm climates, with the lack of polar ice caps and new oceanic ridges promoting high sea levels. Warm seas covered most of the continents providing the shallow-water benthos with luxurious habitats.
Finally a revolution in the trophic chains of the oceans may have provided the building blocks for the radiation. Diversifications in the phytoplankton together with planktotrophic larvae match the diversifications in the suspension feeding animals and the tiering of the benthos.
But there have been a number of other explanations for the event. An extraterrestrial cause ties in the increased flux of asteroids hitting the surface of the Earth, resulting from the breakup of a giant body in the asteroid belt some 470 million years ago, to the biodiversification. The increased frequency of impact craters, asteroid fragments and extraterrestrial chromite in Scandinavia match precisely that huge hike in biodiversity demonstrated by the brachiopod faunas in Putilivo Quarry. Normally we relate such impacts to extinctions rather than diversifications but in this case the impacts were probably relatively minor; the devastation was enough to clear ecospace and provide new habitats for the benthos to return in great numbers a short time after the impacts. This model works well for parts of Baltoscandia but many more sections worldwide will have to be investigated before this can be identified as a global phenomenon.
The Ordovician was also characterized by exceptional volcanic activity together with a possible superplume. Apart from generating a variety of greenhouse gases and contributing to global warming, volcanic eruptions also provided increased amounts of inorganic nutrients to the world’s oceans; these, together with the erosional products of the developing orogenic belts such as the Caledonides, contributed to the base of the food chain during this exceptional event. Finally, oxygen levels continued to rise during the period supporting the growth and metabolism of progressively larger animals.
The oceans would never be the same again
There is currently no single explanation that can account for the GOBE. Rather a coincidence of biological and geological factors combined to drive the biodiversification. Irrespective of its causes, the diversification changed the oceans forever and set a new agenda for marine life.
The cascading increase in biodiversity at species, genus and family levels was apparent at global levels with the high provincialism of early to mid Ordovician faunas, at regional levels with the development of new community types, particularly in deeper water and in and around reefs and thirdly at local levels where more animals were squeezed into existing communities. The oceans were no longer sterile expanses of water, being filled now by phyto and zooplankton, punctuated by blooms, and including larvae and animals such as the graptolites. Community structures were better organized and more densely packed with the expansion of the number of so called ecological guilds, signalling a range of new feeding strategies and life modes. Tiering structures developed both above and within the substrates while the bioerosion and encrustation of hard surfaces offered a new range of ecological opportunities. The Paleozoic evolutionary fauna was relatively stable, surviving the end Ordovician and late Devonian extinctions, for some 200 million years. The end Permian extinction event destroyed its suspension feeding structures and a new ecosystem, based on the detritus feeding ecosystem of the Modern evolutionary fauna, diversified during the Triassic.
How numerically important was the GOBE? Most current biodiversity curves for the Phanerozoic show a slow increase in numbers of families during the Cambrian, a marked hike during the Ordovician reaching a plateau, the so called Palaeozoic Plateau, in the late Ordovician. Following the end Permian extinction event, diversity shows a steep and continuous rise during the Mesozoic and Cenozoic. The shape of this curve has been much debated, perhaps the diversity dynamics of the Cambrian, Paleozoic and Modern evolutionary faunas were different or perhaps families are not particularly sensitive proxies for true diversity change. The reason may be much simpler. More recent fossil biotas, particularly those from the Cenozoic, are known from many more sites than those from older rocks. John Alroy and his colleagues at the Paleobiology Database in Santa Barbara have standardized the Phanerozoic biodiversity curve according to a fixed number of collections for each interval. The curve is quite different. The marked biodiversification during the GOBE is now rarely exceeded during the subsequent stratigraphic record, establishing this as truly one of the really major events in Earth history.