At the start of the Cambrian Period animal life began to diversify from that of the Ediacaran world. For the first time sediments on the seafloor were explored for sustenance, leading to a variety of burrows that disrupted fine depositional layers. The basal Cambrian sandstones found in Britain and elsewhere are pervasively bioturbated: good evidence for the start of a ‘Worm world’ that marks the Precambrian-Phanerozoic boundary. That is probably a misnomer for the shallow seabed of that time, as fossils of burrowers with a variety of hard parts turn up in the oldest Cambrian sequences. Also appearing for the first time are tooth-like microfossils that took on such a range of bizarre shapes that they have long been used for correlating sedimentary strata in the absence of larger creatures. Some of these conodonts have been attributed to early vertebrates akin to modern lampreys and hag fish, but others may have been the grasping mouth-spines of a group of predatory worms which also survive to the present: chaetognaths. Apart from these oral spines chaetognaths lack hard parts, so anatomical details of ancient ones are only found in sites of exquisite preservation or lagerstätten. In such rare, tranquil places soft tissues such as muscles may be preserved by phosphatisation during decay.
Reconstruction of Timorebestia koprii showing its musculature, nerve system and mouthparts, It probably propelled itself by fluttering its outer and rear flaps, much like a modern flatfish. Credit: Park et al., Fig 4
One of the earliest Phanerozoic lagerstätten (Sirius Passet) occurs in northern Greenland. It is curiously named after the Sirius Dog Sled Patrol, an elite pair of naval troops with a sledge and 12 dogs that enforces Danish sovereignty over the Greenlandic shore of the Arctic Ocean. The Sirius Passet fauna includes a monstrous chaetognath over 30 cm long (Park, T.-Y. S. and 12 others 2024. A giant stem-group chaetognath. Science Advances, v. 10 article eadi6678; DOI: 10.1126/sciadv.adi6678). It is called Timorebestia koprii (Timorebestia is Latin for ‘terror beast’) and was related to the living, but tiny, arrow worms that prey on zooplankton in modern oceans. This description and moniker may seem to be somewhat hyperbolic, but Timorobestia outranks in size any Early Cambrian predatory arthropods. It was probably high in the Early Cambrian trophic pyramid, but was soon relegated by the later Cambrian rise of trilobites and then of cephalopods and eventually jawed vertebrate fishes in the Silurian. One specimen contained shells of a swimming arthropod whose protective spines did not deter the ‘terrible’ chaetognath from swimming them down.
The Cretaceous-Palaeogene mass extinction is no longer an event that polarises geologists’ views between a slow volcanic driver (The Deccan large igneous province) and a near instantaneous asteroid impact (Chicxulub). There is now a broad consensus that both processes were involved in weakening the Late Cretaceous biosphere and snuffing out much of it around 66 Ma ago. Yet is still no closure as regards the details. From a palaeontologist’s standpoint the die-off varied dramatically between major groups of animals. For instance, the non-avian dinosaurs disappeared completely while those that evolved to modern birds did not. Crocodiles came through it largely unscathed unlike aquatic dinosaurs. In the seas those animals that lived in the water column, such as ammonites, were far more affected than were denizens of the seafloor. But much the same final devastation was visited on every continent and ocean. However, lesser and more restricted extinctions occurred before the Chicxulub impact.
Scientists from Norway, Canada, the US, Italy, the UK and Sweden have now thrown light on the possibility that climate change during the last half-million years of the Cretaceous may have been eroding biodiversity and disrupting ecosystems (Callegaro, S. et al. 2023. Recurring volcanic winters during the latest Cretaceous: Sulfur and fluorine budgets of Deccan Traps lavas. Science Advances, v. 9, article eadg8284; DOI: 10.1126/sciadv.adg8284). Almost inevitably, they turned to the record of Deccan volcanism that overlapped the K-Pg event, specifically the likely composition of the gases that the magmas may have belched into the atmosphere. Instead of choosing the usual suspect carbon dioxide and its greenhouse effect, their focus was on sulfur and fluorine dissolved in pyroxene grains from 15 basalts erupted in the 10 Formations of the Deccan flood-basalt sequence. From these analyses they were able to estimate the amounts of the two elements in the magma erupted in each of these 10 phases.
Exposed section through a small part of the Deccan Traps in the Western Ghats of Maharashtra, India. (Credit: Gerta Keller, Princeton University)
The accompanying image of a famous section through the Deccan Traps SE of Mumbai clearly shows that 15 sampled flows could reveal only a fraction of the magmas’ variability: there are 12 flows in the foreground alone. The mountain beyond shows that the pale-coloured sequence is underlain by many more flows, and the full Deccan sequence is about 3.5 km thick. Clearly, flood-basalt volcanism is in no way continuous, but builds up from repeated lava flows that can be as much as 50 m thick. Each of them is capped by a red, clay-rich soil or bole – from the Greek word bolos (βόλος) meaning ‘clod of earth’. Weathering of basalt would have taken a few centuries to form each bole. Individual Deccan flows extend over enormous areas: one can be traced for 1500 km. At the end of volcanism the pile extended over roughly 1.5 million km2 to reach a volume of half a million km3.
Fluorine is a particularly toxic gas with horrific effects on organisms that ingest it. In the form of hydrofluoric acid (HF) – routinely used to dissolve rock – it penetrates tissue very rapidly to react with calcium in the blood to form calcium fluoride. This causes very severe pain, bone damage and other symptoms of skeletal fluorosis. The 1783-4 eruption of the Laki volcanic fissure in Iceland emitted an estimated 8,000 t of HF gas that wiped out more than half the domestic animals as a result of their eating contaminated grass. The famine that followed the eruption killed 20 to 25% of Iceland’s people: exhumed human skeletons buried in the aftermath show the distinctive signs of endemic skeletal fluorosis. This small flood-basalt event had global repercussions, as the Wikipedia entry for Laki documents. Volcanic sulfur emissions in the form of SO2 gas react with water vapour to form sulphuric acid aerosols in a reflective haze. If this takes place in the stratosphere as a result of powerful eruptions, as was the case with the 1991 Pinatubo eruption in the Philippines, the high-altitude haze lingers and spreads. This results in reduced solar warming: a so-called ‘volcanic winter’. In the Pinatubo aftermath global temperatures fell by about 0.5°C during 1991-3. Unsurprisingly, volcanic sulfur emissions also result in acid rainfall. Moreover, inhaling the sulphur-rich haze at low altitudes causes victims to choke as their respiratory tissues swell: an estimated 23,000 people in Britain died in this way when the 1783-4 Laki eruption haze spread southwards Sara Calegaro and colleagues found that the fluorine and sulfur contents of Deccan magmas fluctuated significantly during the eruptive phases. They suggest that fluorine emissions were far above those from Laki, perhaps leading to regional fluorine toxicity around the site of the Deccan flood volcanism but not extinctions. Global cooling due to sulphuric acid aerosols in the stratosphere is suggested to have happened repeatedly, albeit briefly, as eruption waxed and waned during each phase. Magmas rich in volatiles would have been more likely to erupt explosively to inject SO2 to stratospheric altitudes (above 10 to 20 km). The authors do not attempt to model when such cooling episodes may have occurred: data from only 15 levels in the Deccan Traps do not have the time-resolution to achieve that. They do, however, show that this large igneous province definitely had the potential to generate ‘volcanic winters’ and toxic episodes. Time and time again ecosystems globally and regionally would have experienced severe stress, the most important perhaps being disruption of the terrestrial and marine food chains.
The US city of Los Angeles, originally known as El Pueblo de Nuestra Señora la Reina de los Ángeles (The Town of Our Lady the Queen of the Angels), was founded in 1781 by 44 Spanish settlers. It remained a small cattle-centred town after the annexation of California from Mexico by the USA in 1847. Once it was reached by the transcontinental Southern Pacific railroad in 1876 it had the potential for growth. But it took the discovery of oil within its limits in 1892 for its population to increase rapidly. The Los Angeles City Oil Field became the top producer in California with 200 separate oil companies crammed cheek by jowl by 1901. Now only one remains, producing just 3.5 barrels per day. That crude oil was there for the taking was pretty obvious as bitumen seeps had long been exploited by native people and the original Spanish colonists. The oilfield was developed near one such seep: the Rancho La Brea tar pits.
Rancho La Brea tar pit and derricks of the Los Angeles City Oil Field in 1901
By 1901 perfectly preserved bones of a huge variety of animals – 231 vertebrate species – as well as plants and invertebrates began to be collected from the continually roiling pond of bitumen. Thousands of specimens have been collected since then, both predators and prey of all sizes. Famous for mastodons and sabre-toothed cats, La Brea is a repository of almost the entire western Californian fauna through much of the Late Pleistocene: before about 100 ka the area lay beneath the Pacific Ocean. Tar pits are traps for unwary animals of any kind, especially as shallow water often hides the danger. Carnivores seeking easy, abundant food end up trapped too.
Because of the anaerobic nature of bitumen, bacterial decay is suppressed. Many of the bones still contain undegraded collagen: the most abundant protein in mammals, which can be dated using the radiocarbon method. So, despite the lack of stratigraphy in the tar pits, it is possible to track the history of the ecosystem by painstaking dating of individual fossils (OKeefe, F.R and 18 others 2023. Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift. Science, v. 381, article eabo3594; DOI: 10.1126/science.abo3594). Robin OKeefe and colleagues dated 169 specimens of eight large mammal species most commonly found in the bitumen: sabre-toothed cat (Smilodon fatalis); dire wolf (Aenocyon dirus); coyote (Canis latrans); American lion (Panthera atrox); ancient bison (Bison antiquus); western horse (Equus occidentalis); Harlan’s ground sloth (Paramylodonharlani); and yesterday’s camel (Camelopshesternus).
The authors focussed on precisely dated specimens spanning the 15.6 to 10.0 ka time range. This would allow the disappearance times of individual species to be compared with stages in the rapid change in the Californian climate during post glacial maximum warming, those during the Younger Dryas abrupt cooling (12.9 to 11.7 ka) and the earliest Holocene warming that succeeded it. The first to go extinct were the camels and giant sloths about 13.6 ka ago. At 13.2 ka the other mammals declined very rapidly, the two remaining herbivores vanishing more quickly than the four predators. By 12.9 ka the only surviving species of the chosen eight was the coyote. So seven members of the Pleistocene mammalian megafauna became extinct before the onset of the Younger Dryas cold millennium.
Part of the team examined pollen from a core through sediments deposited in a lake 100 km south of La Brea. They found that flora, and probably climate, had not changed at the time of camel and sloth extinctions around 13.6 ka. However a 300 year period between 13.2 and 12.9 ka witnessed a collapse in deciduous tree species while conifers, grasses and drought-tolerant shrubs increased. A woodland ecosystem had been replaced by semi-arid chaparral. Another feature of the lake-bed sediments was that charcoal fragments increased explosively during that 300-year episode that ended both the woodland ecosystem and the megafauna that exploited it: undoubtedly three centuries of regular wildfires. What remained was the chaparral ecosystem based on drought-tolerant, fire-adapted plants.
Were the megafauna collapse and a change in ecology results of a climatic harbinger for the Younger Dryas cool millennium, or some other cause? Interestingly, tangible evidence for the Clovis hunting culture of North America, which has long been implicated in the faunal ‘extirpation’, does not appear until 12.9 ka, and in California neither does any implicating other human groups. Yet evidence is accumulating for much earlier entry of humans into North America. Occupation sites are very rare on land, but human presence here and there implies such earlier migration, probably along the west coast that avoided the frigid interior further north than California. The question posed by OKeefe et al. is, ‘Were the fires ignited by humans over a 300 year period just before the Younger Dryas’? It remains to be confirmed … First human arrivals coinciding with evidence for wildfires in Australia, New Zealand and a few other areas do suggest that it is a possibility. There needs to be a motive, such as producing lush clearings in forest to attract game, or removing cover to make hunting easier. In this case, the fires immediately preceded a global climatic downturn with terrestrial drying, so they may have had natural causes: the potentially incendiary chaparral flora had been increasing steadily beforehand and decreased rapidly after the evidence for wildfires
See also: Price, M. 2023. Death by fire. Science, v. 381, p. 724-727; DOI: 10.1126/science.adk3291
Along with algae, jellyfish, oak trees, sharks and nearly every organism that can be seen with the naked eye, we are eukaryotes. The cells of every member of the Eukarya, one of the three great domains of life, all contain a nucleus – the main location of genetic material – and a variety of other small bodies known as organelles, such as the mitochondria of animals and the chloroplasts of plant cells. The vast bulk of organisms that we can’t see unaided are prokaryotes, divided into the domains of Bacteria and Archaea. Their genetic material floats around in their cells’ fluid. The DNA of eukaryotes shares some stretches with prokaryotes, but no prokaryotes contain any eukaryote genetic material. This suggests that the Eukarya arose after the Bacteria and Archaea, and also that they are a product of evolution from prokaryotes, probably by several combining in symbiotic relationships inside a shared cell membrane. Earth-logs has followed developments surrounding this major issue since 2002, as reflected in some of the posts linked to what follows.
While prokaryotes can live in every conceivable environment at the Earth’s surface and even in a few kilometres of crust beneath, the vast majority of eukaryotes depend on free oxygen for their metabolism. Logically, the earliest of the Eukarya could only have emerged when oxygen began to appear in the oceans following the Great Oxidation Event around 2.4 billion years ago. That is more than a billion years after the first prokaryotes had left their geological signature in the form of curiously bulbous, layered carbonate structures (stromatolites), probably formed by bacterial mats. The oldest occur in the Archaean rocks of Western Australia as far back as 3.5 Ga, and disputed examples have been found in the 3.7 Ga Isua sediments of West Greenland. The oldest of them are thought to have been produced through the anoxygenic photosynthesis of purple bacteria (See: Molecular ‘fossils’ and the emergence of photosynthesis; September 2000), suggested by organic molecules found in kerogen from early Archaean sediments. Later stromatolites (<3.0 Ga) have provided similar evidence for oxygen-producing cyanobacteria.
Acritarchs are microfossils of single-celled organisms made of kerogen that have been found in sediments up to 1.8 billion years old. Features protruding from their cell walls distinguish them from prokaryote cells, which are more or less ‘smooth’: acritarchs have been considered as possible early eukaryotes. Yet the oldest undisputed eukaryote microfossils – red and green algae – are much younger (about 1.0 Ga). A means of estimating an age for the crown group from which every later eukaryote organism evolved – last eukaryotic common ancestor (LECA) – is to use an assumed rate of mutation in DNA to deduce the time when differences in genetics between living eukaryotes began to diverge: i.e. a ‘molecular clock’. This gives a time around 2 Ga ago, but the method is fraught with uncertainties, not the least being the high possibility of mutation rates changing through time. So, when the Eukarya arose is blurred within the so-called ‘boring billion’ of the early Proterozoic Eon. A way of resolving this uncertainty to some extent is to look for ‘biomarker’ chemicals in the geological record that provide a ‘signature’ for eukaryotes.
A new study has been undertaken by a group of Australian, German and French scientists to analyse sediments ranging in age from 635 to 1640 Ma from Australia, China, Asia, Africa, North and South America (Brocks, J.J and 9 others 2023. Lost world of complex life and the late rise of the eukaryotic crown. Nature, v. 618, p. 767–773; DOI: 10.1038/s41586-023-06170-w; contact for PDF). Their chosen biomarkers are sterols (steroids) that regulate eukaryote cell membranes. Some prokaryotes also synthesise steroids but all of them produce hopanepolyols (hopanoids), which eukaryotes do not. The key measures for the presence/absence of eukaryote remains in ancient sea-floor sediments is thus the relative proportions of preserved steroids and hopanoids, together with those for the breakdown products of both – steranes and hopanesthat are, crudely speaking, carbon ‘skeletons’ of the original chemicals.
Proportions of biomarkers in sediments from present to 1.64 Ga. Cholesteroids – reds; ergosteroids – blues; stigmasteroids – greens; protosteroids magentas, hopanoids – yellows; unsampled – grey. Snowball glaciations are shown in pale blue. (Credit: Simplified from Figure 3 in Brocks et al.)
Interpretation of the results by Jochen Brocks and colleagues is complicated, and what follows is a summary based partly on an accompanying Nature News & Views article(Kenig, F. 2023. The long infancy of sterol biosynthesis. Nature, v. 618, p. 678-680; DOI: 10.1038/d41586-023-01816-1). The conclusions of Brocks et al. are surprising. First, the break-down products of steroids (saturated steranes) that can be attributed to crown eukaryotes (left on the figure above) are only present in sediments going back to about 200 Ma before the first Snowball Earth event (~900 Ma). Before that only hopanes formed by hopanoid degradation are present: a suggestion that LECA only appeared around that time – the authors suggest sometime between 1 and 1.2 Ga. That is far later than the time when eukaryotes could have emerged: i.e. once there was available oxygen after the Great Oxidation Event (~2.4 to 2.2 Ga). So what was going on before this? The authors broke new ground in analysis of biomarkers by being able to detect signs of the presence of actual hopanoids and steroids of several different kinds. Steroids were present as far back as 1.6 Ga in the oldest sediments that were analysed.
Steroids of crown eukaryotes are represented by cholesteroids, ergosteroids and stigmasteroids. All three are present throughout the Phanerozoic Eon and into the time of the Ediacaran Fauna that began 630 Ma ago. In that time span they generally outweigh hopanoids, thus reflecting the dominance of eukaryotes over prokaryotes. Back to about 900 Ma, only cholesteroids are present, together with archaic forms that are not found in living Eukarya, termed protosteroids. Before that, only protosteroids are found. Moreover, these archaic steroids are not present in sediments that follow the Snowball Earth episodes (the Cryogenian Period).
Thus, it is possible that crown group eukaryotes – and their descendants, including us – evolved from and completely replaced an earlier primitive form (acritarchs?) at around the time of the greatest climatic changes that the Earth had experienced in the previous billion years or more. Moreover, the Cryogenian and Ediacaran Periods seem to show a rapid emergence of stigmasteroid- and ergosteroid production relative to cholesteroid: perhaps a result of explosive evolution of the Eukarya at that time. The organisms that produced protosteroids were present in variable amounts throughout the Mesoproteroic. Clearly there need to be similar analyses of sediments going back to the Great Oxygenation Event and the preceding Archaean to see if the protosteroid producers arose along with increasing levels of molecular oxygen. The ‘boring billion’ (2.0 to 1.0 Ga) may well be more interesting than previously thought.
Enormous events occurred between 460 and 435 Ma around the mid-point of the Palaeozoic Era and spanning the Ordovician-Silurian (O-S) boundary. At around 443 Ma the second-most severe mass extinction in Earth’s history occurred, which eliminated 50 to 60% of all marine genera and almost 85% of species: not much less than the Great Dying at the end of the Permian Period. The event was accompanied by one of the greatest biological diversifications known to palaeontology, which largely replaced the global biota initiated by the Cambrian Explosion. Centred on the Saharan region of northern Africa, Late Ordovician glacial deposits also occur in western South America and North America. At that time all the current southern continents and India were assembled in the Gondwana supercontinent, with continental masses that became North America, the Baltic region, Siberia and South China not far off: all the components that eventually collided to form Pangaea from the Late Silurian to the Carboniferous.
The mass extinction has troubled geologists for quite a while. There are few signs of major volcanism having been involved, although some geochemists have suggested that very high mercury concentrations in some Late Ordovician marine sediments bear witness to large, albeit invisible, igneous events. No large impact crater is known from those times, although there is a curious superabundance of extraterrestrial debris, including high helium-3, chromium and iridium concentrations, preserved in earlier Ordovician sedimentary rocks, around the Baltic Sea. Another suggestion, poorly supported by evidence, is destruction of the atmospheric ozone layer by a gamma-ray burst from some distant but stupendous supernova. A better supported idea is that the oceans around the time of the event lacked oxygen. Such anoxia can encourage solution of toxic metals and hydrogen sulfide gas. Unlike other mass extinctions, this one was long-drawn out with several pulses.
The glacial epoch also seems implicated somehow in the mass die-off, being the only one known to coincide with a mass extinction. It included spells of frigidity that exceeded those of the last Pleistocene glacial maximum, with the main ice cap having a volume of from 50 to 250 million cubic kilometres. The greatest of these, around 445 Ma, involved a 5°C fall in global sea-surface temperatures and a large negative spike in δ13C in carbon-rich sediments, both of which lasted for about a million years. The complex events around that time coincided with the highest ever extinction and speciation rates, the number of marine species being halved in a short space of time: a possible explanation for the δ13 C anomaly. Yet estimates of atmospheric CO2 concentration in the Late Ordovician suggests it was perhaps 8–16 times higher than today; Earth should have been a warm planet then. One probable contributor to extreme glacial conditions has been suggested to be that the South Pole at that time was well within Gondwana and thus isolated from the warming effect of the ocean. So, severe glaciation and a paradoxical combination of mass extinction with considerable biological diversification present quite an enigma.
A group of scientists based in Beijing, China set out to check the palaeogeographic position of South China between 460 and 435 Ma and evaluate those in O-S sediments at locations on 6 present continents (Jing, X., Yang, Z., Mitchell, R.N. et al. 2022. Ordovician–Silurian true polar wander as a mechanism for severe glaciation and mass extinction. Nature Communications, v. 13, article 7941; DOI: 10.1038/s41467-022-35609-3). Their key tool is determining the position of the magnetic poles present at various times in the past from core samples drilled at different levels in these sedimentary sequences. The team aimed to test a hypothesis that in O-S times not only the entire lithosphere but the entire mantle moved relative to the Earth’s axis of rotation, the ‘slippage’ probably being at the Core-mantle boundary [thanks to Steve Rozario for pointing this out]. Such a ‘true polar wander’ spanning 20° over a mere 2 Ma has been detected during the Cretaceous, another case of a 90° shift over 15 Ma may have occurred at the time when Snowball Earth conditions first appeared in the Neoproterozoic around the time when the Rodinia supercontinent broke up and a similar event was proposed in 1994 for C-O times albeit based on sparse and roughly dated palaeomagnetic pole positions.
Xianqing Jing and colleagues report a wholesale 50° rotation of the lithosphere between 450 and 440 Ma that would have involved speeds of about 55 cm per year. It involved the Gondwana supercontinent and other continental masses still isolated from it moving synchronously in the same direction, as shown in the figure. From 460 to 450 Ma the geographic South Pole lay at the centre of the present Sahara. At 445 Ma its position had shifted to central Gondwana during the glacial period. By 440 Gondwana had moved further northwards so that the South Pole then lay at Gondwana’s southernmost extremity.
Palaeogeographic reconstructions charting true polar wander and the synchronised movement of all continental masses between 460 and 440 Ma. Note the changes in the trajectories of lines of latitude on the Mollweide projections. The grey band either side of the palaeo-Equator marks intense chemical weathering in the humid tropics. Credit Jing et al. Fig 5.
As well as a possible key to the brief but extreme glacial episode this astonishing journey by a vast area of lithosphere may help account for the mass extinction with rapid speciation and diversification associated with the O-S boundary. While the South Pole was traversing Gondwana as the supercontinent shifted the ‘satellite’ continental masses remained in or close to the humid tropics, exposed to silicate weathering and erosion. That is a means for extracting CO2 from the atmosphere and launching global cooling, eventually to result in glaciation over a huge tract of Gondwana around 445 Ma. Gondwana then moved rapidly into more clement climatic zones and was deglaciated a few million years later. The rapid movement of the most faunally diverse continental-shelf seas through different climate zones would have condemned earlier species to extinction simultaneous adaptation to changed conditions could have encouraged the appearance of new species and ecosystems. This does not require the catastrophic mechanisms largely established for the other mass extinction events. It seems that during the stupendous, en masse slippage of the Earth’s lithosphere plate tectonic processes still continued, yet it must have had a dynamic effect throughout the underlying mantle.
Yet the fascinating story does have a weak point. What if the position of the magnetic poles shifted during O-S times from their assumed rough coincidence with the geographic poles? In other words, did the self-exciting dynamo in the liquid outer core undergo a large and lengthy wobble? How the outer core’s circulation behaves depends on its depth to the solid core, yet the inner core seems only to have begun solidifying just before the onset of the Cambrian, about 100 Ma before the O-S events. It grew rapidly during the Palaeozoic, so the thickness of the outer core was continuously increasing. Fluid dynamic suggests that the form of its circulation may also have undergone changes, thereby affecting the shape and position of the geomagnetic field: perhaps even shifting its poles away from the geographic poles …
The closest land to the North Pole is Peary Land in northern Greenland. Today, much of it is a polar desert and is bare of ice, so field geology is possible during the Arctic summer. It is one of the last parts of the northern hemisphere to have been mapped in detail. The bedrock ranges in age from the Mesoproterozoic to Upper Cretaceous, although the sequence is incomplete because of tectonic events and erosion during the Phanerozoic Eon. Its complex history has made Peary Land a draw for both structural geologists and stratigraphers. Apart from glacial tills the youngest rocks are estuarine sediments deposited in the early Pleistocene, between two glacial tills. They define one of the earliest known interglacials, roughly between 1.9 and 2.1 Ma, which lasted for an estimated 20 ka. Late Pliocene (3.4 Ma) sediments from around the Arctic Ocean have yielded rich fossil fauna and flora that suggest much warmer conditions – 10°C higher than those at present – before repeated glaciation began in the Northern Hemisphere. The sediments in Peary Land are fossiliferous, plant remains indicating a cover of coniferous trees, but animal fossils are restricted to small invertebrates: the tangible palaeontology offers slim pickings as regards assessing environmental conditions and the ecosystem.
One means of exploring faunal and floral diversity is through sampling and analysing DNA buried in sediments and soils rather than in fossils – plants shed pollen while animals spread their DNA via dung and urine. This approach has met with extraordinary success in revealing megafaunas that may have been decimated by humans newly arrived in the Americas. Even more remarkable was the ability of environmental DNA from cave sediments to reveal the former presence of individual humans who once lived in the caves and thus assess their numbers and relatedness. Such penetrating genetic ‘fingerprinting’ only became possible when new techniques to extract fragments of DNA from sediments and splice them to reconstruct genomes had been developed. But to apply them to material some two million years old would be a big ask; The oldest known DNA sequence had been recovered in 2021 from the molar of a 1.1 Ma old mammoth preserved in permafrost – a near-ideal source. A large multinational team under the supervision of Eske Willerslev (currently of Cambridge University, UK) took on the challenge, despite two million years of burial being likely to have degraded genetic material to minuscule fragments absorbed on the surface of minerals (Kjær, K.H. and 38 others 2022. A 2-million-year-old ecosystem in Greenland uncovered by environmental DNA. Nature, v 612, p. 283–291; DOI: 10.1038/s41586-022-05453-y). But it transpired that quartz grains have a good chance of ‘collecting’ bits of DNA and readily yielding them to the extraction media. The results are extraordinary.
Reconstruction of an American mastodon herd by American painter of large extinct fauna Charles R. Knight
The DNA extraction turned-up signs of 70 vascular plants, including poplar, spruce and yew now typically found at much lower latitudes, alongside sedges, shrubs and birch-tree species that still grow in Greenland. The climate was substantially warmer than it is now. The fauna included elephants – probably mastodons (Mammut) but not mammoths (Mammuthus) and caribou, as well as rabbits, geese and various species of rodents. There were even signs of ants and fleas. The overall assemblage of plants has no analogue in modern vegetation, perhaps because of the absence of anthropogenic influences, such as fires, the smaller extent of glaciations, their shorter duration and less established permafrost during the early Pleistocene. The last factor could have allowed a quicker and wider spread of coniferous-deciduous woodland, found today in NE Canada. In turn this spread of vegetation would have drawn in herds of large herbivores, later mastodons being known to have been wide-ranging forest dwellers. Willerslev suggests that the study has a potential bearing on how ecosystems may respond to climate change.
Among the strange early animals of the latest Precambrian, known as the Ediacaran fauna, is the slug-like Kimberella. Unlike most of its cohort, which are impressions in sediment or trace fossils, Kimberella is a body fossil in which can be seen signs of a front and back, i.e. mouth and anus (See also: A lowly worm from the Ediacaran?). In that respect they are the same as us: bilaterians both. Indeed, Kimberella may be one of the oldest of our broad kind that we will ever be able to see. Rare examples have fans of grooves radiating from their ‘front’. It may have grated its food, a bit like a slug does, but drew it in to its mouth. Some enthusiasts have likened the little beasty to a JCB digger, able to rotate and rake stuff into its mouth. In that case, Kimberella would have moved ‘backwards’ while feeding. If it can be likened to any modern animals, it may be a simple mollusc.
A Kimberella fossil, about 10 centimetres long, and a speculative reconstruction showing its feeding apparatus.
Other Ediacaran animals show no such mouth-gut-anus symmetry. Some have tops and bases, but most show no symmetry at all, being flaccid bag-like creatures. Palaeontologists provisionally suggest that they are primitive sponges, ctenophores, placozoans and cnidarians. Such animals excrete through pores on their surfaces and draw food in either through a simple mouth or their skins. The early bilaterians probably ‘grazed’ on bacterial or algal mats, but until now that has been conjectural. Ilya Bobrovskiy of the Australian National University and colleagues from Russia and Australia have managed to extract and analyse biomarker chemicals contained in well-preserved specimens of three Ediacaran animals from strata on the White Sea coast of Russia (Bobrovskiy, I. et al. 2022. Guts, gut contents, and feeding strategies of Ediacaran animals. Current Biology, v. 32, ; DOI: 10.1016/j.cub.2022.10.051). Biomarkers are molecules, such as fatty acids, phospholipids, triglycerides, hopanes and steranes, that definitively indicate metabolic processes of once living organisms, sometimes referred to as ‘molecular fossils’. Their varying proportions relative to one another are key to recognising the presence of different groups of organisms.
Specifically, hopane molecules are the best indicators of the former metabolism of bacteria whereas steranes (based on linked chains of carbon atoms bonded in rings) are typical products of degradation of sterols in eukaryotes. One sterane group involving 27 carbon atoms (C27 steranes) are typically formed when and animal dies and decays. C28 and C29 steranes likely form when algae decay, as when they are digested in the gut of a herbivore. Specimens of one of the Ediacaran animals analysed by the team – Dickinsonia – contained far more C27 steranes than C28 and C29, a sign of biomarkers associated with its decay. It probably absorbed food, weirdly, through its skin. Kimberella and a worm-like animal – Calyptrina – had sterane proportions which suggested that they digested algae or bacteria in a gut, as befits bilaterians. Simple as they may appear, these are among the earliest ancestors of modern animals, including us: of course!
In September 2022 Earth-logs highlighted how greening of the continents affected the composition of the continental crust. It now seems that was not the only profound change that the first land plants wrought on the Earth system. Beginning in the Silurian, the spread of vegetation swept across the continents during the Devonian Period. From a height of less than 30 cm among the earliest species by the Late Devonian the stature of plants went through a large increase with extensive forests of primitive tree-sized conifers, cycads, horsetails and sporiferous lycopods up to 10 m tall. Their rapid evolution and spread was not hampered by any herbivores. It was during the Devonian that tetrapod amphibians emerged from the seas, probably feeding on burgeoning terrestrial invertebrates. The Late Devonian was marked by five distinct episodes of extinction, two of which comprise the Devonian mass extinction: one of the ‘Big Five’. This affected both marine and terrestrial organisms. Neither flood volcanism nor extraterrestrial impact can be linked to the extinction episodes. Rather they marked a long drawn-out period of repeated environmental stress.
Phytoplankton bloom off the east coast of Scotland ‘fertilised’ by effluents carried by the Tay and Forth estuaries.
One possibility is that a side effect of the greening of the land was the release of massive amounts of nutrients to the seas that would have resulted in large-scale blooms of phytoplankton whose death and decay depleted oxygen levels in the water column. That is a process seen today where large amounts of commercial fertilisers end up in water bodies to result in their eutrophication. Matthew Smart and others from Indiana University-Purdue University, USA and the University of Southampton, UK, geochemically analysed Devonian lake deposits from Greenland and Scotland to test this hypothesis (Smart, M.S. et al. 2022. Enhanced terrestrial nutrient release during the Devonian emergence and expansion of forests: Evidence from lacustrine phosphorus and geochemical records. Geological Society of America Bulletin, v. 134, early release article; DOI: 10.1130/B36384.1).
Smart et al. show that in the Middle and Late Devonian the lacustrine strata show cycles in their abundance of phosphorus (P an important plant nutrient) that parallel evidence for wet and dry cycles in the lacustrine basins. The cycles show that the same phosphorus abundance patterns occurred at roughly the same times at five separate sites. This may suggest a climatic control forced by changes in Earth’s orbital behaviour, similar to the Milankovich Effect on the Pleistocene climate and at other times in Phanerozoic history. The wet and dry intervals show up in the changing ratio between strontium and copper abundances (Sr/Cu): high values signify wet conditions, low suggesting dry. The wet periods show high ratios of rubidium to strontium (Rb/Sr) that suggest enhanced weathering, while dry periods show the reverse – decreased weathering.
When conditions were dry and weathering low, P built up in the lake sediments, whereas during wet conditions P decreases; i.e. it was exported from the lakes, presumably to the oceans. The authors interpret the changes in relation to the fate of plants under the different conditions. Dry periods would result in widespread death of plants and their rotting, which would release their P content to the shallowing, more stagnant lakes. When conditions were wetter root growth would have increased weathering and more rainfall would flush P from the now deeper and more active lake basins. The ultimate repository of the sediments and freshwater, the oceans, would therefore be subject to boom and bust (wet and dry) as regards nutrition and phytoplankton blooms. Dead phytoplankton, in turn, would use up dissolved oxygen during their decay. That would lead to oceanic anoxia, which also occurred in pulses during the Devonian, that may have contributed to animal extinction.
Every organism that you can easily see is a eukaryote, the vast majority of which depend on the availability of oxygen molecules. The range of genetic variation in a wide variety of eukaryotes suggests, using a molecular ‘clock’, that the first of them arose between 2000 to 1000 Ma ago. It possibly originated as a symbiotic assemblage of earlier prokaryote cells ‘bagged-up’ within a single cell wall: Lynn Margulis’s hypothesis of endosymbiosis. It had to have happened after the Great Oxygenation Event (GOE 2.4 to 2.2 Ga), before which free oxygen was present in the seas and atmosphere only at vanishingly small concentrations. Various single-celled fossil possibilities have been suggested to be the oldest members of the Eukarya but are not especially prepossessing, except for one bizarre assemblage in Gabon. The first inescapable sign that eukaryotes were around is the appearance of distinctive organic biomarkers in sediments about 720 Ma old. The Neoproterozoic is famous for its Snowball Earth episodes and the associated multiplicity of large though primitive animals during the Ediacaran Period (see: The rise of the eukaryotes; December 2017).
The records of carbon- and sulfur isotopes in Neo- and Mesoproterozoic sedimentary rocks are more or less flat lines after a mighty hiccup in the carbon and sulfur cycles that followed the GOE and the earliest recorded major glaciation of the Earth. The time between 2.0 and 1.0 Ga has been dubbed ‘the Boring Billion’. At about 900 Ma, both records run riot. Sulfur isotopes in sediments reveal the variations of sulfides and sulfates on the seafloor, which signify reducing and oxidising conditions respectively. The δ13C record charts the burial of organic carbon and its release from marine sediments related to reducing and oxidising conditions in deep water. There were four major ‘excursions’ of δ13C during the Neoproterozoic, which became increasingly extreme. From constant anoxic, reducing conditions throughout the Boring Billion the Late Neoproterozoic ocean-floor experienced repeated cycles of low and high oxygenation reflected in sulfide and sulfate precipitation and by fluctuations in trace elements whose precipitation depends on redox conditions. By the end of the Cambrian, when marine animals were burgeoning, deep-water oxic-anoxic cycles had been smoothed out, though throughout the Phanerozoic eon anoxic events crop up from time to time.
Atmospheric levels of free oxygen relative to that today (scale is logarithmic) computed using combined carbon- and sulfur isotope records from marine sediments since 1500 Ma ago. The black line is the mean of 5,000 model runs, the grey area represents ±1 standard deviations. The pale blue area represents previous ‘guesstimates’. Vertical yellow bars are the three Snowball Earth events of the Late Neoproterozoic (Sturtian, Marinoan and Gaskiers). (Credit: Krause et al., Fig 1a)
The Late Neoproterozoic redox cycles suggest that oxygen levels in the oceans may have fluctuated too. But there are few reliable proxies for free oxygen. Until recently, individual proxies could only suggest broad, stepwise changes in the availability of oxygen: around 0.1% of modern abundance after the GOE until about 800 Ma; a steady rise to about 10% during the Late Neoproterozoic; a sharp rise to an average of roughly 80% at during the Silurian attributed to increased photosynthesis by land plants. But over the last few decades geochemists have devised a new approach based on variations on carbon and sulfur isotope data from which powerful software modelling can make plausible inferences about varying oxygen levels. Results from the latest version have just been published (Krause, A.J. et al. 2022. Extreme variability in atmospheric oxygen levels in the late Precambrian. Science Advances, v. 8, article 8191; DOI: 10.1126/sciadv.abm8191).
Alexander Krause of Leeds University, UK, and colleagues from University College London, the University of Exeter, UK and the Univerisité Claude Bernard, Lyon, France show that atmospheric oxygen oscillated between ~1 and 50 % of modern levels during the critical 740 to 540 Ma period for the origin and initial diversification of animals. Each major glaciation was associated with a rapid decline, whereas oxygen levels rebounded during the largely ice-free episodes. By the end of the Cambrian Period (485 Ma), by which time the majority of animal phyla had emerged, there appear to have been six such extreme cycles.
Entirely dependent on oxygen for their metabolism, the early animals faced periodic life-threatening stresses. In terms of oxygen availability the fluctuations are almost two orders of magnitude greater than those that animal life faced through most of the Phanerozoic. Able to thrive and diversify during the peaks, most animals of those times faced annihilation as O2 levels plummeted. These would have been periods when natural selection was at its most ruthless in the history of metazoan life on Earth. Its survival repeatedly faced termination, later mass extinctions being only partial threats. Each of those Phanerozoic events was followed by massive diversification and re-occupation of abandoned and new ecological niches. So too those Neoproterozoic organism that survived each massive environmental threat may have undergone adaptive radiation involving extreme changes in their form and function. The Ediacaran fauna was one that teemed on the sea floor, but with oxygen able to seep into the subsurface other faunas may have been evolving there exploiting dead organic matter. The only signs of that wholly new ecosystem are the burrows that first appear in the earliest Cambrian rocks. Evolution there would have ben rife but only expressed by those phyla that left it during the Cambrian Explosion.
There is a clear, empirical link between redox shifts and very large-scale glacial and deglaciation events. Seeking a cause for the dramatic cycles of climate, oxygen and life is not easy. The main drivers of the greenhouse effect CO2 and methane had to have been involved, i.e. the global carbon cycle. But what triggered the instability after the ‘Boring Billion’? The modelled oxygen record first shows a sudden rise to above 10% of modern levels at about 900 Ma, with a short-lived tenfold decline at 800 Ma. Could the onset have had something to do with a hidden major development in the biosphere: extinction of prokaryote methane generators; explosion of reef-building and oxygen-generating stromatolites? How about a tectonic driver, such as the break-up of the Rodinia supercontinent? Then there are large extraterrestrial events … Maybe the details provided by Krause et al. will spur others to imaginative solutions. See also: How fluctuating oxygen levels may have accelerated animal evolution. Science Daily, 14 October 2022
Most people are familiar with the term ‘blood diamonds’, meaning diamonds clandestinely exported from areas infested by the lethal activities of military and paramilitary forces. Indeed such conflicts are often fuelled by the large profits to be made from trading diamonds. One such source was in Sierra Leone during the civil war of 1991-2002. Others include Liberia, Côte d’ Ivoire, Angola and the Democratic Republic of Congo. Like illicit money, gemstones can be ‘laundered’ and find their way into conventional trade. To some extent the blood diamond trade has been slowed down by a programme of certification of packaged uncut diamond ‘rough’ by bona fide producers, and banning the sale of uncertified rough. The Kimberley Programme has been criticised because certificates can be issued in corrupt ways, so that blood diamonds probably still make their way to the international diamond markets: certification may hold no fears for those who force people to ine at gun point. However, because diamonds often show geochemical signatures and minute inclusions of other minerals that are unique to individual pipe-like intrusions of kimberlite that carry deep-mantle material to the surface. So, it is technically possible – but costly – to check for suspect rough. Such controls do not apply to other gemstones. A major source of very-high value gems is Myanmar (formerly Burma), whose widely condemned military dictatorship may be engaged in their unethical trade, including smuggling to neighbouring Thailand and China to avoid scrutiny.
Foot of bird chick preserved in Cretaceous amber from Kachin, Myanmar. Credit: Pinterest, Xing Lida, China University of Geosciences)
Myanmar is well endowed with sedimentary deposits that contain amber, the solidified resin from a variety of now extinct trees. Oddly, completely clear amber has low intrinsic value: it is semi-precious, albeit attractive. But it often contains inclusions of vegetation fragments, insects, feathers and small vertebrates, of interest to palaeontologists. Myanmar amber is especially interesting as it is dated to the Middle Cretaceous (~130 Ma), older than that found around the Baltic Sea (Eocene ~44 Ma), which was the main source for European jewellery since the 12th century, and that from Canada (Upper Cretaceous ~80 Ma). Myanmar amber has been used decoratively and medicinally in China since the 3rd century CE, and in Europe since prehistoric times. It is attractive but quite common, so historically amber never commanded high prices but was widely used as a trade item. Since the publicity attending the supposed extraction of dinosaur DNA from the bodies of reptile parasites to resurrect dinosaurs in Steven Spielberg’s 1993 film Jurassic Park, public and scientific interest in amber has boomed. It is primarily the exquisite preservation of encased organisms that piques the interest of palaeontologists. Papers that rely on the Myanmar amber have grown in number over the last ten years, despite the country being infamous for military repression of tribal and religious groups in its rural areas.
One of the most conflict-riven areas is the northern state of Kachin where the most interesting amber to palaeontologists is collected by the Kachin people of the Hukawng Valley. Government forces have been in conflict with the Kachin Independence Army since the 1960s, most particularly for control of the amber industry. A recent paper has focussed on the ethical issue of publications based on fossil-bearing amber from the area (Dunne, E.M. et al. 2022. Ethics, law, and politics in palaeontological research: The case of Myanmar amber. Communications Biology, v. 5, article no. 1023; DOI: 10.1038/s42003-022-03847-2).
In 2010 the military began forcibly to take over mines in Kachin. Between 2014 and 2021 the annual number of publication underwent a tenfold growth from between 10 to 15 to over 150, despite the fact that in 2015 the government in Yangon prohibited removal of fossils from the country. But the export laws exempt gemstones, so the growing demand for fossiliferous amber is clearly reflected in its supply to foreign scientists. Rare specimens that include vertebrate remains command prices up to US$100,000. The Myanmar amber trade is now estimated at around US$ 1 billion per annum. The Myanmar military took over all the mines in 2017, and is clearly the main supplier to palaeontologists.
In the seven-year period, only 3 papers out of 872 included contributors from Myanmar, which also suggests an element of ‘parachute science’: unsurprisingly Myanmar-based scientists also find it difficult to visit the Kachin area. Before 2014 most of the 69 publications involved scientists in the US; since then, the top spot has been occupied by Chinese scientists who have amassed 417. It seems clear that there is a web of contacts linking together the source of Myanmar amber, its market and science. In 2020 the Society of Vertebrate Paleontology called for a moratorium on publishing data from Kachin sources. But since then there is little sign that palaeontologists have taken any notice.
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