Numenius tenuirostris: The Slender-billed Curlew declared extinct.

The Earth's biodiversity is considered to be facing a crisis at a global level, with many experts believing that species may be going extinct at a rate as high as during the great extinctions recorded in the fossil record. Despite this, it is very hard to determine how many species are going extinct, in part because many species have never been documented, but also because it is generally impossible to tell whether a species is extinct, unless it can be confidently asserted that all populations were being monitored prior to extinction. The best recorded organisms tend to be Vertebrates, and one of the most extensively monitored Vertebrate groups are the Birds, a group which tend to be highly visible, and which are often recorded extensively by non-professional citizen-science groups. Despite this attention, many species of Birds have not been seen for a long time, and this cannot always be taken as evidence that they have become extinct: for instance, the Black-browed Babbler, Malacocincla perspicillata, was rediscovered in 2020 after not being observed in 180 years. The apparent extinction of species can be problematic, causing conservationists to cease efforts to protect a species which is close to extinction; conversely failure to realise that a species has become extinct can lead to efforts being dedicated to preserving it which could otherwise have been directed towards other species which might still be saved.

In a paper published in the journal Ibis on 17 November 2024, Greame  Buchanan of the RSPB Centre for Conservation Science, Ben Chapple of the Centre for Biodiversity and Environment Research at University College London, Alex Berryman of BirdLife International, Nicola Crockford of the Royal Society for the Protection of Birds, Justin Jansen of the Naturalis Biodiversity Center, and Alexander Bond of the Bird Group at the Natural History Museum, formally declare the Slender-billed Curlew, Numenius tenuirostris, to be extinct.

The International Union for the Conservation of Nature's Red List of Threatened Species currently lists the Slender-billed Curlew as Critically Endangered, on the assumption that the total population is less than 50 Birds and declining. The species is believed to be restricted to the Palaearctic biogeographical region, breeding in central Asia and migrating to Europe, North Africa, the Middle East, and the Arabian Peninsula. Slender-billed Curlews are thought to breed to the east of the Ural Mountains, in the area around Omsk in southern Russia. Isotope studies of museum specimens suggest that the species may also have bred further south, in northern Kazakhstan, while some eggs assigned to the species, also in museum collections, indicate that the species may also have bred to the west of the Urals. Outside of the breeding season the Birds ranged west as far as Western Europe and the Atlantic coast of North Africa, being known from across the Mediterranean Region, the Middle East, the Arabian Peninsula, and the Pannonian Plains of southeast Central Europe.

Once widespread in Europe, Slender-billed Curlews were last observed in Brittany, France, in February 1968, and in North Yemen in January 1984. A colony, thought to be the last, was known on the Atlantic coast of Morocco in the 1990s, with the last reported sighting in the winter of 1997/8, although a flock of Slender-billed Curlews was photographed in southern Italy in  March 1995. No subsequent observations of the species have been made, despite extensive searches across its former range, including the Middle East and Central Asia.

The last known photograph of a group of Slender-billed Curlews, taken in southern Italy in March 1995. Marco Basso in van den Berg (1995).

The Slender-billed Curlew was first observed breeding in 1912 by Russian ornithologist Valentin Ushakov, who first noted that the species appeared to be in decline. The possibility that Slender-billed Curlews might be at risk of extinction was first raised in 1943 by German ornithologists Erwin Stresemann and Hermann Grote, but it was not until 1988 that the species was identified as Threatened by the International Union for the Conservation of Nature. The species was listed as Critically Endangered in 1994, following a re-assessment of its status, and an action plan for its recovery was produced in 1996. 

Based upon this long absence from its known range, and the extensive, yet unproductive, searched which have been made in the last three decades, combined with the declining number of sightings recorded throughout the twentieth century, Buchanan et al. carried out a statistical analysis of the probability that the species might still exist. Based upon this, they conclude that is most likely that the species became extinct in the 1990s, with only a miniscule possibility that the species might still exist in the 2020s.

If this analysis is correct, then the Slender-billed Curlew is only the third species of Bird known to spend a large part of its annual cycle in the Western Palaearctic to have become extinct since 1500, joining the Great Auk, Pinguinus impennis, last reported in 1844, and the Canarian Oystercatcher, Haematopus meadewaldoi, last observed in 1913. 

A preserved museum specimen of a juvenile male Slender-billed Curlew, Numenius tenuirostris. Naturalis Biodiversity Center/Wikimedia Commons.

It is difficult to directly assess the cause of the extinction of the Slender-billed Curlew, since it is likely that the species finally died out around the time that an action plan for its survival was first drawn up. When that plan was produced, it identified that the species was threatened by habitat loss across its range, and potentially being hunted in some areas as well. 

The only records of breeding by the Slender-billed Curlew are those made by Valentin Ushakov in southern Russia in the early twentieth century. It is possibly that the main breeding area for the species was further south, on the steppes of northern Kazakhstan, which were extensively converted to croplands by the Russian Empire and Soviet Union in the nineteenth and twentieth centuries, in the process destroying extensive wetlands, which might have served as breeding grounds for Slender-billed Curlews, however, without further evidence, this is purely speculative. 

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Rafetus swinhoei: Last known female Swinhoe's Softshell Turtle has died, bringing the species to the brink of extinction.

The last known female Swinhoe's Softshell Turtle, Rafetus swinhoei, has died. The Turtle, which was 1.56 m long and weighed 93 kg was found dead at Hoan Kiem Lake in Hanoi, Vietnam, on Sunday 23 April 2023, by a volunteer with a local Turtle conservation program. A necropsy will be carried out to determine the cause of death (the term 'autopsy' is reserved for investigations of Human deaths). The Hoan Kiem Turtle was confirmed to be female during a capture-release survey in 2020, bringing hopes that it might be possible to start a breeding program for the species; the previous last known female died in a zoo in China in 2016, having never laid any eggs, despite mating with a male owned by the zoo several times.

The Hoan Kiem Lake Swinhoe's Softshell Turtle in 2016. Asian Turtle Program.

Swinhoe's Softshell Turtles were formerly known from two geographic regions, the Yangtze River Basin in China and the Red River Basin in Vietnam (fossil specimens suggest the species had a wider range in the Pleistocene and Early Holocene), but the population has been effectively wiped out in both areas, due to hunting, egg collecting, and habitat modification by Humans. The global population now comprises a single male population in Suzhou Zoo in Jiangsu Province, China, and a second male known only from environmental DNA in Xuan Khanh Lake near Hanoi.

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Using mercury to assess the role of Central Atlantic Magmatic Province volcanism in the End Triassic Extinction.

The End Triassic Extinction event is considered to be the fourth of the Big Five extinction events recorded in the fossil record of the Phanerozoic Eon. On land it wiped out many large Vertebrate groups, paving the way for the Dinosaur-dominated ecosystems of the Jurassic and Cretaceous, but its effects were more severely felt in the oceans, with the total collapse of almost all carbonate reef ecosystems and the extinction of the Conodonts, a jawless Vertebrate group that first appeared in the Cambrian. The extinction is associated with a sharp rise in carbon dioxide levels, thought to have caused significant ocean acidification. This has in turn been linked to the eruption of the Central Atlantic Magmatic Province, the volcanic event that began the break-up of the supercontinent of Pangea and the formation of the Atlantic Ocean, which occurred in three or four phases over a period of about 700 000 years. However all known rocks directly associated with the Central Atlantic Magmatic Province were laid down in either terrestrial or deep marine environments, making it hard to make a direct link between the timing of these volcanic deposits and the collapse of shallow marine reef ecosystems.

In a paper published in the journal Nature Comunications on 6 April 2016, Alyson Thibodeau of the Department of Earth Sciences at the University of Toronto and the Department of Earth Sciences at Dickinson College, Kathleen Ritterbush of the Department of Geology and Geophysics at the University of Utah, Joyce Yager and Joshua West of the Department of Earth Sciences at the University of Southern California, Yadira Ibarra of the Department of Earth System Science at Stanford University, David Bottjer and William Berelson, also of the Department of Earth Sciences at the University of Southern California, Bridget Bergquist, also of the Department of Earth Sciences at the University of Toronto and Frank Corsetti, again of the Department of Earth Sciences at the University of Southern California, examine mercury concentrations across the Triassic-Jurassic boundary in shallow marine deposits at Muller Canyon in Nevada as a proxy for volcanism.

Mercury reaches the Earth's surface almost exclusively through volcanism, with major eruptions leading to significant amounts of the volatile metal entering the atmosphere, and mercury in rocks being found predominantly in volcanic deposits, and fluvial and oceanic sediments derived from such rocks. Atmospheric mercury tends to bind strongly to organic material and clay particles, tending to find its way into marine sediments within at most a few million years of erupting.

The Muller Canyon rock sequence comprises a Late Triassic Bivalve-dominated carbonate reef, which is succeeded by an Early Jurassic sedimentary sequence dominated by siltstone, with rare microscopic Gastropods and Sponge spicules. Importantly the last occurrence of the Ammonite Choristoceras crickmayi, which is used to mark the end of the Triassic, occurs seven meters below the first occurrence of the Ammonite Psiloceras spelae, which is used to mark the beginning of the Jurassic, suggesting that a good sequence across the Triassic-Jurassic boundary has been preserved.

Thibodeau et al. found that mercury levels rose sharply with the onset of the extinction event, and while they drop back after this they remain significantly higher than pre-extinction levels throughout the strata with an impoverished fauna, with several smaller peaks in mercury concentration during this time. Only when mercury levels drop back to pre-extinction levels does the biota begin to recover, and a significant Jurassic ecosystem starts to develop.

Summary of key features of the Triassic-Jurassic interval, Muller Canyon, Nevada. Panels compare (a) Mercury chemostratigraphy, (b) Ammonite species diversity; (c) benthic palaeoecology and microfacies; and (d) ecosystem state for Muller Canyon, Nevada, in association with lithology and key dates. These comparisons show that significant biotic recovery follows the mercury anomalies and provide evidence that biotic recovery began after the cessation of Central Atlantic Magmatic Province magmatism. Thibodeau et al. (2016).

This strongly suggests that the End Triassic Extinction was associated with the onset of volcanism in the Central Atlantic Magmatic Province, and that the fauna did not begin to recover until after volcanism ceased. This contrasts with previous studies, which have shown the End Triassic extinction as an abrupt event, hard to reconcile with the more drawn-out Central Atlantic Magmatism. Furthermore, while the initial extinction event was accompanied by a spike in carbon dioxide levels, likely to have caused a significant ocean acidification event, the ocean fauna remained impoverished significantly after carbon dioxide levels had returned to normal, not recovering for around two million years after the initial event, or a million years after the end of volcanism, compared to a predicted recovery timescale of 10-100 000 years for an ocean acidification event.

This suggests that while ocean acidification may have played a role in the initial extinction event, it was not the only factor involved. Thibodeau et al. note that at their highest mercury levels in the Muller Canyon deposits reach 600 parts per billion relative to organic carbon. This compares to levels of around 200 parts per billion in sediments in parts of the San Fransisco Bay area considered to be heavily polluted today as a result of the use of mercury in gold mining in California in the nineteenth century. Mercury is a powerful neurotoxin, and known to be harmful to a wide range of organisms, being highly deleterious to modern ecosystems at high levels. However Thibodeau et al. do not go as far as to claim that mercury was directly responsible for the extinction event rather than an indicator of a volcanically induced deteriorating ecosystem.

See also...

Evidence for a Middle Permian extinction event from Spitzbergen Island.                      In recent years considerable evidence has been exposed in South China for a major extinction event in the Middle Permian. This event, known as...

Evidence of an Ice Age at the start of the Middle Jurassic.                                           The Jurassic lasted approximately 65.3 million years, from about 201.3 million years ago till about 145 million years ago, and is generally considered to have been...

The reaction of marine invertebrates to global warming during the Early Jurassic Toarcian Extinction Event.                                        About 183 million years ago a major eruptive episode in the Karoo-Ferrar Large Igneous Province of South Africa lead to an abrupt rise in global atmospheric and...

 

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Evidence for a Middle Permian extinction event from Spitzbergen Island.

In recent years considerable evidence has been exposed in South China for a major extinction event in the Middle Permian. This event, known as the Capitanian Extinction, could potentially have been on a scale similar to the 'Big Five' events of geological history, wiping out much of the Earth's marine life and leaving an impoverished fauna ahead of the more famous End Permian Extinction, which could help to explain the severity of that event. On the other hand it could have been a strictly local event, triggered by volcanism in southwest China and affecting only tropical fauna in a portion of the Tethyan Ocean, with no wider implications.

In a paper published in the Geological Society of America Bulletin on 14 April 2015, David Bond of the Department of Geography, Environment and Earth Sciences at the University of Hull, Paul Wignall of the School of Earth and Environment at the University of Leeds, Michael. Joachimski of the GeoZentrum Nordbayen at the Universität Erlangen-Nürnberg, Yadong Sun, also of the Geozentrum Nordbayen at the Universität Erlangen-Nürnberg, and of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Ivan Savov, also of the School of Earth and Environment at the University of Leeds, Stephen Grasby of the Geological Survey of Canada and the Department of Geoscience at the University of Calgary, Benoit Beauchamp, also of the Department of Geoscience at the University of Calgary and Dierk Blomeier of the Norwegian Polar Institute, describe evidence from the Kapp Starostin Formation of Spitsbergen Island that suggests the Capitanian Extinction also affected the Middle Permain Boreal Ocean.

The majority of the Kapp Starostin Formation comprises Brachiopod and Bryozoan dominated limestone beds, however this breaks down in the upper third of the formation, with a series of events removing much o the fauna. The initial event causes the loss of 32 of the 40 Brachiopod species present, along with 8 of 15 Bivalves. This is followed by a period of recovery, with the replacement of the Brachiopod-dominated fauna with one dominated by Molluscs, reminiscent of the faunas that came to dominate Mesozoic seas as they recovered from the End Palaeozoic Extinction. This fauna persists for much of the remainder of the Kapp Starostin Formation, but then disappears abruptly and does not reappear. In a final stage all siliceous Sponges disappear, along with a soft-bodied fauna which left numerous trace fossils.

Brachiopods from the Kapp Starostin Formation at Kapp Starostin. (A) Brachythyrina sp.; (B) Cancrinella spitsbergiana; (C, D) indet. Echinochoncids; (E, F) Haydenella sp.; (G) Lissochonetes superba; (H) Lissochonetes superba showing papillose areas either side of the centrum; (I, J) Stenocisma sp. 2; (K, L, M) Waagenoconcha sp. Scale bar is 1 cm. Bond et al. (2015).

Dating in the Kapp Starostin Formation is imprecise, however the final extinction phase, in which Sponges and soft-bodied tracemakers are lost, coincides with a major carbon isotope excursion and is thought to represent the End Permian Extinction. Bond et al. examined oxygen and strontium isotope ratios and magnitostratigaphy across the Kapp Starostin Formation in order to try to match events to those in South China. 

Oxygen and strontium isotopes ratios reflect conditions such as temperature other environmental conditions, and generally reflect local rather than global conditions, however while ratios vary from area to area overall trends are more likely to be global, i.e. a sharp increase in the proportion of oxygen-18 in South China is also likely to be seen in Spitzbergen, and by looking at ratios of two elements enough similarities should be seen to make a comparison. 

Magnitostratgraphy uses traces of ancient magnetic fields preserved in iron minerals in rocks to trace ancient pole reversals; the poles only have two possible orientations (north pole in the north/south pole in the south or south pole in the north/north pole in the south) and these occasionally flip, with the poles exchanging positions. Pole reversals happen more-or-less at random, with periods between reversals occurring at intervals ranching from tens of thousands to millions of years, and reflected across the globe. This creates a pattern of magnetic reversals in sedimentary rocks that can be matched in different rocks across the globe.

Using these methods Bond et al. were able to establish a close match between the Spitzbergen and South China deposits, strongly suggesting that the Brachiopod extinction event seen in the Kapp Starostin Formation is the Capitanian Extinction seen in South China. They further note that a similar extinction event can be seen in the Wegener Halvø Formation of Greenland and Sverdrup Basin of Arctic Canada, suggesting that the Capitanian Extinction can be traced across the Boreal Ocean.

Bivalves from the Kapp Starostin Formation at Kapp Starostin. (A) Indet. Buchid; (B) Etheripecten keyslingiformis; (C) Grammatodon (Cosmetodon)? suzuki; (D) Palaeolima sp.; (E) Palaeoneilo sp.; (F) Retroceramus sp.; (G) Streblopteria winsnesi; (H) Streblopteria winsnesi overlain by a fragment of Cassianoides; (I, J) Vorkutopecten svalbardensis; (K) Vorkutopecten sp. Scale bar is 1 cm. Bond et al. (2015).

The cause of this extinction is harder to determine. The loss of tropical fauna in South China during the Capitanian Extinction has led to the suggestion that this could be a cooling event, preferentially affecting warm water organisms unable to cope with an abrupt drop in temperature. Such an event would be expected to affect cold-water Boreal Ocean species far less severely, as such organisms would presumably be pre-adapted to deal with cooler conditions, indeed they might be expected to expand into new areas as the cooling waters create new opportunities by wiping out warm water species. However the event appears to have affected cold water species in the Boreal Ocean just as much as warm water species in the Tethys, suggesting another cause may be responsible.

Bond et al. were able to find pyrite framboids (raspberry-shaped iron pyrite crystals) in parts of the Kapp Starostin Formation thought to have been laid down in deeper water. Such framboids only form in the absence of oxygen, raising the possibility that the Capitanian Extinction may have been linked to an anoxic ocean event. Such events have been seen elsewhere in the fossil record and are thought to be linked to break-downs in ocean circulation, which inhibit the ability of oxygen to reach deeper water. However the extinction appears to have affected shallow water species as severely as deep water species. There is no evidence of framboids in shallower water, and anoxia is a much less likely explanation for shallow water extinctions, as even when ocean circulation breaks down oxygen can be replaced in shallow water by free exchange with the atmosphere, suggesting that the cause(s) of the Capitanian Extinction are likely to be more complex.

See also...

Five new species of Proetoid Trilobites from northeastern Oman.                                                 Trilobites dominated the seas of the early Palaeozoic, but suffered a major loss of diversity in the Devonian extinction and became extinct...

The fate of soil microbes during the End Permian Extinction.                                                    The End Permian Extinction is the most severe extinction event recorded in the fossil record, with the loss of around 96% of all known species, and many dominant Palaeozoic groups of organisms. The event led to the effective resetting of the Earth’s biosphere, enabling the emergence of a radically different Mesozoic Biota...

The cause of the end-Permian extinction.    The Earth has been dominated by multi-cellular life forms (animals, plants etc.) for a little over half a billion years, but life has not had it easy throughout all of...

 

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The fate of soil microbes during the End Permian Extinction.

The End Permian Extinction is the most severe extinction event recorded in the fossil record, with the loss of around 96% of all known species, and many dominant Palaeozoic groups of organisms. The event led to the effective resetting of the Earth’s biosphere, enabling the emergence of a radically different Mesozoic Biota. The event is thought to have been caused by massive volcanism in the Siberian Traps, which not only produced huge emissions of lava and volcanic gasses at the surface, but ignited vast areas of buried Palaeozoic coals and hydrocarbons, leading to massive emissions of acidic and halogenic gas, which in turn resulted in bouts of severe acid rain and the breakdown of the ozone layer, allowing harmful ultraviolet light to reach the Earth’s surface.

A widespread increase in the mutation rate seen in Lycopsid microspores at the end of the Permian has long been seen as evidence of the mutagenic influence of ultraviolet radiation on End Permian ecosystems, but widespread soil acidification is harder to detect. Such an event would be expected to wash acid-soluble metallic plant nutrients such as aluminium, calcium and magnesium out of soils, as well as cause a rapid increase in erosion (and marine sedimentation) rates, due to a loss of soil cohesion; however while both of these have been recorded, they could both also be caused by a variety of other phenomena, making them inconclusive evidence.

In a paper published in the journal Geology on 7 January 2015, Mark Sephton and Dan Jiao of the Department of Earth Science and Engineering at Imperial College London, Michael Engel of the School of Geology and Geophysics at The University of Oklahoma, Cindy Looy of the Department of Integrative Biology and Museum of Paleontology at the University of California–Berkeley and Henk Visscher of the Laboratory of Palaeobotany and Palynology at the Department of EarthSciences at Utrecht University, describe the results of an investigation into the breakdown of lignin at the Vigo Meano Section in northern Italy.

The Vigo Meano Section is thought to provide the most detailed record of the molecular composition of solvent-extractable organic matter from across the Permian/Triassic boundary. The section comprises organic-rich marls (calcium-rich clays, likely to have been formed in an inshore marine environment with high ground runoff) that are not thought to have been influenced by any subsequent metamorphic heating. This section has been used in several prior studies of geochemistry across the Permian/Triassic boundary, and has extremely well constrained dates.

Lignin (the major component of plant fibres) is largely broken down by enzymes excreted into the environment by Fungi and Bacteria. This breakdown process results in lignin breaking down to vanillin (4-hydroxy-3-methoxybenzaldehyde), which then breaks down to vanillic acid (4-hydroxy-3-methoxybenzoic acid), which is further broken down into protocatechuic acid (3,4-dihydroxybenzoic acid), which can be broken down further into a variety of products by a range of soil microbes. Because vanillin is widely used as flavouring in the food industry (vanilla), this breakdown process has been extensively studied in investigations into food spoiling and is very well understood.

Soil bacteria are known to be very sensitive to fluctuations in pH (in chemistry the pH is a reflection of the acidity or alkalinity of a substance, with neutral substances such as pure water having a pH of 7 and more acid substances having lower pHs), with even small changes in acidity leading to major differences in species composition, and much poorer and less diverse bacterial flora found in acid soils. Fungi are far more tolerant, with many common soil species able to survive large fluctuations in soil pH. However the enzymes used to break down vanillin are at their most effective when the soil pH is about 8, and cease activity at about pH 4, with the effect that few soil fungi can survive long at pHs below about 4.5.

Sephtonet al. reasoned that since this is the case, it should be possible to detect any sudden and dramatic increase in soil acidity acid during the end-Permian biotic crisis due to an increase in vanillin and vanillic acid in sediments at this time. Moreover since both compounds are readily biodegradable even under anaerobic conditions, they should not persist for long in the marine environment, making for a close relationship between spikes in soil acidity and sediment composition.

Results mass spectrography analysis for vanillin and vanillic acid across the Permian/Triassic boundary at Vigo Meano show a number of peaks in presence of the two chemicals, including a sustained peak across the major extinction episodes and Permian/Triassic boundary, which suggests the occurrence of pulses of soil acidification so severe that an almost complete cessation of biodegradation must have occurred not just within the soil, but during the transportation and sedimentation process, strongly supporting the idea that the soils were exposed to bouts of rainfall with pHs as low as 4 and possibly lower than 2 (strong enough to cause acid burns to exposed skin) during the end Permian biotic crisis.

Ratios of vanillic acid to vanillin [acid to aldehyde ratio, (Ad/Al)v] in latest Permian and earliest Triassic organic matter assemblagesfrom the VigoMeano section (southern Alps, Italy), providing proxy evidence for pulses of soil acidification (pH < 4 ).VG, Val Gardena Formation; PTB, approximate positionof the Permian-Triassic boundary; extinctions, interval of principal marine extinction and floral turnover in southern Alps; d13C, position ofend-Permian negative carbon-isotope shift in southern Alps. Letters at bottom of stratigraphic column, from left to right, correspond to clay, silt, fine sand, medium sand, and coarse sand, respectively. Sephton et al. (2015).

See also…

How changes in Plant ecology shed light on the End Cretaceous Extinction Event.             One of the two main theories that seeks to explain the extinction event at the end of the Cretaceous postulates that a large bolide (extra-terrestrial object such as a comet or asteroid) smashed into the Yucatan Peninsula in Mexico close to the modern town of Chicxulub, resulting in a devastating explosion and long term climate change. Such an event would have led to a...

The cause of the end-Permian extinction.

The Earth has been dominated by multi-cellular life forms (animals...

The nature of the Chicxulub impactor.           65 million years ago, at the end of the Cretaceous, the Earth underwent the last of the five great mass extinctions recorded in the fossil record. While this is by no means the largest of these events, it is the most familiar to the general public, as it was responsible for the extinction of, amongst other things, the non-Avian Dinosaurs and the large marine Reptiles of...

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The demise of the Megashark.

The Giant Shark, Carcharocles megalodon, is one of the more charismatic creatures of the recent fossil record, a relative of modern Mackerel Sharks that is thought to have been able to reach about 18 m in length, known from the Middle Miocene to the end of the Pliocene, with some claims of the species persisting into the Pleistocene. It is interpreted to have had a life-style similar to the modern Great White Shark, which preys primarily on Marine Mammals, and teeth of Carcharocles megalodon are frequently found with fossil Whales from the Miocene and Pliocene.

A set of jaws of the Giant Shark, Carcharocles megalodon, from the National Aquarium of New Zealand. Wikipedia.

Large predators are thought to have a profound effect on marine ecosystems, and it has been shown in modern habitats that the removal of such top predators often leads to a chain reaction which can completely reshape the local biological community. Given the size of Carcharocles megalodon, its removal from marine ecosystems can be expected to have had a major impact, however it is difficult to determine when fossil species actually went extinct, making it hard to detect changes in marine ecosystems associated with the loss of particular species.

In a paper published in the journal PLoS One on 22 October 2014, Catalina Pimiento of the Florida Museum of Natural History and Department of Biology at the University of Florida and Christopher Clements of the Institute ofEvolutionary Biology and Environmental Studies at The University of Zurich, use a statistical method called the Optimal Linear Estimation model to attempt to determine exactly when Carcharocles megalodonwent extinct.

The Optimal Linear Estimation model was developed by conservation biologists studying extinction in modern species. It uses a statistical method based around recorded sightings of a species to determine the probability of an endangered species still being alive, or when it is most likely to be extinct. Since Carcharocles megalodon is known only from fossil specimens and has never been seen by human eyes fossil records were used as a proxy for sightings. Like all Sharks Carcharocles megalodon lacked a bony skeleton (Shark skeletons are formed entirely of cartilage) but produce new teeth throughout it life, losing some when attacking prey and shedding others as they become worn. This makes for a very good fossil record (tooth is denser than bone, and preserves more easily as a fossil) for all Sharks, particularly large and distinctive species that have lived in the not-to-distant past.

Using the Optimal Linear Estimation model Pimento and Clements determined that the extinction of Carcharocles megalodon is most likely to have occurred 2.6 million years ago exactly at the end of the Pliocene. This would coincide with a dramatic expansion of Baleen Whales (filter-feeding Whales) at the beginning of the Pleistocene; up until this point these Whales were a minor part of marine ecosystems and were quite small compared to modern species, but in the early Pleistocene they began to diversify rapidly and grow dramatically in size, and today include the largest species of animal ever known to have lived (the Blue Whale, Balaenoptera musculus).

The association between the demise of Carcharocles megalodon and the rise of the great Baleen Whales has been made in the past, but has been hard to prove or to compare to the alternative theory that the expansion of the Baleen Whales was driven by a re-organization of plankton diversity at the end of the Pliocene driven by the closure of the Panama Seaway, which connected North and South America, and which in turn led to a major re-organization of the Earth’s ocean currents.

Temporal distribution of the inferred dates of extinction of Carcharocles megalodon using the Optimal Linear Estimation model bootstrapped 10,000 times. The orange area shows the distribution of inferred dates of extinction through time, whereas the green line shows the cumulative frequency of inferred dates of extinction. The modal peak represents the point in time by which the species was most likely to have gone extinct (2.6 million years ago). Approximately 50% of simulations fell before the modal peak of inferred dates of extinction (2.6 million years ago), whereas the remaining 50% are roughly evenly distributed between the mode and the present day. The two vertical dashed lines indicate the most recent and oldest inferred dates of extinction (160,000 years in the future and 3.5 million years ago respectively). The horizontal bars represent the time range of each fossil occurrence. The blue bars are the occurrences used in the Optimal Linear Estimation model analysis. The grey bar represents the occurrences that failed the age evaluation process and were not used in the analysis. Pimento & Clements (2014).

Pimento and Clements also estimated that there is a 0.06% chance that Carcharocles megalodon has not gone extinct at all, and is still found in today’s oceans; however they strongly emphasize that this is an artefact of the statistical methodology used, and that they do not believe that there is any chance of the species still being in existence. While surviving Megasharks makes for a good movie plot, it is highly unlikely that an 18 m Shark could have escaped detection into the twenty-first century, no matter how cryptic its behaviour. In order for this to be the case the Shark would not just have to avoid detection by all observers (including military sonar systems developed during the Cold War which looked for things far smaller than 18 m), it would have to avoid leaving any distinctive marks on prey species from attacks (bite marks several meters across would be likely to be recorded), and would need to avoid losing any teeth in places where humans could find them; as noted before Sharks lose teeth throughout their lives, and Sharks’  teeth are widely collected by humans for ornamental purposes, making it highly unlikely that recent teeth of Carcharocles megalodon would escape detection.

(Left) A tooth from Carcharocles megalodon, and (right) pendants made from modern Sharks teeth. Wikipedia/ebay.

See also…

Satellite tagging Whale Sharks in the Red Sea. 

Whale Sharks (Rhincodon typus) are the largest extant Shark species, and indeed the largest living Fish...

A new species of Sawshark from the Philippines.

Sawsharks (Pristiophoridae) are highly specialized Sharks related to Skates and Rays (Batoids). They...

Deepwater Sharks from the Early Miocene of Slovakia.

Sharks appear in the fossil record between 450 and 420 million years ago (all possible specimens older than... 

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Opportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic Event.

The Toarcian Oceanic Anoxic Event is an extinction event that took place in the Early Jurassic, about 183 million years ago. It took place in four phases, thought to have been related to Milankovitch Cycles. During each phase the temperature of the global ocean is thought to have risen abruptly by as much as 13℃, leading to a depletion in oxygen levels in the oceans, followed by an extinction event. Warmer waters are less able to retain oxygen, and this is thought to have been made worse by an increase in runoff from the continents due to higher rainfall, and a breakdown in ocean currents caused by the warming of deep oceanic waters. Each of these phases is marked by a distinct shift in Carbon, Oxygen and Strontium isotope ratios, an extinction event in the fossil record and the deposition of vast amounts of organic matter which has led to extensive hydrocarbons deposits at these levels in many places around the world. The Early Jurassic was considerably warmer than today, and it is thought that the warmest points on the Milankovitch Cycles (which are driven by cyclic variations in the Earth's orbit) raised global temperatures above a tipping point which led to runaway warming.

In a paper published in the journal Geology on 6 September 2013, Bryony Caswell of the School of Environmental Sciences at the University of Liverpool and Angela Coe of the Department of Environment, Earth and Ecosystems at the Centre for Earth, Planetary, Space and Astronomical Research at the The Open University, examine the behavior of populations of two species of opportunistic Bivalve, Bositra radiata and Pseudomytiloides dubius during the Toarcian Oceanic Anoxic Event in deposits at Whitby near North Yorkshire, England.

During the run up to the initial event the deposits were dominated by the Bivalve Bositra radiata, a Posidoniid Clam related to modern Scallops, quickly came to dominate to fauna, forming monospecific pavements and growing to sizes not achieved prior to the onset of the event. 

Bositra radiata shell pavement from Hawsker Bottoms, North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

During the remaining three events the Inoceramid Clam Pseudomytiloides dubius (also related to modern Scallops) quickly came to dominate the faunas, again reaching larger sizes during the event. The two species appear to compete during the onset of the third event, but Pseudomytiloides dubius quickly came to dominate, suggesting that the two species competed for some resource (probably food) and that Pseudomytiloides dubius was able to outcompete Bositra radiata.

Pseudomytiloides dubius from Port Mulgrave in North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

Bositra radiata and Pseudomytiloides dubius are both thought to have been opportunistic species. Both were small (even when reaching exceptional sizes), and are thought to have had short generation times and high larval production rates, probably comparable to the modern Mulinia lateralis, a small Surf Clam which can breed at two months old and seldom lives more than two years, which does well in anoxic conditions. Such organisms can thrive in environments even where they are exterminated each year, since they will have time to reproduce and can recolonize from adjacent areas.

The approximate location of the study area. Google Maps.

The Toarcian Oceanic Anoxic Event is of particular interest today as we are living in time when rising global temperatures, combined with local anoxic zones in parts of the oceans caused by excess runoff due to human activities, for example in the Black Sea and the Gulf of Mexico. This makes it important to understand how organisms cope (or don't) with such events, and what the long-term impact on the global biosphere is likely to be, particularly if these events become larger and more frequent.

See also A new species of Pea Clam from the Miocene of Bosnia and Herzegovina, Iceberg damage in an Antarctic Clam, The evolution of Galeommatoid Bivalves, Symbiosis and the success of Galeommatoid Bivalves and A new species of Scallop from Western Australia.

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Two new species of Cichlid Fish from Lake Victoria.

Cichlid Fish are widespread freshwater Perch (Perciformes), found in Africa, South America, southern North America, parts of the Caribbean, Madagascar, the Middle East and South Asia. There are one of the most diverse Fish, and therefore Vertebrate, families, with over 1600 described species. They are popular in the aquarium trade, which has led to them becoming naturalized in many parts of the world where they are not native.

In a paper published in the journal ZooKeys on 2 January 2013, a team of scientists led by Marnix de Zeeuw of the Naturalis Biodiversity Center in Leden describe two new species of Cichlid Fish from the waters of southern Lake Victoria. Neither of these is, strictly speaking, new to science, but neither has been formally described before.

The first species described is named Haplochromis argens, it has appeared previously in numerous scientific publications under the name Haplochromis 'argens' (argens meaning silver) from the 1970s onwards, but has never actually been properly described. Haplochromis argens is a 53-78 mm, slender Cichlid Fish. Males have a blue-to-purple sheen on their upper sides and a yellow-to-green sheen on their flanks, as well as reddish fins with spots on the anal fin. They consume zooplankton in the photic zone (top part of the water that light can penetrate), being restricted to the top two meters at night, but foraging deeper at night. The species was abundant in the Mwanza, Speke and Emin Pasha Gulfs and around Kome Island until the 1980s, but suffered a collapse in population due to the introduction of Nile Perch (Lates niloticus) to the lake, and have not recovered despite the success of measures to reduce the Nile Perch population. The species is also well known in the aquarium trade.

Haplochromis argens; (top) male specimen from Emin Pasha Gulf, (middle) male specimen from Mwanza Gulf, (bottom) line drawing. Scale bar is 10 mm. de Zeeuw et al. (2013).

The second Cichlid Fish is named Haplochromis goldschmidti; it has previously been known as Haplochromis 'dusky argens', and is now named after Tijs Goldschmidt, the scientist who first referred to the Fish, and an expert on the Cichlids of Lake Victoria who has studied both their ecology and evolution, and the extinction event they suffered as a result of the introduction of the Nile Perch. It is similar to H. argens, but darker and with a curved profile. It is known only from Emin Pasha Gulf; it is unclear if the species survives.

Haplochromis goldschmidti; (top and middle) male specimens, (bottom) line drawing of male specimen. Scale bar is 10 mm. de Zeeuw et al. (2013).

Southern Lake Victoria. (A) The known distribution of Haplochromis argens (crosshatched area). (B) Area where Haplochromis argens specimens were collected in Mwanza Gulf. Letters indicate the locations of research stations. (C) Locations where specimens were collected in Emin Pasha Gulf. Circles are for H. argens, triangles for H. goldschmidti. de Zeeuw et al. (2013).

See also A new species of tree-dwelling Fighting Fish from Thailand, A new species of Cichlid Fish from Lake Tanganyika, A new species of Cichlid Fish from Lake Malaŵi, Two new species of Sandperch from the South China Sea and Boney Fish on Sciency Thoughts YouTube.

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