Sunday, 13 June 2021

Aplonis opaca: The Såli (or Micronesian Starling) holds on despite the presence of the invasive Brown Treesnake on Guam.

Invasive species present one of the biggest challenges for conservation biologists, with many endemic species threatened by the arrival of invasive predators or competitors. Such organisms typically arrive in an area where local species are naïve to their behaviour, and undergo an explosive ecological release, being freed from their own predators and pathogens whilst at the same time able to exploit local prey which have no defences against them. Species on oceanic islands are often particularly vulnerable to the arrival of invasive predators, and are also often home to highly endemic species, not found anywhere else, which means that the arrival of a novel predator on such an island can quite often lead to a cascade of extinctions. Among Vertebrates, island-dwelling Bird species appear to be particularly vulnerable to such effects, with widespread loss of native Bird species across island systems such as Hawai’i, New Zealand, and the Mascarenes.

However, where species are not completely wiped out, they can respond positively to predator control programs, with the recovery of Bird species recorded on many small islands where invasive Mammal species have been removed. Such programs are difficult to run on larger islands however, and in such cases species recovery is often limited to fenced-off areas where the predators have been removed, although the presence of wild populations in such areas greatly increases the availability of recruits to repopulate the rest of the island, should this later become possible.

The predatory Brown Treesnake, Boiga irregularis, was introduced to Guam, an island in the Mariana Archipelago, shortly after the end of World War II, since which time nine of the island's eleven indigenous Bird species have been wiped out. By the early 1990s, there were an estimated 5000-10 000 Brown Treesnakes per square kilometre on Guam, with 1-2 million thought to be living on island in total. Despite this, some Birds have managed to survive on the island, notably the endangered Yåyaguak (or Mariana Swiftlet), Aerodramus bartschi, which is a cave-roosting Bird, difficult for the Snakes to attack, and the locally endangered Såli (or Micronesian Starling), Aplonis opaca, a cavity-nesting Bird found across the Marianas. The omnivorous Såli is an important seed-disperser, and therefore critical to the ecology of the island, where it was formerly found in all habitats, but underwent a catastrophic decline after the introduction of the Brown Treesnake, with the last recorded survey of the species, carried out in the early 1990s, finding only 60-120 Birds remaining on Guam, almost all of which resided on Andersen Air Force Base on the northeast of the island.

Despite a lack of formal assessment since this time, the Såli is known to still be present on Guam, with the population around Andersen Air Force Base thought to have expanded due to Snake control measures. The species is known to be suffering high fledgeling mortality, again due to the Snakes, but has recently been seen in urban areas in northern and central Guam, where it has not been seen since the 1980s, leading to hopes that these Birds may be expanding back into parts of their former range, despite a lack of Snake control.

In a paper published in the journal Bird Conservation International on 1 March 2021, Henry Pollock of the School of Global Environmental Sustainability at Colorado State University, Martin Kastner of the Department of Ecology, Evolution, and Organismal Biology at Iowa State University, Gary Wiles of Olympia in Washington State, Hugo Thierry, also of the Department of Ecology, Evolution, and Organismal Biology at Iowa State University, Laura Barhart Dueñas of the Division of Aquatic and Wildlife Resources at the Guam Department of Agriculture, Eben Paxton of the Pacific Island Ecosystems Research Center of the U.S. Geological Survey. Nicole Suckow, also of the School of Global Environmental Sustainability at Colorado State University, Jeff Quitugua also of the Division of Aquatic and Wildlife Resources at the Guam Department of Agriculture, and Haldre Rogers, again of the Department of Ecology, Evolution, and Organismal Biology at Iowa State University, present the results of a study of an island-wide survey of the distribution and abundance of the Såli on Guam, 

At 541 km², Guam is the largest island in Micronesia, as well as the most populated, with a population of about 160 000 people in 2010, and the most developed, with about 20% of the . The northern part of the island is covered by a limestone plateau, with karst forest, and the majority of the island's Human population and urbanised areas, while the southern part is dominated by volcanic geological features, with areas of savanna and ravine forest, and a more sparse Human population.

Previous studies have established that Såli roosting on Andersen Air Force Base range widely throughout the forested areas along the eastern and southern perimeter of the base during the day, but return to the developed area around 3.00 pm, where they are relatively sedentary and easier to count. In 2017-8 over 350 Birds from the Air Force Base population were colour-banded, providing a basis for future population estimates based upon resighting of these Birds; a number of Birds were also radio-tagged, all of which used forest extensively and travelled off-base but returned to the core roosting area at night.

 
The study area on Andersen Air Force Base and the search areas used for the standardised area searches. The developed areas of Andersen Air Force Base were divided into 28 search areas, comprising three types of habitats (forest search areas FO₀₁, FO₀₂, and FO₀₃ were included in the closest adjacent search area): urban (UR), residential housing (HW, HE, HN), and golf course (GC). Inset depicts the island of Guam, with the study area indicated by the white rectangle. Pollock et al. (2021).

Pollock et al. divided the main developed area of Andersen Air Force Base into 28 search areas of roughly similar size, comprising three habitat types: urban, residential housing, and golf course. Every day two areas were chosen at random, and each of these was observed by two fieldworkers, who traversed the search area together, to increase overall detection probability and the accuracy of colour-band identifications. Adjacent search areas were never searched on the same day, to minimise the risk of double-counting Birds. Observers also remained in constant contact during surveys, and communicated movements of any Birds throughout a given search area. All Birds detected in the search areas were logged, along with data on their ages and any banding information; the open nature of the landscape made it relatively easy to approach and visually observe Birds detected by sound.

In order to study the distribution of the Såli away from Andersen Air Force Base, Pollock et al. combined data from transect surveys with recorded opportunistic sightings of the Birds. They excluded the small, but stable, population of Såli known to be present on the nearby island of Cocos. Three data sources were consulted for data on opportunistic sightings, the eBird website, which enables members of the public to record Bird sightings anywhere in the world, a database if sightings maintained by the Guam Division of Aquatic and Wildlife Resources, and a personal database maintained by Martin Kastner. All of the sightings in the Division of Aquatic and Wildlife Resources database were confirmed by biologists from the department, and all of the sightings recorded in Martin Kastner's database were made by scientists familiar with the species.

 
A Såli (or Micronesian Starling), Aplonis opaca, on Cocos Island, Guam. Joseph Mancuso/eBird.

A total of 46 transect surveys were carried out in April–May 2018. Each transect route was visited once, with 19 of the routes having been previously used in a Division of Aquatic and Wildlife Resources survey in 1985, ten surveys were carried out in rural areas with little development (two northern, three central, and five southern), and nine in suburban areas within a kilometre of forest (five northern and four central). The transects were an average of 5189 m in length, with ten-minute surveys being carried out at ten points spaced roughly evenly along these lines. A further 27 transects were carried out in areas where sightings of Såli had recently been recorded by the Division of Aquatic and Wildlife Resources; these surveys were only 500 m in length, and did not overlap with the longer surveys. All but one of these short surveys were carried out within 500 m of an area of forest, with surveys carried out at six points roughly 100 m apart on the transect.

Pollock et al. found 16 previous studies that mentioned either the abundance or distribution of Såli on Guam published between 1901 and 1995. Twelve of these were published prior to 1970, and invariably described the Såli as very common. By 1978–1979 the species was rare on the southern part of the island, and uncommon on the northern and central parts. The first attempt at assessing the population of the Birds on Guam was carried out in 1981, and counted 1667 Såli in a series of surveys, which was extrapolated to a total of 15 132–18 602 Birds on the island. By this time Såli were completely absent from the southern part of the island, and in the central part, only a single small population was found, around the village of Hagåtña. This survey only found an estimated 231 individuals living at and around Andersen Air Force Base.

Subsequent surveys found almost no Såli on the island, and by the early 1990s it was estimated that only 60–120 Birds remained, including 50–100 living on Andersen Air Force Base, and the nearby areas of Mt. Santa Rosa and Gayinero, Yigo. Smaller groups of Birds, totalling no more than five individuals, were present at the Conventional Weapons Storage Area (now called ‘Munitions Storage Area’), and Naval Computer and Telecommunications Area Master Station (now called ‘Naval Base Guam Telecommunications Site’), as well as a scattering of solitary Birds along the southern coast.

 
Såli distribution on the island of Guam during the last three population surveys. Panel (a) indicates results from the 15 search areas surveyed in 1981. Panel (b) indicates results from the island-wide population assessment conducted between 1992-1994. Panel (c) indicates the current distribution on the island as derived from opportunistic sightings and the Andersen Air Force Base area search in 2018. Pollock et al. (2021).

During their three week-long surveys of Såli around Andersen Air Force Base, Pollock et al. counted 683, 609, and 844 Birds, respectively. However, in forests along the southern and eastern peripheries of the base they only counted 3–6 Birds each week. The Såli appeared to be concentrated towards the centre of the base’s main developed area, with less Birds in peripheral search areas adjacent to forest edge. Less than 5% of the Birds counted were banded, including 42 unique individuals, with 13 re-sightings of Birds seen in week one in week 2, and four re-sightings of Birds seen in weeks one or two in week 3. Extrapolating from this, Pollock et al. conclude about 50 ringed Birds remain on Andersen Air Force Base, out of a total population of about 1391, 91.1% of which are adults and subadults.

Pollock et al. compiled records of sightings of Såli at 64 unique locations across Guam from 2005 to 2019, these were largely concentrated in villages of northern and central Guam, with a few sightings around the southernmost tip of the island. A total of 64 sightings were recorded in 12 of the island's 19 villages, representing 156 Birds. The Birds were more common in urbanised areas, including the island’s main business districts (particularly at large malls and shopping centres), as well as urban parks and residential areas. No Birds were sighted more than 2 km from a built-up area or major road. The Birds were most frequently seen perched on power lines, power poles, buildings, and trees. Ten nests were observed outside the Air Base, all on lamp posts or power poles.

Duromg the transect surveys, Pollock et al. made 91 observations of Såli on 20 of the 29 surveys. All sightings were on the northern and central parts of the island, with the majority around the Air Force Base and the island of Yigo. No Såli were detected in the southern villages of Merizo or Umatac, despite these being the closest points to the island of Cocos, with its own population of the Birds. 

 
Satellite imagery of the island of Guam showing the locations of, panel (a), both long (white) and short (orange) transect surveys and, panel (b), the island’s 19 villages. Panel (b) lists the villages where Såli were detected (pink polygons) or not (red polygons) during transect surveys. Pollock et al. (2021).

Based upon the results of the sightings records and survey results, Pollock conclude there are currently around 1450–1490 Såli living on the island of Guam. This includes 30–40 individuals living around the villages of Yigo and Dededo on northern Guam, 20–30 Birds in Hagåtña, and 10–20 in Tamuning-Tumon-Harmon, in the central part of the island, as well as up to ten further Birds living outside of these areas; the remainder of the population being resident at Andersen Air Force Base.

Pollock et al. have produced the first update on the distribution and abundance of Såli on Guam since the 1990s. They record a 15-fold increase in population size since the last population survey took place, with the population up from about 100 to about 1500, although 93-96% of the population is concentrated at a single location, Andersen Air Force Base. Despite this uneven distribution, the Såli do appear to be in the process of recolonising urbanised areas elsewhere in northern and central Guam, with a few isolated Birds being seen on southern parts of the island. 

Away from Andersen Air Force Base almost all sightings of Såli occur in urbanised areas, with the overwhelming majority of such sightings. It is, however, unclear how well established the Birds are in areas away from the Base, as while the majority of these sightings were of pairs or small groups of Birds, indicating a potential for breeding, very few nests or juveniles were seen. This is further complicated by the fact that, apart from at Hagåtña, almost all of these sightings occurred at shopping malls, which might indicate that the (highly mobile) Birds are exploiting a new food source rather than settling in these areas.

For any Bird to survive on Guam today, they need to be able to avoid Brown Treesnakes. This means there are two major factors likely to influence the long-term survival of the Såli on Guam; Snake control measures, and the Birds increasing adaptation to urban areas. Snake control measures have been in place at Andersen Air Force Base since 1993, with thousands of Snakes being removed from the site each year, which has clearly benefitted the Såli. Similar Snake eradication programs are in place at other military installations on the island, but these sites are all much smaller, and do not seem to have created suitable safe environments.

Brown Treesnakes avoid brightly lit area, brightly lit areas, and open spaces such as lawns and car parks, and urbanised areas seem to have become refugia for the Såli on Guam. Recorded nesting by the Birds occurs in solitary trees, building cavities, lamp posts and artificial nest boxes. 

Snake eradication programs on Guam are largely restricted to military instillations, and Guam International Airport, and although other urban areas clearly present some refuge from the Snakes, most also include patches of vegetation, where the Snakes are found. Most urban areas are known to have fairly high Snake populations, with the largest Snakes often found in such areas, where there is an abundance of prey. However, even the limited protection presented by these areas appears to be beneficial, with the Birds apparently becoming re-established here.

Whatever the current successes of the Såli, the Snake control measures currently present on Guam are unlikely to protect the species in the long run, as these are intended to protect the island's electrical infrastructure and prevent the spread of the Snakes to new islands, rather than to eradicate them. The number of Snakes captured at Andersen Air Force Base has remained steady since the 1990s, indicating the overall population of Snakes has been uneffected, and a constant supply of new recruits exists to replace any Snakes removed. In addition, studies have shown that, even on Andersen Air Force Base, very few fledgling Såli survive to adulthood, due to predation by both Brown Treesnakes and Domestic Cats, leaving the Bird population unusually skewed towards older Birds. The Såli have failed to recolonise the extensive suburban areas on the east-central Guam, despite the high Human population here. The Birds are also largely absent from the sparsely-populated central and southern parts of the island. This strongly suggests that the Birds will be unable to repopulate the island properly without a more extensive Snake control program being implemented. 

One action that has clearly proven beneficial to the Såli has been the provision of nest boxes. These provide additional nesting opportunities for cavity-nesting Birds such as the Såli, and can offer protection from both the elements and predators. Such boxes have been placed on Andersen Air Force Base since the 1990s, with at least 50 predator-resistant boxes in place at any one time since 2015, which is thought to have allowed the fledging of about 900 Birds. However, fledgelings still suffer very high predation rates from both Brown Treesnakes and Domestic Cats, and there are still more nests in natural cavities than in nest boxes, so it is unclear how much of an impact this program has on the overall population.

The survival of the Såli is considered essential for ecosystem functioning on Guam. These omnivorous Birds are the only surviving native frugivores on the island, and as such are vital for the distribution of the seeds of many Plants, and subsequently the ability of the indigenous forests to regenerate. The ability of these Birds to survive in urban areas is beneficial for the species itself, but clearly of limited value to the island's forests. Thus, plans for rewilding efforts on Guam will require extensive application of Snake-control measures if they are to resume their natural ecological function.

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Sunday, 6 June 2021

Annular Eclipse to be visible from parts of the Arctic Circle on 10 June 2021.

An annular eclipse of the Sun (eclipse in which the Moon passes in front of the Sun, but does not completely block it, leaving visible ring of light) will be visible from parts of the Arctic Circle, including areas of Canada, Greenland, and Russia, on Thursday 10 June 2021, with a partial eclipse visible from much of the rest of Canada and Greenland, Iceland and the North Atlantic, most of Europe and parts of Central Asia and Morocco.

 
The path of the 10 June 2021 Annular Solar Eclipse. The annular eclipse will be visible along the central dark grey path. A partial eclipse will be visible from the shaded areas; in the lighters area the full eclipse will not be visible as it will have started before dawn (west) or will continue after sunset (east). The solid red lines are the Equator and the Greenwich Meridian, the dotted red lines are the Topics of Cancer (north) and Capricorn (south). HM Nautical Almanac Office.

Eclipses are a product of the way the Earth, Moon and Sun move about one-another. The Moon orbits the Earth every 28 days, while the Earth orbits the Sun every 365 days, and because the two Sun and Moon appear roughly the same size when seen from Earth, it is quite possible for the Moon to block out the light of the Sun. At first sight this would seem likely to happen every month at the New Moon, when the Moon is on the same side of the Earth as the Sun, and therefore invisible (the Moon produced no light of its own, when we see the Moon we are seeing reflected sunlight, but this can only happen when we can see parts of the Moon illuminated by the Sun). 
 
The relative positions of the Sun, Moon and Earth during a Solar eclipse. Not to scale. Starry Night.
 
An Annular Eclipse is a type of Solar Eclipse, in which the Moon passes between the Earth and the Sun while the moon is close to aphelion (when it is furthest from the Earth). The Moon has a variable orbit, getting considerably closer and further from the Earth at different times, which alters its size as seen from the Earth. Thus when it is at its furthest from the Earth it appears considerably smaller than the Sun so an eclipse occurring at this time will produce a ring of sunlight, rather than a period of darkness. A Partial Annular Eclipse resembles a regular Partial Eclipse, in that the light of the Sun will be partially blocked by the Moon passing in from of it, though the disk of the Moon will be smaller.

An Annular Eclipse on 20 May 2013, photographed from Middlegate, Nevada. Wikipedia.

However the Moon does not orbit in quite the same plane as the Earth orbits the Sun, so the Eclipses only occur when the two orbital planes cross one-another; this typically happens two or three times a year, and always at the New Moon. During Total Eclipses the Moon entirely blocks the light of the Sun, however most Eclipses are Partial, the Moon only partially blocks the light of the Sun.
 
How the differing inclinations of the Earth and Moon's orbits prevent us having an eclipse every 28 days. Starry Skies.
 
Although the light of the Sun is reduced during an Eclipse, it is still extremely dangerous to look directly at the Sun, and eclipses should only be viewed with specialized equipment.

Animation showing the shadow of the Moon at five minute intervals on Wednesday 10 June 2021. Andrew Sinclair/HM Nautical Almanac.

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Eleven injured in fire at oil refinery in Tehran.

Eleven people were injured fighting a fire at an oil refinery in the south of Tehran, Iran, this week. The fire broke out on Wednesday 2 June 2021, and was brought under control and then extinguished within 24 hours. It was initially thought to have been triggered by a leak from a natural gas pipeline, but is now thought to the leak occurred from a waste storage tank, with a second tank also being engulphed by the blaze. The state-owned Tondgooyan Petrochemical Company, which operates the site, has ruled out sabotage as a cause of the blaze, with an investigation into the precise origin of the fire still ongoing. 

 
Fire at an oil refinery in Tehran on 2 June 2021. Vahid Ahmadi/Tasnim News Agency/AFP/Getty Images.

Iran has suffered a number of serious oil fires in recent years, caused by a combination of a decaying infrastructure, and deliberate attacks by opponents of the regime. However, on this occasion the cause of the fire appears to have been more mundane, with fire the fire likely to have been triggered by volatile fumes leaking from the tank being ignited by the heat of an Iranian summer day.

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Two confirmed dead and five still missing following flood and collapse at Mexico coal mine.

Two miners have now been confirmed dead, and five more are still missing, following a flood at a coal mine at Murquiz in Coahuila State in northeastern Mexico on Friday 4 June 2021. The mine, which is described as a narrow, steep-sided, open pit, is reported to have undergone a sudden inrush of water and collapse, burying the workers there under several tonnes of wood, rock and metal. Rescue attempts have concentrated on trying to pump water out of the collapsed mine, although specialist rescue teams with dogs are on hand, and have begun to enter the mine and look for survivors.

 
Rescue teams at a coal mine in Coahuila State, Mexico, which underwent a flood and collapse in Friday 4 June 2021. La Guardia Nacional/Twitter.

Floods and inrushes typically occur when miners accidently break through into pockets of water and gas trapped within rocks. Since such buried waters are often under high pressure due to the weight of rocks above them, they tend to escape into the mine rapidly, and on occasion explosively, leading to a highly dangerous situation in which miners are often rapidly overwhelmed. Such inrushes can also occur when miners encounter flooded disused mineworkings, a danger in areas where mining has occurred for a long time but good records have not been kept.

However on this occassion it has been suggested that the flood was caused by the collapse of a poorly maintained tailings dam (dam on a pool holding water drained from a mine, which typically contains a high proportion of fine silt and clay particles, which take time to settle out of the water) close to the mine head. Poor maintainence at small scale mines is a serious issue in Coahuila State, according to Miguel Agustin Pro Human Rights Center, which reports that mine-owners in the state routinely flout safety regulations without facing official sanction.

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Friday, 4 June 2021

Understanding the role of sediment-gravity flows in the formation of the Cambrian Burgess Shale Lagerstätte.

The fossil record has been used to reconstruct a history of the evolution of life on Earth, but itself preserves a rather incomplete record. Almost all fossils are of biomineralised or other hard tissues, with only rare sites, known as Konservat Lagerstätten, preserving soft tissues or the bodies of organisms which lack hard tissues. Estimates of the preservation potential of modern organisms suggest that 30% of marine megafuana, and 80% of megafauna overall, would leave no fossil record in normal deposits. Much of our understanding of the emergence of modern Animals comes from Cambrian Konservat Lagerstätten, such as the Burgess Shale, where it is estimated that 86% of the preserved fauna would not be preserved under normal conditions. This makes understanding the processes which led to the creation of these deposits particularlt important. It is often assumed that the Burgess Shale represents the near-faithful preservation of an intact biological community, and these fossils have been used to reconstruct food webs and community structures, but it is unclear whether the conditions which led to the preservation of these fossils did indeed preserve a high fidelity impression of a living community, or created biases which we have not detected and have worked into our understanding of how these ancient communities worked, and thus how modern communities developed from those early, Cambrian examples.

The preservation of soft tissues in fossils is rare, and thus not always as readily understood by palaeontologists as the preservation of more familiar hard tissues. Studies of the decay of modern organisms can help palaeontologists understand these preserved soft tissue fossils, but only a limited number of such studies have ever been undertaken. In particular, there are very few studies of the post-mortem transportation of soft bodied organisms, and how this effects the preservation process. 

The Burgess Shale Lagerstätte is considered to be one of the most important fossil faunas known, producing a large range of soft bodied organisms from an outer shelf Middle Cambrian environment. Understanding rhe mechanisms that led organisms to be preserved here is a key to understanding the biodiversity of the original ecosystem, as well as those of the forty plus other Cambrian sites around the world which show 'Burgess Shale-type preservation'. However, there has been a long-standing debate about the role of sediment transportation in the preservation of the Burgiss Shale Fauna, with the sediments that host the fossils originally diagnosed as having been produced in dilute turbidity currents, in which organisms were transported and then buried, then to have been in situ organisms which were buried by the turbidites. More recently, the deposits have been re-interpreted to be mud-rich slurry flows, an interpretation which, if right, would have profound implications for any soft bodied organism caught up in them.

In a paper published in the journal Communications Earth & Environment on 2 June 2021, Orla Bath Enright of the School of the Environment, Geography, and Geosciences at the University of Portsmouth, and the Institute of Earth Sciences at the University of Lausanne, Nicholas Minter, also of the School of the Environment, Geography, and Geosciences at the University of Portsmouth, Esther Sumner of the National Oceanography Centre at the University of Southampton, and Gabriela Mángano and Luis Buatois of the Department of Geological Sciences at the University of Saskatchewan, present the results of a study which used flume experiments to understand how transportation would impact the degradation of samples of the King Ragworm, Alitta virens, a modern marine species lacking hard tissues.

Field observations of the Burgess Shale exposure at Walcott Quary in British Colombia revealed that the deposits are made up of a series of silt and clay beds, with 'floating' quartz grains which average 100–500 μm in size, but can reach up to 1000 μm. These beds tend to have scoured bases, and structures such as parallel laminations, which are considered to be indicative of sediment transport. Bath Enright et al. interpret these structures as having been laid down by transitional cohesive flows, with both turbulent and laminar characteristics.

Bath Enright et al. next developed an index of degradation for Alitta virens, using specimens decayed for 0, 24, or 48 hours, and then subjected to conditions similar to those they interpret for the deposition of the Burgess Shale in a flume tank, for 25, 225, or 900 minutes. As a control measure, specimens were subjected to static decay for similar periods of time. Specimens that were partially decayed prior to transport produced a variety of results, ranging from whole, but shrivelled, to an unsupported gut with fluid escape and a general flattening of the body. The specimens tended to decay more rapidly at their posterior ends and mid-section; pre-decayed and then transported specimens tended to be more damaged towards their posterior ends. 

Next, Bath Enright et al. examined 197 specimens of Polychaete Worms from the Burgess Shale, 154 specimens of Burgessochaeta, and 43 specimens of Canadia. Very few of these showed any sign of degradation, and few of those that did showed little preference in where this occurred, although in these the preference was for the posterior to be more decayed.

 
Increasing states of Polychaete degradation. Alitta virens (right) and comparable states in the fossil, Burgessochaeta (left). (A) State 1-complete Polychaete, entire body segment intact (ROM–64913). (B) State 2-damage towards the mid-section and posterior transforms into tangled remains caused by the combination of transport and decay. The body remains intact as one segment (ROM–64916). (C) State 3-remains of the trunk and setae. The body structure has deteriorated significantly (ROM–64914). (D) State 4-remains of loose setae are attached to minute segments of cuticle and jaw elements only are recovered (ROM–64915). Bath Enright et al. (2021).

Based upon the analysis of the flume tank specimens, and comparison to specimens from Walcott Quarry, Bath Enright et al. conclude that the specimens of the Burgess Shale could have been transported in quasi-laminar flows for more than 20 km before being burried. This long distance preservation in quasi-laminar flows implied in this is in contrast to the situation seen in turbulent flows, which are more widely studied, and in which the amount of damage seen tends to increase with the distance of transport. However, Bath Enright et al. also note that this is not the case for specimens heavily decayed before transportation.

From these observations, Bath Enright et al. conclude that the specimens of the Burgess Shale were mostly un-decayed before being caught in the flows. Many of the Burgess Shale Polychaetes are compressed, but have no preferred orientation, and are typically found within the beds, rather than at their tops or bottoms. This suggests that they were transported within the flow, rather than being buried by it. At the end of Bath Enright et al.'s experiments, the sediments remained in a soupy state for some time, and living Worms were rapidly able to escape. Assuming the same applied at in the Burgess Shale, then any Worms remaining in the sediment must have been dead. Interpretation of the oxidation state of the Burgess Shale is complex, but it has been suggested that the oxic/anoxic boundary happened at the sediment surface, and the deposits are free of signs of burrowing or other bioturbation that would be expected if Worms were living in the sediment.

 

Schematic flow reconstruction for the Walcott Quarry in the Burgess Shale. (A) Schematic Representation in which the laminar plug extends towards the base of the flow and changes to a transitional plug regime. A turbulent cloud of sediment is suspended in the water column above the plug flow. The soft-bodied organisms (labelled 1, 2, and 3) have been picked up along the flow path, potentially kilometres apart from one another. (B) Bed A from the Greater Phyllopod Bed of the Walcott Quarry. (C) Graphic log showing Bed A; soft-bodied organisms (1, 2, and 3) from the flow type above (A) will become mixed in the deposit. (D) Thin-section scan from Bed A showing parallel laminae, erosive, scoured bases, and 'floating' quartz grains (Q). White arrows indicate transitional cohesive flow deposits. Bath Enright et al. (2021).

Burgess Shale-type lagerstätten are traditionally viewed as ‘windows’ into the biology and ecology of past life and are used to reconstruct in-life communities. However, Bath Enright et al.'s  study casts doubt on the validity of this, suggesting that the sediments present may have been transported very long distances, potentially collecting organisms from a variety of habitats before finally settling. In particular the Cambrian Chengjiang and Qingjiang biotas of China, and Upper Ordovician Beecher’s Trilobite Bed of the USA are widely interpreted to been emplaced by flows, and might therefore also have interpreted organisms from multiple environments. 

Konservat Lagerstätten provide us with a great deal of information about past life, due to their unique preservation of soft tissues and soft bodied organisms. These deposits tell us a great deal of information about the anatomy of long dead organisms, and have helped us to understand the origins of many living groups of organisms. However, Bath Enright et al. caution against attempts to reconstruct ancient biological communities based upon these deposits, without clear indication that the fossils have been preserved in their life-environment.

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Wednesday, 2 June 2021

Was Stonehenge originally located at Waun Mawn in the Preseli Hills of Wales.

Around 1036 AD, Geoffrey of Monmouth wrote that the stones of Stonehenge came originally from Giants’ Dance on the legendary Mount Killaraus in Ireland, and that they were brought to England by the wizard Merlin, who had defeated the Irish and stolen their stones. Stonehenge itself was erected by an army of 15 000 workers, as a memorial to the Ancient Britons murdered by the treacherous Saxons during peace talks at Amesbury. This version of events is no longer viewed as being completely accurate; the Saxons did not arrive in England until after 300 AD, long after Stonehenge was erected, and none of the stones come from Ireland. However, in Geoffrey of Monmouth's time much of southwest Wales, from where the stones have been shown to originate, was considered to be Irish territory, which has led to some speculation that the story could contain a grain of truth, with the bluestones of Stonehenge having originally stood at a site somewhere in the Preseli Hills of west Wales, and subsequently having been moved to their current position on Salisbury Plain.

In a paper published in the journal Antiquity on 12 February 2021, Mike Parker Pearson of the Institute of Archaeology at University College London, Josh Pollard of the Department of Archaeology at the University of Southampton, Colin Richards of the Archaeology Institute at the University of the Highlands & Islands, Kate Welham of the Department of Archaeology & Anthropology at Bournemouth University, Timothy Kinnaird of the School of Earth and Environmental Sciences at the University of St Andrews, Dave Shaw of Allen Archaeology Ltd, Ellen Simmons of the Department of Archaeology at the University of Sheffield, Adam Stanford of Aerial-Cam Ltd, Richard Bevins of the Department of Natural Sciences at the National Museum of Wales, Rob Ixer, also of the Institute of Archaeology at University College London, Clive Ruggles of the School of Archaeology & Ancient History at the University of Leicester, Jim Rylatt of Past Participate, and Kevan Edinborough of the Faculty of Medicine, Dentistry and Health Sciences at the University of Melbourne, describe the results of the ‘Stones of Stonehenge’ project's attempts to find the Welsh origin of the Stonehenge stones. 

The standing stones of Waun Mawn, in the Preseli Hills of Pembrokeshire, comprise four monoliths, three now recumbent, which originally stood in an arc. These were identified in 1925 as the remains of a stone circle, but subsequently have been seen as ‘doubtful or negative’ and ‘destroyed or unrecognisable’. However, recent excavations have identified bluestone megalith quarries at Craig Rhos-y-felin and Carn Goedog in the Preseli Hills as dating from 3400–3000 BC, making the Waun Mawn site a much more likely candidate for the original site of Stonehenge.

 
Location of the dismantled stone circle of Waun Mawn (red-ringed circle), as well as the bluestone sources of Carn Goedog (spotted dolerite), Craig Rhos-y-felin (rhyolite) and Cerrigmarchogion (unspotted dolerite). The locations of the Neolithic causewayed enclosure of Banc Du and palisaded enclosure of Dryslwyn (black-ringed circles), as well as Early Neolithic portal tombs (black squares), are also shown. Mike Parker Pearson in Parker Pearson et al. (2021).

The Waun Mawn site was first identified as being of interest in 2010, but subsequent magnetometer and earth-resistance surveys carried out in 2011 failed to find any evidence to support this. Thus, Waun Mawn received no further attention at that time, while several other circular monuments in the area were surveyed and excavated between 2012 and 2017, none of these proved to be Neolithic in origin.

In 2017, Parker Pearson et al. returned to Waun Mawn, carrying out excavations at either end of the arc of stones, which revealed two stoneholes without stones which had not been detected by magnetometric investigation. Further surveys using earth resistance, ground-penetrating radar, and electromagnetic induction, also failed to reveal any indication of ancient activity, which led Parker Pearson et al. that, due to the minimally magnetic and conductive properties of the substrate, only physical excavation would be able to reveal further stoneholes in the non-magnetic substrate of glacial drift deposits at the site.

 
The arc of former standing stones at Waun Mawn during trial excavations in 2017, viewed from the east. Only one of them (third from the camera) is still standing. Recumbent stone 13 is in the foreground. Adam Stanford in Parker Pearson et al. (2021).

In September 2018, Parker Pearson et al. began a new series of excavations at Waun Mawn, carrying of further work beyond each end of the arc, and opening up further small trenches to the west, south-west and south, which followed the projected circumference of the circle. The 2017 and 2018 excavations revealed a total of twelve sub-surface features, six of which were identified as stoneholes with emptied sockets from which standing monoliths had been removed, and two as stoneholes of two of the fallen stones at the ends of the arc. These holes suggest a former stone circle with a diameter of 110 m, which may have comprised as many as 30–50 stones.

 
Waun Mawn during excavation in 2018, viewed from the north. The stone circle sits on the side of the hill Cnwc yr Hˆy (‘the hillock of the deer’) at 311m OD, with distant views of Ireland to the west and the mountains of Snowdonia to the north. Adam Stanford in Parker Pearson et al. (2021).

The majority of the stoneholes comprised shallow pits, 0.80–1.20m in diameter and 0.30–0.50m deep, with a shallow ramp up to 0.50m long. These contained stone packing around an emptied socket, which had subsequently filled with sediment following the removal of each standing stone. At the base of each socket an imprint of the monolith that once stood in it can be seen, preserving each stone’s basal shape and size. The largest of these (stonehole 91) preserves a pentagonal imprint, with four of the other holes preserve square of rectangular imprints.

 
(a) Waun Mawn: the excavation trenches (in red) showing the locations of the four remaining standing stones (in red and black), the additional stoneholes (in green and black) and other features (in blue). From the centre of the circle, the midsummer solstice sun rose within the entrance formed by stoneholes 9 and 21; (b) Stonehenge stage one (beginning in 3080–2950 BC and ending in 2865–2755 BC). Stonehenge’s enclosing ditch and bank were constructed in 2995–2900 BC (at 95% probability). Kate Welham & Irene de Luis in Parker Pearson et al. (2021).

Several stone tools were uncovered at Waum Mawn, including a flint scraper, a flint chip and a trimmed, circular mudstone disc. None of this material was directly datable, but the mudstone disc was made from a type of rock accessible within Neolithic levels at the Carn Goedog megalith quarry, which is about 5 km to the east of Waum Mawn. This is fairly typical of stone circles, which can be very hard to date, due to the small amount of associated material, and the fact that what material there is tends to be of types that do not lend themselves to radiometric dating. This problem is particularly acute at Waum Mawn, where the acid soil prevents the preservation of material made from bone. A small amount of charcoal, a material amenable to radiometric dating, was recovered by sediment flotation, was recovered, but the largest of these was under 4 mm long, making it likely that this material could have been moved from its original stratigraphic position by bioturbation. To attempt to remedy this, Parker Pearson et al. combined radiocarbon dating of these samples with optically stimulated luminescence dating of the sediments from which they were recovered. Optically stimulated luminescence dating can reveal the date at which a sample was last exposed to light; this potentially gives the date of burial, but can be reset if an object is subsequently exposed, making it unreliable on its own, but providing a plausible test for radiometric dating results.

 
Stonehole 7, after removal of sediment filling the emptied socket, but with the stone packing still in place (viewed from the east). The packing stones were created from a single boulder, split into pieces before being packed against the side of the monolith. Its imprint in the base of the stonehole reveals that this monolith had a square cross-section. Mike Parker Pearson in Parker Pearson et al. (2021).

Optically stimulated luminescence dating of samples from 11 feature profiles revealed a complex depositional history, with material from several exposures, possibly relating to the emplacement and removal of the megaliths, found within the sockets. Nevertheless, the material good internal stratigraphic coherence, with material likely to have been deposited in the Neolithic or Early Bronze Age.

 
A 3D photogrammetric image of stonehole 91 after excavation of the socket left by the standing stone’s removal, viewed from the north. The imprint of this stone (in the right half of the stonehole) reveals that the base of this stone had a pentagonal cross-section. The ramp, along which the stone was erected and removed, is at the top of the picture. Adam Stanford in Parker Pearson et al. (2021).

The sediments from the holes yielded dates of between 6980 BC and 1900 AD, although the primary fills of the four sampled stoneholes, i.e. the deposits associated with the erection of the stone circle, are likely to have been deposited around 3530 BC, and the secondary fills, associated with the removal of the stones, were probably deposited around 2120 BC, although this date will not reflect the actual date of removal of the stones, but rather material that infilled the holes later, potentially at any time in the subsequent centuries or even millennia.

Radiocarbon dates were obtained from 43 charcoal samples recovered from Waun Maen, 31 from the stoneholes and 12 from other features. Most of these yielded Mesolithic dates, from the the ninth to fifth millennia BC, making them too old to be associated with the site, and were therefore discarded. Several more dates were rejected because they were too young, dating from the second and first millennia BC, i.e. the Bronze and Iron Ages. However, seven charcoal fragments, four of them from the postholes, yielded dates of 3600-3000 BC, i.e. the end of the Early Neolithic and beginning of the Middle Neolithic, dates which could plausibly relate to the construction of the site.

Such a date would place Waun Mawn amongst the earliest stone circles in Britain, alongside Long Meg and her Daughters in Cumbria (109m diameter) and the stone circle beneath the passage tomb of Bryn Celli Ddu on Anglesey (18m diameter). Radiometric dates obtained from a sample of Hazel charcoal recovered from one of the stoneholes of Long Meg and her Daughters dates to 3340–3100 BC, and cremated human remains obtained from pits associated with stoneholes at Bryn Celli Ddu have given radiocarbon dates of 3500–3100 and 3310–2900 BC, As no stone circle in Britain is currently thought to date to more than 3400 BC, Parker Pearson et al. suggest that the Waun Mawr stone circle was probably erected between 3400 and 3200 BC, and most likely towards the end of this range.

The three remaining stones at Waun Mawn are comprised of an unspotted dolerite, which may have come from the outcrops 3km to the south-east at Cerrigmarchogion on the Preseli ridge. A flake of rock left in the hole left by the standing stone with the pentagonal base is also made of this material.

 
(Left) a flake of unspotted dolerite from stonehole 91 was recovered from the junction of the empty socket and the ramp; (top right) stone 62 is one of the three unspotted dolerite pillars at Stonehenge; (bottom right) stone 62’s basal cross-section matches the imprint of the pillar that once stood in stonehole 91 at Waun Mawn. Sophie Laidler & Adam Stanford in Parker Pearson et al. (2021).

Three of the stones of Stonehenge (stones 44, 45 & 62), are also made from unspotted dolerite bluestone, and one of these (stone 62) has a pentagonal cross-section at the turf line, which is similar in shape and dimensions to the imprint in stonehole 91 at Waun Mawn. Stones 44 and 45, which are undressed and form part of the outer circle at Stonehenge, are about 2 m high, similar in size to the single standing stone at Waum Mawn, at 1.20 m, but smaller than the two fallen stones, which are about 3.20 m long; stone 62, which stands about 2 m above the ground, would probably be almost as large as the fallen stones if its full length were exposed. The stones of Waum Mawn are therefore reasonably comparable with the stones of Stonehenge in terms of size.

Two of the stoneholes at Waum Mawn show no signs of having packing stones or ramps. One of these formerly held a short monolith 1.20m long, 0.90m wide and 0.25m thick, which is now recumbent at the east end of the arc, the second holds no stone, and is about 1.3 m to the east of the first. The long sides of these stones would have been perpendicular to the circumference of the circle, rather than parallel with it, forming what Parker Pearson et al. describe as a 'gunsight' on the north-east side of the circle, which possibly served as an entrance. To somebody standing in the centre of the circle during the Neolithic period, the Sun would have risen within this entrance on the Summer Solstice. 

 
(Top) recumbent stone 013 lying beside its stonehole (9), viewed from the west. It formed the west side of the stone circle’s north-east-facing entrance. Although the top of this pillar (left) is broken off, its weathered surface indicates that this probably occurred long before the Neolithic; (bottom) stonehole 21 in half-section, viewed from the east. With its ‘gunsight’ arrangement, perpendicular to the circumference of the stone circle, the removed pillar would once have formed the east side of the north-east-facing entrance. Mike Parker Pearson in Parker Pearson et al. (2021).

With a diameter of 110 m, Waum Mawn is the joint third largest stone circle in Britain (with the outer circle at Stonehenge), behind the outer circle at Avebury, at 331 m, and Stanton Drew, at 113 m, and ahead of Long Meg and her Daughters at 109 m, the Ring of Brodgar at 104 m, and the north and south circles at Avebury, at 104 m each. The inner circle of bluestone monoliths at Stonehenge was 87 m in diameter, with the stones carefully placed 4.5 m apart. This is quite different from the situation at Waum Mawn, where the stones are spaced irregularly, and where there are gaps in the perimeter where no stones were erected, particularly on the northwest side, although whether this was always the intention or whether the circle was not completed for some reason is unclear. Either way, the construction of Waum Mawn and Stonehenge appear to have been carried out with different emphasis and perspectives, with the regularity and homogeneity seen at Stonehenge being an inovation which was not present during the construction of the (older) Waum Mawn.

There are, however, still parallels with Stonehenge, notably the sighting of the putative entrance to Waum Mawn, in line with the Summer Solstice, which corelates with the setting of Stonehenge, which is positioned at the south-west end of a geomorphological landform of parallel ridges that coincidentally align on the solstitial axis. However, Stonehenge also appears to have an entrance aligned broadly with the northernmost major Moonrise, something which does not appear to have been marked at Waum Mawn.

The common diameter of Stonehenge and Waum Mawn, combined with the match between the cross section of stone 62 at Stonehenge and hole 91 at Waum Mawn, as well as the common building material (unspotted dolerite) used at both sites, strongly hints at a connection between the two ancient monuments.

Parker Pearson et al. believe there is a strong case for at least some of the material used at Stonehenge having originated at Waum Mawn, although they doubt that the stone circle at Waum Mawn ever contained 56 standing stones, the number suggested for Stonehenge by the Aubrey Holes. It is thought that about 80 bluestones were brought from Wales to Salisbury Plane in the Neolithic, with 56 going to Stonehenge and about 25 to the nearby Bluestonehenge circle. During Stonehenge stage two (2740–2505 BC) the site is thought to have comprised a double arc of standing stones, which were rearranged into an inner and outer circle at the onset of stage 3 (beginning in about 2400–2220 BC). During this last phase all of the original stones were re-organised, and the stones which had previously stood at Bluestonehenge incorporated into the new monument.

While the geology of the Waum Mawn stones matches that of some of the Stonehenge stones, this is not the case for the majority. Only three of the 44 bluestone stones at Stonehenge today are unspotted dolerite, compared to 27 spotted dolerite stones. Thus, while some of the Stonehenge material may have come from Waum Mawn, it is likely that the majority of the 80 original stones came from elsewhere. The Alter Stone at Stonehenge clearly did not come from Waum Mawn, or anywhere else in Preseli, and is thought most likely to have cone from the Devonian Senni Sandstone, about 100 km to the east. Two other sandstone pillars are thought to be of Palaeozoic origin, and to have come from the area to the north and east of Preseli. It is quite possible that these stones once formed part of other stone circles, which were dismantled in order to provide material for the construction of Stonehenge, as were other stones at the site with different lithologies.

It is unlikely that Waum Mawn is the Giants’ Dance described by Geoffrey of Monmouth. The common features seen at Waum Mawn and Stonehenge are sufficient to suggest that the stones were moved from one site to the other by people with a common purpose, not carried off as trophies of war following a military victory.

Recent strontium isotopic analysis on 25 of the approximately 60 cremation burials from Stonehenge has shown that four of these individuals had probably lived the last decades of their lives on the Ordovician/Silurian rocks of south-west Wales, the area that includes the Preseli Hills, with the remaining remains consistent with the individuals having lived on the chalk of Salisbury Plain or on the surrounding Mesozoic strata. Bone is remodelled repeatedly throughout our lives, with all our bone being replaced over a period of about a decade. This means that it is impossible, using bone as a test material, to detect the difference between a migrant who has lived in an area for more than a decade and a local who has lived their all their lives, so it is impossible to say if any of the other burials represent individuals who had moved to the area more than ten years before dying.

It is notable that the four individuals identified as being of southern Welsh origin have all been radiocarbon dated to around the very beginning of construction at Stonehenge, at about 3000 BC, when it is thought that the Bluestones were first erected, suggesting that these individuals might have relocated to the Salisbury Plain with the stones, while later burials reflect descendants of these migrants living on local chalk. This may even have applied to livestock as well as people. A sample of dentine from the mandible of an elderly Cow found in Stonehenge’s enclosing ditch has been dated to around 3350–2920 BC, and is has a strontium isotope signature consistent with the Animal having been reared in South Wales (unlike bone, dentine is laid down shortly before a tooth erupts, an is not replaced again within an Animal's lifetime.

No material at Waum Mawn has been dated to more recent than about 3000 BC, and very few items from the Preseli region in general. This is in spite of decades of research at Neolithic sites in the area. Waum Mawn did not go on to become the core of a larger monument complex, as happened to older stone circles at sites such as the Ring of Brodgar, Avebury and Stonehenge. Instead, the site seems to have been developed as a major centre in the earlier Neolithic, but then dismantled and abandoned. It is, however, unlikely that the area was completely depopulated, and the remaining stones at the circle may have remained in use in some way.

Stonehenge appears to have been built by Neolithic migrants from South Wales, whether on their own, or in collaboration with the local population. These migrants appear to have brought the large stones of the circle with them, possibly as a physical manifestation of their ancestral identities to be recreated in their new home. This brining of sacred stones to a new, mystically significant location may have helped to unify the people of southern Britain during the first phase of activity at Stonehenge. 

Stonehenge lies upon a north–south line of henges, stone circles and cursuses (elongated parallel-sided enclosures) from the Thames Valley to the south coast of England, which broadly forms a geographic divide between different regional traditions in earlier Neolithic material culture, as well as variations in genetic ancestry between east and west, a location which has previously been cited as evidence for a unifying role for the monument.

 
The location of Stonehenge and other monument complexes of the Middle to Late Neolithic (roughly 3400–2450 BC) that may have formed a neutral zone or territorial boundary between the west and south-east of Britain. Irene de Luis in Parker Pearson et al. (2021).

The discovery of evidence for a potential migration of people and livestock from Wales to Salisbury Plain at the same time as the stones were relocated raises new questions about Stonehenge's origins and purpose. Why did these people move at this time? Was the movement driven by a change in climatic conditions, or was it driven by some economic, social or political change? Did people leave adverse conditions in Wales, or simply take advantage of some new opportunity on Salisbury Plane? Did they take over an older sacred site at Stonehenge? And if they did, was this an act of conquest, or unification (or both)?

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