Showing posts with label Bioerosion. Show all posts
Showing posts with label Bioerosion. Show all posts

Tuesday, 19 July 2022

Understanding the incorporation of older Human bodyparts into Bronze Age burials in Britain.

Improvements in dating techniques have led archaeologists to conclude that many artefacts placed in graves in Bronze Age Britain were already old when they were deposited. This possibly made them heirloom objects buried with people of significant standing, and retained till that point as an indicator of relationships to an older generation. Burials from this period also frequently include partial Human remains as well as the main burials, which raises the prospect that these objects too might have been heirloom objects prior to their burial.

Partial remains, including articulated and disarticulated fragments of bodies, are frequent occurrences in Chalcolithic and Early Bronze Age burials in Britain, and have traditionally been interpreted as evidence for the disturbance of earlier remains at sites which were re-used due to some spiritual significance. From about 2100 BC onwards, the cremation of remains prior to burial became the predominant funerary practice, although these cremation burials appear to have often only comprised fragments of the deceased, and on occasion fragments of more than one individual.

In a paper published in the European Journal of Archaeology on 25 May 2022, Joanna Brück of the School of Archaeology at University College Dublin, and Thomas Booth of the Skoglund Laboratory at the Francis Crick Institute, explore the possibility that the partial remains found in many Bronze Age British burials may be the result of the deliberate placement of these remains as artefacts in their own right, rather than the accidental disturbance of earlier remains.

A number of recent studies have found evidence of bodies in Bronze Age tombs in Britain being mummified before they were deposited. The most famous of these is the mummy from Cladh Hallan on South Uist, which was deposited beneath the floor of a Late Bronze Age/Early Iron Age roundhouse, but the practice has also been recorded from Chalcolithic and Early Bronze Age tombs. It is therefore not unreasonable to wonder if partial remains, articulated, disarticulated or cremated, from Bronze Age tombs in Britain might also have been mummified prior to their placement within the burial sites.

Brück and Booth collated 81 radiocarbon dates from fifteen graves at thirteen different locations; sixty four of these were from previous studies and seventeen are new to this publication. Of the thirteen burial sites, eleven were Chalcolithic or Early Bronze Age in origin. Eleven of the graves contained only unburnt bone, while the remaining four contained cremated remains. In the case of graves containing unburnt bones from both whole and partial bodies, the whole skeletons were used to provide an assumed date of burial, while other remains were dated separately and compared to these. In the case of graves containing cremated remains, each bone was dated separately. All but two of the graves were modern excavations, with osteological examination of the site being used to exclude the possibility of bones from the same individual being dated. Two of the sites included in the study were the subject of earlier excavations, though in both cases Brück and Booth are confident that the records provided by the archaeologists who carried out these digs were good enough to prevent double-sampling of the same individuals.

Bones which can be shown to have been old at the time of their burial could have been placed deliberately or accidentally, but the high proportion of burials in which older bones are incorporated makes accidental inclusion unlikely, as would any consistent relationship between the age of the main burial and the age of other bones included within a grave. Brück and Booth's data suggests that many of the partial remains incorporated into Bronze Age burials in Britain are about two generations older than the main burials; i.e. they had apparently been looked after elsewhere for decades, but not centuries. Other partial remains incorporated into burials showed no age difference to the main burial, but it is still possible that they were older, just not sufficiently so for the techniques used to detect the difference. 

Brück and Booth note that a high proportion of seafood in a persons diet can have an impact on the isotopic signature of their bones, but also observe that numerous previous studies have suggested that seafood was at most a very minor component of the British Bronze Age diet.

Histological examination of bones can also tell us a great deal about how they are treated after death. When individuals are buried in a dry, but well aerated, environment (such as that of most Bronze Age tombs), their bones will tend to suffer a great deal of bacterial bioerosion. On the other hand, bones of individuals who are prepared in some way prior to burial in the same environment, for example by mummification or excarnation (removal of the flesh from the bones), will show much less bone damage.

Seven of the thirteen sites examines were found to contain partial remains significantly older than the main burials. These remains all show less bioerosion than the main burials, suggesting that they were subjected to different treatment prior to their incorporation within the graves, and that this different treatment had begun immediately after death. 

At one site, Melton Quarry in East Yorkshire, the disarticulated and incomplete remains of an infant were found between the legs and torso of a complete and articulated adult. Radiocarbon dates obtained from the two sets of remains showed that the infant was in fact probably between 189 and 348 years older than the adult, and had relatively low levels of bioerosion to its bones, suggesting that the child had been stripped of their flesh shortly after death. Brück and Booth suggest that this infant may then have been kept in some sort of organic bag, possibly worn by a person, until the time of their eventual burial. 

Another example comes from a slab-lined grave found on the Cnip Headland on the Isle of Lewis. This grave contained one set of incomplete and partially articulated remains, thought to be those of an adolescent male, as well as the disarticulated remains of at least two adults. The adolescent male was placed on his right side, and it is thought that he was in an advanced state of putrefaction when he was buried; several vertebrae, the left fibula, and the right humorous were all out of position, suggesting that these parts of the body were skeletallised by the time burial occurred. There were also spaces between the head and torso, and the torso and lower body, which suggest the individual may have been in several pieces. However, the bones of the left hand are well articulated, which makes it likely that the hand was intact and fleshed at the time of deposition. The right arm, and both feet, are completely missing. The bones of this individual show little bioerosion, which may imply he was excarnated and then buried before complete skeletonisation occurred. However, a disarticulated bone from a layer depositionally below the adolescent produced a younger radiocarbon age, implying that his bones were already old when they were placed into the tomb. It is possible that the remains had been buried elsewhere and then excavated and placed in the Cnip Headland tomb, however calculations made using the OxCal radiocarbon calibration program suggest that between three and 82 years had passed between the death of the youth and his burial in the tomb, and again his bones show relatively low levels of bioerosion, suggesting that some form of treatment of the remains happened soon after death.

Cnip Headland, Isle of Lewis: plan of the partially articulated burial. Brück & Booth (2022).

Another burial of note is that at Windmill Fields in North Yorkshire. Here the primary burial is that of an adult woman, whose skeleton was articulate but heavily bioeroded, consistent with her having been buried whole and fully fleshed. In front of this individual had been stacked the disarticulated remains of another three individuals, an adult male, an adult female, and a probable adolescent female. Approximate dates were obtained from the crania of the two adults, suggesting that they are between 59 and 179 years older than the principle burial. Close to this burial was a pit with dark staining, consistent with a wooden structure such as a coffin having once been present. Further disarticulated bones were found within this structure, which were found to be of a similar age to the two crania, suggesting that this was the original location of the disarticulated remains found with the main burial.

Windmill Fields, Ingleby Barwick, Stockton-on-Tees: inhumation burial accompanied by a carefully arranged stack of disarticulated bone. Tees Archaeology in Brück & Booth (2022).

It has been known for a long time that Bronze Age burial chambers in Britain were frequently reopened to place additional remains within them, and more recently it has also become clear that remains were also removed from them. 

The South Dumpton Down site in Kent contains two complete individuals buried in the Early Bronze age, one of which is also accompanied by a detached Human mandible. This mandible was not found to be anonymously older than the two intact individuals, but showed a level of decomposition which suggests that it was removed from a body that had died some time before. It is unclear whether this implies removal from a tomb of from remains kept elsewhere, but the revisiting of burials is known to have occurred in this area. One nearby site consists of a shaft with five bodies on it, which had been deposited sequentially. Several of these bodies were missing their skulls, suggesting that pieces of the skeletons were being removed as well as new bodies added. It is quite possible that the mandible was removed from one of these bodies, although if they are less than decades older than the other burial the dating technique used would be unable to detect the age difference.

A pit at Cotswold Community near Ashton Keynes in Wiltshire yielded fragments of burned bone from both Humans and Animals, as well as pieces of beaker pottery, charcoal, burnt stone and plant remains. Dates were obtained from a fragment of Human femur and a piece of Animal bone, with the femur proving to be between five and 175 years older than the Animal. It has been suggested that such pits represent settlements, due to the presence of possible domestic contents (charcoal, burnt Animal remains, pottery fragments), but these sites could also represent locations revisited periodically, possibly for annual festivals or other such events. If that is the case then it is also quite possible that the remains, or partial remains, of the dead might have been brought to these sites, either specifically, or as part of a wider pattern of carrying portions of the dead with a semi-mobile community. 

Analysis of the locations examined by Brück and Booth suggests that certain bones from grave sites may have been predominantly chosen for redeposition elsewhere, notably long limb-bones and skulls, However, they also caution that their sample size is small, and for the most part they are reliant on data collected by other archaeologists, who were not looking for data on this topic.

The comparison of burned and unburned bone using isotope dating methods is problematic, and Brück and Booth have sought to avoid this, including only four sites with burned bone in their study. Two of these sites again produced anonymously old bones, and although Brück and Booth are less confident of these findings, this does appear consistent with the data from non-cremation burials.

The Trelowthas burial in Cornwall comprises a stone cist filled with cremated bones from numerous individuals. The site also contained an urn containing further cremated remains, from at least two individuals, which appears to have been placed their at a later date. However, analysis of the remains in the urn suggests that they were between three and 72 years older than the other remains. Brück and Booth believe this is indicative of these remains being kept elsewhere between their cremation and their eventual placement within the tomb, possibly in an environment where they would be encountered, and possibly even handled, by the living on a regular basis.

Another pit burial was found in the middle of a stone circle at Whitton Hill in Northumberland. This yielded 21.6 kg of bone from at least 24 individuals, including both adults and children. Only a single individual could be sexed, being found to be female. All of the bones here appear to have deposited in a single event, but the three bones which could be dated yielded different ages, with the older skeletons being between three and 115 and between three and 37 years older than the youngest skeleton. This has previously been interpreted as remains from an older cremation being accidentally incorporated into the deposit, although there is no evidence for the site having been revisited. Brück and Booth suggest that, in the light of evidence from other sites of a similar age, this is likely to be another example of older remains being deliberately included into a burial.

Brück and Booth present a large body of evidence for the incorporation of older remains into Bronze Age burials (and in particular Early Bronze Age) in Britain. Some of these appear to have been recovered from other grave sites specifically for re-internment within the new burial, but others show signs of practices such as mummification and excarnation not seen in primary burials, strongly suggesting that these bodies were treated differently from the time of death. Brück and Booth strongly suspect that this may have involved carrying portions of the dead with the living, possibly as a form of personal ornamentation, a practice known from some modern Human groups.

Brück and Booth suggest that the incorporation of older Human remains into Bronze Age burials is likely to indicate kinship between these remains and the principle occupiers of the graves, given that the occupants of other multi-occupancy grave-sites from the same period are typically related. 

This can be confirmed for one site, the Boscombe Bowmen burial at Boscombe Down in Wiltshire. Here several sets of adult and child remains were found together, with one adult male being accompanied by a bundle of bones representing the partial remains of at least four other individuals; two adult males, a subadult male and a juvenile. The bones in the bundle were principally long bones (i.e. arm or leg bones) from the left side of the body, but the skeleton also had two further crania and a partial mandible at his feet. DNA was recovered from both the male skeleton and one of the skulls (an adult male, 25-30 years old), enabling comparison of the relationships between these two individuals. This revealed that this two came from different lineages on their maternal side, but paternally were related, potentially being half-siblings, cousins, an uncle (or great uncle) and nephew, or grandfather (or great grandfather) and grandson. Unfortunately this skull was not radiocarbon dated, but one of the bones from the bundle, a femur, was, and was found to be significantly older than the intact skeleton.

The Boscombe Bowmen, Wiltshire: plan. Wessex Archaeology in Brück & Booth (2022).

Furthermore, strontium isotope analyses of tooth enamel from the skeleton and the two detached skulls suggests that all three undertook similar childhood journeys (strontium isotopes in water vary with local geology, and are incorporated in tooth and bone, providing a record of where people have lived). This implies that either all three had undertaken a the same journey as children, presumably together as living contemporaries, or all three had lived at a similar location away from the burial site, and been transported to that site after death. Either case would imply that the individuals involved shared some measure of shared life-history as well as a genetic relationship, and that this is likely to be reflected in the decision to bury them together.

Next Brück and Booth examined the age gap between partial remains found within graves and the principal occupants of those graves, finding that the principle occupants were on average about 95 years younger than the partial remains incorporated into their burials. They suggest this may represent a rough upper limit on the cultural memory of these older individuals as living people.

Not all ancient skeletons can be confidently sexed, particularly when dealing with partial remains, but of the partial remains incorporated into younger burials which could be identified, four were male and two female, implying that gender was not considered important when selecting remains for this purpose. Therefore, if these burials do represent the inclusion of remains of significant relatives with the recently deceased, then perception of who was a significant relative was apparently not related to gender.

The age of these older relatives also appears relatively unimportant. Of the examined remains for which an age could be determined, four were adults, one a subadult, one an adolescent, and one was an infant aged 2-4 months. Views on who represents an adult are known to have changed significantly over written history, making it unlikely that the views of Bronze Age Britons were identical to those of their modern descendants, but this is unlikely to have included babes-in-arms, making it plausible that age at death was unrelated to the status of individuals, when determining significant relationships.

Brück and Booth also note that perceived kinship is unrelated to biological relationships, which may present difficulties when establishing relationships between individuals within ancient burials.

Finally, Brück and Booth mention the burial of an adult male at Wilsford in Wiltshire, dated to between 1950 and 1970 BC, who was found to have among his grave goods a whistle made from a Human femur. It has been suggested that this was the grave of a shaman or other ritual specialist. Brück and Booth were able to date the whistle from this grave, finding it was not significantly older that the skeleton it was buried with, and therefore that the two individuals could have been known to one-another in life, as well as having both been known by the people who placed the whistle in the grave with the body. Though the relationship between the two individuals could not be determined, there are clearly other possibilities than the two being relatives, for example the whistle could have been made from the bone of another ritual specialist, possibly a previous holder of the same role within the community, or a person perceived as an enemy by the deceased or whole community.

Bronze Aged peoples are known to have valued goods made from certain materials, such as jet or amber, and treated these as being significant and powerful items. It is not an unreasonable supposition that goods made from Human remains would be seen as being similarly significant, and that the ownership of such items may have conferred social status. The incorporation of Human remains into burials as grave goods may also be an indication of status, although this is difficult to unravel from funeral practices intended to reflect ritual, familial, or emotional relationships between the dead. Brück and Booth note that the tendency to finally deposit relics made from Human remains at about the time when the individuals from whom these were made would have been disappearing from the collective memory of the group may also be significant.

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Saturday, 22 January 2022

Languidipes lithophagus: A rock-boring Mayfly from the Bago River system of Myanmar.

Rock-borings in marine environments have been extensively studied, both in modern and fossil environments, with the organisms which make these borings known to include Molluscs, Crustaceans, Polychaete and Sipunculid worms, Sea Urchins, Sponges, and Bryozoans. These organisms are considered to be major shapers of their environments, extensively modifying rocky marine substrates and playing an important role in coastal erosion and the evolution of coastal profiles. In contrast to this, freshwater rock-borers have been very little studied. Most known freshwater rock-borers are members of marine rock-boring groups which have adapted to the freshwater environment. For example, bioerosion in the shells of freshwater Molluscs in Southeast Asia, India, and Sri Lanka is known to be caused by Caobangiid Worms, which are Sabellid Polychaetes. Freshwater Mollusc shell borings are also known to be caused by endolithic Cyanobacteria in North America and Argentina, and shell borings of unknown origin have been reported in subfossil and recent freshwater Bivalve shells from North America. Macroborings in rocks from freshwater environments are well recorded in the fossil record, predominantly from calcareous hardground environments, although these are less well studied than their marine counterparts. 

The first modern Invertebrate boring into silicate rocks in a freshwater environment was not discovered until 2018, when the Pidock (rock boring Bivalve) Lignopholas fluminalis was discovered in the middle reaches of the Kaladan River in Myanmar. This species had previously been recorded from estuarine environments with variable salinities, but had not been recorded in a fully freshwater environment till this time. In 2019 a second Bivalve, the Shipworm Lithoredo abatanica, was found boring into limestone in the lower reaches of a small river on the Bohol Island, the Philippines. This species does appear to be found exclusively in freshwater environments, although its closest relatives are all marine. The larvae of a number of Insect species, including Mayflies, Ephemeroptera, Nonbiting Midges, Chironomidae, and Caddisflies, Trichoptera, have been reported burrowing into firmgrounds such as claystones, sandstones, shales, and marls.

In a paper published in the journal Materials Degradation on 20 January 2022, Ivan Bolotov and Alexander Kondakov of the Laverov Federal Center for Integrated Arctic Research oVasily Yapaskurtf the Ural Branch of the Russian Academy of Sciences a d the Northern Arctic Federal University, Grigory Potapov, also of the Laverov Federal Center for Integrated Arctic Research, Dmitry Palatov of the Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences, Nyein Chan of the Myanmar Programme of Fauna & Flora International, Zau Lunn, also of the Myanmar Programme of Fauna & Flora International, and of the Biology Department at the University of New Brunswick, Galina Bovykina, also of the Laverov Federal Center for Integrated Arctic Research and the Northern Arctic Federal University, Yulia Chapurina, Yulia Kolosova, Elizaveta Spitsyna, Vitaly Spitsyn, and Artyom Lyubas, again of the Alexander Kondakov of the Laverov Federal Center for Integrated Arctic Research, Mikhail Gofarov and Ilya Vikhrev, again of the Federal Center for Integrated Arctic Research and the Northern Arctic Federal University,  of the Faculty of Geology at the Lomonosov Moscow State University, Andrey Bychkov, also of the Faculty of Geology at the Lomonosov Moscow State University, and of Vernadsky Institute of Geochemistry and Analytical Chemistry at the Russian Academy of Sciences, and Oleg Pokrovsky of Geosciences and Environment Toulouse, and the BIO-GEO-CLIM Laboratory at Tomsk State University, describe a new species of Mayfly with rock-boring larvae from the Bago River basin of Myanmar.

Bioerosion was observed in siliceous rocks at two locations on the Bago River, one on the middle reaches and one on the upper part, and both where the water was freshwater and free from tidal influence. The site on the middle reaches of the river was about 40 m above sealevel, where the river crosses a continuous outcrop of Miocene siltstone. Borings can be seen on the right bank, and across the river bottom.

 
Study area and sampling localities. (a) Map of Myanmar with location of the study area. (b) Map of the Bago River basin (light red filling). The red stars indicate freshwater macrobioerosion sites. Bolotov et al. (2022).

The borings are elongate tunnel-like structures, penetrating 20-30 mm into the rock, oriented horizontally, sharing two circular end openings (apertures) at the outer surface. Many of the borings contained Mayfly larvae, others were empty, and often filled with clay. The borings were lined with a silky material. The rock surface was covered by elongate groves, apparently formed by the erosion of burrows. These groves extended onto the surface of the youngest river terrace, above the level of the present floodplain.

 
Examples of macrobioerosion in the Bago River, Myanmar. (a) Right shore with continuous siltstone outcrop at the middle reaches of the river. The red arrows show the location of submerged rocks with bioerosion traces. (b) Submerged part of the siltstone rock outcrop (indicated by the red arrows) at the middle reaches of the river. (c) Emerged siltstone rocks with bioerosion traces (indicated by the red arrows) at the upper reaches of the river. (d) Close-up view of siltstone outcrop with multiple bioerosion traces at the upstream section of the river. (e) Siltstone fragment densely covered by sub-recent bioerosion traces from the upper reaches of the river (plan view). The yellow arrows show crusts of a freshwater Sponge species. (f) Siltstone surface with a dense network of sub-recent bioerosion traces from the upper reaches of the river (plan view). The blue arrows show the macroborings filled with clay. The green arrow shows a pea clam in a bioerosion groove (Bivalvia: Sphaeriidae). Scale bars are 10 mm (e(, (f). Mikhail Gofarov & Ilya Vikhrev in Bolotov et al. (2022).

The upper section of the river is 195 m above sealevel, and cuts through Miocene clays, with outcrops of siltstone rock. These were exposed during the dry season, when the site was visited, but would have been covered in the rainy season when waters were higher. The surfaces of these rocky outcrops were completely covered by grooves and borings similar to those seen at the lower site. These burrows were mostly abandoned, although some contained dead Insect larvae, Eroded burrows had often been occupied by an unknown freshwater Sponge. Some burrow-bearing surfaces were covered by clay, suggesting the burrows on them were of some age.

 
Siltstone rock substrate with borings and living nymphs of the rock-boring Mayfly species Languidipes lithophagus from the middle reaches of the Bago River, Myanmar. (a) Rock fragment with borings (lateral view). (b) Rock fragment showing apertures (circular openings) and partly eroded borings (plan view). (c) Rock surface with apertures and partly eroded abandoned borings. (d) Living nymphs in their borings. (e) Living nymph in its boring. The white arrow shows a fragment of a silky substance covering the inner side of the boring. Scale bars are 10 mm (a), (c), 20 mm (b), and 5 mm (d), (e). Ilya Vikhrev in Bolotov et al. (2022).

Genetic analysis of the larvae confirmed that they belong to the genus Languidipes, but not within either of the two species currently assigned to that genus; Languidipes corporaali, which has wood boring nymphs and which has been recorded from Indonesia, Malaysia, and Thailand, and Languidipes taprobanes, which is known from India and Sri Lanka, and which also has wood-boring nymphs. A third, undescribed, species of Languidipes is also known from a single female specimen collected in Assam, India, over a century ago. The genus Languidipes was split from the African wood-boring genus Povilla in 1984, upon the basis of morphological features; Bolotov et al.'s genetic study confirmed that these genera are related, but clearly distinct.

 
Maximum likelihood phylogeny of boring and burrowing Mayflies (Ephemeroptera) based on the COI gene sequences. Bolotov et al. (2022).

The new species is named Languidipes lithophagus, where 'lithophagus' means 'rock-eater'. The description is based entirely upon mature nymphs, with the adults being unknown. These are 11.3–17.9 mm in length, with a head semicircular in shape when seen from above, with bristles on its labrum (upper lip) and mandibles. The abdomen is also covered in setae (bristles), and the limbs are biramous (branching), with the upper branches forming gills, as in many Crustaceans. The nymphs are pale yellow or white in colour, with two brown stripes on the upper part of the head, and black mandibles. 

 
Nymph of the rock-boring Mayfly species Languidipes lithophagus from Bago River, Myanmar. (a) Holotype RMBH N54_8 (dorsal view); (b) holotype (lateral view); (c) left and (d) right mandible (dorsal view). Scale bars are 5 mm (a), (b) and 1 mm (c), (d). Elizaveta Spitsyna in Bolotov et al. (2022).

The nymphs are filter feeders, obtaining food from water pumped through their burrows. The species is restricted to the Bago River Basin, which is impacted by a variety of anthropogenic loads arising from agricultural and urban land use, deforestation, sewage inputs, garbage littering, and general habitat degradation, causing Bolotov et al. to suggest that it may be Endangered under the terms of the International Union for the Conservation of Nature’s Red List of Threatened Species.

 
Morphological features and neoichnological model of the rock-boring Mayfly species Languidipes lithophagus traces from the middle section of the Bago River, Myanmar. (a) Close-up view of the apertures (plan view). The white arrows show minute fractures tracing the upper layer of the borings. The yellow arrows show larger fractures corresponding to the initial stage of rock erosion leading to the origin of grooves (i.e. borings, the upper layer of whose was lost). (b) Plan view of a groove with concave bottom originated via partial erosion of the boring. The groove bears fragments of a silky substance produced by the tracemaker (shown by white arrows). (c) Transverse cross-section of the borings showing their circular tunnel-like shape. Neoichnological model of the boring: lateral view of the longitudinal section (d) and plan view of the two apertures at the rock surface (e). Orange filling indicates the boring. Scale bars are 10 mm (a) and 5 mm (b)–(e). Artyom Lyubas and Ivan Bolotov in Bolotov et al. (2022).

As far as Bolotov et al. are aware, this is the first recorded instance of freshwater Insect larvae tunnelling into hardground substrates anywhere in the world; burrowing into firmground substrates has been recorded, but such substrates soften when wet, unlike to rocky siltstones of the Bago River Basin. Languidipes lithophagus also appears to be the first known rock-boring freshwater Animal not descended from a rock-boring marine Animal anywhere in the world, with both previously recorded species with this habit being Bivalves belonging to primarily marine groups. 

 
Siltstone outcrop at the midstream bioerosion site of the Bago River. The insert illustrates a rock surface with bioerosion traces of the rock-boring mayfly species Languidipes lithophagus  (indicated by the yellow arrow) at the youngest (sub-recent) river terrace. Mikhail Gofarov in Bolotov et al. (2022).

The relatives of Languidipes lithophagus do, however, do share some ecological similarities with the relatives of the two Bivalve species. The other members of the genus Languidipes bore into wood and bamboo, as do the closest relatives of Lignopholas fluminalis, which comes from a group of primarily wood-boring Piddocks (most Piddocks are rock-boring), and the Shipworm Lithoredo abatanica (almost all Shipworms are wood-boring). The shift from wood-boring, which requires considerable mechanical strength, to rock boring, which requires even more, is plausibly easier than switching directly from free-living to wood-boring.

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