Sunday, 27 September 2020

Sixteen fatalities caused by mine fire in Chongqing, China.

Sixteen miners have died and one is being treated in hospital following a fire at a coal mine in the Municipality of Chongqing in southwest China on Sunday 27 September 2020. The fire is understood to have broken out at the Songzao Mine on a conveyer belt used to carry coal from the excavation front to the surface. This ignited coal dust in the mine, which is more or less pure carbon, which reacted with oxygen in the atmosphere to produce deadly carbon monoxide gas (carbon usually produces carbon dioxide when it burns, but within coal mines, where there is abundant carbon, in a powdered form, and a limited supply of oxygen, then incomplete combustion often occurs, resulting in the formation of the much more deadly carbon monoxide). The cause of the incident is still being investigated, but it is understood that the mine had previously been fined for breached of safety regulations.

 
Rescue workers at the Songzao Coal Mine in Chongqing on Sunday 27 September 2020. Associated Press.

Coal is formed when buried organic material, principally wood, in heated and pressurized, forcing off hydrogen and oxygen (i.e. water) and leaving more-or-less pure carbon. Methane is formed by the decay of organic material within the coal. There is typically little pore-space within coal, but the methane can be trapped in a liquid form under pressure. Some countries have started to extract this gas as a fuel in its own right. When this pressure is released suddenly, as by mining activity, then the methane turns back to a gas, expanding rapidly causing, an explosion. This is a bit like the pressure being released on a carbonated drink; the term 'explosion' does not necessarily imply fire in this context, although as methane is flammable this is quite likely.

Coal is also comprised more or less of pure carbon, and therefore reacts freely with oxygen (particularly when in dust form), to create carbon dioxide and (more-deadly) carbon monoxide, while at the same time depleting the supply of oxygen. This means that subterranean coal mines need good ventilation systems, and that fatalities can occur if these break down.

China gains 70% of its energy from coal-burning power stations, which places the country under great pressure to maintain coal supplies. This has led to a poor safety record within the mining sector, particularly in the private sector, where there is a culture of seeking quick profits in poorly regulated (and sometimes officially non-existent) mines.  However, the Chinese authorities have been making efforts to remedy this situation, introducing safety regulations and closing (or at least attempting to close) mines that fail to comply. Annual deaths in Chinese mines have steadily fallen from 6995 in 2002 to 316 in 2019.

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Magnitude 2.7 Earthquake shakes Cape Town.

The South African Council for Geosciences recorded a Magnitude 2.7 to the northeast of the city of Cape Town in Western Cape Province, slightly after 8.40 pm local time (slightly after 6.40 pm GMT) on Saturday 26 September 2020. There are no reports of any damage or casualties associated with this event, though many people reported feeling it in the Cape Town area. The city was hit by a second, Magnitude 2.3 event, slightly after 9.10 am local time (slightly after 7.10 am GMT) the following morning, again causing no damage or injuries, but causing considereable consternation in the usuall Earthquake-free city.

 
The approximate location of the 26 September 2020 Cape Town Earthquake. USGS.

Earthquakes of any size are relatively rare in South Africa and because of this rarity it is hard to make precise judgements about the cause of quakes in South Africa, due to a paucity of data. Northwestern South Africa is close the southern end of the Great Rift Valley exits the continent and passes out under the Indian Ocean on the coast of Mozambique. The Great Rift Valley is slowly splitting the African Plate in two allow a line from the Red Sea through Ethiopia, and which includes the great lakes and volcanoes of east-central Africa. This has the potential to open into a new ocean over the next few tens of millions of years, splitting Africa into two new, smaller, continents; Nubia to the west and Somalia to the east. However, this is a long way from Cape Town, and while Earthquakes are occasionally recorded in the Western Cape, the reason for these are not entirely clear.

 
Movement on the African Rift Valley, with associated volcanoes. Rob Gamesby/Cool Geography.

Witness statements can help geologists to understand Earthquakes and the geological processes that cause them; if you felt an Earthquake in South Africa  you can report it to the South African Council for Geoscience here.

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Bhavania annandalei: The identity and distribution of a a Hillstream Loach endemic to the Western Ghats of India.

The Hillstream Loach, Bhavania annandalei, was described by Sunda Lal Hora in 1920 from Tenmalai, in what was then Travancore State (current day southern Kerala), and suggested that the species occurs throughout the southern Western Ghats in the Nilgiris, Malabar, and Travancore. Hora diagnosed Bhavania annandalei from its only known congener, Bhavania australis by a combination of characters; the most prominent of which included a broad snout (rather than a pointed one), interrupted lower lip (as opposed to a continuous one), caudal-lobes equal (rather than a longer lower lobe), and presence of a pair of papillae on the lower lip (rather than thier absence). Hora’s description of Bhavania annandalei was however, based on a single adult female specimen collected by Thomas Nelson Annandale from Travancore, Kerala. Though, Hora (1920) seemed to have access to additional juvenile specimens collected by Captain Robert Sewell from the Nilgiris (Cherambadi) and Wayanad (Nellimunda, Mananthavady, and near Vythiri), he did not examine them or provide other details. Subsequently, Hora extended the distribution of the species to Mysore, based on four specimens collected by M.S. Bhimachar from a stream between Kottigehar and Balehonnur (in what was then Mysore State, i.e. the current day Tunga River System in Karnataka). No details of the specimens were provided. 

In his review on ‘Homalopterid fishes from Peninsular India’, Hora synonymized Bhavania annandalei with Bhavania australis, after examining specimens from throughout its distribution range including Kallar/South Travancore (current day Vamanapuram River, Kerala); Pampadumpara/North Travancore (current day Periyar River, Kerala); Sethumadai Hills/Mysore (current day Anamalai hills near Pollachi, Tamil Nadu); and Kottigehar/Mysore (current day Tunga River, Karnataka), and realising that his description of Bhavania annandalei was based mainly on immature specimens. This synonymy was subsequently adopted by Ambet Gopalan Kutty Menon in his review of the Homalopterid Loaches of India, but without examining the type (or fresh topotypes) of Bhavania annandalei, or the topotypes of Bhavania australis. Later workers followed this synonymy and considered Bhavania to be monotypic. ‘Bhavania arunachalensis’, described in 2007 from Naodhing drainage in Arunachal Pradesh, is considered to be a species of doubtful identity and uncertain placement, and is most likely a species of the genus Balitora

Given their hill-stream adaptations (widespread paired fins, flattened ventral surfaces with body suckers and rasping mouths on their ventral surface allowing them to firmly grasp rock or gravel surfaces necessary in the mountain torrents), and the fact that the type locality of Bhavania annandalei (Tenmalai) and Bhavania australis (Walayar) are at least 300 km apart and separated by two significant biogeographic barriers - the Palghat Gap and the Shencottah Gap, it is highly unlikely that the two are conspecific.

In a paper published in the journal Threatened Taxa on 26 July 2020, Remya Sundar of the Center for Aquatic Resource Management and Conservation at the Kerala University of Fisheries and Ocean Studies, VK Anoop and Arya Sidharthan of the School of Ocean Science and Technology, also at the Kerala University of Fisheries and Ocean Studies, Neelesh Dahanukar of the Indian Institute of Science Education and Research and the Zoo Outreach Organization, and Rajeev Raghavan, also of the Center for Aquatic Resource Management and Conservation and School of Ocean Science and Technology, and of the Department of Fisheries Resource Management, at the Kerala University of Fisheries and Ocean Studies, present a re-description of Bhavania annandalei, based upon newly collected specimens.

Six specimens of putative topotypic Bhavania annandalei were collected from Palaruvi falls at Tenmala (Kallada River), Kerala, and six specimens of putative topotypic Bhavania australis were collected from near the Kavarakund falls, upstream of Malampuzha Reservoir, Kerala, India. Samples were collected using a hand net/scoop net during early morning hours, fixed in 10% formalin and transferred to 70% ethanol for permanent voucher storage in the museum collections of the Kerala University of Fisheries and Ocean Studies. Gill tissues were obtained from fresh specimens and preserved in absolute ethanol.

 
Collection localities of putative topotyes of Bhavania annandalei and Bhavania australis. Sundar et al. (2020).

Bhavania annandalei is distinguished from its only known congener Bhavania australis by a combination of characters: low density and sparsely distributed tubercles on dorsal surface of head, especially on operculum, (whereas Bhavania australis has a high density of tubercles on dorsal surface of head and operculum); gape of mouth comparatively farther from snout tip, as a result the rostral barbels reaching anterior border of upper lip, (in Bhavania australis the gape of mouth is closer to snout tip, and rostral barbels reaching posterior border of upper lip); rostral flaps between the rostral barbels fleshier than in Bhavania australis; fewer post-dorsal scales (34–36 compared to 38–41); fewer scales above the lateral line (11–12 compated to 14–15); and caudal peduncle stout with its depth to width ratio 1.8–2.3 (Bhavania australis has a laterally compressed caudal peduncle with depth to width ratio 2.8–3.6).

 
Putative topotypes: (a) Bhavania annandalei; (b) Bhavania australis in life (specimens not preserved). Sundar et al. (2020).

The body of Bhavania annandalei is elongate, dorso-ventrally depressed anteriorly, laterally compressed posteriorly; dorsal profile convex, deepest at dorsal-fin origin. Body wider than its depth at dorsal-fin origin, deeper than wide at anus. Head small, rounded, less than one-fourth of standard length; depressed, longer than broad, with minute sparsely distributed indistinct tubercles on dorsal surface of head. Eyes small, dorso-laterally positioned, not visible from underside of head. Snout pointed in lateral view, round in dorsal view. Nostrils positioned dorsally, closer to anterior border of eye than to snout tip, anterior nostril situated inside a skin flap covering the posterior nostrils. Mouth inferior. Lips fleshy. Gape of mouth less than three times maximum head width. Barbels three pairs, two rostral: outer rostral barbels shorter than inner ones; one pair of maxillary barbels, situated slightly anterior to the angle of mouth. Three fleshy rostral flaps interspaced between rostral barbels. Gill opening small, restricted above the base of the pectoral fin.

 
Dorsal, lateral, and ventral images of putative topotypes: (a) Bhavania annandalei; (b) Bhavania australis. Sundar et al. (2020).

Body with scales except chest and belly. Lateral line complete, with 68–72 small scales. Caudal peduncle slender, its length almost three times its depth. Dorsalfin originating slightly behind the pelvic-fin origin, closer to tip of snout than to caudal-fin base; with two unbranched, followed by seven branched and a simple ray. Pectoral fin elongated, longer than head, with six unbranched, followed by 10 branched and a simple ray. Pelvic-fin length almost equal to head length; fin origin closer to snout tip than to end of caudal peduncle, its posterior end not reaching anus, with two unbranched and eight branched rays. Anal fin with two unbranched and five branched rays. Caudal fin forked, with 19 principal rays. 

Dorsal and ventral view of head: (a), (c) Bhavania annandalei; (b), (d) Bhavania australis. Sundar et al. (2020).

In life Bhavania annandalei is body is chestnut brown on dorsal and lateral sides, creamish-white on chest and belly; 3–4 prominent broad dark brown ventral bands; two broad ventral bands on the dorsalfin base. There are three black-coloured bands on the dorsal fin, 6–7 bands on the pectoral, three bands on the pelvic, 1–2 bands on the anal, and four bands across the caudal fin.

Morphometric analysis is a tool used by palaeontologists, archaeologists, anthropologists and forensic pathologists to analyse and compare specimens. It relies on taking numerous measurements of an object such as a bone or shell, and comparing both these measurements and ratios between measurements to those obtained from other specimens in order to establish relationships between them. 

Morphometric measurements were taken for 37 characters (measured to the nearest 0.1mm using digital calliper) and meristic values were determined for 10 characters using a stereo-zoom microscope. For meristic counts, values in parenthesis after the count respresent its frequency. For statistical analysis of morphometric data, subunits of body were taken as percentage of standard length and subunits of head were taken as percentage of head length. Principal component analysis was performed to check whether the two species formed distinct clusters in multivariate space using correlation matrix. Null hypothesis that the clusters are not significantly different from each other was tested using analysis of similarities employing Euclidian distances and 9999 permutations. Statistical analysis was performed in PAST 4.02.

Using size-adjusted characters, the two species clustered separately on the first two principal component analysis axes. The clusters were significantly different from each other, indicating that the species formed distinct clusters in multivariate space. While lengthlength relationships for most characters showed similar trends for both the species, there were two relationships that showed marked differences. Length-length relationship between caudal peduncle depth and width suggested that width increased rapidly with increasing depth in the case of Bhavania annandalei compared to Bhavania australis. Similarly, length-length relationship between head length and head depth at nape suggested that head depth increased rapidly with increasing head length in the case of Bhavania annandalei compared to Bhavania australis.

Genetic sequences of mitochondrial partial cytochrome oxidase subunit 1 (cox1) of topotypic Bhavania annandalei and Bhavania australis were obtained from the collected specimens. Additional sequences were downloaded from GenBank database. Gene sequences were aligned using MUSCLE and raw genetic distance was estimated using MEGA 7. Data were partitioned into three codon positions of cox1 gene. Partition analysis (a statistical method) and ModelFinder were used to find the right partitioning scheme and nucleotide substitution model for the partition scheme employing minimum Bayesian information criterion. Maximum likelihood analysis was performed in IQ Tree with best partition scheme and ultrafast bootstrap support for 1000 iterations. Phylogenetic tree was edited in FigTree v1.4.2.

Partition analysis and model selection identified separate nucleotide substitution models for all three codon positions, TNe+I for first codon, F81+F for second codon, TN+F+G4 for third codon position of cox1 gene. Maximum likelihood phylogenetic tree based on best partition scheme and model selection recovered Bhavania annandalei and Bhavania australis as a clade sister to Southeast Asian congeners of Balitoridae. Topotypic Bhavania annandalei (MT002520) differed from topotypic Bhavania australis (MT002518) with a raw genetic distance of 6.4% in the cox1 gene.

 
Maximum likelihood phylogenetic tree based on mitochondrial cytochrome oxidase subunit 1 gene using best partition scheme and model selection (lnL of consensus tree = -2631.97). Indoreonectes keralensis (Nemacheilidae) is used as an outgroup. Values along the nodes are percentage bootstraps based on 1,000 iterations. Sundar et al. (2020).

Bhavania annandalei is known with certainty from the Kallada, Vamanapuram, and Neyyar river systems in southern Kerala, India. These river systems drain the western slopes of the Agasthyamalai Hill ranges, south of the Shencottah Gap. It is highly likely that the species also occurs on the eastern slopes of the Agasthyamalai Hills particularly in the Tambaraparini River system in Tamil Nadu, but detailed surveys and voucher specimens are required to confirm this. In this context, Sundar et al. believe that previous records of Bhavania australis from several tributaries of the Tambaraparini, Manimuthar, and Chittar draining the eastern slopes of the Agasthyamalai, could most likely represent Bhavania annandalei.

The density of chromatophores in Bhavania is likely to be dependent on the micro-habitat as well as the colour and type of substratum it inhabits. Other ecological factors that may influence body colour are forest/canopy cover, intensity of light, turbidity, water flow and water temperature. This is reflected in the different body colours shown by the two species in different habitats and locations, an observation which was also made by Sunda Lal Hora. 

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Saturday, 26 September 2020

Eight-year-old girl killed by Crocodile in Uttarakhand.

An eight-year-old girl has died in a Crocodile attack in Haridwar District of Uttarakhand State India on Friday 25 October 2020. The girl identified as Radhika, from the village of Kudi Baghwanpur, had gone with her sister to collect flowers, when she was snatched by the Animal, which emerged from an overgrown waterway. and dragged her in. A search by divers and local police recovered the girls body, which had visible Crocodile bires on one arm, but were unable to find the Crocodile itself.

 
Forest department officials and police officers searching an overgrown waterway for an eight-year-old girl who was snatched by a Crocodile on Friday 25 September 2020. Hindustan Times.

There are three species of Crocodilians found in India, the Saltwater Crocodile, Crocodylus porosus, the Mugger Crocodile, Crocodylus palustris, and the Gharial, Gavialis gangeticus. Of these the Gharail is unlikely to be found in a waterway of this type, and is generally a Fish-eater, not prone to attacking Humans.  Muggers are smaller that Saltwater and Nile Crocodiles, with males reaching about 3.5 m and females about 2.5 m, and are generally less considered less dangerous, as their preferred prey is animals smaller than Humans. This can, however, sometimes include children, who are at risk.

 
A Mugger Crocodile, Crocodylus palustris. Paul Asman and Jill Lenoble/Flikr/Wikimedia Commons.

Mugger Crocodiles were formerly found across South Asia from Iran to Myanmar and Nepal to Sri Lanka, but they are thought to be extinct in Myanmar, Bhutan, and Bangladesh, and the species is considered to be Vulnerable under the terms of the International Union for the Conservation of Nature’s Red List of  Threatened Species. Attacks such as the one in Haridwar District this week can lead to  reciprocal actions by local villagers, with regular reports of Crocodiles being killed (illegally) in the area. This conflict appears to have been driven by rising Human populations in the area, which means that there are more Humans living close to the Crocodiles, and that there is more water being taken from the river for domestic and industrial use, reducing the available habitat for Crocodiles and their main food source (Fish).

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Comet 88P/Howell reaches perihelion.

Comet 88P/Howell reaches its perihelion (the closest point on its orbit to the Sun) at 8.24 pm GMT on Monday 28 September 2020, when it will be approximately 1.36 AU from the Sun (i.e. 136% of the distance between the Earth and the Sun, and some way inside the orbit of Mercury). At this time the comet will be 1.40 AU from the Earth, in the constellation of Scorpius, having a magnitude of 15.58, making it impossible to spot without a fairly good telescope, and not visible north of the Tropic of Cancer.

 
Image of Comet 88P/Howell taken from Valencia in Spain on 5 September 2020. José Chambó/Cometografía.

Comet 88P/Howell was discovered by American astronomer Ellen Howell on 29 August 1981; the name 88P/Howell implies that it was the 88th comet discovered, that it is a periodic comet, and that it was discovered by Ellen Howell.

 
The orbit and current position of Comet 88P/Howell. In The Sky.
 
Comet 88P/Howell has an orbital period of 2003 days (5.48 years) and a highly eccentric orbit tilted at an angle of 4.38° to the plain of the Solar System, that brings it from 1.40 AU from the Sun at perihelion (140% of the distance between the Earth and the Sun, and slightly inside the orbit of the planet Mars); to 4.86 AU from the Sun at aphelion (4386 times as far from the Sun as the Earth or slightly inside the orbit of the planet Jupiter). As a comet with a period of less than 20 years with an orbit angled at less than 30° to the plane of the Solar System, 88P/Howell is considered to be a Jupiter Family Comet.
 
Calculations have shown that 88P/Howell formerly orbited on a path further than the Sun, with a perihelion of 1.9 AU (i.e. it was 1.9 times as far from the Sun as the planet Earth its closest), until a close encounter with the planet Jupiter in 1978 shifted it onto its current course. The comet has regular close encounters with Jupiter, with the last having happened in June 2002, and the next predicted for May 2061. The Comet is also predicted to come close to the planet Mars in September 2031. It is predicted that another close encounter with Jupiter in October 2168 will cause it to shift its orbit again, so that moves onto a new orbital path that will bring it close to the Earth, with close encounters between 88P/Howell and the Earth predicted in July 2170, July 2181, and July 2192. 

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Friday, 25 September 2020

Palaeotreta shannanensis & Palaeotreta zhujiahensis: Brachiopods from the Early Cambrian Shuijingtuo Formation of South China.

Linguliform Brachiopods first appeared at the beginning of the Cambrian, and they represent one of the key components of benthic communities during the Cambrian evolutionary radiation. The Linguliform order Acrotretida, which is uniquely characterised by a diminutive shell size and coniform ventral valve, had a relatively late origination, but subsequently diversified rapidly and became extremely abundant with a cosmopolitan distribution during the Cambrian and Ordovician. However, many questions concerning their early origin, divergence and phylogenetic relationships with other Brachiopod groups remain uncertain. To date, only six Acrotretide genera, including Eohadrotreta, Linnarssonia, Prototreta, Vandalotreta, Kuangshanotreta and Hadrotreta, are known from Cambrian Series 2. Recently published works show that Eohadrotreta rapidly achieved a wide distribution over key palaeocontinents (South China, East Gondwana, West Gondwana, North China), soon after their first appearance in Cambrian Epoch 2. Two species of Eohadrotreta (Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis) were originally described from South China, but the diagnostic features to discriminate these taxa were not clearly defined, making it difficult to evaluate the biostratigraphy and phylogeny of this important genus. A subsequent detailed quantitative analysis of Brachiopod ontogeny demonstrated that the developmental pattern of the metamorphic shell was quite different in these two species, revealing heterochrony in the ontogeny of the earliest acrotretide representatives during the Cambrian radiation. Numerous morphological differences between Eohadrotreta? zhujiahensis and Eohadrotreta zhenbaensiss indicate that they may belong to separate genera.

In a paper published in the Journal of Systematic Palaeontology on 7 August 2020, Zhiliang Zhang of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, and Department of Geology at Northwest University, the Department of Biological Sciences at Macquarie University, and Institute of Earth Sciences at Uppsala University, Lars Holmer, also of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, and Department of Geology at Northwest University, and the Institute of Earth Sciences at Uppsala University, and Feiyang Chen and Glenn Brock, again of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, and Department of Geology at Northwest University, and the Department of Biological Sciences at Macquarie University, describe a new Acrotretide genus from the Cambrian Series 2 Shuijingtuo Formation from southern Shaanxi and western Hubei provinces of South China.

Neoproterozoic to Palaeozoic sedimentary successions are well developed and distributed on the northern margin of the Yangtze Platform, South China. The Acrotretide Brachiopods described here come from the lower part of the Shuijingtuo Formation of South China. In southern Shaanxi, Precambrian to Cambrian strata are well exposed at the Xiaoyangba section, approximately 106 km south-east of Hanzhong City, including (in ascending order) the Ediacaran Dengying Formation, the Cambrian Xihaoping Member of Dengying Formation, the Shuijingtuo Formation and the Shipai Formation. The Shuijingtuo Formation (unconformably overlying the Xihaoping Member of the Dengying Formation) is characterised by organic-rich and nodular limestones at the lower boundary. The 31 m thick lower Shuijingtuo Formation is comprised of dark, thin silty limestones, while the 85 m thick upper part consists of siltstones interbedded with thin limestones. Large quantities of phosphatised shelly fossils have been reported from the lower part of the Shuijingtuo Formation at the Xiaoyangba section, which are dominated by the Eodiscoid Trilobite, Hupeidiscus orientalis, the Hyolith, Paramicrocornus zhenbaensis and Bradoriids. Associated fossils include the Brachiopods, Eohadrotreta, Palaeobolus, Botsfordia, and Lingulellotreta, the Tommotiid, Kelenella, Hyoliths, and Molluscs, along with Chancelloriids, Echinoderm plates, and Sponge spicules.

 
Palaeogeographic map and fossil localities in southern Shaanxi and western Hubei, South China, noting the Xiaoyangba and Aijiahe sections. Zhang et al. (2020).

In the Three Gorges area of western Hubei, the Aijiahe section, approximately 25 km north-west of Yichang City, has been well studied. The 68 m thick Shuijingtuo Formation (unconformably overlying the Yanjiahe Formation) is composed of three parts. The 10 m thick basal part consists of condensed black shale intercalated with black limestone concretions, yielding the oldest known Trilobite, Tsunyidiscus actus, from this region. The 40 m thick middle part of the succession is dominated by alternating black shale and thin limestones, while the 18 m thick upper part consists of greyish-black argillaceous limestones. Abundant phosphatised shelly fossils have been described from the middle and upper parts of the Shuijingtuo Formation at the Aijiahe section with the Acrotretide Brachiopod, Eohadrotreta zhenbaensis, the Eodiscoid Trilobite, Tsunyidiscus actus, and Hyoliths are the most dominant groups. Other associated fossils include the Brachiopods, Palaeobolus, Spinobolus and Lingulellotreta, Archaeocyaths, Molluscs, Bradoriids, Chancelloriid sclerites and Sponge spicules.

All specimens described and illustrated by Zhang et al. are from the Shuijingtuo Formation (Cambrian Series 2) of South China. The specimens were obtained through acetic acid (10%) maceration of samples from thin-bedded limestones following standard techniques.

The new genus is given the name Palaeotreta, which derives from 'Palaeo', meaning ‘old’, from the Greek palaiós, describing an ancestral representative of Acrotretide Brachiopods, with the ending '–treta', meaning ‘perforated’ from the Greek trētos.

The shells of Palaeotreta are ventribiconvex, sub-circular to transversely oval, ornamented with evenly distributed pits, about 1 mm in diameter covering the metamorphic shell, and with finely concentric growth lines on the postmetamorphic shell. Ventral valve cap-like to low conical, pseudointerarea vestigial, from catacline to apsacline, with very short intertrough. Incipient pedicle notch is open during the long pedicle foramen-developing stage. Adult pedicle foramen located outside of the metamorphic shell. Growth lines developed on a separate area between the metamorphic shell and pedicle foramen. Apical process vestigial, very close to the pedicle foramen. Cardinal muscle scars and vascula lateralia weakly impressed. Dorsal valve slightly convex. Pseudointerarea orthocline, short with narrow subtriangular median groove. Median buttress generally well developed, while median septum vestigial. Cardinal muscle scars weakly impressed. Secondary shell layer columns relatively short.

The new genus is established for Acrotretide species that are similar to Eohadrotreta and Linnarssonia. However, the new genus differs from all other (especially from Cambrian Epoch 2) Acrotretides in having the following characters: (1) a catacline to apsacline inclination of ventral pseudointerarea with short intertrough; (2) a pedicle foramen that is located mostly outside the metamorphic shell; (3) a vestigial apical process and median septum; and (4) weakly impressed cardinal muscle scars. Eohadrotreta, Prototreta, Vandalotreta, Hadrotreta and Kotylotreta mainly have procline pseudointerarea with a relatively long intertrough. Although the Apsacline ventral pseudointerarea and internal structures (apical process, median septum) of Palaeotreta can be compared with Cambrian Epoch 3 taxa, such as Aphelotreta and the Furongian, Linnarssonella, the positions of pedicle foramen and ventral pseudointerarea are different. The ventral apical process and dorsal septum of Palaeotreta are very weakly defined, while Linnarssonia develops a high, boss-like apical process and has a high median septum. Eohadrotreta has a relatively more well-developed apical process and median septum. Vandalotreta has an apical process that forms a boss-like thickening anterior to the internal foramen and Hadrotreta develops apical pits that are situated close to the apical process. Kuangshanotreta is difficult to compare with the new genus in the absence of data on shell morphology and ornamentation.

Palaeotreta is the third genus of the Acrotretida that has been described from the Cambrian Series 2 of South China, and this region has a higher diversity of coeval Acrotretides as compared to South Australia, Laurentia, Antarctica and Siberia. The most unique character of Palaeotreta is the position of the pedicle foramen, which is located mostly outside of the metamorphic shell from the T2 ontogenetic stage. In this feature it has similarities to some Cambrian Ceratretidae and Ordovician Scaphelasmatidae species; but to date, this character is not known from early Cambrian Acrotretidae.

The first species assigned to the new genus is Palaeotreta shannanensis, a new species from the Shuijingtuo Formation (layer S4-3, 0.7 m above the base) at Xiaoyangba section in Zhenba County, south-eastern Shaanxi. A total of 21 ventral valves and nine dorsal valves were recovered from this locality. The name 'shannanensis' means 'from Shannan' in reference to the occurrence of the species in Shannan, southern Shaanxi Province.

The shell of Palaeotreta shannanensis is ventribiconvex, sub-circular in outline with round posterior margin. 1.1 μm hemispherical pits evenly distributed on the whole metamorphic shell surface without overlapping, while the post-metamorphic shell is covered by finely circular growth lines and drape structures. Shell structure consists of thin-lamella (2 μm) primary layer and thin-lamina (5 μm) secondary columnar layers.

 
Ontogenetic development of ventral valve of Palaeotreta shannanensis from the Shuijingtuo Formation of southern Shaanxi. (A)–(H) ventral valves demonstrating pedicle foramen forming stage (T1); (A)–(D) juvenile with unrestricted pedicle notch, ELI-XYB S4-3AU-06; (E)–(H) small valve with pedicle opening, ELI-XYB S4s-3 AV-04; (I)–(P), ventral valves demonstrating pedicle foramen-enclosing stage (T2); (I)–(L), adult with enclosed pedicle foramen, ELI-XYB S4-3 AU-01; (L) interior view, noting vascula lateralia (tailed arrow); (M)–(P) larger valve with very short intertrough, ELI-XYB S4-3 AU-08. Zhang et al. (2020).

Ventral valve sub-circular, on average 89% as long as wide with maximum width at mid-length. The valve is convex, with a cap-like shape, on average 21% as deep as long, with maximum height almost at midvalve. Metamorphic shell pronounced at the apex, occupying 13% of valve length. Pseudointerarea weakly developed, catacline to apsacline. Intertrough poorly defined, very short, occupying on average about 2% of the length and 6% of the width of the valve. Apical process vestigial, only observed in adult
specimens, close to pedicle foramen, occupying 20% of valve length. Enclosed pedicle foramen almost circular, about 50 μm in diameter, located mostly directly outside of the metamorphic shell, until valve reaches about 1100 μm in length. Growth lines developed t the posterior margin of the metamorphic shell and the lateral side of the pedicle foramen. Cardinal muscle scars and vascula lateralia weakly impressed.

 
Ontogenetic development of dorsal valve of Palaeotreta shannanensis from the Shuijingtuo Formation of southern Shaanxi. (A)–(D) juvenile with rudiment median buttress, ELI-XYB S4-3 AV-19; (E)–(G) larger valve with developed median buttress, ELI-XYB S4-3 AU-12; (G) oblique lateral view showing weakly developed median septum (arrow); (H)–(K) adult valve with weakly developed median septum, ELI-XYB S4-3 AV-18. Zhang et al. (2020).

Dorsal valve sub-circular, on average 87% as long as wide, with maximum width almost at mid-valve. It is slightly convex, on average about 19% as deep as long. Pseudointerarea small, orthocline, occupying 6% of the valve length and 36% of valve width. Median groove sub-triangular, short, about 31% of the pseudointerarea width. Median buttress generally developed, fading anteriorly. Median septum vestigial, only developed in adult valve, extending anteriorly for 59% of valve length. Cardinal muscle scars weakly impressed, occupying 17% of the length and 45% of the width of the valve.

 
Ornamentation and ultrastructure of the earliest ontogeny of Palaeotreta shannanensis from the Shuijingtuo Formation of southern Shaanxi. (A) enlarged ventral apex, note pronounced halo (arrow) and drape structures (tailed arrow), ELI-XYB S4-3 AU-08; (B) oblique lateral view of A, showing the pedicle foramen (arrow) and protegulum (tailed arrow); (C) posterior view of ventral apex, show enclosed pedicle foramen outside of metamorphic shell, ELI-XYB S4-3AU-01; (D) enlarged dorsal apex, note pronounced halo (arrow) and drape structures (tailed arrow), ELI-XYB S4-3 AV-15 E, obliquely lateral view of (D) showing the protegulum (tailed arrow) and two pairs of larval setal sacs (arrows); (F) evenly distributed pitting structures on metamorphic shell, ELI-XYB S4-3AU-13; (G) enlargement of (F); (H) growth lines and drape structures on post-metamorphic shell, ELI-XYB S4-3 AU-13. Zhang et al. (2020).

Palaeotreta shannanensis shares a similar morphology in outline with Eohadrotreta zhenbaensis (from the same locality). However, the new species has a low cap-like shape, small pseudointerarea with very short intertrough, apsacline pseudointerarea in the adult valve, prolonged pedicle foramen-forming stage and a pedicle foramen that is located outside of the metamorphic shell. Furthermore, the new species has a less developed intertrough as compared with Eohadrotreta zhenbaensis; in the latter species the intertrough is becoming remarkably prominent in stage T3, which does not happen in the new species. The shell structure of Palaeotreta shannanensis is similar to that of Eohadrotreta zhenbaensis. However, the former species has a very thin primary layer about 2 μm thick, and very thin secondary columnar layers. The thickness of columns in Palaeotreta shannanensis is variable in different parts of the shell, ranging from 4 μm to 8 μm, and are quite short compared to the columns of Eohadrotreta zhenbaensis. The secondary layer insignificantly increases the overall thickness of the ventral valve, being composed of one to two columnar laminae, resulting in a low cap-like shape in Palaeotreta shannanensis.

 
Internal morphology and ultrastructure of Palaeotreta shannanensis from the Shuijingtuo Formation of southern Shaanxi. (A) Enlarged ventral posterior end, showing cardinal muscle scars (arrows) and paired vascula lateralia (tailed arrows), ELI-XYB S4-3 AU-07; (B) oblique lateral view of (A); (C) enlarged ventral posterior end, ELI-XYB S4-3 AV-07; (D) lateral view of dorsal posterior end, noting cardinal muscle scars by arrows, XYB S4-3 AV-18; (E) enlarged dorsal pseudointerarea and median buttress, ELI-XYB S4-3AU-12; (F) enlargement of the terminal of median buttress of (E), note weakly developed median septum; (G) fine pores on the exterior of columnar lamella by exfoliation of the primary layer, ELI-XYB S4-3AU-09; (H), (I), one layer of the secondary columnar structures on valve margin, ELI-XYB S4-3 AV-09, XYB S4-3 AU-07; (J), (K) remaining base of columns after exfoliation of the covering lamella of (H); (L) one lamella of very short secondary columns, ELI-XYB S4-3AU-08; (M) re-crystallisation of the columns, note hollow in the centre, ELI-XYB S4-3 AV-13. Zhang et al. (2020).

Palaeotreta shannanensis differs from Vandalotreta djagoran and Linnarssonia rowelli in having a lower ventral valve, weakly developed apical process and median septum, apsacline ventral pseudointerarea, as well as a pedicle foramen that is mostly located outside the metamorphic shell. Palaeotreta shannanensis has a similar inclination of the ventral pseudointerarea as Aphelotreta khemangarensis, while the ornamentation of the metamorphic shell, pedicle foramen size and the median groove allow the distinction of the two species. 

 
Ontogenetic development of pedicle foramen of Palaeotreta shannanensis from the Shuijingtuo Formation of southern Shaanxi. (A), (B) Juvenile with unrestricted pedicle notch, note raised propareas (arrows), ELI-XYB S4-3 AV-09; (C), (D) juvenile with unrestricted pedicle notch, ELI-XYB S4-3 AV-17; (E)–(G) semicircular pedicle foramen soon to be enclosed, ELI-XYB S4-3 AV-05; (H) raised propareas (arrows), ELI-XYB S4-3 AV-11; (I)–(L) pedicle foramen, just enclosed with very short intertrough, XYB S4-3AU-07; (M), (N) adult with enclosed pedicle foramen, showing the successive growth of propareas at the posterior margin of the metamorphic shell (arrow), ELI-XYB S4-3 AU-01; (O) enclosed pedicle foramen is mostly outside the metamorphic shell, ELIXYB S4-3 AV-07. Zhang et al. (2020).

At the Xiaoyangba section, Palaeotreta shannanensis occurs at the base of the Shuijingtuo Formation, while Eohadrotreta zhenbaensis first occurs 60 cm higher. Therefore, Palaeotreta shannanensis represents the oldest known Acrotretide Brachiopod in southern Shaanxi. The diachronous nature of the stratigraphic hiatus at the base of Cambrian Series 2 across southern Shaanxi and western Hubei makes the biostratigraphic correlation between these regions imprecise, and more index fossils from new sections are required to better constrain the time gap and better resolve correlation.

The ontogeny of Palaeotreta shannanensis includes the formation of an enclosed pedicle foramen that is mostly located outside of the metamorphic shell, as well as an apsacline ventral pseudointerarea during later ontogenetic stages, which are unlike any other Acrotretides from Cambrian Epoch 2. The problematic Eohadrotreta? zhujiahensis is also referred to the new genus Palaeotreta. The new information on the development of the pedicle foramen, pseudointerarea, apical process and median septum is also shown to be important for deciphering phylogeny. Three ontogenetic stages (e.g. pedicle foramen-forming stage (T1), pedicle foramen-enclosing stage (T2) and intertrough-increasing stage (T3)) were identified for Eohadrotreta? zhujiahensis and can also be identified in Palaeotreta, with the species thus being renamed Palaeotreta zhujiahensis.

The shell of Palaeotreta zhujiahensis ventribiconvex, transverse oval in outline with slightly straightened posterior margin. 1.3 μm hemispherical pits evenly distributed on the whole metamorphic shell surface without overlapping while post-metamorphic shell covered by finely circular growth lines and drape structures. Shell structure consists of thin-lamella (2 μm) primary layer and thin-lamina (5–10 μm) secondary columnar layers.

 
Ontogenetic development of ventral valve of Palaeotreta zhujiahensis from the Shuijingtuo Formation of western Hubei. (A)–(F) Ventral valves demonstrating pedicle foramen forming stage (T1); (A) oblique dorsal view of a very small conjoined specimen showing unrestricted pedicle notch, ELI-AJH 8-1-2-B AF12; (B) lateral view of a small conjoined specimen, ELI-AJH 8-2-3 AD2-07; (C) posterior view, ELI-AJH S05 AG07; (D) interior view of (C); (E) oblique view of a larger juvenile, ELI-AJH 8-2-3 AC-27; (F) posterior view of (E), note ‘U’-shaped pedicle notch; (G) lateral view, indicating pedicle foramen-enclosing stage (T2), showing enclosed pedicle foramen outside of the metamorphic shell, ELI-AJH 8-2-D AD2-12; (H)–(N) intertrough-increasing stage (T3); (H) posterior view, ELI-AJH S05 AF-16; (I)–(N) adult with short intertrough, ELI-AJH 8-2-1 CE-03; (I) exterior view; (J) interior view; (K) oblique lateral view; (L) lateral view; (M) posterior view, note the posterior migration of enclosed pedicle foramen, outside the metamorphic shell; (N) oblique anterior view. Zhang et al. (2020).

Ventral valve sub-circular, on average 83% as long as wide with maximum width at the posterior half of valve. It is convex, with a low conical shape, on average 28% as deep as long, with a maximum height almost at mid-valve. Metamorphic shell pronounced at the apex, occupying 31% of the valve length. Pseudointerarea weakly developed, almost catacline, divided by a very short intertrough, which is on average about 5% of the length and 11% of the width of the valve. Apical process weakly developed, occupying on average 30% of valve length, close to pedicle foramen. Pedicle foramen is relatively large, about 90 μm in diameter, enclosed and located directly outside the metamorphic shell until valve reaches about 650 μm in length. Growth lines distinctively developed at the posterior margin of the metamorphic shell. Cardinal muscle scars and vascula lateralia weakly impressed.

 
Ontogenetic development of pedicle foramen of Palaeotreta zhujiahensis from the Shuijingtuo Formation of western Hubei. (A) Enlarged pedicle notch; (B) juvenile with unrestricted pedicle notch, showing raised propareas (arrows), ELIAJH 8-2-3 AC-11; (C) posterior view; (D) ‘U’shaped pedicle notch, note the growth of propareas at the posterior margin of metamorphic shell (arrow); (E), (F) ‘U’-shaped pedicle foramen is soon to be enclosed, note the growth of propareas (arrow); (G) enclosed pedicle foramen with short intertrough, ELI-AJH 8-2-D AD2-12; (H) larger adult showing pedicle foramen outside the metamorphic shell, ELI-AJH 8-2-3 CD2-02; (I) enlargement of propareas growing at the posterior margin of metamorphic shell and lateral sides of pedicle foramen of (G); (J) lateral view of (I), note propareas growth (arrow); (K) posterior view, showing pedicle foramen mostly located outside the metamorphic shell, box indicates the area shown in (L), ELI-AJH 8-2-3 AC-22; (L) enlarged view showing propareas growth (arrow); (M) enlargement of propareas at the posterior margin of metamorphic shell, note pedicle foramen (arrow), ELI-AJH 8-2-1 AE-09; (N) pitting structures on metamorphic shell, ELI-AJH 8-2-1 AE-09; (O), (P) enlarged secondary columnar layer, ELI-AJH 8-2-1 AE-09, ELI-AJH 8-2-1 CE-03. Zhang et al. (2020).

Dorsal valve transversely oval, on average 82% as long as wide, with maximum width almost at mid-valve. Slightly convex, on average 17% as deep as long. Pseudointerarea small, orthocline, occupying about 7% of valve length and 38% of valve width. Median groove subtriangular, short, on average 44% of pseudointerarea width. Median buttress moderately developed, fading anteriorly. Median septum vestigial, only developed in adult valve, extending anteriorly at mid-valve. Cardinal muscle scars gently impressed, occupying 22% of the length and 51% of the width of the valve.

Based on their similar morphology, Palaeotreta zhujiahensis was originally considered to represent a second species of Eohadrotreta. However, there are significant differences in ontogenetic growth between these two species. New material collected from western Hubei demonstrates that Palaeotreta zhujiahensis has a lower ventral valve, straightened posterior margin, relatively larger pedicle foramen, late enclosure of the pedicle foramen, smaller ventral pseudointerarea, much shorter intertrough, weakly developed growth lines, apical process and median septum, thinner secondary layers and more weakly impressed cardinal muscle scars than those of Eohadrotreta zhenbaensis. Furthermore, the  most characteristic feature of Palaeotreta zhujiahensis is that the pedicle foramen is located directly outside of the metamorphic shell, which fits closely with the diagnosis of the new genus Palaeotreta. The valve shape and ontogenetic development of the pedicle foramen in Palaeotreta zhujiahensis can be compared with that of Palaeotreta shannanensis. However, the former has a catacline inclination of the ventral pseudointerarea and a relatively longer intertrough, which is two times longer than that of Palaeotreta shannanensis.

The shell structure of Palaeotreta zhujiahensis is comparable with that of Palaeotreta shannanensis. Both have a very thin primary layer about 2 mm thick, but Palaeotreta shannanensis has  relatively thinner columnar layers. The thickness of columns in Palaeotreta zhujiahensis is variable in different shell regions, ranging from 5 μm to 10 μm, which is quite short compared to the columns in Eohadrotreta zhenbaensis.

At the Aijiahe section, Palaeotreta zhujiahensis co-occurs with Eohadrotreta zhenbaensis at the middle part of the Shuijingtuo Formation. Compared to the biostratigraphy with southern Shaanxi, Palaeotreta zhujiahensis is slightly younger than Palaeotreta shannanensis. Palaeotreta zhujiahensis is the second species discovered in southern Shaanxi and western Hubei (after Eohadrotreta zhenbaensis), but Eohadrotreta zhenbaensis has a much wider palaeogeographical distribution.

Recent study of the earliest ontogeny of Cambrian Epoch 2 Acrotretide Brachiopods has revealed that the primary larval body plan of Acrotretides is shared with most early linguliforms, and that the metamorphic shell is formed at the end of the planktotrophic stage. The same kind of early ontogenetic characters can also be observed in Palaeotreta shannanensis. The raised lobes at the posterior end of both the ventral and dorsal valves of Palaeotreta shannanensis are about 50 μm in size, which is comparable to the protegula of Eohadrotreta zhenbaensis. They are probably secreted initially as a bivalved organic shell by the larval mantle lobes. During subsequent peripheral growth of the brephic shell at the planktotrophic stage, two pairs of lobes are preserved as imprints of larval setal sacs on the dorsal side of the larval body. Pits with an average diameter of 1 μm evenly cover the whole surface of the metamorphic shell, which may represent imprints of mineralised tablets that potentially protected the larvae against ultraviolet radiation penetrating surface waters. The metamorphic shell is separated from the post-metamorphic shell by a pronounced halo when the shell reached about 200 μm in width, which likely represents the completion of larval metamorphosis. A narrow belt about 20 μm wide, outside the metamorphic shell, lacking drape structures indicates the secretion of the neanic shell. Subsequently, the mature shell is developed by accretionary growth outside the neanic shell, exhibiting all major characters of the adult shell including drape structures and a three-layer shell. The growth of the mature shell indicates the development of marginal mantle after completion of metamorphosis. Detailed investigation of the earliest ontogeny of Palaeotreta zhujiahensis has previously been documented. The closely similar features of metamorphosis between Eohadrotreta and Palaeotreta imply that Palaeotreta probably had a life cycle similar to the early Cambrian Acrotretide Eohadrotreta and Lingulide Eoobolus, which are interpreted to have had a relatively prolonged free-swimming stage.

For the later post-metamorphic ontogeny, a strong allometric growth pattern has been demostrated in Eohadrotreta zhenbaensis by quantifying the ontogenetic variations in size and shape in successive ontogenetic stages. This also revealed that Palaeotreta zhujiahensis experienced three stages during the entire ontogenetic sequence, which can be well compared with the almost coeval Eohadrotreta zhenbaensis. When compared to the ontogeny of Eohadrotreta zhenbaensis, however, only two ontogenetic stages (T1–T2) can be distinguished during the growth of Palaeotreta shannanensis. During the T1 pedicle foramen-forming stage (pedicle opening is not enclosed as a foramen and the ventral intertrough is not developed), the shape of the ventral valve remains low and cap-like and the cardinal muscle scars, apical process and median septum are vestigial, while the incipient pedicle notch changes from semicircular to circular. In the subsequent T2 pedicle foramen-enclosing stage (enclosure of pedicle notch to form a foramen and development of an incipient apical process and median septum), the marginal accretionary growth increases the length and width of both valves, while the ventral valve maintains a low cap-like form. At the same time, cardinal muscle scars are weakly impressed, while the apical process and median septum are weakly developed. The inclination of the ventral pseudointerarea changes from catacline to apsacline, and the accelerated shell growth along the posterior margin of the metamorphic shell produces a posterior migration of the enclosed pedicle foramen. It is important to note that the stage T2 of Palaeotreta shannanensis is much delayed until valves reach about 1100 mm in length, which is twice the size compared to Eohadrotreta zhenbaensis. Furthermore, the boundary delineating the pedicle foramen from the metamorphic shell is developed early in stage T1, and successive growth lines of the propareas near the posterior margin of the metamorphic shell and lateral sides of pedicle foramen are well developed in stage T2. These remarkable ontogenetic variations are also well demonstrated in the entire three ontogenetic stages of Palaeotreta zhujiahensis

The morphology of juvenile shells is quite similar between Palaeotreta and Eohadrotreta, which makes it difficult to distinguish them when a complete ontogeny is lacking. However, the heterochronic process probably occurred in later ontogenetic stages, resulting in quite different morphologies in adults of Palaeotreta and Eohadrotreta. A strong allometric growth pattern was verified in Eohadrotreta zhenbaensis especially during the T3 intertrough-increasing stage (complete development of ventral intertrough, apical process, median septum and imprint of vascula lateralia) by the rapid increase in the ventral intertrough length. Nonetheless, this ontogenetic variation in stage T3 for Palaeotreta shannanensis is completely lacking, resulting in a very small ventral pseudointerarea and short intertrough. As the growth of the intertrough is an important event in Acrotretide Brachiopods, the relatively slow development of the intertrough (about 1.6% length of ventral valve) in Palaeotreta shannanensis results in a change in inclination of the ventral pseudointerarea from catacline to apsacline, producing a low, cap-like valve shape. In contrast, the ventral valve of Eohadrotreta zhenbaensis becomes more conical with a very long intertrough (about 24.5% length of ventral valve) and the inclination of the ventral pseudointerarea changes from catacline to procline during stage T3. Compared with the slightly older Palaeotreta shannanensis, Palaeotreta zhujiahensis demonstrates a relatively rapid growth during ontogenetic stage T1, as the pedicle foramen is enclosed at an earlier time when the ventral valve reaches 650 μm in length, contrasting with 1100 μm in Palaeotreta shannanensis. Thus, there is an earlier termination of shape change in the younger Palaeotreta zhujiahensis during ontogenetic stage T1. Moreover, the growth of the ventral pseudointerarea greatly decreases during ontogenetic stage T3 (increasing 0.8% of the length and 3.5% of the width of the ventral valve), producing a cap-like shape, catacline ventral pseudointerarea and posterior migration of the pedicle foramen outside of the metamorphic shell as well. The morphological similarity between the adult valves (650–1600 μm) of Palaeotreta zhujiahensis and juvenile valves (smaller than 1110 μm) of Palaeotreta shannanensis indicates paedomorphosis (progenesis) of Palaeotreta zhujiahensis. Thus, heterochrony plays an important role in the differentiation of species in the new genus Palaeotreta.

Changes in shape through ontogeny and the different growth patterns in the two species of Palaeotreta may correspond to subtle modification in life mode during the gradual cumulative increase in size. However, the life habit of extinct acrotretides is still not well understood. Rare but exquisitely preserved fossils may suggest that Acrotretides attached to shell fragments or even to other benthic animals like sponges to achieve secondarily tiered niches. The low cap-like shape and apsacline ventral pseudointerarea indicate that Palaeotreta shannanensis is likely to have had a mode of life similar to Treptotreta with a thin pedicle anchorage at the apex of the ventral valve. The development of a slightly longer intertrough whilst maintaining a catacline pseudointerarea in Palaeotreta zhujiahensis may have aided stable attachment to the substrate.

The phylogenetic relationships of the Order Acrotretida to other groups within the Class Lingulata are still poorly resolved, though acrotretides possess many important unique characters, such as diminutive body size, conical shape of the ventral valve with enclosed pedicle foramen, simplified muscular system, developed apical process and often a raised median septum in the dorsal valve. Recent studies have revealed that bearing an incipient pedicle notch in the juvenile, pitted metamorphic shell and the presence of columnar shell structures in Acrotretides demonstrates a close relationship with the oldest known Lingulide Brachiopods. As the position of the developing pedicle foramen in relation to the metamorphic shell is one of the most taxonomically important characters in Acrotretides, investigation of the ontogeny of the pedicle foramen provides a new way of considering Acrotretide phylogeny. The incipient pedicle notch in juvenile specimens of Palaeotreta during ontogenetic stage T1 is interpreted to represent a similar ancestral type of pedicle opening between the ventral and dorsal valves that occurs in Linguliform Brachiopods. The weakly developed ventral pseudointerarea is comparable with the raised pseudointerarea above the valve floor in older lingulides such as Eoobolus. In forming a high conical ventral valve, the cardinal muscles move posteriorly to the raised pseudointerarea in Acrotretides. During later ontogeny, the slow growth of the intertrough results in a catacline pseudointerarea in Palaeotreta zhujiahensis, whilst the lack of major growth of the intertrough changes the pseudointerarea from catacline to apsacline in Palaeotreta shannanensis. In contrast, the rapid growth of the intertrough in Eohadrotreta zhenbaensis results in a significant gradual transition of the ventral valve pseudointerarea from catacline to procline. The new evidence presented by Zhang et al. supports the hypothesis that the ventral intertrough in Acrotretide Brachiopods evolved by the gradual and variable ‘rolling up’ of the Lingulide propareas along the posterior margin of the ventral valve in association with changes in the inclination of the ventral intertrough. A distinct evolutionary lineage from basal Botsfordiids that developed an open delthyrium and vestigial ventral pseudointerarea to more derived Acrothelids which have an enclosed pedicle foramen and raised ventral pseudointerarea probably indicates that this monophyletic clade is derived from the Lingulides.

 
Ontogenetic scheme of Palaeotreta and Eohadrotreta from Cambrian Series 2 of South China, demonstrating transitions of important characters during different ontogenetic stages. Zhang et al. (2020).

There are three main positions of the pedicle foramen in the fossil record of Acrotretides concerning its position related to the metamorphic shell. Type 1 is not enclosed in the metamorphic shell, Type 2 is completely enclosed in the metamorphic shell and Type 3 is mostly located outside of the metamorphic shell. The first type, where the pedicle foramen is partly enclosed in the metamorphic shell, occurred widely across genera beginning in Cambrian Epoch 2, such as Linnarssonia, Prototreta (or Homotreta), Eohadrotreta, Hadrotreta and Vandalotreta. The second and third types (Type 2–3), where accelerated shell growth along the posterior margin of metamorphic shell resulted in anterior or posterior migration of the pedicle foramen respectively, become common later in Cambrian Epoch 3 and the Ordovician (e.g. Apsotreta and Scaphelasma). However, this is the first discovery of the Type 3 during Cambrian Epoch 2 in Palaeotreta from South China. 

The morphological diversity amongst Acrotretide Brachiopods in Cambrian Stage 2 of South China includes great variation in the position of the pedicle foramen, shell growth patterns and inclination of the ventral pseudointerarea. The low cap-like shape, vestigial apical process and median septum, very short intertrough and longer pedicle foramen-forming stage (T1) probably imply that Palaeotreta shannanensis is one of the ancestral stock of Acrotretides. This is also supported by its early occurrence in the Shuijingtuo Formation of southern Shaanxi. Compared with the other two species Palaeotreta zhujiahensis and Eohadrotreta zhenbaensis, Palaeotreta shannanensis has a more restricted distribution and lower abundance, which may indicate this ancestral acrotretide was not suited to life in distal mixed clastic platforms. By contrast, Eohadrotreta zhenbaensis has more derived characters exemplified by faster growth rates during accretionary shell growth, especially during stage T3, enabling a wider geographical distribution and increased abundance in shallow carbonate environments. The evidence presented here suggests evolutionary modification to ontogenetic changes resulted in the early divergence of Palaeotreta and Eohadrotreta, and played an important role in the early evolution of Acrotretides. The early Cambrian widespread Acrotretides Hadrotreta and Prototreta, developing a long ventral intertrough and dorsal median septum, may have a similar heterochronic process to Eohadrotreta zhenbaensis during the T3 intertrough increasing stage. Heterochrony also probably drove rapid morphological variation in acrotretides, explaining their subsequent radiation and optimal adaptation across a wide range of shallow marine depositional environments. These adaptations contributed to the success of younger Acrotretide Brachiopods, especially during the transition from Cambrian Epoch 2 to Epoch 3 and their subsequent radiation during the Great Ordovician Biodiversification Event.

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