Magnitude 1.7 Earthquake to the south of the Isle of Mull.

The British Geological Survey recorded a Magnitude 1.7 Earthquake at a depth of about 10 km to the south of the Isle of Mull, in Argyll and Bute, Scotland, slightly after 5.20 am Brittish Summertime (slightly after 4.20 am GMT) on Wednesay 1 July 2020. This was not a major event, and presented no threat to human life or property, but it may have been felt locally..

 

The approximate location of the 1 July 2020 Isle of Mull Earthquake. Google Maps.

 

Earthquakes become more common as you travel north and west in Great Britain, with the west coast of Scotland being the most quake-prone part of the island and the northwest of Wales being more prone  to quakes than the rest of Wales or most of England.

 

The precise cause of Earthquakes in the UK can be hard to determine; the country is not close to any obvious single cause of such activity such as a plate margin, but is subject to tectonic pressures from several different sources, with most quakes probably being the result of the interplay between these forces.

 

Britain is being pushed to the east by the expansion of the Atlantic Ocean and to the north by the impact of Africa into Europe from the south. It is also affected by lesser areas of tectonic spreading beneath the North Sea, Rhine Valley and Bay of Biscay. Finally the country is subject to glacial rebound; until about 10 000 years ago much of the north of the country was covered by a thick layer of glacial ice (this is believed to have been thickest on the west coast of Scotland), pushing the rocks of the British lithosphere down into the underlying mantle. This ice is now gone, and the rocks are springing (slowly) back into their original position, causing the occasional Earthquake in the process.

 

(Top) Simplified diagram showing principle of glacial rebound. Wikipedia. (Bottom) Map showing the rate of glacial rebound in various parts of the UK. Note that some parts of England and Wales show negative values, these areas are being pushed down slightly by uplift in Scotland, as the entire landmass is quite rigid and acts a bit like a see-saw. Climate North East. 

  

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. If you felt this quake, or were in the area but did not (which is also useful information) then you can report it to the British Geological Survey here.  

 

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Eptatretus wandoensis: A new species of Hagfish from the Korea.

Hagfish, Myxinidae, are currently classified into six genera and 81 species worldwide. They are characterised by an Eel-like body shape and 1–16 pairs of gill apertures and gill pouches; however, they have no jaws, eyes, or fins. Recent research using morphological and molecular characteristics revealed that hagfishes comprise three subfamilies: Eptatretinae, Myxininae, and Rubicundinae. There have been several unresolved issues regarding the number of recognized genera in the subfamily Eptatretinae; however, its genera were recently reorganised taxonomically based on morphological and molecular data. Currently, the Eptatretinae includes a single genus, Eptatretus, which is characterised by the presence of more than two pairs of gill apertures; notably, Eptatretus is the most species-rich Myxinid genus, currently comprising 51 valid species in the northwestern Pacific Ocean (e.g. Korea, Taiwan, and Japan) and coastal waters around Asia (e.g. China, Philippines, and Vietnam). Surveys of the deep sea and other hard-to-reach areas using special-purpose submarines are increasingly revealing new or cryptic species worldwide. 

In a paper published in the journal ZooKeys on 13 April 2020, Young Sun Song and Jin-Koo Kim of the Department of Marine Biology at Pukyong National University, describe a new species of Eptatretus, based on examinations of both morphological and genetic characteristics of Hagfish specimens from the southwestern Sea of Korea.

The new species is called Eptatretus wandoensis, where 'wandoensis' means 'from Wando' in reference to Wando County in South Korea; the new species is described from three specimens caught in Fish traps at depths of 60-80 m off the coast of Yeoseo Island, which forms part of Wando County. 

Sampling location of Eptatretus wandoensis in Korea. Song & Kim (2020).

The body of Eptatretus wandoensis is elongated; laterally compressed at the trunk and strongly compressed at the tail. The rostrum is slightly blunt and round. Nasal-sinus papilla are absent. Eyespots are present. The pre-eyespot area is shorter than the branchial region. There are three pairs of barbels on the head, the first and second barbels are nearly equal in size; the third barbel is longer. The tips of the third barbels extend at the mouth. There are five pairs of gill pouches and apertures; the gill apertures are arranged in a regularly spaced straight line. Teeth are comb-like, arranged in two rows with tips sharp and curved rearward; in the outer row there are 3 multicusped and 7–8 unicusped teeth; in the inner row, there are 2 multicusped and 8–9 unicusped teeth; the total number of cusps is 40–43. The dental muscle is thick and long, the posterior tip of the dental muscle is located in first the first gill pouuch. There are 14-18 prebranchial slime pores, 4 branchial slime pores, 46-49 trunk slime pores, and 9-11 tail slime pores, for a total of 74-82. The osterior-most efferent branchial duct is confluent with the pharyngocutaneous duct on the left side, forming a larger aperture. All efferent branchial ducts are equal in length. The ventral aorta consists of two side branchial arteries and one medial section, bifurcating at approximately the third or fourth gill pre. The first through third pairs of afferent branchial arteries, which cannot be regarded as branches of the ventral aorta, branch from side branchial arteries; however, the fourth and fifth afferent branchial arteries on left and right branch from the medial section of the ventral artery. The ventral fin-fold is weakly developed or vestigial, beginning approximately at the middle of the body and extending to the cloaca. The caudal fin-fold is weakly developed, beginning posterior to the cloaca and extending around the tail to the dorsal surface.

Overall view of Eptatretus wandoensis, (A) holotype, PKU 62167, 292.0 mm in total length, (B) paratype, PKU 62169, 202.0 mm total length, (C) paratype, PKU 62171, 290.0 mm total length, (D) paratype, PKU 62173, 275.0 mm total length, photographed prior to preservation. Scale bars are 1 mm. Song & Kim (2020).

The body is uniformly dark brown or purplish dorsally and white ventrally; a white mid-dorsal line is conspicuous, beginning from the upper region of the first prebranchial slime pore to around the tail. The eyespots are conspicuous; the whole barbels are pale, as is the area around mouth. Each gill aperture and pharyngocutaneous duct aperture has a white margin; most slime pores are blackish, except for those of the tail region, which are the same as the surrounding skin in colour. The area around the cloaca is white; the ventral fin-fold has a white line along the ventral midline; the posterior margin of the caudal fin is pale.

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Comet C/2020 F3 (NEOWISE) approaches perihelion.

Comet  C/2020 F3 (NEOWISE) reaches its perihelion (the closest point on its orbit to the Sun) at 4.20 pm GMT on Friday 3 July 2020, when it will bet  approximately 0.29 AU from the Sun (i.e. 29% of the distance between the Earth and the Sun, and some way inside the orbit of Mercury). At this time the comet will be 1.22 AU from the Earth, in the constellation of Taurus, having a magnitude of 2.00, making it potentially naked-eye visible, although the closeness to the Sun means that it will be visible only shortly before dawn and/or after dusk (depending on geographical location on Earth).

Comet C/2020 F3 (NEOWISE) imaged on 10 June 2020 from Swan Hill in Victoria, Australia. Stars in the background are slightly elongate due to the two minute exposure used to get the image. Michael Mattiazzo/Seiichi Yoshida's Comet Page.

Comet C/2020 F3 (NEOWISE) was discovered on 27 March 2020 by the NEOWISE system on the Wide-field Infrared Survey Explorer satellite. The designation C/2020 F3 (NEOWISE) implies that it is a comet (C/), that it was the third comet-like body (3) discovered in the second half of March 2020 (period 2020 F - the year being split into 24 half-months represented by the letters A-Y, with I being excluded), and that it was discovered by the NEOWISE system.

The orbit and current position of C/2020 F3 (NEOWISE). The Sky Live 3D Solar System Simulator.

C/2020 F3 (NEOWISE) has an estimated orbital period of 6765 years and a highly eccentric orbit tilted at an angle of 129° to the plain of the Solar System, that brings it to 0.29 AU from the Sun at perihelion (29% of the distance between the Earth and the Sun); to 715 AU from the Sun at aphelion (715 times as far from the Sun as the Earth or 24 times as far from the Sun as the planet Neptune, and some way beyond the Kuiper Belt). As such it is considered to be a Long Period Comet (comet with a period of over 200 years).

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Sinogaleaspis shankouensis: New material of a Silurian Jawless Fish sheds new light on its anatomy.

Sinogaleaspis, the type genus of the family Sinogaleaspidae, comprises two species, Sinogaleaspis shankouensis and ‘Sinogaleaspis’ xikengensis. The monophyly of the Sinogaleaspidae remains controversial largely due to the poorly known nature of Sinogaleaspis shankouensis whose description was based on two incomplete specimens from the Lower Silurian of Xiushui in Jiangxi Province. Among about 80 described galeaspid species, the sensory canal system of Sinogaleaspis shankouensis is peculiar in bearing three pairs of median transverse canals, which contrasts with the known pattern of other Galeaspid Fish. Considering the unusual condition known from two poorly preserved specimens, it has been pointed out that the present interpretation of the sensory canal system in Sinogaleaspis shankouensis needs further corroboration from additional materials.

In a paper published in the journal Vertebrata PalAsiatica on 1 March 2020, Gai Zhi-Kun of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment, and the University of the Chinese Academy of Sciences, Shan Xian-Ren, also of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and of the College of Earth Science and Engineering at the Shandong University of Science and Technology, Sun Zhi-Xin, also of the College of Earth Science and Engineering at the Shandong University of Science and Technology, Zhao Wen-jin, again of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment, and the University of the Chinese Academy of Sciences, Pan Zhao-Hui, again of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the University of the Chinese Academy of Sciences, and Zhu Min. once again of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment, and the University of the Chinese Academy of Sciences, present a redescription of Sinogaleaspis shankouensis, based upon eleven new specimens collected from the type locality of the species in a series of expeditions since 2003.

The new material of Sinogaleaspis shankouensis was collected from the type locality of the Xikeng Formation in Taiyangshen Town, Xiushui County, Jiangxi Province, South China. The Fish-bearing Xikeng Formation belongs to the Upper Red Beds in South China, which is equivalent of the Huixingshao Formation in Chongqing and Guizhou, and the Maoshan Formation in Jiangsu and Zhejiang. Although the precise age of Upper Red Beds in western part of the Yangtze Platform is difficult to determine, an age of the middle-late Telychian is proposed in light of the evidence from the underlying Xiushan Formation with its invertebrate fauna and sequence stratigraphic analyses.

Geological backround of Sinogaleaspis shankouensis. (A) Location map of the fossil site in Xiushui, Jiangxi, South China, (B) stratigraphical section of the Xikeng Formation at the type locality, (C) cross section of the Fish layer showing a large sum of muddy gravels in grey-green pelitic siltstone. Gai et al. (2020).

The new material of Sinogaleaspis shankouensis includes 11 head-shields (IVPP V 25135.1–11) from the type locality of the Xikeng Formation in the town of Taiyangshen, in Xiushui county, Jiangxi Province, South China. All these specimens are permanently housed and accessible for examination in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences. The holotype and paratype of Sinogaleaspis shankouensis, GMC V 1751 and GMC V 1752 were also examined for comparison and measurement; these are housed in the collections of the Geological Museum of China. All fossil specimens were prepared mechanically using a vibro tool with a tungsten-carbide bit or a needle. They were measured with a digital vernier calliper, studied under optical zoom, and photographed with a Canon EOS 5D Mark III camera coupled with a Canon macro photo lens (MP-E 65 mm 1:2.8 1-5×).

Sinogaleaspis shankouensis is a is a small-sized sinogaleaspid jawless fish with a subtriangular head-shield. The rostral margin of the head-shield is arciform without a rostral process. The measurements of 11 specimens of Sinogaleaspis shankouensis indicate that the size of the head-shield is quite stable with the maximum length of head-shield varying from 15.5 to 18.4 mm, the maximum width of head-shield varying from 17.2 to 20.8 mm, and the length of head-shield in midline varying from 9.6 to 12.7 mm. There is about 10% variation in the size of headshield. The head-shield is wider than it is long. Caudally, the head-shield protrudes into a pair of cornual and inner cornual processes. The cornual processes, which are completely preserved in specimens V 25135.1, 3, 4, and 9, are oriented caudo-laterally (or postero-laterally), short and rapidly tapered off. The inner cornual processes, which are completely preserved in specimens V 25135.1, 3, 6, and 9, are small and spine-like, projecting caudally. The inner cornual processes are much smaller than the cornual processes.

Photographs (A), (C) and interpretative drawings (B), (D) of Sinogaleaspis shankouensis. (A), (B) IVPP V 25135.10; (C), (D) IVPP V 25135.6. Abbreviations: a, the first lateral transverse canal or median transverse canal issuing from infraorbital canal; 1–4, the first to fourth lateral transverse canal or median transverse canal issuing from lateral dorsal canal; cc, central canal; cp, cornual process; ic, inner cornual process; ifc, infraorbital canal; ldc, lateral dorsal canal; ltc, lateral transverse canal; mdc, median dorsal canal; md.o, median dorsal opening; mtc, median transverse canal; orb, orbital opening; pi, pineal opening; poc, preorbital commissure; soc2, posterior supraorbital canal. Gai et al. (2020).

The median dorsal opening is fairly long, wedge-shaped or elliptic in outline with long axis aligned with the rostro-caudal axis. The length of the long axis of median dorsal opening varies from 3.7 to 4.2 mm, and the length of the short axis varies from 0.6 to 1.2 mm. The length of the long axis of median dorsal opening is about 5 times the length of the short axis. The distance between the anterior end of the median dorsal opening and the rostral margin of the head-shield is rather short (about 1 mm). The posterior end of the median dorsal opening extends posteriorly beyond the level of the center of the orbital opening, but in front of the level of the posterior margin of the orbital openings.

Photographs of Sinogaleaspis shankouensis from the Silurian of Xiushui showing the morphology of the head-shield. (A), (B) External (A) and internal (B) moulds of a nearly complete head-shield, IVPP V 25135.1a, b; (C) internal mould of an incomplete head-shield, V 25135.3; (D) internal mould of an incomplete head-shield, V 25135.7b; (E) internal mould of an incomplete head-shield, V 25135.11; (F) close-up of granular tubercles (box region of E). Abbreviations: soc1, anterior supraorbital canal; for other abbreviations as above. Scale bars are 2 mm. Gai et al. (2020).

The orbital opening has a dorsal position on the head-shield. The orbital opening is round in outline with a diameter ranging from 1.5 to 2.2 mm in 11 specimens, roughly accounting for1/5 of the length of the head-shield in midline. The distance between paired orbital openings is range from 3.7 to 4.6 mm in 11 specimens.

Photographs of Sinogaleaspis shankouensis from the Silurian of Xiushui showing the sensory canal system. (A) An incomplete external mould of the head-shield IVPP V 25135.4a; (B) a complete external mould of the head-shield V 25135.9a; (C) an incomplete internal mould of the head-shield V 25135.8; (D) an incomplete internal mould of the head-shield V 25135.5; (E) an incomplete external mould of the head-shield V 25135.7b; (F) an incomplete external mould of the head-shield V 25135.2b. Abbreviations see as above. Scale bars are 2 mm.

The pineal opening is well preserved in specimen V 25135.3, which is located posterior to the level of the posterior margin of the orbital opening in the midline of the head-shield. The pineal opening is tiny and oval in outline with the length of the long axis about 0.54 mm, and the length of the short axis about 0.41 mm. Along the midline, the length of the pre-pineal region of head-shield ranges from 5.4 to 6.7 mm and the length of the postpineal region ranges from 4.2 to 6.1 mm in 11 specimens.

The sensory canal system of Sinogaleaspis shankouensis is comprehensively reconstructed based on 10 specimens. The well-preserved sensory canal system displays an Eugaleaspid pattern in distribution which consists of anterior supraorbital canals, posterior supraorbital canals, infraorbital canals, lateral dorsal canals, lateral transverse canals, median dorsal canals, preorbital commissure, median transverse canals and a short central canal. The paired anterior supraorbital canals slightly diverge posteriorly from the rostral margin, but do not connect with the posterior supraorbital canal. The paired posterior supraorbital canals are V-shaped and converge posteriorly to the pineal opening.

The characterised median dorsal canal joins with the posterior supraorbital canal in about 45 degree angle at the level of the pineal opening, and posteriorly converges with the opposite one to form a U-shaped trajectory. A short previously unnamed longitudinal canal issuing from the middle of the U-shaped median dorsal canal, here, is termed as the central canal, The infraorbital canals are positioned lateral to the orbital opening. Posteriorly, the infraorbital canal continues to extend as the lateral dorsal canal, which is the main longitudinal canal on the head-shield. There are six pairs of lateral transverse canals and median transverse canals observed in our new specimens. In addition to four pairs of lateral transverse canals running from the lateral dorsal canal two pair of additional lateral transverse canals are observed issuing from the infraorbital canals in specimen V 25135.7b. There are six pairs of median transverse canals. In addition to the originally described 3 pairs of median transverse canals, three pairs of additional median transverse canals are observed in our new specimens. The most anterior pair of short transverse canals unite the infraorbital and posterior supraorbital canals in front of orbital openings, which probably represent the first median transverse canal, the preorbital commissure.

The lateral margin of the head-shield is smooth and the exoskeleton of the head-shield is ornamented with closely set, tiny granular tubercles. The tubercles in the central part of the head-shield are bigger than those in the lateral part and the cornual process. There are about 10–15 tubercles per square millimeter in the center, whereas about 20–25 tubercles per square millimeter in the region of the cornual process.

Restoration of Sinogaleaspis shankouensis (A) dorsal view; (B) anterior-lateral view. Gai et al. (2020).

Sinogaleaspis shankouensis can be referred definitely to the Eugaleaspiformes by the longitudinal oval median dorsal opening and typical eugaleaspid-pattern sensory canal system. The three species assigned to Sinogaleaspis do not form a monophyletic group, but a paraphyletic group. The type species of Sinogaleaspis, Sinogaleaspis shankouensis has a closer relationship to Yunnanogaleaspis and higher eugaleaspids than to ‘Sinogaleaspis.’ zhejiangensis and ‘Sinogaleaspis.’ xikengensis, whereas ‘Sinogaleaspis.’ zhejiangensis is resolved as the deepest branching of the Eugaleaspiformes in later phylogenetic analyses. Based on abundant new materials, especially the 3D preserved neurocrania, a new genus, Shuyu was erected for ‘Sinogaleaspis’ zhejiangensis in 2011. By contrast, the nature of ‘Sinogaleaspis.’ xikengensis still needs more data for corroboration as the original description is mainly based on one poorly known specimen, especially its sensory canal system remains unknown. Sinogaleaspis shankouensis is similar to Shuyu zhejiangensis, Meishanaspis lehmani, and Anjiaspis reticularis from the Lower Silurian of Zhejiang Province in the subtriangular head-shield with spine-like cornual and inner cornual processes, but clearly different from them in the shape and position of median dorsal opening, the margin of head-shield, and pattern of sensory canals. The sensory canal system of Shuyu zhejiangensis and Meishanaspis lehmani are not of the typical Eugaleaspid-pattern by being deficient of the median dorsal canal, probably representing a plesiomorphic condition of the Eugaleaspiformes. 

By contrast, Sinogaleaspis shankouensis exhibits a mélange of primitive and derived characters in the pattern of sensory canals. It possesses the U-shaped medial dorsal canal which is regarded as a derived character found in Eugaleaspiformes, V-shaped posterior supraorbital canals which is a derived character found in Polybranchiaspiformes and Huananaspiformes, while it has more than one median transverse canals which is regarded as a primitive character found in plesiomorphic taxa such as Dayongaspidae, Hanyangaspidae and Xiushuiaspidae. In addition to four pairs of lateral transverse canals issuing from the lateral dorsal canal, the new materials of Sinogaleaspis shankouensis also reveal two pairs of lateral transverse canals on infraorbital canals, a condition is strikingly similar to that of Shuyu and Meishanaspis. Ususally, there are 3–4 pairs of lateral transverse canals issuing from the infraorbital canal in the plesiomorphic taxa of Galeaspids such as Dayongaspis, Hanyangaspis, and Changxingaspis. It has been suggested that the number of lateral transverse canals has a decreasing tendency from six pairs to three pairs in Eugaleaspiformes. The vestiges of the lateral transverse canals can be observed on the infraorbital canals in some members of Eugaleaspiformes (e.g. Eugaleaspis changi and Eugaleaspis xujiachongensis) and Polybranchiaspiformes (e.g. Polybranchiaspis liaojiaoshanensis and Laxaspis qujingensis), which can be regard as a corroborative evidence for this evolutionary tendency.

Time-callibrated cladogram of the Eugaleaspiformes. Eugaleaspiformes experienced two diversifications in Silurian and Devonian (Solid columns represent known time ranges, thin lines represent ‘ghost lineages’). Gai et al. (2020).

The most noteworthy feature of Sinogaleaspis shankouensis and Anjiaspis reticularis is the presence of much more than two pairs of median transverse canals (6 pairs in the former, 8 pairs in the latter), which is odd among about 80 described galeaspid species and has been questioned in several previous studies. In addition to the originally described 3 pairs of median transverse canals, additional three pairs of median transverse canals are observed in our new specimens of Sinogaleaspis shankouensis. The six pairs of transverse canals (median transverse canals coupled with lateral transverse canals) form a grid distribution on the dorsal side of head-shield by intersecting with four longitudinal pairs (infraorbital canals, lateral dorsal canals, posterior supraorbital canals, and median dorsal canals). The most anteiror pair of median transverse canals, the preorbital commissures which link the infraorbital and posterior supraorbital canals in front of orbital openings, are also observed in some members of Polybranchiaspiformes and Huananaspiformes such as Polybranchiaspis, Laxaspis and Sanchaspis. In addition, a short previously unnamed longitudinal canal of Sinogaleaspis shankouensis, the central canal is also observed in Changxingaspis, Polybranchiaspis, Laxaspis and Sanchaspis issuing from the middle of dorsal commissure. In sum, the sensory canal system of Sinogaleaspis shankouensis exhibits mosaic characters of plesiomorphic taxa, Eugaleaspiformes, Polybranchiaspiformes and Huananaspiformes, and probably represents plesiomorphic condition of Vertebrates.

The fossil-bearing strata of Sinogaleaspis shankouensis also yield ‘Sinogaleaspis.’ xikengensis, Xiushuiaspis jiangxiensis, and Xiushuiaspis ganbeiensis, which are together named the Sinogaleaspis-Xiushuiaspis Fauna or the Maoshan Assemblage, because similar galeaspid species including Shuyu zhejiangensis, Meishanaspis lehmani, Anjiaspis reticularis and Changxingaspis gui were found in the Maoshan Formation of Zhejiang Province. The Sinogaleaspis-Xiushuiaspis Fauna together with other plesiomorphic taxa of Galeaspids (Dayongaspis, Hanyangaspis, Kalpinolepis, Xiyuaspis, and Microphymaspis) and the most primitive Polybranchiaspiforme (Platylomaspis) from the Silurian of the South China and Tarim probably represent the first diversification of Galeaspids in the Telychian, Llandovery of the Silurian, but they underwent a mass extinction during the Ludlow except for some Eugaleaspiformes and Polybranchiaspiforme survived until . the late Pragian of the Devonian. The Fish-bearing strata are referred to Silurian ‘upper red beds’ which reflect a nearshore, very shallow deltaic environment with the low nutrient, euphotic zone and rich oxidation. According to the evidence from the paleosalinity, and other geochemical data, it has been suggested that the sedimentary paleoenvironment of the Xikeng Formation at Xiushui, Jiangxi Province was suggestive of a brackish water body probably caused by a large deal of fresh water pouring into the sea from nearby rivers. In particular, most of specimens were collected from the Layer 13, which was composed of grey-green pelitic siltstone interbedded with a large deal of muddy gravels, strongly suggesting a short distance of potamic transportation. This indicates that the Sinogaleaspis-Xiushuiaspis Fauna may live in a fresh water environment in the river rather than their burial area in the sea.

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Biomineralising Tunicates from the Late Triassic of Italy.

Biologically controlled biomineralisation is a fundamental process in the evolution of the animal kingdom. Though it was sporadically achieved already in the latest Precambrian, the so-called 'Cambrian Explosion' around 540 million years ago, marks the general advent of Animals with hard parts in the fossil record. It is well known, that this event was not an instantaneous process, but that it was accomplished in a time span of about 20–25 million years. The acquisition and evolution of hard skeletons within the great majority of phyla and classes are the basic prerequisites for the reconstruction of animal form and function through time. Once hard parts were created in order to protect and stabilize soft tissues and organs, this evolutionary advantage indeed developed a great variety of morphologies, but in spite of this diversity and modification, solid skeletons were generally maintained as such until extinction of the equivalent groups.

In a paper published in the Journal of Paleontology on 24 January 2020, Jobst Wendt of the Fachbereich Geowissenschaften der Universität Tübingen, describes and decrypts an unusual case of biomineralisation in fossils of the rather poorly known class Ascidiacea (Sea Squirts) of the subphylum Tunicata.

The reduction of a hard skeleton into a mere softbodied stage seems to have been achieved in some Coleoid Cephalopods and Opisthobranch Sea Slugs, but this accordance is based on notional palaeontological evidence. An alleged late acquisition of a hard skeleton inherited from soft-bodied ancestors is suspected in Bryozoans and Scleractinian Corals, but in contrast to Ascidians, once these groups had acquired calcified skeletons, they maintained this protective and stabilizing advantage until Recent. Thus, the very late acquirement of a solid calcareous skeleton among Tunicates and its later loss in favor of an almost soft-bodied stage is a very rare phenomenon in the geological record.

From their form and mode of life, Ascidiacea (Sea Squirts) appear as rather simple-structured organisms that superficially and functionally are evocative of Porifera. But the presence of a tubular dorsal nerve cord and a notochord in their larval stage and pharyngeal clefts allocates them amuch higher position within the Animal Kingdom as a subphylum of the Chordata. Ascidians are an artificial, polyphyletic group that comprises three orders (Aplousobranchiata, Stolidobranchiata, Phlebobranchiata) with a total of about 2940 living species. They generally lack hard parts, thus minimizing the possibility of preservation. Only a few genera of the orders Aplousobranchiata and Stolidobranchiata segregate tiny isolated spicules, embedded in the mantle (tunica) or other organs, but their origin remains unknown. Tunica is the Latin word for mantle. Therefore, many zoologists regard both terms as synonymous. Others, however, distinguish an inner layer (tunic), in which the spicules are secreted, from an outer one (mantle) In order to guard against misunderstandings, Wendt uses both terms as synonyms because, with regard to fossils, the differential application of both terms would appear arbitrary.

Spicules of cpossible Tunicates have been found in rocks as old as lower Liassic. Generally, ascidian spicules consist of aragonite, more rarely of vaterite or other minerals. Unlike some siliceous sponge spicules, they are never fused together and thus could not form solid skeletons. The discovery of complete soft-bodied ascidians (lacking spicules, however) in the lower Cambrian of China has revealed that ascidians existed already at an early stage of Metazoan evolution, but this finding does not allow to reconstruct their relationships to Mesozoic to recent representatives of this class. A new evolutionary branch among Ascidians is the recent discovery of calcareous Tunicate exoskeletons in the Upper Triassic and their ancestors in the Permian. 

Wendt suggests that these very unusual skeletons, which are composed of irregular aragonitic plates, can be assigned to no phylum other than the Tunicates. This systematic position of these previously unknown organisms to the class Ascidiacea has been the fundamental clue for the systematic attribution of the new discoveries described by Wendt. The fact, however, that they are endoskeletons, opens an unusual insight into a new aspect of the evolution of this poorly known class which has no counterpart among their living representatives. Tunicate exoskeletons have long been known from Permian deposits in east Asia and Europe, but until recently they were erroneously attributed to Rugose Corals. These skeletons are composed of a varying number (2 to about 35) of irregular plates that consist of acicular aragonite. This very unusual construction and mineralogy were crucial for their systematic attribution, though similar living representatives of this class are unknown. Comparably organized endoskeletons now add a new aspect to the fossil record of this largely ignored subphylum of the Chordata.

Endoskeletons are widespread in the Animal Kingdom and display a great variety of shape, function, and mineralogical composition. They are typical for the major phyla of Deuterostomia (Echinodermata and Vertebrata, including Conodontophorida) and many Protista (Foraminifera and Radiolaria), which exhibit a stunning variety of shapes and geometries. Endoskeletal spicules (sclerites) of different mineralogical composition evolved independently among several phyla and classes (Porifera, Octocorallia, Vermes, Holothuroidea, Tunicata). Apart from the calcareous octocoral Tubipora musica, only some Porifera only some Porifera (Hexactinellida and Lithistida), produced siliceous spicules, which are fused or articulated into compound endoskeletons forming cubic or irregular meshworks, thus giving more stability to the soft body. Among other phyla, compact solid calcareous endoskeletons occur only among the Mollusca. Composite calcareous endoskeletons consisting of numerous plates with flexible boundaries are characteristic for Echinodermata and for the newly discovered fossil Tunicata.

Wendt describes two incomplete specimens from the the Cassian Formation (lower Carnian) of the Dolomites (northern Italy). Extensive searches for additional, either biostratigraphically older, contemporaneous, or younger specimens from European and North American reef specialists and collections were unsuccessful. The two specimens were discovered among far over a million of skeletal remains from the the Cassian Formation. The number of taxa and the diversity of skeletal remains collected from the Cassian Formation during almost two centuries are indeed impressive and assign this rock unit a singular rank among the Fossil-Lagerstätten in the geological record. As of 2019, 1429 species have been described so far from this formation. A special feature of this unrivaled fauna is their often excellent state of preservation, exemplified by diagenetically almost unaltered aragonitic microstructures, which are among the oldest in earth history. In this state of perfection and considering its age, the Cassian Fauna can even be regarded as unique.

Fossil record of Tunicates. Black dot, soft-bodied; black triangles, calcareous exoskeletons; black square, calcareous endoskeletons; asterisks, spicules. Wendt (2020).

The fossils are placed in a new order and family of Ascidians, named the Cassianosomidae and Cassianomorpha respectively; both names refer to the Cassian Formation. Both are placed in the new genus Toscanisoma, which is named in honour of Maria Luigia Toscani, who collected the fossils upon which the study is based. Each of the two fossils is described as a separate species within the new genus.

The first species described is named Toscanisoma multipartitum, where 'multipartitum' means 'consisting of several parts'. This is a sessile colonial species consisting of seven branching tubes (zooids) composed of irregular plates that are joined by straight or curved boundaries on the outer and zigzag ones on the inner side. The only complete zooid of the colony is closed at the top by six plates that are smaller than the remaining ones. The other zooids are incomplete, but were probably slightly larger. Spicules have not been observed.

Toscanisoma multipartitum holotype (GPIT/TU 82). (1) Lateral view (drawing); (2) opposite side; (3) view from top; (4) base. Asterisks mark complete zooids, arrows indicate zigzag sutures on inner surfaces. Scale bar 10 mm. Wendt (2020).

The only specimen of Toscanisoma multipartitum available is not complete, lacking the basal portion, which probably served as a holdfast. The seven zooids, which bud from the flattened base, have different shapes ranging from almost circular to elongate or flattened in cross section. Only the smallest zooid is complete, 25 mm long and consists of about 10 plates with very faint and barely recognisable outer boundaries. The inner plate boundaries of the incomplete zooids show zigzag sutures similar to contemporaneous representatives of the order Khmeriamorpha. The zooids fit closely together at the base and are separated by small open interspaces higher up.

The second new species is named Toscanisoma triplicatum, meaning 'triplicate'. as the only known specimen consists of three zooids. This species is distinguished from Toscanisoma multipartitum by the presence of spicules, the different kind of budding, in which the separation into three individual zooids starts at a distance of about 2 cm above the base. Thus, in their early growth stage, the individual zooids share the outer walls of the adjacent ones, but higher up than in Toscanisoma multipartitum.

Toscanisoma triplicatum holotype (GPIT/TU 83). (3) Lateral view; (4) view from base; (5) view from top; 1, 2, 3 mark cavities of three incomplete zooids. Scale bar 15 mm. Wendt (2020).

Because of the fragileness of the specimen, the inner side of the basal cup and the individual zooids could not be sufficiently prepared, but it is assumed that the inner plate boundaries show zigzag sutures similar to Toscanisoma multipartitum. Due to the incomplete state of preservation and the lack of comparable material, speculations about the final growth form and the number of zooids are premature.

The morphological and mineralogical features described by Wendt raise the pivotal question of the systematic attribution and the functional morphology of these enigmatic remains. Solid skeletons (whether exo- or endo-) composed of irregular plates that consist of acicular aragonite crystals are unknown in the Animal Kingdom, with one exception: Permian/Triassic Ascidian Tunicates with a compound calcareous skeleton, which appeared in the early(?) Permian and became extinct during the Late Triassic. These remains reveal a certain relationship to some living Ascidians (e.g. Chelyosoma and Forbesella), although these lack any hard parts. But they are partially composed of irregular soft plates that can be moved by muscles. The hinge-like sutures on the inner plate surfaces of the newly described fossil endoskeletons also suggest a certain flexibility of the compound skeleton, which is indispensable for their here proposed assignment to Ascidian Tunicates. The latter are filter-feeders in which a steady flow of seawater moves through an atrial and a branchial siphon. If we assign a similar function to the fossil counterparts described by Wendt, it must be required that one or two of the top plates could be opened by muscles during the life-time of the organism because this is the case in the genera Chelyosoma and Forbesella. Unfortunately, the upper part of the examined specimens generally is not preserved. But one zooid of Toscanisoma multipartitum is complete and exhibits a mosaic of tiny plates, which probably could be opened by muscles to allowaccess for the protruding siphons.

At a first glance, the presence of a solid calcareous endoskeleton in Cassianomorpha might be surprising. However, one should bear in mind that, apart from Arthropoda (which have a totally different kind of growth), all invertebrate classes with a calcareous exoskeleton show well-developed growth lines reflecting an intermittent growth at the edge of the skeleton-secreting tissue. This is not the case in the Cassianomorpha in which the skeleton is formed within the mantle (tunica) starting more or less simultaneously at several nucleation points. Faced with these observations, it is less surprising that the extinct Cassianomorpha developed an endoskeleton, as did the other main Deuterostome phyla or classes (Echinodermata, Vertebrata, Jurassic to recent Ascidiacea), than the fact that the other contemporaneous fossil Ascidian order Khmeriamorpha strangely developed an exoskeleton.

By mid-Cambrian times, representatives of almost all Metazoan phyla and subphyla had reached a high degree of biomineralisation in such away to enable a reliable reconstruction of their evolution. Why is this not the case with Tunicates, which must have existed contemporaneously? It is possible to speculate about a change in seawater chemistry or a possible global perturbation of the carbon cycle near the Carboniferous/Permian boundary, but such speculations about the appearance of mineralised (aragonitic) tunicate skeletons at this interval appear rather theoretical, because the time-span of an 'aragonitic ocean' (Mississippian to Middle Jurassic) is not consistent with the existence of aragonitic Ascician skeletons.

A possible answer to this fundamental question is that compound pre-Permian tunicate skeletons might have existed in earlier Paleozoic times, but they have not yet been discovered or recognized as such. It can also not be totally dismissed that they are hidden among the great number of previously described fossil Problematica or those of incertae sedis. The discovery of rare spicules embedded in the solid skeleton clearly points to a new and successful attempt of Tunicate biomineralisation in the Late Triassic, which persisted until recent. However, post-Triassic survivors of Tunicates with compound calcareous, either endo- or exoskeletons, are unknown from the fossil record. Wendt speculates that post-Triassic Ascidians developed other protective strategies that served as a defensive function (e.g., secretion of indigestible chemicals), which, of course, cannot be detected in fossil remains. Not taking into account these theoretical considerations, the total loss of a compound calcareous skeleton in Ascidians during the Late Triassic in favor of a much less stable one consisting of isolated spicules only remains an unresolved question.

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