New Chroniosuchian materials from the Permian and Triassic of Xinjiang Province, China

The Chroniosuchians were an enigmatic clade of non-Amniotic Tetrapods with uncertain phylogenetic position. This group can be divided into two families, Bystrowianidae and Chroniosuchidae. The earliest representatives of both families are known from the middle Permian Dashankou Fauna of Gansu Province, China. The latest representative of Chroniosuchidae is known only from a single Triassic taxon, Madygenerpeton pustulatus, from the Ladinian or Carnian of Kyrgyzstan; while those of Bystrowianidae are known from Middle Triassic of Russia and Germany.

This group was first reported from the late Permian Jiyuan fauna in China, i.e. Bystrowiana sinica; later, two more species were named and referred to Bystrowianidae for this fauna (Jiyuanitectum flatum and Dromotectum largum). Some postcranial bones were identified as Bystrowianid from the Permo-Triassic Guodikeng Formation of Jimusar in Xinjiang Province, China, but no definite Triassic Chroniosuchian is known from China up to now. Recent fieldwork has shown that Chroniosuchians were a diverse and persistent group in Xinjiang: they survived from the late Permian to at least the Early Triassic here.

In a paper published in the journal Vertebrata PalAsiatica in October 2020, Liu Jun 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 Center for Excellence in Life and Paleoenvironment, and the College of Earth and Planetary Sciences of the University of the Chinese Academy of Sciences, reports three new Chroniosuchian specimens from Xinjiang, including the first definite Triassic Chroniosuchian from the Jiucaiyuan Formation.

 

Permian to Triassic stratigraphic sequence within Turpan-Hami Basin, Xinjiang, showing the horizons of studying specimens. Liu (2020).

The first specimen described, IVPP V 26539, is a left femur from the upper part of the middle Permian Quanzijie Formation at the Dalongkou section in Santai in Jimusar, Xinjiang. 

This femur is slightly dorsoventrally compressed, and is not so curved as the femur of IVPP V 23295. It is slender and waisted as other Chroniosuchians. The anterior margin is more concave than the posterior margin. The length of the bone is 40 mm, and the width is 13, 15 mm respectively for the proximal and distal ends. Both expanded ends are incompletely ossified. The unfinished proximal articular surface is convex in outline dorsally and slightly concave ventrally when viewed mesially. It extends anteroventrally, forms a continuous surface with the unfinished internal trochanter. The proximal articular surface decreases in dorsoventral height backwards and forms a pointed angle posteriorly.

 

Chroniosuchian left femur (IVPP V 26539) from the Quanzijie Formation in Jimusar, Xinjiang in extensor (A), anterior (B), flexor (C), proximal (D), and distal (E) views Abbreviations: ab. adductor blade; ac. anterior (tibial) condyle; adc. adductor crest; ff. fibula fossa; icg. intercondylar groove; it. internal trochanter; itf. intertrochanteric fossa; pa. popliteal area; pc. posterior (fibular) condyle. Liu (2020).

The dorsal surface of the femur bears striations on the proximal side. The dorsal surface of the distal end of the femur is divided by a relatively broad intercondylar groove. The anterior condyle extends slightly wider than the posterior one. Due to the incomplete ossification, the anterior condyle projects distally similar to the posterior condyle. The anterodorsal surface of the anterior condyle, near the middle, bears distinct longitudinal ridges for ligaments.

The adductor blade is low, directed posteroventrally, forming the anterior margin of the large intertrochanteric fossa. The anterior surface of the adductor blade is rugose. Distal to the adductor blade, a low but distinct adductor crest is slightly curved, runs distally near the posterior margin of the shaft on the ventral surface, and ends by a crack. Distal to the crest, there is a low ridge to a point close to the triangular popliteal area. However, it is likely separated from the adductor crest. Posterior to it, another crest runs on the posterior side of the shaft ending on the distal posterior corner. Two crests frame the small fibula fossa on the posteroventral corner of the distal end.

The shape of this femur is primitive among Tetrapods. However, it is slender, similar to that of Temnospondyls like Trimerorhachis or Chroniosuchians, rather than robust as in many other basal Tetrapod groups. As in IVPP V 23295 and Bystrowiella schumanni, the adductor crest follows a diagonal course, ending distally close to the posterior margin of the shaft. In most Permian Tetrapods, adductor crest generally runs towards, not posterior to the popliteal area. This could be a diagnostic character of Chroniosuchians.

IVPP V 26539 also shares the following features with other chroniosuchian femurs (V 23295 and SMNS 96948): the adductor blade is directed posteroventrally, and the posterior end of the proximal articular surface is pointed. So this specimen can be referred to Chroniosuchia.

This specimen has unfinished ends, as in the much longer femur of SMNS 96948, may indicated juvenile state of both specimens. Meanwhile, a medium sized femur of V 23295 is well-ossified, indicated that species has a smaller adult size.

The second specimen described, IVPP V 26540, comprises four vertebrae, one rib, and several scutes, from the base of the upper Permian Guodikeng Formation at the Taoshuyuan (Taoxigou) section,  at Turpan in Xinjiang. 

There are four articulated and one isolated vertebrae, two of them are nearly complete. They are identified as the vertebrae around sacral region. The vertebrae have a basic shape of Bystrowianidae. 

 

Bystrowianid specimen (IVPP V 26540) from the Guodikeng Formation in Turpan, Xinjiang (A)–(B) Four continuous vertebrae in ventral (A) and right lateral (B) views; (C)–(D) Isolated vertebra in anterior (C) and posterior (D) views; (E) An incomplete rib; (F)–(G) The impression of dorsal surface of three osteoderms (F) and two osteoderms (G) Abbreviations: dp. diapophysis; ic. intercentrum; ns. neural spine; pc. pleurocentrum; pnc. paraneural canal; poz. postzygapophysis; pp. parapophysis; prz. prezygapophysis; tp. transverse process; vp. ventral process of osteoderm; vr. ventral ridge. Lui (2020).

Only two articulated intercentra are exposed. The anterior one shows smooth periosteal bone on ventral and lateral surfaces. Its lateral surface bears part of the parapophyses for articulation with the capitulum near the posterior margin, and ventral surface has no haemal arch. In lateral view, its length relative to the pleurocentrum is similar to IVPP V 23295, much narrower than in Chroniosaurus dongusensis.

The lengths of the pleurocentra are approximately 10 mm, while the heights are about 9 mm. The pleurocentra are massive with a round cross-section, and they are not perforated by the notochord. The ventral surface is relatively flat (slightly convex on the anterior one, slightly concave on the posterior one) with faint ventral ridges on three articulated larger pleurocentra. Two longitudinally aligned, low ridges are clear on the narrow ventral face of the smaller pleurocentrum of the isolated vertebra. This feature has been proposed as a Bystrowianid characteristic. The neural arches are fused to the pleurocentra, but their suture is clear; so this specimen looks like in a younger stage than V 23295, in which the suture is absent. This suture runs across the diapophysis in three articulated vertebrae, indicating the pleurocentrum participates in formation of the diapophysis. On two posterior pleurocentra, the anterior half of the ventrolateral surface participates in formation of the parapophysis, which is separated from the diapophysis by a narrow groove. So they are identified as the sacral and the first caudal vertebrae, as in Kotlassia and Proterogyrinus.

The transverse processes are very short and massive, directed ventrolaterally. The facets of the diapophyses are strongly enlarged in the sacral and first caudal. In the isolated vertebra, the transverse processes are completely formed by the neural arch. It is probably derived from the anterior tail region.

The neural spines are only nearly complete in the sacral vertebra and one isolated caudal vertebra. The neural spines are anteroposteriorly slightly shorter than the pleurocentrum. In the sacral, its anterior and posterior margins are nearly parallel. Its height is less than twice the height of the pleurocentrum. Its dorsal tip is connected with the osteoderm by interdigitating sutures. In the isolated caudal, the neural spine widens dorsally near the tip then narrows. Its dorsal tip does not carry an osteoderm.

A curved incomplete rib measures more than 5 cm in original length. The proximal side is not preserved.

There are two pieces of impression of the dermal surface of the osteoderms, one with three osteoderms, and the other with two osteoderms. Neither of the osteoderms is complete, so it is unsure on the width of the osteoderms. The distinct dermal sculpturing is composed of ridges and depressions. A mid-ridge is preserved. The pits have different sizes on two sides of the ridge, and they likely arrange in posterolaterally radiated line. No prominent parasagittal ridge is observed.

This specimen is diagnosed as a Bystrowianid chroniosuchian for the presence of the sculptured dorsal osteoderms which are sutured with the neural spine, paired deep paraneural canals on anterior and posterior surfaces of the neural arch. It cannot be further diagnosed for the poor preservation of the osteoderms.

The final specimen described, IVPP V 26541, is an incomplete dorsal osteoderm from the Lower Triassic Jiucaiyuan Formation, at a location 4 km east to the Dalongkou section at Santai in Jimusar, Xinjiang. 

This single osteoderm is slightly convex dorsally. Most of the margins are broken other than right anterolateral margin The complete width should be approximately 3 cm. The dermal sculpture is not symmetric, and it is dominated by different-sized polygonal pits formed mainly by longitudinally oriented and oblique ridges. Transversely elongated sculptural depressions are only present near the posterior area. The sculpture has no axial crest or any parasagittal ridge, as in Synesuchus muravjevi or Dromotectum spinosum. The right anterior wing is preserved its posterior portion, with a smooth dorsal facet for the facies alaris (ventral facets) of the preceding osteoderm. The right accessory process is also partially preserved. The maximum width of one anterior wing is estimated as 9 mm, slightly smaller than the width of accessory processes.

 

Bystrowianid osteoderm (IVPP V 26541) from the Jiucaiyuan Formation in Jimusar, Xinjiang in dorsal (A), ventral (B), and posterior (C) views Abbreviations: am. area marginalis; ap. accessory process; aw. anterior wing; cl. lateral crista; cm. medial crista; co. crista obliqua; df. dorsal facet; dv. ventral depression; fa. facies alaris; pv. ventral process; sa. sulcus articularis. Liu (2020).

The incomplete median articular plate measures 10 mm in width. It lies posteriorly and slightly ventral to the sculptured dorsal plate. The median crista decreases in width backwards and is triangular in shape in dorsal view. The broad lateral cristae are longer than and located at the same level as the median crista. Most parts of these cristae are formed by tapered extensions of the posterior margin of the sculptured dorsal plate. As most Bystrowianids, two deep longitudinal grooves (sulci articulares) lies between the median and lateral crests, for accommodation of the accessory processes and associated ligaments of the successive osteoderm. The area marginalis (marginal zone) is incompletely preserved on the right side. It extends anteriorly ventral to the dorsal plate as in Bystrowiana and Jiyuanitectum.

On the ventral surface, the shallow median depression, between the anterior wings back to the base of the ventral process, is preserved for most of the right side and nearly posterior half of the left side. The ridge extends from the right accessory process is distinct. The sulcus medius (median groove) should be narrow. A broad, low but distinct crista obliqua (oblique crest) extends along the ventral surface of the osteoderm posteromedially from the anterior wings toward the region of the ventral process, and continuous to the margin zone of the median articular plate. Lateral to the ventral margin, a shallow depression on the posterior part of the ventral surface is the ventral facet that overlapped the dorsal facet of the anterior wing of the succeeding osteoderm.

The base of the ventral process is located in the posterior half of the osteoderm, and its posterior margin extends posteriorly beyond the boundary of the sculptured surface of the osteoderm. The process is ovate in transverse section and lacks anterior and posterior extensions.

IVPP V 26541 can be referred to Bystrowianidae based on the unpaired posterior articular plate and weakly expanded plate bearing broadly separated anterior facets. IVPP V 26541 represents a new taxon closely related to Dromotectum. However, it is not named by Liu, who feels it should be named from more complete material. Its close relationship with Dromotectum is not surprise, because Dromotectum existed in the upper Permian of Henan, China.

The new specimens described here increase the diversity of Permo-Triassic Tetrapods. Previously, only one Tetrapod species, Kunpania scopulusa, was reported from the top of the Quanzijie Formation. Also, only one Chroniosuchian specimen was reported from the Guodikeng Formation. Although Chroniosuchian should have existed from middle Permian in Xinjiang based on their known distributions, this is the first evidence which confirmed its existence from the Quanzijie Formation. The bystrowianian Chroniosuchian specimens from the base and the top of the Guodikeng Formation and the Jiucaiyuan Formation demonstrated that this group survived in the end-Permian mass extinction here in Xinjiang, together with Lystrosaurus.

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Dendrochernes mahnerti: A new species of Pseudoscorpion from Xinjiang Province, China.

The Chernetidae is the most speciose family in the Pseudoscorpiones and is composed of over 650 species in more than 110 genera, but only 10 genera and 16 species have been reported from China. The chernetid genus Dendrochernes was erected by Max Beier in 1932, and at present comprises only four known species: the type species Dendrochernes cyrneus is widespread in central Asia (Pakistan, Kazakhstan and Kyrgyzstan), north Africa and Europe; while the remaining three species occur in north America: Dendrochernes crassus, Dendrochernes morosus, and Dendrochernes instabilis.

In a paper published in the journal Arthropoda Selecta on 19 June 2020, Zhizhong Gao of the Department of Biology and Wutai Mountain Institute of Resource and Environment at Xinzhou Teachers University, and Feng Zhang of the Key Laboratory of Zoological Systematics and Application at Hebei University, describe a new species of Dendrochernes from Xinjiang Province, China.

Kanas Lake, located in a valley in the Altai Mountains in the Burqin County of Altay Prefecture, northwestern China, is China’s deepest freshwater alpine lake, with a high biodiversity. It borders with the country of Kazakhstan, Mongolia and Russia.

While examining Pseudoscorpion specimens collected from Xinjiang, two chernetid specimens presenting the generic characters of Dendrochernes were examined: eye spots lacking or indistinct; tergites divided except the last one and finely to moderately granulate; setae of body and palps broadened and thickened, denticulate and short, never strongly clavate; rallum with 4 (but occasionally 3) blades; palps stout, finely to moderately granulate; accessory teeth of chelal fingers numerous; tactile seta sub-terminal of movable finger nearer to sub-basal than to terminal; fixed finger with tactile seta interior sub-terminal at the level of or very little distal to the level of exterior sub-terminal; tarsus IV with a tactile seta located distal to the midpoint of the tarsus, being 0.6 to 0.7 of the length of the tarsus from the proximal margin of the segment; the spermatheca of the female with two long, slender tubules, of uniform diameter throughout, greatly coiled, and without a terminal enlargement or bulb.

The new species is named Dendrochernes mahnerti, in honour of Volker Mahnert, a famous zoologist and arachnologist, who has made significant contribution and leaves behind an outstanding scientific legacy on Pseudoscorpions. It is described from two female specimens, collected by Feng Zheng in August 2006 from under tree bark, at an altitude of 1456 m above sealevel, in the Kanas National Nature Reserve in Burqin  County in the Xinjiang Uygur Autonomous Region of China.

 

Dendrochernes mahnerti, (a) female holotype habitus, dorsal view; (b) female paratype habitus, dorsal view. Gao & Zhang (2020).

The colour of Dendrochernes mahnerti is mostly reddish brown, carapace, legs brown, tergites and palps darker, remaining parts (sternites and pleural membranes) light yellowish brown. Most setae slightly apically denticulate.

The surface of the carapace is evenly granular. Slightly longer than broad (1.07–1.17 times), eyespots indistinct, with two regularly granular transverse furrows, both of them very distinct, the subbasal one slightly nearer to the posterior margin than to median furrow, a longitudinal shallow groove in metazone. With about 110 setae, including 6 on anterior margin and 12–13 on posterior margin. All setae short and apically dentate.

All tergites of the abdomen are widely divided except the last one. Lateral keels absent; weakly scale-shaped sculpture. Half-tergites with about 8–11 setae, tergite VI with 13–16 setae (include 2 long tactile setae), anus with 2 simple and acuminate setae. Manducatory process with 5 setae. All sternites distinctly divided except VI, weakly scaly sculptured, setae simple and acuminate, chaetotaxy (IV–XI): 6(5)–6: 13–13(16): 15(16)–15: 14(17)–14(15): 11(15)–12(15): 11(15)–11(16): 9(11)–10(11): 12 (include 4 long tactile setae): 2 (simple and acuminate setae). Coxae of pedipalps scale-shaped sculpture, with about 21, coxae I with about 15, II 20, III about 25 setae, IV numerous setae. Anterior genital operculum with about 31–32 simple and acuminate setae, 21–22 setae in a row along the posterior margin of the posterior operculum.

Spermatheca with 2 long tubules, both of them without distinct terminal enlargements or bulbs.

The chelicera have weakly scale-shaped sculpture. Five setae in basal part, all of them simple and acuminate; with 2 lyrifissures on the dorsal face of palm; fixed fingers with a few different sized teeth in the end. Movable finger with a seta in midway of terminal of finger; serrula exterior with 23–24 lamellae. Rallum usually with 4 blades, but occasionally with 3, anterior blade slightly denticulate. Galea with 6 short branches.

Palp slender, most segments normally granulate; except for chelal fingers, which are finely gaping; setae acuminate and weakly apically dentate; without tactile setae on femur, patella or hand; trochanter with distinct rounded dorsal hump; proportions (based on two specimens): trochanter 1.41–1.70 times as long as broad; femur 2.63–2.71 times as long as broad; patella 2.18–2.23 times as long as broad; chela with pedicel 2.63–2.75 times as long as broad, chela without pedicel 2.45–2.54 times as long as broad, hand with pedicel 1.56–1.59 times, without pedicel 1.37–1.41 times, as long as broad. Movable finger 0.77–0.78 times as long as hand with pedicel, and 0.88 times without pedicel. fixed finger with 36–37 teeth, 11–12 acuminate accessory teeth on lateral side, 7–8 accessory teeth on interior side; movable finger with 40–44 teeth, 10–11 acuminate accessory teeth on lateral side, 6–7 accessory teeth on interior side; nodus ramosus closer to terminal than to sub-terminal. 

Venom apparatus only present in moveable chelal fingers, venom duct slender, extend over trichobothrium terminal in movable fingers.

Leg I has a typical facies, with numerous terminal slightly denticulate and acuminate setae, tactile setae absent, weakly scale-like sculpture, claws simple; proportions: trochanter 1.00–1.31 times; femur 1.45–1.47 times; patella 2.30–2.35 times; tibia 3.43–3.54 times; tarsus 4.20–4.56 times as long as deep. Subterminal tarsal seta simple, acuminate and curved, arolium shorter than claws. 

Leg IV has a surface weakly scale-like sculptured; tibia and tarsus with numerous terminal slightly denticulate and acuminate setae, trochanter 1.68–1.71 times; femur+patella 3.23–3.48 times; tibia 4.44–4.53 times; tarsus 3.75–3.83 times as long as deep. An acute tactile seta in terminal third of tarsus and almost the half long as tarsus; arolia undivided and shorter than the simple and large claws, subterminal seta simple, acuminate and curved.

Both chelicerae of the holotype and paratype were examined in different orientations to obtain an 'all-round-view' of each rallum. Although most had four blades, which is typical of the genus, the left rallum of the holotype consisted of only three blades.

The right tubule of the spermatheca in the holotype possesses a short branch, which is unusual, we subsequently dissected the spermatheca of the paratype and there is no any modification on both tubules, so Gao and Zhang prefer to think that the presence of a short branch on the right spermatheca of the holotype is an anomaly.

Dendrochernes cyrneus is widespread across Europe and Asia, with vast literatures citations. It is difficult to identify a specimen to species level only based on immature individuals which little diagnostic characteristic can be used, for instance, extraordinarily dark coloring of the carapace, the palps and the tergites is absent in juveniles. The record from Kyrgyzstan are based on only a deutonymph; although it is possible that Dendrochernes cyrneus distributed in Kyrgyzstan, the record is still unreliable.

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Sharks from the Early Silurian of the Tarim Basin, Xinjiang Province, China.

Lower Silurian Vertebrate assemblages of the Tarim and South China tectonic blocks are dominated by Galeaspid Agnathans (Jawless FIsh) and putative Chondrichthyans (Sharks) and are united together within the recently erected Zhangjiajie Vertebrate Fauna. The Chondrichthyans are represented by two enigmatic groups, the Mongolepidida, and an as yet unnamed group. Sampling in South China has revealed these groups in Telychian strata of the Xiushan Formation in Guizhou Province and similar faunas from the Ymogantau Formation on the northwestern margin of the Tarim Basin in the Xinjiang Uygur Autonomous Region, of northwest China. The Xiushan Formation material contains a number of Mongolepid and Mongolepid-like taxa (Xinjiangichthys, Shiqianolepis, Rongolepis, and Chenolepis), whilst only Xinjiangichthys and indeterminate Chondrichthyes scales have previously been documented from the Ymogantau. Despite these and other studies on the Vertebrate fossils of the Tarim Basin, a large portion of the material collected from the area in the 1990’s has remained undescribed until now. These new specimens provide supporting evidence for the stratigraphic resolution of the Tarim Basin red beds, an issue that has proved problematic over the last 60 years or so.

In a paper published in the journal PLoS One on 13 February 2020, Plamen Andreev of the Research Center of Natural History and Culture at Qujing Normal University, and the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Wenjin Zhao, 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 University of the Chinese Academy of Sciences, and the Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment, Nian-Zhong Wang, 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, Moya Smith of the Department of Earth Sciences at the Natural History Museum, and the Faculty of Dentistry, Oral & Craniofacial Sciences, at King's College London, Qiang Li, also of the Research Center of Natural History and Culture at Qujing Normal University, Xindong Cui, 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, Min Zhu, 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 University of the Chinese Academy of Sciences, and the Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment, and Ivan Sansom of the School of Geography, Earth and Environmental Sciences at the University of Birmingham, describe a series Early Silurian Chondrichthyan fossils from the Tarim Basin of Xinjiang Province, China.

The taxa identified by Andreev et al. are placed within the total-group Chondrichthyes and add to the burgeoning diversity of Lower Palaeozoic scales that are Shark-like in their overall appearance, growth and histology. These include, in approximate stratigraphic order from the Darriwilian (Middle Ordovician) through to the Lower Devonian, Tantalepis, Tezakia and Canyonlepis, Mongolepids, Elegestolepids, and Tuvalepis. The relationship of these taxa to conventionally defined Chondrichthyans remains contentious despite recent progress in integrating scale-based trees into the phylogenetic framework of early jawed Gnathostomes. This comes at a time of renewed evaluation of the Chondrichthyan stem, following the introduction of what have previously been regarded as crown-group taxa (e.g. Doliodus and Pucapampella) into the phylogenetic space occupied by ‘Acanthodians’.

Silurian strata outcropping on the northwestern margin of the Tarim Basin form a continuous depositional sequence reaching approximately 2000m in thickness. Comprised of predominately red sandstones and mudstones, this sequence has been subdivided into four lithostratigraphic units: the Kalpintag, Tataertag, Ymogantau and Kezirtag formations. Of these, the Tataertag and Ymogantau have yielded the abundant Fish fossils that are the subject of Andreev et al.'s study.

Lower Silurian Vertebrate sites and lithostratigraphic Formations in the Tarim Basin (Xinjiang, China) sampled for this study. Map depicting the locations of the sections in Kalpin and Bachu Counties (A) and a summary log of both sections correlated with Silurian chronostratigraphy. Andreev et al. (2020).

The mostly continental shelf deposits of the Kalpintag Formation grade rapidly upwards into the grey, greyish-white siltstones, sandstones and mudstones intercalated with light-purple, purplish-red siltstones and marlstones of the Tataertag Formation. The two members of the Formation (lower and upper) are exposed only in Kalpin County where they are about 190 m thick. These sequences are thought to represent fluctuating shoreface and neritic environments.

Rare invertebrates are represented by Gastropods and Brachiopods, whereas Vertebrate macrofossils are common in the calcareous siltstone beds. A diverse array of Fish are known from the Tataertag Formation including the galeaspids Nanjiangaspis zhangi, Nanjiangaspis kalpinensis, Kalpinolepis tarimensis, Microphymaspis pani, Platycaraspis tianshanensis, Hanyangaspis guodingshanensis, Hanyangaspis sp., and the putative Chondrichthyan spine genera Sinacanthus and Neoasiacanthus. The presence of these forms has been used to interpret Tataertag as coeval with the early Telychian Rongxi Formation from the middle and lower reaches of the Yangtze River in the South China block. This agrees with Llandovery dates proposed for the Tataertag based upon Acritarchs, Scolecodonts, Cryptospores and Plant cuticles.

The Bachu and Kalpin sections of the overlying Ymogantau Formation are about 160 m and 520 m thick respectively, with strata divided into lower and upper member. These consist of purplish-red tuffaceous and argillaceous siltstones and mudstones intercalated with greyish-green tuffaceous fine-grained sandstones and siltstones and are thought to represent predominant tidal flat deposition. Invertebrate macrofossils are rare with only a few Lingulid Brachiopods (Lingula sp.), Gastropods and Bivalve Molluscs found. However, a diverse fossil Fish fauna has been described, including the Galeaspids Pseudoduyunaspis bachuensis, Hanyangaspis guodingshanensis, Hanyangaspis sp. and the putative Chondrichthyans Sinacanthus wuchangensis, Sinacanthus triangulatus, Tarimacanthus bachuensis and Xinjiangichthys pluridentatus.

Given the absence of age diagnostic taxa, dating of the Ymogantau Formation has been contentious. The main suggestions are as follows: (1) Late Devonian based upon the sedimentologic and tectonic character; (2) Early Devonian based on the Conodont Ozarkodina denckmanni collected from the lower part of the Ymogantau Formation near the Mukuleke village in the Bachu area; (3) middle Aeronian to Wenlock according to the presence of the Conodonts Ozarkodina cf. edithae, Ozarkodina sp. A and Ligonogina silurica; (4) Telychian based on Galeaspid and Sinacanthid macrofossils and the presence of the Mongolepid Xinjiangichthys, also reported from the Xiushan Formation in Shiqian, Guizhou Province.

Andreev et al.'s samples from the Ymogantau Formation contain the Conodont Ozarkodina guizhouensis together with the Mongolepids Rongolepis cosmetica, Shiqianolepis hollandi and Chenolepis asketa. These also occur in the lower Member of Xiushan Formation and suggest that the upper part of the Ymogantau sequence is of middle Telychian age.

The first fossils described are placed in the Mongolepidid family Mongolepididae and assigned to the species, Rongolepis cosmetica. These are ovate to trapezoid scales with a low-profile crown manifesting parallel odontode rows ornamented by a raised medial ridge. Along its anterior margin the crown develops a crescent-like field of densely packed tubercles that represent the exposed portions of secondary odontodes. The posterior of the lower crown surface is devoid of ornament and can exhibit numerous pores arranged in loose rows. Scale bases are rhombic in outline and develop a spur-like central protrusion.

Scale morphology of Rongolepis cosmetica. (A) Crown view of a trunk scale (IVPP V 11954.4) with a broad crown. (B) Trunk scale (IVPP V 11954.5) in anterior crown view showing the crown-base junction. (C), (F) Trunk scale (IVPPV11954.1) in crown (C) and lateral (F) view exposing the low-profile of the crown. (D), (E) Trunk scale (IVPPV11954.2) in crown (D) and anterior view (E). (G), (H) Trunk scale (IVPPV11954.3) with coarse tuberculate ornament in crown (G) and base view (H). Anterior to the left in (F) and to the bottom in (A)–(D), (G), (H). Andreev et al. (2020).

The second group of scales are also placed in the Mongolepidid family Mongolepididae, and assigned to a new species, which is named Taklamakanolepis asiaticus, where 'Taklamakanolepis' means 'Taklamakan-scale', in reference to the Taklamakan Desert, and 'asiaticus' means 'Asian'. All specimens of aklamakanolepis asiaticus display trunk scale morphologies characterized by horizontal, low profile crowns with a considerable posterior extension. Crowns are elliptical to ovoid and transition into the base without forming a clear neck. Primary odontodes extend the crown’s length as elongate arched elements with sub-parallel orientation. Odontode ornament exists in the form of a prominent medial ridge and markedly shallower lateral ridges aligned to it. In most specimens the ridges break down into strings of tubercles that become particularly pronounced towards the anterior. A cluster of small lanceolate to irregular secondary odontodes overlaps the ends of primary elements at the anterior crown margin. The lower crown surface appears grooved and devoid of canal openings/pores. Scale bases are rhombic with abraded, flat profiles.

Scale morphology and histology of Taklamakanolepis asiaticus. (A)–(D) Holotype, trunk scale (IVPP V 11952.6) in crown (A), basal (B), lateral (C) and posterior base view (D) revealing the narrow profile and crown architecture of Taklamakanolepis scales. (E)–(L) Crown and (M) base views of scales ((E) IVPP V 11952.7, (F) IVPP V 11952.8, (I) IVPP V 11952.9, (J) IVPP V 11952.10, (K) IVPP V 11952.11, (L), (M) IVPP11952.1) demonstrating a range of crown morphologies. (N) Longitudinally sectioned scale (IVPP V 11952.5) showing the structure of the basal bone and the lamellin crown. (N1), (N2) Detail views of (N) depicting secondary odontodes at the crown’s anterior and (N2) primary odontodes near the junction with the base. Arrowheads point at contacts between primary or secondary odontodes. Anterior to the left in (C), (N), (N1), N2) and to the bottom in (A), (B), (D)–(M). Andreev et al. (2020).

The crown tissue in ths scales of Taklamakanolepis asiaticus is atubular dentine (lamellin), demonstrating phases of lamellar and globular mineralisation in primary odontodes. Rudiments of pulp canals are seen in the latter but absent from the smaller secondary odontodes. Crown ridges consist of overlapping generations of primary odontodes with the ontogenetically oldest elements occupying an apical position. The scale bases exhibit acellular bone with a layered structure resulting from fibre-bundle arrangement into apically arched lamellae.

The third group of scales are placed in the Mongolepidid family Shiqianolepidae, and assigned to the species Shiqianolepis hollandi. These trunk scales have rhombic to ovate crowns with a narrow neck transitioning to a flared scale base. The flattened crown has principal odontodes exposed and arranged in ridge-like rows covered by denticulate ornament. Deeply incised furrows divide the principle odontode rows and these converge to meet a field of tubercles capping the secondary odontodes at the anterior crown margin. The underside of the crown is without ornament and shows a number of pores. The base is slightly excavated and noticeably smaller than the crown.

Scale morphology of Shiqianolepis hollandi and Xinjiangichthys pluridentatus. (A)–(E) Shiqianolepis hollandi. (A), (B), (E) Two trunk scales, IVPP V 11951 (A), (B) and IVPP V 11951.3 (E), with crowns at late stage of development in crown (A), (E) and basal view (B). (C), (D) Asymmetrical trunk scale (IVPP V 11951.2), with an incipient crown. (F)–(N) Xinjiangichthys pluridentatus. (F)–(H) Trunk scales with broad crowns, IVPP V 11664.4 (F) and 11664.3 (G), (H), in crown (F), (G) and basal view (H). (I), (J) Trunk scales with compact crowns, IVPP V 11664.5 (I) and IVPP V 11664.6 (J), in crown (I) and lateral view, demonstrating stubby secondary odontodes and posterior curvature. (K)–(N) Asymmetrical trunk scales, IVPP V 11664.7 (K), IVPP V 11664.8 (L), (M) and IVPP V 11664.9 (N) in crown (K), (M), (N) and basal view (L). Anterior to the right in (J) and towards the bottom in (A)–(I) and (K)–(N). Andreev et al. (2020).

The next group of fossils are also placed in the Mongolepidid family Shiqianolepidae, and assigned to the species Xinjiangichthys pluridentatus. Scale crowns in this species are trapezoid to rhombic with a well-defined neck and a pronounced posterior curvature in lateral profile. They consist of numerous needle-like odontodes arranged in closely packed rows, whose lower surface is pitted by pores in proximity the crown neck The base is irregular-shaped to rhombic with a concave profile.

The next group of fossils are also placed in the Mongolepidid family Shiqianolepidae, and assigned to another new species. This is named Tielikewatielepis sinensis, where 'Tielikewatielepis' means 'Tielikewatie-scale', in reference to the Tielikewatie section, from which the specimens from which the species is described were collected, and 'sinensis' means 'from China'. 

The head scales of Tielikewatielepis sinensis are distinguished by absence of a clear separation of crown and base. They are low profile elements with an antero-posterior polarity indicated by diverging rows of arrowhead-shaped odontodes. Scale bases have a flattened appearance and extend beyond the crown perimeter.

Scale morphology and histology of Tielikewatielepis sinensis. (A)–(C) Trunk scale (IVPP V 11950.10) with an elongated crown, in crown (A), lateral (B) and basal (C) view, holotype. (D) Trunk scale (IVPP V 11950.11) with an incipient crown, in crown (D) view. (E)–(G) Trunk scale (IVPPV11950.1) with a well-developed base, in crown (E), basal (F) and lateral (G) view. (H)–(K) Asymmetrical trunk scales, IVPPV11950.5 (H), (I) and IVPP V 11950.12 (J), (K), with pronounced necks and incipient crowns, in crown (H), (K), lateral (I) and basal (J) view. (L) Head scale (IVPPV11950.6) with a low-profile crown in anterior crown view. (M), (N). Transverse and longitudinal sections of two trunk scales, IVPP V 11950.13 (M) and IVPP V 11950.15 (N), showing the growth lamellae and fibre spaces of the cellular basal bone and crown architecture. Arrowheads point at contacts between primary odontodes. Anterior to the left in (B), (G), (N) to the right in (I), (J) and towards the bottom in (A), (C), (D)–(F), (K), (L). Andreev et al. (2020).

The trunk scales of Tielikewatielepis sinensis possess rhombic crowns with a pronounced neck developed along the contact with the base. The crowns display arrowhead-shaped primary odontodes arranged in, presumably, mature specimens in a pair or pairs of rows oriented at an angle to the principle medial row. These flanking rows diverge posteriorly and quickly lose contact due to deepening of the furrows between them. Lanceolate secondary odontodes with a strong central ridge form along the crown’s anterior and these partially overlap the oldest portions of the main odontocomplexes. The sub-crown surface bears no ornament but is marked by discontinuous longitudinal furrows leading to pore-like openings. The crown neck flares out to extend over the entire upper portion of the basal tissue leaving exposed only its smooth lower surface. The latter has a slightly convex to bulbous profile and margins generally conforming to the outlines of the crown.

The scale tissues manifest a lack of large vascular spaces/canals, including absence of distinct pulp cavities within crown odontodes. The sole component of the odontodes is atubular dentine (lamellin) with lamellar and globular patterns of mineralisation. Along the length of odontocomplex rows odontode height grows in posterior direction whilst uninterrupted contact between odontodes is maintained at their overlap. The basal bone harbours flattened cell lacunae within a lamellar matrix characterized by parallel fibre spaces that propagate apically through the tissue.

The next group of fossils are also placed in the Mongolepidid family Shiqianolepidae, and assigned to another new species. This is named Xiaohaizilepis liui, where 'Xiaohaizilepis' means 'Xiaohaizi-scale', in reference to the Xiaohaizi section, where the specimens were collected, and 'liui' in honour of palaeontologist Liu Hsienting for his contributions the study of Chinese fossil Fish.

These trunk-type scales have well-delineated crowns with more or less rhomboidal outlines. The main constituents of the crown are primary odontodes organized into sub-parallel rows within which the anterior most elements are those with the greatest surface exposure. A conspicuous medial ridge which bifurcates close to the anterior crown margin is a prominent feature of the primary odontodes. Diminutive pyramidal secondary odontodes are commonly found on the anterior crown margin, and are particularly numerous in ontogenetically mature specimens.

Scale morphology and histology of Xiaohaizilepis liui. (A)–(C) Asymmetrical trunk scale, IVPP V 11949.1, in crown (A), basal (B) and posterior (C) view. (D) Elongate trunk scale (IVPP V 11949.2) in crown view. (E)–(G) Trunk scale (IVPP V 11949.9) in anterior crown (E), lateral (F) and basal (G) view. (H), (I) Trunk scale (IVPP V 11949.3) in basal (H) and crown (I) view, holotype. (J)–(M) Trunk scales with broad crowns, IVPP V 11949.5 (J, K) and IVPP V 11949.4 (L), (M), produced by increase of odontode rows, in crown (L), (K), posterior (M) and basal (J) view. (N) Longitudinally sectioned trunk scale (IVPP V 11949.10) showing the relationships between secondary and primary odontodes. (O) Transversely sectioned trunk scale (IVPP V 11949.11), not depicted in full, showing the triangular crosssection of primary odontodes and orientation of fibre-bundles in the base. Arrowhead points at a contact between primary odontodes. Anterior to the left in (F), (N) and towards the bottom in (A), (B), (D), (E), (G)–(M). Andreev et al. (2020).

The sub-crown surface in Xiaohaizilepis liui bears grooves leading to gaps between odontocomplexes at the point of their separation at the posterior of the crown. The crown/base transition exhibits a pronounced constriction (neck) with a series of horizontally distributed openings. The scale base is rhombic in shape with a central protuberance.

Crown odontodes are formed of lamellin-type tissue with globular as well as lamellar texture, especially prominent in the mineralisation lines around the rudimentary pulp cavities. The anterior crown margin bears wedge-like secondary odontodes that overlap the ends of primary odontocomplex rows, each composed of several odontode generations.

The basal bone of scales harbours compressed cell lacunae aligned to the tissue’s lamellae. Fibre spaces penetrate the thickness of the bony base, which shows little evidence for the presence of vascular canals.

The next group of fossils are also placed in the Mongolepidid family Shiqianolepidae, and assigned to the species Chenolepis asketa. Individual scales of Chenolepis asketa have an ovate to oblong appearance and demonstrate rows of odontodes diverging from a point near the anterior of the crown. The odontodes are reclined posteriorly and at their tip develop a prominent conical cusp that issues from the main body of each element. In the posterior odontodes, this cusp is seen flanked by several pairs of accessory ‘cusplets’. Wedge-shaped secondary odontodes, with an ornament of tubercles, form the crown anterior to the primary odontode rows. The crown attaches via a well-defined neck to an anteriorly offset base; the latter is polygonal to elliptical and possesses a slightly hollowed lower surface.

Scale morphology and histology of Chenolepis asketa. (A), (B) Trunk scale (IVPP V 13773.1) demonstrating the typical for the genus cuspidate primary odontodes, in crown (A) and basal (B) view. (C) Trunk scale (IVPP V 13773.2) with an elongated crown, in crown view. (D) Trunk scale (IVPP V 13773.3) with small number of odontode rows in crown view. (E) Trunk scale (IVPP V 13773.4) with a broad crown in crown view. (F) Longitudinally sectioned trunk scale (IVPP V 13773.5) showing a crown composed of lamellin. Arrowheads point at contacts between primary or secondary odontodes. Anterior to the right in (F) and towards the bottom in (A)–(E). Andreev et al. (2020).

The scale crown is constructed of atubular dentine demonstrating the calcospherites and scalloped mineralisation lines of lamellin. In longitudinal section the posterior generations of primary odontodes are seen to contain vestiges of pulp cavities that are closed off in the smaller secondary odontodes. The basal bone possesses fusiform cell lacunae embedded in a matrix of slightly arched lamellae of fibre bundles following the outline of the lower base surface. The bone tissue is penetrated by vertical fibre spaces that converge apically.

Andreev et al. also create a new order of Chondrichthyans, containing a single family, the previously described Sinacanthidae. Theese age given the designatios 'Spine morphology A' and 'Spine morphology B' rather than being assigned to a species or genus, as spines are difficult to connect to a species described from scales, unless body specimens containing both are found.

Spines of 'Spine morphology A' carry a strong ornament of flat-topped ridges with corrugated margins that in part are divided into rhombic/lanceolate segments. Ridges exhibit subparallel orientation and uneven spacing on the flattened lateral sides of spines  The spines have a narrow cross section with a shallow posterior indentation representing the sulcus of the posterior edge.The tissue structure of sectioned specimens can be discerned in the ridges of the spine ornament that comprise of atubular dentine formed around a central canal

Morphology and histology of Sinacanthid spines. (A)–(D), (K) Spine morphology A. (A) Lateral wall of a spine fragment (IVPP V 13774.1) showing the width and distribution of ornamenting ridges, lateral view. (B) Supposed basal fragment of a spine (IVPP V 13774.2) showing strong segmentation of ridges, lateral view. (C) Apical portion of a partial spine (IVPP V 13774.3), anterior lateral view. (D) Spine fragment (IVPP V 13774.4) demonstrating the rhombic secondary ornament of ridges. (E)–(I) Spine morphology B. (E) Apical spine fragment (IVPP V 13775.1) in lateral view. (F) Incomplete spine (IVPP V 13775.2) demonstrating the characteristic for the type strong keel and narrow ornamenting ridges, lateral view. (G) Detailed depiction of the main ornamenting ridges and the tuberculate ornament of the keel of an incomplete spine (IVPP V 13775.3), lateral view. (H) Apical fragment of a spine (IVPP V 13775.4) in lateral view. (I) Apical fragment of spine (IVPP V 13775.5) showing a nodose ornament along its anterior edge. (J) Transversely sectioned spine (IVPP V 13775.6) demonstrating the atubular dentine of the spine trunk and formation of large denteons (arrows) inside the ornamenting ridges. (J1) Detailed view of (J) showing the relationship between the lamellar dentine and the calcified cartilage formed inside the spine’s central cavity. (K) Part of a lateral wall of a transversely sectioned spine fragment (IVPP V 13774.5). Abbreviations: AD; atubular dentine; CC, calcified cartilage. Dotted line marks the lamellar dentine/calcified cartilage boundary. Arrows point at denteons in J and K. Anterior to the left in (C), (E), (F), (H), (J), (K), to the right in (A), (B), *G) and towards the top in (J1). Andreev et al (2020).

'Spine morphology B; includes recurved spines that broaden in their profile at the base. Spine surfaces bear evenly spaced subparallel ridges with triangular cross section and corrugated margins. The ridges break down into tuberculate/nodose ornament along the anterior edge where the spine develops a laterally compressed keel that widens towards the base. The posterior spine margin is marked by a deep sulcus. 

Spines consist of a type of atubular lamellar dentine with traces of calcospheritic mineralisation, lined internally by an optically less distinct globular calcified cartilage. The centre of spine ridges is occupied by a large denteon distinguished by concentric lamellae formed around a vascular canal. The denteon tissue extends into the inter-ridge spaces in a continuous manner around the spine’s perimeter. On its inner surface the dentine is bounded by remnants of optically faint calcified cartilage with globular microstructure, evidenced by mineralised spherites and wavy precipitation lines (Liesegang waves).

The histological signature of sinacanthid spines, an outer sculpted layer of atubular dentine and an inner layer of globular calcified cartilage, is also found in the morphology B spines. A notable feature of morphology B is the lamellin-like appearance of the dentine tissue, with a mineralisation pattern akin to that of mongolepid scale crowns. In Sinacanthus, and likely in Neosinacanthus and Tarimacanthus, the dentine layer has a distinctly globular texture inside the ridges and becomes lamellar only interior of the ornament. Other characteristics of morphology B spines not reported previously in sinacanthids are the development of a strong keel and ridges with a triangular cross section.

In the absence of articulated material, it is unclear what portion of the taxa assigned to the order Sinacanthida need to be synonymised, given that a number of more completely known stem-group Chondrichthyans show a similar range of spine morphologies within individual specimens (e.g. Parexus recurvus, Climatius reticulatus and Doliodus problematicus). With this in mind, and in order to avoid creating a series of sinacanthid morphotaxa, Andreev et al. have adopted open nomenclature for the spines they have described.

Finally, Andreev et al. describe a new Chondrichthyan species from seventeen isolated trunk scales and three thin sectioned scales, which they assign to a new species and genus, but not to any higher taxonomic group narrower than the Chondrichthyans. This is named Yuanolepis bachunensis, where 'Yuanolepis' means 'Yuan's scale' in honour of FL Yuan, one of the first geologists to study the early Vertebrates of Xinjiang, and 'bachunensis' means 'from Bachu' in reference to the Bachu fossil locality, where the specimens from which it is described were found. 

All specimens of Yuanolepis bachunensis display scale crowns with a neck-like constriction and an antero-posterior polarity. The crowns are elliptical to deltoid with serrated/corrugated posterior margins. Primary crown odontodes bear strong ridges and tubercles along their periphery. On the crown surface they appear as deltoid/elliptical overlapping elements deposited in a growth series (odontocomplex), with smaller heavily ornamented secondary odontodes forming along the crown’s anterior. The sub crown is smooth and devoid of canal openings. Scale bases are rhombic with a slight central protrusion.

Scale morphology and histology of Yuanolepis bachunensis. (A), (B) Trunk scale (IVPP V 17709.1) at an early stage of development in crown (A) and lateral view (B), holotype. (C), (D) Trunk scale (IVPP V 17709.2) with an elongated crown, in crown (C) and basal (D) view. (E), (F) Trunk scale (IVPP V 17709.3) at late stage of development, in crown (E) and lateral (F) view. (G) Asymmetrical trunk scale (IVPP V 17709.6) in crown view. (H), (I) Trunk scale at early stage of development (IVPP V 17709.7) in crown (H) and basal (I) view. (J) Elongated trunk scale (IVPP V 17709.8) in crown view. (K) Trunk scale (IVPP V 17709.9) with a broad crown in anterior crown view. (L) Longitudinal section of a trunk scale (IVPPV17709.4) demonstrating the arrangement of primary of secondary odontodes, shown in part. (L1) Detail of (L) showing the structure of lamellin at the posterior of the crown. (M) Transverse section of a trunk scale (IVPP V 17709.10). Arrowheads point at contacts between primary odontodes. Anterior to the left in (B), (F), to the right in (L), (L1) and towards the bottom in (A)–(E), (G)–(K). Adnreev et al. (2020).

The crown’s primordial odontode sits at the apex of the base, being the smallest component of a primary odontocomplex within which odontode size increases towards the posterior. Odontodes consist of an atubular dentine tissue demonstrating extensive globular mineralisation and absence of clearly recognizable pulp cavity spaces. The basal bone has a distinctly lamellar appearance with spindle-shaped cell spaces distributed throughout the tissue. Spaces for fibre bundles run across the thickness of the base in a subparallel manner, assuming slightly undulating trajectories along their course.

Yuanolepis scale crowns have a lamellin-like histology but are readily distinguished from known mongolepid species in form and structure. Instead of the multiple odontode rows characteristic of Mongolepidida, Yuanolepis scales have single-odontocomplex scales with an appositional growth pattern. This crown architecture occurs in the Chondrichthyes sensu lato within ‘Acanthodians’ and Euchondrichthyans (e.g. Seretolepis, Kathemacanthus, Parexus, Wodnika) and is indicative of affinity to the clade. The overall geometry of the crown in these specimens suggests they are trunk scales.

The Xinjiang taxa described by Andreev et al. expand the Mongolepidida to eleven formally described genera, adding to records from North America, Mongolia and South China. The Mongolepids are the most widely distributed (in stratigraphic and palaeogeographic senses) scale-based components of the earliest chondrichthyan faunas, with microvertebrate assemblages from the Siberian Platform hinting that their diversity in the Silurian (Llandovery–Wenlock) might be greater than currently recognised. The material from the Tataertag and Ymogantau Formations has extended the overlap between the Tarim and South China constituents of the Zhangjiajie Vertebrate Fauna. These data further underscore the impoverished nature of the Zhangjiajie Fauna (Mongolepids, Sinacanthids and Galeaspids) when compared with coeval sites from Siberia and Mongolia and their complement of Mongolepids, Acanthodians, Thelodonts, Eriptychiids and Heterostracans.

As far as can be determined, all Chinese Mongolepids come from Telychian Vertebrate assemblages (from the Xiushan, Tataertag and Ymogantau Formations) where they co-occur with a variety of Sinacanthid spines. This has prompted some authors to suggest grouping together Mongolepids and Sinacanthids on the basis of shared atubular dentine and the absence of other associated Chondrichthyan-like remains. The presence of Mongolepid scales in other Sinacanthid-bearing Formations (such as the Rongxi and Fentou from South China) is yet to be determined as these have not been sampled for micro-remains, and the abundance of associated Sinacanthid fossils is not a feature of Mongolian and North American Mongolepid assemblages. In the absence of definitive proof for a co-association of mongolepid scales and Sinacanthid spines, the discovery of Yuanolepis has bearing on the affinities of the Mongolepidida and their relationship to the Sinacanthidae. Although falling outside of the current definition of a Mongolepid, developmentally the scales of Yuanolepis are distinctly Chondrichthyan. It may be reasonable to suggest an expanded grouping for lamellin-forming taxa including Mongolepids and taxa with the Yuanolepis type scale crown architecture. Their single- and poly-odontocomplex patterns of odontode addition also appear in more derived components of the Chondrichthyan stem (e.g. the Ctenacanthus and Seretolepis morphogenesis types, in non-acanthodid taxa). The arrays of paired spines possessed by a number of these species (e.g. Doliodus, Kathemacanthus, Climatius and Parexus). This leads us to view lamellin-forming Chondrichthyans as possessing modes of morphogenesis previously recorded only in a subset of stem Chondrichthyans. Our present understanding of the former points towards them being on a branch supporting the Euchondrichthyan node, crownward of ‘Acanthodians’ retaining the plesiomorphic box-in-box mechanism of odontode addition.

Stratigraphic ranges of lamellin-forming chondrichthyans. Diagram of all formally described Mongolepids, Yuanolepis and Sinacanthids. Ranges of taxa represented by dark grey bars and circles. Abbreviations of generic names: E, Eosinacanthus; H, Hunanacanthus; N, Neosinacanthus; S, Sinacanthus; T, Tarimacanthus. Andreev et al. (2020).

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Magnitude 6.4 Earthquake in western Xinjoang Province, China.

The China Earthquake Networks Center recorded a Magnitude 6.4 Earthquake at a depth of 16 km in the Tian Shan Mountains of northwestern Xinjiang Province, China, slightly after 9.20 pm local time (slightly after 1.20 pm GMT) on Sunday 19 January 2020. There are no reports of any damage or injuries associated with this event, but it was felt across a wide area of Xinjiang Province and neighbouring Kyrgyzstan.

 

The approximate location of the 19 January 2020 Xinjiang Earthquake. USGS.

 

The Tian Shan Mountains stretch for 2500 km across Xinjiang, Kazakhstan, Kyrgyzstan and Uzbekistan. The Tian Shan are part of the Himalayan Orogenic Belt, mountains in Central Asia pushed upwards by the collision of India and Asia. The Indian Plate is currently pushing into the Eurasian Plate from the south at a rate of 3 cm per year. Since both are continental plates, which do not subduct, the Eurasian Plate is folding and buckling, causing uplift in the Himalayas and other mountains of Central Asia. This is not a smooth process, the rocks will remain effectively stationary for log periods of time while pressure builds up, then give suddenly, releasing large amounts of energy in the form of Earthquakes.

The movement of India relative to Asia, and the blocks within the eastern part if the Eurasian Plate. University of Wollongong.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

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