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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