The Silurian -Devonian armored Galeaspids were a prevalent and diverse clade of jawless stem-Gnathostomes that exhibited traits thought to belong to jawed Gnathostomes. They contribute to our understanding of the conformation of the Gnathostome body, which is significant to Vertebrate evolution. The family Sinogaleaspidae of the Eugaleaspidiformes within the Galeaspida, from the Lower Silurian of Xiushui, Jiangxi province and Changxing, Zhejiang province, is an important early clade possessing the characteristics that demonstrate the step-by-step transitions from jawless to jawed vVertebrates. Synchrotron Radiation X-ray Tomographic Microscopy provides an example of the cranial anatomy of Shuyu, a Sinogaleaspid, and other important characteristics that may be compared with other early Vertebrate groups. However, the phylogeny and morphology of its constituents are still disputed.
The family Sinogaleaspidae includes the species Sinogaleaspis shankouensis, Meishanaspis and Anjiaspis, and `Sinogaleaspis.' xikengensis, and `Sinogaleaspis' zhejiangensis. It remains unknown whether Sinogaleaspidae is a monophyletic group; it has been suggested that that the three species assigned to Sinogaleaspis form a paraphyletic group instead of a monophyletic group. Sinogaleaspis shankouensis is probably more closely related to Yunnanogaleaspis and higher Eugaleaspids than to `Sinogaleaspis.' xikengensis and `Sinogaleaspis' zhejiangensis, whereas `Sinogaleaspis' zhejiangensis was determined as the sister to all other Eugaleaspididiforms in later phylogenetic analyses. Based upon this `Sinogaleaspis' zhejiangensis has been reassigned to the new genus Shuyu, as Shuyu zhejiangensis, based on novel material, especially the three-dimensional images of the neurocrania. However, the systematic position of `Sinogaleaspis.' xikengensis is still unresolved due to its poor preservation and large amounts of missing data, especially related to its sensory canal system.
In a paper published in the journal PeerJ on 15 May 2020, Xianren Shan 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 College of Earth Science and Engineering at the Shandong University of Science and Technology, Min Zhu and 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, as well as the Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment, and the University of the Chinese Academy of Sciences, Zhaohui Pan, 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, Pingli Wang, also of the College of Earth Science and Engineering at the Shandong University of Science and Technology, and Zhikun Gai, 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, describe a new species of Sinogaleaspid Fish from the Early Silurian of Jiangxi Province, China, as well as proposing a new generic designation for `Sinogaleaspis.' xikengensis.
Phylogenetic placement (A) and interrelationships (B )-(D) of Galeaspids. (A) Galeaspids are attributed to the major armored, jawless fossil Vertebrates (or `Ostracoderms', purple bar), (B)-(D) summary of previous hypotheses of Galeaspid phylogeny showing controversy on the monophyly of Sinogaleaspidae. Shan et al. (2020).
Five excavations of the Lower Silurian region of Xiushui, Jiangxi province have been organised since 2003. This location is the primary site of `Sinogaleaspis.' xikengensis discovery and an abundance of Silurian fish remains have been found here, including Sinogaleaspids, Xiushuiaspids, and sclerites of Dayongaspids and Hanyangaspids.
The Silurian strata in the northwestern Jiangxi province are subdivided into six formations: the Lishuwo, Dianbei, Qingshui, Xiajiaqiao, Xikeng, and Xiaoxi formations. New sinogaleaspid material was collected from two fossil sites in the Xikeng formation at Taiyangsheng Town, Xiushui County, Jiangxi province near Xikeng village and a newly discovered location on the side of Shipan Reservoir. The Xikeng Formation is mainly composed of medium- to thin-bedded yellow-green and purple siltstone and mudstone intercalated with fine sandstone. It is conformably underlaid by the Xiajiaqiao Formation and unconformably overlaid by the Xiaoxi Formation. The Galeaspids from the Xikeng Formation include Sinogaleaspis shankouensis, `Sinogaleaspis.' xikengensis, Xiushuiaspis jiangxiensis, and Xiushuiaspis ganbeiensis. The early vertebrate fossil assemblage was referred to as either the Sinogaleaspis -Xiushuiaspis assemblage or the Maoshan Assemblage and it is consistent with the assemblage found in the Maoshan Formation of the northwestern Zhejiang Province. The Fish-bearing Xikeng Formation is known as the Upper Red Beds and is the equivalent of the Huixingshao Formation in Chongqing and Guizhou, and the Maoshan Formation in the Jiangsu and Zhejiang provinces. Although the precise age of the Upper Red Beds in the western part of the Yangtze Platform is difficult to determine, it is thought to be from the middle-late Telychian due to evidence from the underlying Xiushan Formation with its invertebrate fauna and sequence stratigraphic analyses. The age of the Fish-bearing Xikeng Formation is thought to be from the middle-late Telychian Age of the Llandovery Epoch during the Silurian Period like those of the Huixingshao and Maoshan formations in South China.
Maps of the two fossil localities of Rumporostralis (A) and the Fish-bearing lithological column (B) in Xiushui County, Jiangxi Province, China. Shan et al. (2020).
Shan et al. create a new genus, Rumporostralis, to accomodate `Sinogaleaspis.' xikengensis and the new species. The name 'Rumporostralis' derives from 'Rumpo' Latin, state of being dehiscent or split; and 'rostralis', Latin, snout, in referring to the rostral margin of the head-shield split by the anterior end of median dorsal opening.
The newly discovered sinogaleaspid material includes four head-shields of Rumporostralis xikengensis (IVPP V25136.1 -4), and one head-shield of Rumporostralis shipanensis (IVPP V26114). All specimens are permanently housed in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and are accessible for examination. The holotypes of Sinogaleaspis shankouensis (GMC V1751) and Rumporostralis xikengensis (GMC V1753) are permanently housed in the collections of the Geological Museum of China and were used for comparison and measurement.
All specimens were prepared mechanically using a Vibro-tool with a tungsten-carbide bit or a needle. Some specimens were reversed in latex casts. Specimens 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.' xikengensis is redescribed as Rumporostralis xikengensis. This is a small-sized Sinogaleaspid with a subtriangular head-shield. The rostral margin of the head-shield is disrupted by the anterior end of the median dorsal opening. The measurements of 4 specimens of Rumporostralis xikengensis indicate that the size of the head-shield is consistent. The head-shield is longer than it is wide with a length-to-width ratio of about 1:2. The head-shield protrudes caudally into a pair of cornual and inner cornual processes. The cornual processes are oriented caudo-laterally (or postero-laterally) and are short and rapidly taper off in the holotype and the newly discovered specimen IVPP V25136.1. The inner cornual processes, which are completely preserved in the holotype and new specimen IVPP V25136.1. are small, spine-like, and caudally-oriented. The inner cornual processes are much smaller than the cornual processes.
Photographs of Rumporostralis xikengensis. A nearly complete external (A) and internal (B) mould of head-shield, holotype, GMC V1753A, B. (C) Close-up of coarse granular tubercles. (D) A nearly complete external mould of the head-shield, IVPP V25136.2a. (E) Close-up of the anterior part of head-shield. (F) Close-up of the posterior part of head-shield. (G) An incomplete internal mould of the head-shield, IVPP V25136.4. Abbreviations: br.c, branchial chamber; c, cornual process; ic, inner cornual process; md.o, median dorsal opening; nc.p, pore for the passage of the neural canal; orb, orbital opening; pi, pineal opening; pb.w, postbranchial wall; va.p, subcutaneous vascular plexus; vr, ventral rim. Shan et al. (2020).
The median dorsal opening is fairly long and wedge-shaped or longitudinally elliptic in outline along the midline. The length-to-width ratio of the opening is less than 6. The anterior end of the median dorsal opening disrupts the rostral margin of the head-shield and its posterior end is positioned anterior to the level of the orbital opening.
Photographs (A) and interpretative drawing (B) of Rumporostralis xikengensis, IVPP V25136.1 (C) close-up of postbranchial wall and pore on it for passage of the neural canal. Abbreviations: c, 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; nc.p, pore for passage of the neural canal; orb, orbital opening; pi, pineal opening; pb.w, postbranchial wall; soc₁, anterior supraorbital canal; soc₂, posterior supraorbital canal. ifc, infraorbital canal; ldc, lateral dorsal canal; ltc, lateral transverse canal; mdc, median dorsal canal. Shan et al. (2020).
The orbital openings are dorsally positioned on the head-shield and are round with an average diameter of about 1.5 mm among the four specimens. The orbital opening on the left side of specimen IVPP V25136.1 is longitudinal oval, which may be due to a deformation caused during preservation.
Comparison of Sinogaleaspis shankouensis (A) and Rumporostralis xikengensis (B). Xiaocong Guo in Shan et al. (2020).
The pineal opening is clearly preserved in specimen IVPP V25136.2. It is level with the posterior margin of the orbital opening in the midline of the head-shield. The pineal opening is small and round with a diameter of 0.7 mm. The ratio of the length of the pre-pineal and post-pineal region is about 1:2.
The sensory canal system is difficult to reconstruct in Rumporostralis xikengensis because it is preserved in only one specimen (IVPP V25136.1). The identified sensory canals consist of posterior supraorbital canals, infraorbital canals, lateral dorsal canals, lateral transverse canals, median dorsal canals, and median transverse canals. The posterior supraorbital canals are V-shaped. These canals originate from the anterior margin of the orbital opening, extend posteriorly along the inner side of the orbital opening, and meet behind the pineal opening. The median dorsal canals are U-shaped and connect anteriorly with the posterior supraorbital canals level with the pineal opening and curve inward to converge with the opposite one on the midline of head-shield. The infraorbital canals are an inverted S-shape. These canals originate on the lateral margin of the head-shield, pass through the lateral side of the orbital opening, and connect with the lateral dorsal canals. There are at least four pairs of lateral transverse canals and three pairs of median transverse canals. The anterior three pairs of lateral transverse canals extend across the lateral dorsal canals to connect with the median transverse canals. The fourth lateral transverse canal is near the posterior edge of the head-shield and extends posterolaterally.
The endoskeletal roof of the oralobranchial chamber was poorly preserved in the internal mold of holotype GMC V1753B, but there are indications of at least 5 pairs of transversely elongated branchial fossae. Impressions for the subcutaneous vascular plexus are also preserved on the endoskeletal roof of the oralobranchial chamber in the internal mold of holotype GMC V1753B. There is an extensive endoskeletal postbranchial wall in specimen IVPP V25136.1, 2, that closes the oralobranchial chamber posteriorly. The postbranchial wall is penetrated by a large pore in the midline of the head-shield for the passage of the neural canal to the body.
The lateral margin of the head-shield is smooth and the surface of the head-shield is ornamented with closely set, coarse, granular tubercles. There are about 10 tubercles per square millimeter.
Rumporostralis shipanensis is a medium-sized sinogaleaspid. The longest known head-shield is 52.4 mm; the widest known head-shield is 63.0 mm, and the length of its head-shield along the midline is 34.5 mm. The rostral margin of the head-shield is unclosed. The holotype of this species is 12.6 mm along the long axis of the median dorsal opening and 4.9 mm along the short axis. The diameter of the orbital opening is 5.8 mm in the holotype. The orbital opening on the left side is a longitudinal oval, which may be due to a deformation during preservation. The distance between the paired orbital openings is 8.0 mm in the holotype. The lateral margin of the head-shield is smooth and the exoskeleton of the head-shield is ornamented with closely set, coarse granular tubercles. There are about 1.5 tubercles per square millimeter.
Photograph and interpretative drawing of Rumporostralis shipanensis gen. et sp. nov. (A) An incomplete internal mould of head-shield, holotype, IVPP V26114.1a, in dorsal view. (B) Interpretative drawing. (C) Close-up of the coarse granular tubercles. Abbreviations: md.o, median dorsal opening; orb, orbital opening. Shan et al. (2020).
The sensory canal system, also called the lateral line system in modern aquatic Vertebrates, is a system of sense organs that serves to detect movements, vibration, and pressure gradients in the surrounding water. It is unique to aquatic vertebrates from Cyclostome Fish (Lampreys and Hagfish) to Amphibians. It is prevalent in the armored jawless Fish such as Galeaspids, Osteostracans, and Heterostracans, and jawed Placoderms during the Silurian-Devonian period. The sensory canal system of galeaspids exhibits a characteristic festooned pattern consisting of two pairs of longitudinal stems and a varied number of transverse canals issuing from the stems. Its general pattern is comparable with other vertebrate groups. For example, most stem canals such as supraorbital canals, median dorsal canals, infraorbital canals, and lateral dorsal canals have their corresponding homologous parts in Lampreys, Heterostracans, Osteostracans, and Placoderms. The number, placement, and branching pattern of the sensory canals in galeaspids varies significantly among different groups, even if the species are closely related. Three patterns of sensory canals are generally recognized in Galeaspids: (1) two median transverse canals with more lateral transverse canals issuing from the infraorbital canals and undeveloped supraorbital canals as in plesiomorphic taxa Dayongaspidae, Hanyangaspidae, and Xiushuiaspidae; (2) a V-shaped posterior supraorbital canal and one median transverse canal (dorsal commissures) as in Huananaspiformes and Polybranchiaspidiformes; (3) the Ushaped median dorsal canals anteriorly fused with the posterior supraorbital canals as in Eugaleaspidiformes.
The sensory canal system in early vertebrates. (A) Heterostracan Anchipteraspis crenulata. (B) Petromyzontid Lampetra fluviatilis. (C) Osteostracan Ateleaspis tessellate. (D) Placoderm Radotina prima. (E)-( J) Galeaspids: (E) Dayongaspis hunanensis; (F) Sinogaleaspis shankouensis; (G) Hanyangaspis guodingshanensis; (H) Laxaspis qujingensis; (I) Eugaleaspis changi; (J) Sanchaspis magalarostrata. Abbreviations: c, cornual process; cc, central canal; ic, inner cornual process; ifc, infraorbital canal; ldc, lateral dorsal canal; lf, lateral field; ltc, lateral transverse canal; mdc, median dorsal canal; md.o, median dorsal opening; mf, median field; mtc, median transverse canal; nhf, naso-hypophysial foramen; no, nasal opening; orb, orbital opening; pi, pineal opening; poc, preorbital commissure; soc, supraorbital canal; soc1, anterior supraorbital canal; soc₂, posterior supraorbital canal; ro, rostral process; v.mdc, vestige of median dorsal canal. Shan et al. (2020).
The sensory canal patterns of Sinogaleaspids are different from those of all other known Galeaspids. The sensory canals of Sinogaleaspids are a typical eugaleaspid-pattern with a U-shaped median dorsal canal which is a dignostic characteristic of Eugaleaspidiformes. The U-shaped median dorsal canals were thought to be lost in Polybranchiaspidiformes and Huananaspidiformes, but their vestiges are sometimes visible as a pair of short canals crossing with the dorsal commissure in Polybranchiaspis, Damaspis, and Laxaspis. Sinogaleaspids also exhibit the mosaic features of two other known patterns. For example, they have two additional lateral transverse canals issuing from the infraorbital canals, which may be regarded as a plesiomorphic characteristic of Galeaspids, since 3-4 lateral transverse canals are found on the infraorbital canal in the plesiomorphic taxa such as Dayongaspidae, Hanyangaspidae, and Xiushuiaspidae. The number of lateral transverse canals tends to decrease in later evolution, but the vestiges of these canals can sometimes be observed on the infraorbital canals in Eugaleaspis changi and Laxaspis qujingensis. Sinogaleaspids bear the typical V-shaped posterior supraorbital canal which is a derived characteristic uniquely shared by Polybranchiaspidiformes and Huananaspidiformes. The preorbital commissure and central canal in sinogaleaspids are also found in some members of Huananaspidiformes and Polybranchiaspidiformes including Laxaspis and Sanchaspis.
The Sinogaleaspid sensory canal system is notable for the presence of more than two pairs of median transverse canals, although this has been questioned. However, newly discovered Sinogaleaspids confirm their presence. Among three genera referred to Sinogaleaspids, Sinogaleaspis has 6 pairs of median transverse canals, Anjiaspis has 8 pairs, and Rumporostralis has at least 3 and more likely 6 pairs, like Sinogaleaspis. Among about 80 described galeaspid species, this feature occurs uniquely in Sinogaleaspids but is very common in Heterostracans observed a general grid-like pattern of the sensory canal system for plesiomorphic Vertebrates composed of 2 3 pairs of longitudinal stems linked by transverse branches; this is a common pattern among the different types of sensory canal systems in various Vertebrate groups. The sensory canal system of Heterostracans is regarded as the ideal model for a general pattern.
The median transverse canals of Sinogaleaspids occur in the post-orbital region of the head-shield and are level with the anterior, central, and posterior margins of the orbital opening as in Anjiaspis and Sinogaleaspis. The grid distribution of the sensory canal system on the dorsal side of the head-shield in Sinogaleaspids is made up of 4 longitudinal canals intercrossed with 3 8 pairs of transverse canals, reflecting the assumed plesiomorphic condition of Vertebrates.
The newly discovered Sinogaleaspids from the Lower Silurian in Jiangxi, China provides a wealth of new data and reliable diagnostic features to assign the new genus, Rumporostralis, to `Sinogaleaspis' xikengensis. Shan et al.'s in-depth morphological study determined that the sensory canal system of sinogaleaspids exhibits the mosaic features of three known Galeaspid patterns. The presence of 3-8 pairs of transverse canals in Sinogaleaspidae suggests that the sensory canal system of Galeaspids probably displayed a grid distribution with transverse canals arranged throughout the cephalic division. An extended phylogenetic analysis of Galeaspida corroborates the monophyly of Sinogaleaspidae, which consists of Sinogaleaspis, Rumporostralis, and Anjiaspis. Shuyu and Meishanaspis were excluded from the Sinogaleaspidae to form the monophyletic group, the family Shuyuidae, which is the sister of all other Eugaleaspididiformes. Shan et al. propose a cladistically-based classification of the Galeaspida.
Life restoration of Sinogaleaspis shankouensis (left) and Rumporostralis xikengensis (right) in a fresh river. Xiaocong Guo in Shan et al. (2020).
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