Sinogaleaspis, the type genus of the family Sinogaleaspidae, comprises two species, Sinogaleaspis shankouensis and ‘Sinogaleaspis’ xikengensis. The monophyly of the Sinogaleaspidae remains controversial largely due to the poorly known nature of Sinogaleaspis shankouensis whose description was based on two incomplete specimens from the Lower Silurian of Xiushui in Jiangxi Province. Among about 80 described galeaspid species, the sensory canal system of Sinogaleaspis shankouensis is peculiar in bearing three pairs of median transverse canals, which contrasts with the known pattern of other Galeaspid Fish. Considering the unusual condition known from two poorly preserved specimens, it has been pointed out that the present interpretation of the sensory canal system in Sinogaleaspis shankouensis needs further corroboration from additional materials.
In a paper published in the journal Vertebrata PalAsiatica on 1 March 2020, Gai Zhi-Kun of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment, and the University of the Chinese Academy of Sciences, Shan Xian-Ren, also of the Key Laboratory of Vertebrate Evolution and Human Origins at the
Institute of Vertebrate Paleontology and Paleoanthropology of the
Chinese Academy of Sciences, and of the College of Earth Science and Engineering at the Shandong University of Science and Technology, Sun Zhi-Xin, also of the College of Earth Science and Engineering at the Shandong University of Science and Technology, Zhao Wen-jin, again of the Key Laboratory of Vertebrate Evolution and Human Origins at the
Institute of Vertebrate Paleontology and Paleoanthropology of the
Chinese Academy of Sciences, the Chinese Academy of Sciences Center for
Excellence in Life and Paleoenvironment, and the University of the
Chinese Academy of Sciences, Pan Zhao-Hui, again of the Key Laboratory of Vertebrate Evolution and Human Origins at the
Institute of Vertebrate Paleontology and Paleoanthropology of the
Chinese Academy of Sciences, and the University of the
Chinese Academy of Sciences, and Zhu Min. once again of the Key Laboratory of Vertebrate Evolution and Human Origins at the
Institute of Vertebrate Paleontology and Paleoanthropology of the
Chinese Academy of Sciences, the Chinese Academy of Sciences Center for
Excellence in Life and Paleoenvironment, and the University of the
Chinese Academy of Sciences, present a redescription of Sinogaleaspis shankouensis, based upon eleven new specimens collected from the type locality of the species in a series of expeditions since 2003.
The new material of Sinogaleaspis shankouensis was collected from the type locality of the Xikeng Formation in Taiyangshen Town, Xiushui County, Jiangxi Province, South China. The Fish-bearing Xikeng Formation belongs to the Upper Red Beds in South China, which is equivalent of the Huixingshao Formation in Chongqing and Guizhou, and the Maoshan Formation in Jiangsu and Zhejiang. Although the precise age of Upper Red Beds in western part of the Yangtze Platform is difficult to determine, an age of the middle-late Telychian is proposed in light of the evidence from the underlying Xiushan Formation with its invertebrate fauna and sequence stratigraphic analyses.
Geological backround of Sinogaleaspis shankouensis. (A) Location map of the fossil site in Xiushui, Jiangxi, South China, (B) stratigraphical section of the Xikeng Formation at the type locality, (C) cross section of the Fish layer showing a large sum of muddy gravels in grey-green pelitic siltstone. Gai et al. (2020).
The new material of Sinogaleaspis shankouensis includes 11 head-shields (IVPP V 25135.1–11) from the type locality of the Xikeng Formation in the town of Taiyangshen, in Xiushui county, Jiangxi Province, South China. All these specimens are permanently housed and accessible for examination in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences. The holotype and paratype of Sinogaleaspis shankouensis, GMC V 1751 and GMC V 1752 were also examined for comparison and measurement; these are housed in the collections of the Geological Museum of China. All fossil specimens were prepared mechanically using a vibro tool with a tungsten-carbide bit or a needle. They were measured with a digital vernier calliper, studied under optical zoom, and photographed with a Canon EOS 5D Mark III camera coupled with a Canon macro photo lens (MP-E 65 mm 1:2.8 1-5×).
Sinogaleaspis shankouensis is a is a small-sized sinogaleaspid jawless fish with a subtriangular head-shield. The rostral margin of the head-shield is arciform without a rostral process. The measurements of 11 specimens of Sinogaleaspis shankouensis indicate that the size of the head-shield is quite stable with the maximum length of head-shield varying from 15.5 to 18.4 mm, the maximum width of head-shield varying from 17.2 to 20.8 mm, and the length of head-shield in midline varying from 9.6 to 12.7 mm. There is about 10% variation in the size of headshield. The head-shield is wider than it is long. Caudally, the head-shield protrudes into a pair of cornual and inner cornual processes. The cornual processes, which are completely preserved in specimens V 25135.1, 3, 4, and 9, are oriented caudo-laterally (or postero-laterally), short and rapidly tapered off. The inner cornual processes, which are completely preserved in specimens V 25135.1, 3, 6, and 9, are small and spine-like, projecting caudally. The inner cornual processes are much smaller than the cornual processes.
Photographs (A), (C) and interpretative drawings (B), (D) of Sinogaleaspis shankouensis. (A), (B) IVPP V 25135.10; (C), (D) IVPP V 25135.6. Abbreviations: a, the first lateral transverse canal or median transverse canal issuing from infraorbital canal; 1–4, the first to fourth lateral transverse canal or median transverse canal issuing from lateral dorsal canal; cc, central canal; cp, cornual process; ic, inner cornual process; ifc, infraorbital canal; ldc, lateral dorsal canal; ltc, lateral transverse canal; mdc, median dorsal canal; md.o, median dorsal opening; mtc, median transverse canal; orb, orbital opening; pi, pineal opening; poc, preorbital commissure; soc2, posterior supraorbital canal. Gai et al. (2020).
The median dorsal opening is fairly long, wedge-shaped or elliptic in outline with long axis aligned with the rostro-caudal axis. The length of the long axis of median dorsal opening varies from 3.7 to 4.2 mm, and the length of the short axis varies from 0.6 to 1.2 mm. The length of the long axis of median dorsal opening is about 5 times the length of the short axis. The distance between the anterior end of the median dorsal opening and the rostral margin of the head-shield is rather short (about 1 mm). The posterior end of the median dorsal opening extends posteriorly beyond the level of the center of the orbital opening, but in front of the level of the posterior margin of the orbital openings.
Photographs of Sinogaleaspis shankouensis from the Silurian of Xiushui showing the morphology of the head-shield. (A), (B) External (A) and internal (B) moulds of a nearly complete head-shield, IVPP V 25135.1a, b; (C) internal mould of an incomplete head-shield, V 25135.3; (D) internal mould of an incomplete head-shield, V 25135.7b; (E) internal mould of an incomplete head-shield, V 25135.11; (F) close-up of granular tubercles (box region of E). Abbreviations: soc1, anterior supraorbital canal; for other abbreviations as above. Scale bars are 2 mm. Gai et al. (2020).
The orbital opening has a dorsal position on the head-shield. The orbital opening is round in outline with a diameter ranging from 1.5 to 2.2 mm in 11 specimens, roughly accounting for1/5 of the length of the head-shield in midline. The distance between paired orbital openings is range from 3.7 to 4.6 mm in 11 specimens.
Photographs of Sinogaleaspis shankouensis from the Silurian of Xiushui showing the sensory canal system. (A) An incomplete external mould of the head-shield IVPP V 25135.4a; (B) a complete external mould of the head-shield V 25135.9a; (C) an incomplete internal mould of the head-shield V 25135.8; (D) an incomplete internal mould of the head-shield V 25135.5; (E) an incomplete external mould of the head-shield V 25135.7b; (F) an incomplete external mould of the head-shield V 25135.2b. Abbreviations see as above. Scale bars are 2 mm.
The pineal opening is well preserved in specimen V 25135.3, which is located posterior to the level of the posterior margin of the orbital opening in the midline of the head-shield. The pineal opening is tiny and oval in outline with the length of the long axis about 0.54 mm, and the length of the short axis about 0.41 mm. Along the midline, the length of the pre-pineal region of head-shield ranges from 5.4 to 6.7 mm and the length of the postpineal region ranges from 4.2 to 6.1 mm in 11 specimens.
The sensory canal system of Sinogaleaspis shankouensis is comprehensively reconstructed based on 10 specimens. The well-preserved sensory canal system displays an Eugaleaspid pattern in distribution which consists of anterior supraorbital canals, posterior supraorbital canals, infraorbital canals, lateral dorsal canals, lateral transverse canals, median dorsal canals, preorbital commissure, median transverse canals and a short central canal. The paired anterior supraorbital canals slightly diverge posteriorly from the rostral margin, but do not connect with the posterior supraorbital canal. The paired posterior supraorbital canals are V-shaped and converge posteriorly to the pineal opening.
The characterised median dorsal canal joins with the posterior supraorbital canal in about 45 degree angle at the level of the pineal opening, and posteriorly converges with the opposite one to form a U-shaped trajectory. A short previously unnamed longitudinal canal issuing from the middle of the U-shaped median dorsal canal, here, is termed as the central canal, The infraorbital canals are positioned lateral to the orbital opening. Posteriorly, the infraorbital canal continues to extend as the lateral dorsal canal, which is the main longitudinal canal on the head-shield. There are six pairs of lateral transverse canals and median transverse canals observed in our new specimens. In addition to four pairs of lateral transverse canals running from the lateral dorsal canal two pair of additional lateral transverse canals are observed issuing from the infraorbital canals in specimen V 25135.7b. There are six pairs of median transverse canals. In addition to the originally described 3 pairs of median transverse canals, three pairs of additional median transverse canals are observed in our new specimens. The most anterior pair of short transverse canals unite the infraorbital and posterior supraorbital canals in front of orbital openings, which probably represent the first median transverse canal, the preorbital commissure.
The lateral margin of the head-shield is smooth and the exoskeleton of the head-shield is ornamented with closely set, tiny granular tubercles. The tubercles in the central part of the head-shield are bigger than those in the lateral part and the cornual process. There are about 10–15 tubercles per square millimeter in the center, whereas about 20–25 tubercles per square millimeter in the region of the cornual process.
Restoration of Sinogaleaspis shankouensis (A) dorsal view; (B) anterior-lateral view. Gai et al. (2020).
Sinogaleaspis shankouensis can be referred definitely to the Eugaleaspiformes by the longitudinal oval median dorsal opening and typical eugaleaspid-pattern sensory canal system. The three species assigned to Sinogaleaspis do not form a monophyletic group, but a paraphyletic group. The type species of Sinogaleaspis, Sinogaleaspis shankouensis has a closer relationship to Yunnanogaleaspis and higher eugaleaspids than to ‘Sinogaleaspis.’ zhejiangensis and ‘Sinogaleaspis.’ xikengensis, whereas ‘Sinogaleaspis.’ zhejiangensis is resolved as the deepest branching of the Eugaleaspiformes in later phylogenetic analyses. Based on abundant new materials, especially the 3D preserved neurocrania, a new genus, Shuyu was erected for ‘Sinogaleaspis’ zhejiangensis in 2011. By contrast, the nature of ‘Sinogaleaspis.’ xikengensis still needs more data for corroboration as the original description is mainly based on one poorly known specimen, especially its sensory canal system remains unknown. Sinogaleaspis shankouensis is similar to Shuyu zhejiangensis, Meishanaspis lehmani, and Anjiaspis reticularis from the Lower Silurian of Zhejiang Province in the subtriangular head-shield with spine-like cornual and inner cornual processes, but clearly different from them in the shape and position of median dorsal opening, the margin of head-shield, and pattern of sensory canals. The sensory canal system of Shuyu zhejiangensis and Meishanaspis lehmani are not of the typical Eugaleaspid-pattern by being deficient of the median dorsal canal, probably representing a plesiomorphic condition of the Eugaleaspiformes.
By contrast, Sinogaleaspis shankouensis exhibits a mélange of primitive and derived characters in the pattern of sensory canals. It possesses the U-shaped medial dorsal canal which is regarded as a derived character found in Eugaleaspiformes, V-shaped posterior supraorbital canals which is a derived character found in Polybranchiaspiformes and Huananaspiformes, while it has more than one median transverse canals which is regarded as a primitive character found in plesiomorphic taxa such as Dayongaspidae, Hanyangaspidae and Xiushuiaspidae. In addition to four pairs of lateral transverse canals issuing from the lateral dorsal canal, the new materials of Sinogaleaspis shankouensis also reveal two pairs of lateral transverse canals on infraorbital canals, a condition is strikingly similar to that of Shuyu and Meishanaspis. Ususally, there are 3–4 pairs of lateral transverse canals issuing from the infraorbital canal in the plesiomorphic taxa of Galeaspids such as Dayongaspis, Hanyangaspis, and Changxingaspis. It has been suggested that the number of lateral transverse canals has a decreasing tendency from six pairs to three pairs in Eugaleaspiformes. The vestiges of the lateral transverse canals can be observed on the infraorbital canals in some members of Eugaleaspiformes (e.g. Eugaleaspis changi and Eugaleaspis xujiachongensis) and Polybranchiaspiformes (e.g. Polybranchiaspis liaojiaoshanensis and Laxaspis qujingensis), which can be regard as a corroborative evidence for this evolutionary tendency.
Time-callibrated cladogram of the Eugaleaspiformes. Eugaleaspiformes experienced two diversifications in Silurian and Devonian (Solid columns represent known time ranges, thin lines represent ‘ghost lineages’). Gai et al. (2020).
The most noteworthy feature of Sinogaleaspis shankouensis and Anjiaspis reticularis is the presence of much more than two pairs of median transverse canals (6 pairs in the former, 8 pairs in the latter), which is odd among about 80 described galeaspid species and has been questioned in several previous studies. In addition to the originally described 3 pairs of median transverse canals, additional three pairs of median transverse canals are observed in our new specimens of Sinogaleaspis shankouensis. The six pairs of transverse canals (median transverse canals coupled with lateral transverse canals) form a grid distribution on the dorsal side of head-shield by intersecting with four longitudinal pairs (infraorbital canals, lateral dorsal canals, posterior supraorbital canals, and median dorsal canals). The most anteiror pair of median transverse canals, the preorbital commissures which link the infraorbital and posterior supraorbital canals in front of orbital openings, are also observed in some members of Polybranchiaspiformes and Huananaspiformes such as Polybranchiaspis, Laxaspis and Sanchaspis. In addition, a short previously unnamed longitudinal canal of Sinogaleaspis shankouensis, the central canal is also observed in Changxingaspis, Polybranchiaspis, Laxaspis and Sanchaspis issuing from the middle of dorsal commissure. In sum, the sensory canal system of Sinogaleaspis shankouensis exhibits mosaic characters of plesiomorphic taxa, Eugaleaspiformes, Polybranchiaspiformes and Huananaspiformes, and probably represents plesiomorphic condition of Vertebrates.
The fossil-bearing strata of Sinogaleaspis shankouensis also yield ‘Sinogaleaspis.’ xikengensis, Xiushuiaspis jiangxiensis, and Xiushuiaspis ganbeiensis, which are together named the Sinogaleaspis-Xiushuiaspis Fauna or the Maoshan Assemblage, because similar galeaspid species including Shuyu zhejiangensis, Meishanaspis lehmani, Anjiaspis reticularis and Changxingaspis gui were found in the Maoshan Formation of Zhejiang Province. The Sinogaleaspis-Xiushuiaspis Fauna together with other plesiomorphic taxa of Galeaspids (Dayongaspis, Hanyangaspis, Kalpinolepis, Xiyuaspis, and Microphymaspis) and the most primitive Polybranchiaspiforme (Platylomaspis) from the Silurian of the South China and Tarim probably represent the first diversification of Galeaspids in the Telychian, Llandovery of the Silurian, but they underwent a mass extinction during the Ludlow except for some Eugaleaspiformes and Polybranchiaspiforme survived until . the late Pragian of the Devonian. The Fish-bearing strata are referred to Silurian ‘upper red beds’ which reflect a nearshore, very shallow deltaic environment with the low nutrient, euphotic zone and rich oxidation. According to the evidence from the paleosalinity, and other geochemical data, it has been suggested that the sedimentary paleoenvironment of the Xikeng Formation at Xiushui, Jiangxi Province was suggestive of a brackish water body probably caused by a large deal of fresh water pouring into the sea from nearby rivers. In particular, most of specimens were collected from the Layer 13, which was composed of grey-green pelitic siltstone interbedded with a large deal of muddy gravels, strongly suggesting a short distance of potamic transportation. This indicates that the Sinogaleaspis-Xiushuiaspis Fauna may live in a fresh water environment in the river rather than their burial area in the sea.
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