A redescription of the Silurian Sinogaleaspis shankouensis(Galeaspida,stem-Gnathostomata) from Jiangxi,China

GAI Zhi-Kun SHAN Xian-Ren SUN Zhi-Xin ZHAO Wen-Jin PAN Zhao-Hui ZHU Min*

(1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences Beijing 100044)

(2 CAS Center for Excellence in Life and Paleoenvironment Beijing 100044)

(3 University of Chinese Academy of Sciences Beijing 100049 * Corresponding author:zhumin@ivpp.ac.cn)

(4 College of Earth Science and Engineering,Shandong University of Science and Technology Qingdao 266590)

Abstract Sinogaleaspis shankouensis is redescribed based on 11 new specimens collected from the type locality of the Xikeng Formation in Xiushui county,Jiangxi Province.The in-depth morphological study indicates that the sensory canal system of S.shankouensis exhibits a mélange of characters of plesiomorphic galeaspid taxa,Eugaleaspiformes,Polybranchiaspiformes and Huananaspiformes.The grid distribution of the sensory canal system on the dorsal side of the head-shield,which comprises four longitudinal canals intercrossed with six pairs of transverse canals in S.shankouensis,probably represents a plesiomorphic condition of vertebrates.Sinogaleaspis shankouensis belongs to the Sinogaleaspis-Xiushuiaspis Fauna or the Maoshan Assemblage which represents the first diversification of galeaspids in the Telychian,Llandovery of the Silurian period.The sedimentary paleoenvironment of the Xikeng Formation in Xiushui,Jiangxi Province is suggestive of a brackish water environment,whereas a large sum of muddy gravels in fish-bearing sandstone beds point to 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.

Key words Xiushui,Jiangxi;Silurian;eugaleaspids,Sinogaleaspis;sensory canal system

1 Introduction

Sinogaleaspis,the type genus of the family Sinogaleaspidae,comprises two species,S.shankouensisand ‘S’.xikengensis(Pan and Wang,1980).The monophyly of the Sinogaleaspidae remains controversial largely due to the poorly known nature ofS.shankouensiswhose description was based on two incomplete specimens from the Lower Silurian of Xiushui,Jiangxi Province (Pan,1986a;Zhu,1992;Gai et al.,2005,2018;Gai and Zhu,2005,Zhu and Gai,2006,Liu et al.,2015).Among about 80 described galeaspid species,the sensory canal system ofS.shankouensisis peculiar in bearing three pairs of median transverse canals,which contrasts with the known pattern of other galeaspids (Liu,1986;Wang,1991;Liu et al.,2015;Gai and Zhu,2017).Considering the unusual condition known from two poorly preserved specimens,Wang (1991) pointed out that the present interpretation of the sensory canal system inS.shankouensisneeds further corroboration from additional materials.Since 2003,the authors have organized five field excursions to the type locality ofS.shankouensis,i.e.Xikeng village of Xiushui,Jiangxi Province,and collected abundant fish remains including 11 new specimens ofS.shankouensis.Based on these new data,we present a detailed redescription ofS.shankouensis,and clarify its sensory canal condition.

2 Geological setting

The new material ofSinogaleaspis shankouensiswas collected from the type locality of the Xikeng Formation in Taiyangshen Town,Xiushui County,Jiangxi Province,South China(Fig.1A).The fish-bearing Xikeng Formation belongs to the Upper Red Beds (URBs) 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 URBs 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 (Bureau of Geology and Mineral Resources of Jiangxi Province,1984;Zhao et al.,2009;Zhao and Zhu,2010;Wang et al.,2018;Rong et al.,2019).The Xikeng Formation in the type locality has been remeasured (Fig.1B) as following:

Xikeng Formation (Layers above Layer 11 are not measured)

11.Grey-green medium-bedded siltstone,intercalated with grey-green thin-bedded silty mudstone 10.30 m

10.Purple medium and thick-bedded pelitic siltstone,intercalated with thin-bedded mudstone 12.11 m

9.Grey-green medium and thick-bedded siltstone,with abundant tiny tubular trace fossils 1.05 m

8.Yellow-green medium to thick-bedded pelitic siltstone 5.04 m

7.Purple medium and thick-bedded pelitic siltstone 1.33 m

6.Grey-green medium-bedded pelitic siltstone,intercalated with yellow-green thin-bedded mudstone 1.81 m

5.Yellow-green medium and thick-bedded siltstone,intercalated with thin and medium-bedded pelitic siltstone 2.37 m

4.Purple thin to thick-bedded pelitic siltstone 4.73 m

3.Grey-green thin and medium-bedded pelitic siltstone interbedded with a large sum of muddy gravels,yielding galeaspids:Sinogaleaspis shankouensis,‘S’.xikengensis,Xiushuiaspis jiangxiensis,andX.ganbeiensis0.93 m

2.Yellow-green medium-bedded siltstone 1.27 m

1.Purple thin and medium-bedded pelitic siltstone,intercalated with thin-bedded mudstone 25.02 m

──── conformity ────

Xiajiaqiao Formation

0.Medium and thick-bedded fine sandstone 1.31 m

Fig.1 Geological backround of Sinogaleaspis shankouensis

3 Material and methods

The new material ofSinogaleaspis shankouensisincludes 11 head-shields (IVPP V 25135.1-11) from the type locality of the Xikeng Formation in Taiyangshen town,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,Chinese Academy of Sciences (IVPP).The holotype and paratype ofS.shankouensis,GMC V 1751 and GMC V 1752 is examined for comparison and measurement,which is housed in the collections of the Geological Museum of China (GMC).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×).

4 Systematic paleontology

(Figs.2−6)

Fig.2 Photographs (A,C) and interpretative drawings (B,D) of Sinogaleaspis shankouensis

Diagnosis (emended)A small-sized sinogaleaspid jawless fish,subtriangular headshield with an arciform rostral margin,maximum length of head-shield about 15-18 mm,maximum width of head-shield about 17-21 mm,wider than long;cornual process developed,projecting caudo-laterally;inner cornual process spine-like,projecting caudally,much smaller than cornual process;median dorsal opening wedge-shaped or longitudinal oval in outline,its anterior end separated from the rostral margin of head-shield by a short distance,and its posterior end extending posteriorly beyond the level of the center of the orbital opening;orbital opening large,round,and dorsally positioned;pineal opening small,oval in shape,and located behind the posterior margin of the orbital opening;sensory canal system sophisticated,V-shaped posterior supraorbital canals,infraorbital canal posteriorly connected with lateral dorsal canals naturally;U-shaped medial dorsal canals joining posterior supraorbital canals anteriorly;6 pairs of lateral transverse canals with two pairs issuing from infraorbital canals;6 pairs of median transverse canals with the first pair the preorbital commissures uniting the infraorbital and posterior supraorbital canals in front of orbital openings;margin of head-shield smooth;ornamentation composed of tiny granular tubercles decreasing gradually from the shield center towards the cornual process.

(Figs.2-6)

Sinogaleaspis shankouensisPan and Wang,1980,p.3,fig.3,pls.I,II

Sinogaleaspis shankouensisLiu,1986,p.248,fig.2A

Sinogaleaspis shankouensisWang,1991,p.54,fig.15

Sinogaleaspis shankouensisGai et al.,2005,p.68

Sinogaleaspis shankouensisZhu and Gai,2006,p.7,fig.4D

Sinogaleaspis shankouensisLiu et al.,2015,p.170,fig.82

Sinogaleaspis shankouensisGai and Zhu,2017,p.227,figs.7-44,7-45

HolotypeA nearly complete head-shield,GMC V 1751.

ParatypeA nearly complete head-shield,GMC V 1752.

Referred specimensTwo nearly complete head-shields,IVPP V 25135.1,V 25135.9,and nine incomplete head-shields,V 25135.2−8,V 25135.10-11.

Type locality and horizonXikeng village,Taiyangshen Town (previously known as Sandu Town),Xiushui county,Jiangxi Province,China;Xikeng Formation,Telychian,Llandovery,Silurian.

DiagnosisThe only known species,diagnosis as that of the genus.Sinogaleaspis xikengensis(Pan and Wang,1980) is removed out ofSinogaleaspisas discussed below (Gai et al.,2005).

MeasurementsSee Table 1.

DescriptionSinogaleaspis shankouensisis a small-sized sinogaleaspid jawless fish with a subtriangular head-shield (Figs.2−6).The rostral margin of the head-shield is arciform without rostral process.The measurements of 11 specimens ofS.shankouensisindicate 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 (Table 1).There are 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 V 25135.1,3,4,9,are oriented caudo-laterally (or postero-laterally),short and rapidly tapered off(cp,Figs.2,3A−C,E,4A,B).The inner cornual processes,which are completely preserved in V 25135.1,3,6,9,are small and spine-like,projecting caudally (ic,Figs.2,3A−C,4A,B).The inner cornual processes are much smaller than the cornual processes.

The median dorsal opening (md.o,Figs.2−6) 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.

Fig.3 Photographs of Sinogaleaspis shankouensis from the Silurian of Xiushui showing the morphology of the head-shield

Table 1 Measurements of Sinogaleaspis shankouensis (mm)

Fig.4 Photographs of Sinogaleaspis shankouensis from the Silurian of Xiushui showing the sensory canal system

The orbital opening (orb,Figs.2−6) 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 (Table 1),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 (Table 1).

The pineal opening is well preserved in specimen V 25135.3 (pi,Fig.3C),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 (Table 1).

Fig.5 Interpretative and synthetic drawings of the sensory canal system of Sinogaleaspis shankouensis

The sensory canal system ofS.shankouensisis comprehensively reconstructed based on 10 specimens (Figs.2−6).The well-preserved sensory canal system displays an eugaleaspid pattern in distribution which consists of anterior supraorbital canals (soc1),posterior supraorbital canals (soc2),infraorbital canals (ifc),lateral dorsal canals (ldc),lateral transverse canals (ltc),median dorsal canals (mdc),preorbital commissure (poc),median transverse canals (mtc) and a short central canal (cc) (Figs.2−6).The paired anterior supraorbital canals(soc1,Figs.4C,E,5C,E,H,I) slightly diverge posteriorly from the rostral margin,but do not connect with the posterior supraorbital canal (soc2,Figs.2−6).The paired posterior supraorbital canals (soc2,Figs.2−6) are V-shaped and converge posteriorly to the pineal opening.

The characterized median dorsal canal (mdc,Figs.2−6) 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 (cc,Figs.2D,3C,4F,5B,F,H,I).The infraorbital canals (ifc,Figs.2−6) are positioned lateral to the orbital opening.Posteriorly,the infraorbital canal continues to extend as the lateral dorsal canal (ldc,Figs.2−6),which is the main longitudinal canal on the head-shield.There are six pairs of lateral transverse canals (ltc,Figs.2−6) and median transverse canals (mtc,Figs.2−6) observed in our new specimens.In addition to four pairs of lateral transverse canals running from the lateral dorsal canal (ltc 1−4,Figs.2,5),two pair of additional lateral transverse canals (ltc a,b,Figs.2,5) are observed issuing from the infraorbital canals in specimen V 25135.7b (Figs.4E,5E).There are six pairs of median transverse canals (mtc,Figs.2−6).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 (poc,Figs.2D,3C,4A,C,E,F,5A,C,E,F,I,7G-I).

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 (Fig.3E,F).

Fig.6 Restoration of Sinogaleaspis shankouensis

Fig.7 The sensory canal systems of galeaspids in different groups

5 Discussion and conclusion

Sinogaleaspis shankouensiscan be referred definitely to the Eugaleaspiformes by the longitudinal oval median dorsal opening and typical eugaleaspid-pattern sensory canal system(Zhu,1992;Gai et al.,2005,2018;Zhu and Gai,2006).Gai et al.(2005) indicated that the three species assigned toSinogaleaspisdo not form a monophyletic group,but a paraphyletic group.The type species ofSinogaleaspis,S.shankouensishas a closer relationship toYunnanogaleaspisand higher eugaleaspids than to ‘S.’zhejiangensisand ‘S.’xikengensis(Gai et al.,2005),whereas ‘S.’zhejiangensisis resolved as the deepest branching of the Eugaleaspiformes in later phylogenetic analyses (Fig.8) (Zhu and Gai,2006;Gai et al.,2018).Based on abundant new materials,especially the 3D preserved neurocrania,Gai et al.(2011) erected a new genusShuyufor ‘S.’zhejiangensis.By contrast,the nature of ‘S.’xikengensisstill 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 shankouensisissimilar toShuyu zhejiangensis,Meishanaspis lehmani,andAnjiaspis reticularisfrom the Lower Silurian of Zhejiang Provincein 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,themargin of head-shield,and pattern of sensory canals (Pan,1986a;Wang,1991;Gai and Zhu,2005;Gai et al.,2011).The sensory canal system ofSh.zhejiangensisandM.lehmaniare not of the typical eugaleaspid-pattern by being deficient of the median dorsal canal (Fig.7D),probably representing a plesiomorphic condition of the Eugaleaspiformes (Gai et al.,2005,2011).

By contrast,S.shankouensisexhibits 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 (mdc,Fig.7E,F),V-shaped posterior supraorbital canals which is a derived character found in Polybranchiaspiformes and Huananaspiformes(soc2,Fig.7G-I),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 (mtc,Fig.7A-C).In addition to four pairs of lateral transverse canals issuing from the lateral dorsal canal,the new materials ofS.shankouensisalso reveal two pairs oflateral transverse canals on infraorbital canals,a condition is strikingly similar to that ofShuyuandMeishanaspis(ltc a,b,Fig.7D).Ususally,there are 3-4 pairs of lateral transverse canals issuing from the infraorbital canal in the plesiomorphic taxa of galeaspids such asDayongaspis,Hanyangaspis,andChangxingaspis(ltc a,b,c,Fig.7A-C).Liu (1986)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 changiandE.xujiachongensis) (ltc a,b,Fig.7E,F) and Polybranchiaspiformes (e.g.Polybranchiaspis liaojiaoshanensisandLaxaspis qujingensis) (ltc a,b,Fig.7G,H),which can be regard as a corroborative evidence for this evolutionary tendency (Liu,1986;Gai et al.,2005).

Fig.8 Time-callibrated cladogram of the Eugaleaspiformes

The most noteworthy feature ofS.shankouensisandA.reticularisis 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(Wang,1991;Liu et al.,2014).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 ofS.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 (poc) which link the infraorbital and posterior supraorbital canals in front of orbital openings,are also observed in some members of Polybranchiaspiformes and Huananaspiformes such asPolybranchiaspis,LaxaspisandSanchaspis(poc,Fig.7G-I).In addition,a short previously unnamed longitudinal canal ofS.shankouensis,the central canal is also observed inChangxingaspis,Polybranchiaspis,LaxaspisandSanchaspis(cc,Fig.7C,G-I) issuing from the middle of dorsal commissure.In sum,the sensory canal system ofS.shankouensisexhibits mosaic characters of plesiomorphic taxa,Eugaleaspiformes,Polybranchiaspiformes and Huananaspiformes,and probably represents plesiomorphic condition of vertebrates as Liu (1986) assumed.

The fossil-bearing strata ofS.shankouensisalso yield ‘S.’xikengensis,Xiushuiaspis jiangxiensis,andX.ganbeiensis,which are together named theSinogaleaspis-XiushuiaspisFauna or the Maoshan Assemblage (P’an and Wang,1983;Zhao and Zhu,2010),because similar galeaspid species includingSh.zhejiangensis,M.lehmani,A.reticularisandC.guiwere found in the Maoshan Formation of Zhejiang Province (Pan,1986a,1988;Wang,1991;Gai and Zhu,2005;Gai et al.,2005).TheSinogaleaspis-XiushuiaspisFauna together with other plesiomorphic taxa of galeaspids (Dayongaspis,Hanyangaspis,Kalpinolepis,Xiyuaspis,andMicrophymaspis) 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 (Fig.8),but they underwent a mass extinction during the Ludlow except for some Eugaleaspiformes and Polybranchiaspiforme survived until the late Pragian of the Devonian (Zhu and Gai,2006;Gai et al.,2018;Liu et al.,2019) (Fig.8).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 (Rong et al.,2012).According to the evidence from the paleosalinity,and other geochemical data,Li(1996) thought 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 (Fig.1B),strongly suggesting a short distance of potamic transportation.This indicates that theSinogaleaspis-XiushuiaspisFauna may live in a fresh water environment in the river rather than their burial area in the sea.

AcknowledgementsWe are grateful to Zhao Ridong,Lin Xianghong,and Sun Baichuan for the great help in the field work,Guo Xiaocong,Yang Dinghua,and Huang Jinling for the illustrations and ecological restoration.This work was supported by Key Research Program of Frontier Sciences,CAS (QYZDB-SSW-DQC040,QYZDJ-SSW-DQC002),the National Natural Science Foundation of China (41572018,41530102,41972006),National Program for Support of Top-notch Young Professionals,and Strategic Priority Research Program of CAS(XDB26000000),State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology,CAS,No.193121) and Innovation Training Programs for Undergraduates,CAS.