New findings of Changxingaspis(Xiushuiaspidae,Galeaspida)from the Silurian of Tarim Basin and Zhejiang Province,China

New discovery of the early Silurian fossil fish Changxingaspis(Xiushuiaspidae,Galeaspida),Changxingaspis nianzhongi sp.nov.and C.gui,are described from the Tataertag Formation in Tarim Basin and the Kangshan Formation in Zhejiang Province,respectively.C.nianzhongi mainly differs from C.gui in the shape of the median dorsal opening that is transverse elliptic with a width/length ratio of about 3.0,the long lateral transverse canals extending to the lateral margin of the headshield,and the second lateral transverse canal with dichotomous branchings.Discovery of C.nianzhongi from the Tataertag Formation and C.gui from the Kangshan Formation provide direct evidence on the specific level for the correlation between these two formations,which further supports the Silurian fish-bearing red beds in northwest Zhejiang belonging to the Silurian Lower Red Beds(LRBs)rather than the Upper Red Beds(URBs).Additionally,as the first record of the Changxingaspis in Tarim Basin,it extends the paleogeographical distribution of this genus from the South China Block to the Tarim Block,providing new evidence to support faunal exchanges between these two blocks and the hypothesis of a united Tarim-South China Block during the early Silurian.

A New Species of Jiangxialepis(Galeaspida)from the Lower Telychian(Silurian)of Jiangxi and its Biostratigraphic Significance

A new species of Shuyuidae(Eugaleaspiformes,Galeaspida),Jiangxialepis jiujiangensis sp.nov.,is described from the lower Telychian(Llandovery,Silurian)Qingshui Formation in Jiujiang,Jiangxi Province,China.The new species differs from the type species J.retrospina from Wuhan,Hubei Province in its sharp and posteriorly positioned median dorsal spine and narrow spine-shaped inner cornual processes.The Silurian strata in Xiushui–Wuning area has provided a standard framework for the correlation of Silurian shallow marine red beds in South China.Thus,the finding of J.jiujiangensis from the Silurian Lower Red Beds(LRBs)in Jiangxi Province bears very important biostratigraphic significance.It can directly compare to Jiangxialepis retrospina from the Fentou Formation in Wuhan,Hubei Province in the genus level.This indicates that the age of the fish-bearing strata in Wuhan is most likely to be the early Telychian rather than middle Telychian as previously assumed.

新疆晚奥陶世鱼形动物

本文记述了新近发现的保存较好、具甲胄确切证据的奥陶纪鱼形动物化石。该化石采自新疆塔里木盆地巴楚县一间房地区依朗里克塔格山上奥陶统恰尔巴克组滨浅海紫红色砂屑灰岩中。对所发现的鱼形动物新属新种——优雅郝氏鱼(Haoina elegantia gen.et sp.nov.)进行了生物学系统描述。郝氏鱼是我国奥陶纪已知保存较完整的古脊椎动物化石,是新科——郝氏鱼科(Haoinaspididae fam.nov.)的科型属。并依据其独特的生物学特征建立新目——郝氏鱼目(Haoinaspidiformes ord.nov.),置于无颌超纲甲胄鱼纲之下。依据牙形石化石带的时代分布,恰尔巴克组属于晚奥陶世早期,即桑比期。新发现的鱼形动物化石包括一个较完整的背侧甲片,化石表面覆盖具有特征纹饰的外骨骼甲胄,大致可分为四种不同的类型;松果区位于头甲中前部;有宽大而深陷的背侧位眶凹,位于松果体稍前的两侧,瘤突组成的嵴在凹壁呈同心圆排列,眶凹呈喇叭状开口。感觉沟系统发育,具眶上感觉沟、主侧线感觉沟和嵴间感觉沟,显示出真皮及皮下组织的管孔系统结构。根据与其他地区鱼形动物的对比和生物特征,建立了具有古特提斯特色的新的鱼形动物古地理分区——奥陶纪郝氏鱼类巴楚鱼类古脊椎动物生物地理区系。这一化石是我国乃至亚洲目前已知发现最早具外骨骼确凿证据的鱼形动物。

A New Family of Galeaspids(Jawless Stem-Gnathostomata) from the Early Silurian of Chongqing, Southwestern China

A new genus and species of agnathan Eugaleaspidiformes(Galeaspida), Yongdongaspis littoralis gen. et sp. nov., is described from the Llandovery(lower Silurian) Huixingshao Formation at Yongdong Town, Xiushan County, Chongqing, southwestern China. This new Telychian taxon morphologically exhibits some transitional states between Sinogaleaspidae and a cluster of higher eugaleaspidiforms containing Tridensaspidae, Eugaleaspidae, Yunnanogaleaspis, and Nochelaspis, which we term here as the ’eugaleaspid cluster’. Phylogenetic analysis of an extended character matrix of Galeaspida reveals Yongdongaspis, on which Yongdongaspidae fam. nov. is erected, as the sister taxon of this ’eugaleaspid cluster’, supported by two synapomorphies, the presence of one median transverse canal, and two lateral transverse canals leaving from the infraorbital canal. As the first fish described from the Llandovery Huixingshao Formation in Chongqing, Yongdongaspis provides new fossil evidence for the subdivision and correlation of the Upper Red Beds in South China.

First Record of Petalodus Owen,1840(Chondrichthyes,Petalodontidae)in the Lower Permian(Cisuralian)of China

The Petalodontiformes are a small intriguing group of Permo–Carboniferous chondrichthyans.Petalodus is the longest known petalodont genus generally considered representative of the order.The first definite fossil record of seven well-preserved Petalodus teeth has been found in the Qianshi limestone in the Lower Permian(Cisuralian)middle–upper Taiyuan Formation in Yangquan City,Shanxi Province,North China.The specimens are characterized by petal-shaped teeth with a spade-like crown,and a long,tongue-shaped root;the crown is circled with a band or cingulum composed of imbricated ridges at the base.All seven teeth are assigned to the species P.ohioenesis because of their vertically narrow cingulum and much longer root.Petalodus is a worldwide genus,with fossil localities mainly concentrated in the Laurussia supercontinent.The occurrence of Petalodus teeth in Yangquan not only is the first fossil record in China,but also only the second record in Asia.The Yangquan fossil site was part of the paleoequatorial North China Craton during the Early Permian,and was isolated from Laurusia and East Gondwana by the Paleo-Tethys Ocean.The successful dispersal of Petalodus from Laurusia to the North China Block along Paleo-Tethys may support the possibility that Petalodus taxa were active free-swimmers rather than bottom dwellers.The new finding increases the petalodont diversity in eastern Asia,and also sheds new light on the distribution and stratigraphic range.

The origin of the vertebrate jaw:Intersection between developmental biology-based model and fossil evidence

The origin of the vertebrate jaw has been reviewed based on the molecular,developmental and paleontological evidences.Advances in developmental genetics have accumulated to propose the heterotopy theory of jaw evolution,i.e.the jaw evolved as a novelty through a heterotopic shift of mesenchyme-epithelial interaction.According to this theory,the disassociation of the nasohypophyseal complex is a fundamental prerequisite for the origin of the jaw,since the median position of the nasohypophyseal placode in cyclostome head development precludes the forward growth of the neural-crest-derived craniofacial ectomesenchyme.The potential impacts of this disassociation on the origin of the diplorhiny are also discussed from the molecular perspectives.Thus far,our study on the cranial anatomy of galeaspids,a 435-370-million-year-old 'ostracoderm' group from China and northern Vietnam,has provided the earliest fossil evidence for the disassociation of nasohypophyseal complex in vertebrate phylogeny.Using Synchrotron Radiation X-ray Tomography,we further show some derivative structures of the trabeculae(e.g.orbitonasal lamina,ethmoid plate) in jawless galeaspids,which provide new insights into the reorganization of the vertebrate head before the evolutionary origin of the jaw.These anatomical observations based on new techniques highlight the possibility that galeaspids are,in many respects,a better proxy than osteostracans for reconstructing the pre-gnathostome condition of the rostral part of the braincase.The cranial anatomy of galeaspids reveals a number of derived characters uniquely shared with gnathostomes.This raises the potential possibility that galeaspids might be the closest jawless relatives of jawed vertebrates.Our study provides an intriguing example of intersection between developmental biology-based model and fossil evidence.

Phylogenetic relationships of Galeaspids(Agnatha)

We present the first parsimony analysis of the agnathan subclass Galeaspida based on the analysis of 53 morph ological characters.Three most parsimonious cladograms(126 steps in length;CI=0.508;RI=0.801)were discovered.An amended classification of the Galeaspida is proposed corresponding to the present analysis.Our results suggest that hanyangaspids,xiushuiaspids and dayon-gaspids from the Llandovery-Wenlock of Silurian are basal galeaspids.Within the remaining galeaspids,three major mono phyletic groups(the Eugaleaspidiformes,the Polybranchiaspidiformes and the Huananaspidiformes)are well sup-ported.It is shown that the dorsal fenestrae of the headshield evolved twice within the Galeaspida,one in the polybranchi-aspidiform lineage,and the other in the huananaspidiform lineage(nested within the Huananaspidae).The chronological distribution of galeaspids highlights two radiations of the group,one for basal galeaspids and eugaleaspids in the Telychian(Llandovery)of Silurian,and the other for polybranchiaspidiforms and huananaspidiforms in the Lochkovian of Early Devonian.