Cranial morphological features of the stem-group sarcopterygian Guiyu oneiros Zhu et al.,2009 provided here include the dermal bone pattern and anatomical details of the ethmosphenoid.Based on those features,we restored,for the first time,the skull roof bone pattern in the Guiyu clade that comprises Psarolepis and Achoania.Comparisons with Onychodus,Achoania,coelacanths,and actinopterygians show that the posterior nostril enclosed by the preorbital or the preorbital process is shared by actinopterygians and sarcopterygians,and the lachrymals in sarcopterygians and actinopterygians are not homologous.The endocranium closely resembles that of Psarolepis,Achoania and Onychodus;however,the attachment area of the vomer possesses irregular ridges and grooves as in Youngolepis and Diabolepis.The orbito-nasal canal is positioned mesial to the nasal capsule as in Youngolepis and porolepiforms.The position of the hypophysial canal at the same level or slightly anterior to the ethmoid articulation represents a synapmorphy of the Guiyu clade.The large attachment area of the basicranial muscle indicates the presence of a well-developed intracranial joint in Guiyu.
QIAO Tuo1,2 & ZHU Min1* 1 Key Laboratory of Evolutionary Systematics of Vertebrates,Institute of Vertebrate Paleontology and Paleoanthropology,Chinese Academy of Sciences,Beijing 100044,China
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.
本文记述了肉鳍鱼类基干类型—梦幻鬼鱼(Guiyuoneiros Zhu et al.,2009)的头部形态特征,包括颅顶的膜质骨型式和顶甲的内颅构造.文中在鬼鱼支系(包括鬼鱼、斑鳞鱼(Psarolepis)和无孔鱼(Achoania))中恢复了颅顶的膜质骨型式.通过和爪齿鱼(Onychodus)、无孔鱼、空棘鱼类以及辐鳍鱼类的比较显示:后外鼻孔后缘被眶前骨或前上颌骨眶前突包围是硬骨鱼类的原始特征,肉鳍鱼类和辐鳍鱼类中的泪骨并非同源结构.鬼鱼的筛蝶区结构和斑鳞鱼、无孔鱼以及爪齿鱼极为相似.发达的颅下肌附着区可能揭示了鬼鱼发育的颅间关节.与杨氏鱼(Youngolepis)和奇异鱼(Diabolepis)相近,犁骨附着区具不规则沟脊.与杨氏鱼和孔鳞鱼类(porolepiforms)一致,鼻眶管靠近鼻囊内侧.垂体孔位于脑颅筛关节稍前处或位于同一水平是鬼鱼支系的共近裔性状.
