摘要
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.
The origin of the vertebrate jaw has been reviewed based on the molecular, developmental and paleontological evidences. Ad- vances 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 naso- hypophyseal 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 north- ern 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.
基金
supported by the National Natural Science Foundation of China(40930208)
the National Basic Research Program of China (2012CB821902)
the Knowledge Innovation Program of the Chinese Academy of Sciences(KZCX2-YW-156)
the CAS/SAFEA International Partnership Program for Creative Research Teams
