Fossil jawless fish from China foreshadows early jawed vertebrate anatomy

• Published in: Nature (London) 2011-08, Vol.476 (7360), p.324-327
• Main Authors: Gai, Zhikun, Donoghue, Philip C. J., Zhu, Min, Janvier, Philippe, Stampanoni, Marco
• Format: Article
• Language: English
• Subjects: 631/136; 631/181/414; 631/601; 692/698/1671/63; Anatomy; Animals; Biological Evolution; China; Earth Sciences; Earth, ocean, space; Evolution; Exact sciences and technology; Fishes - anatomy & histology; Fishes - classification; Fossils; Head - anatomy & histology; Humanities and Social Sciences; Jaw - anatomy & histology; letter; multidisciplinary; Paleontology; Science; Science (multidisciplinary); Sciences of the Universe; Veins & arteries; Vertebrate paleontology; Vertebrates; Vietnam
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature10276

Jawless vertebrate saves face Almost all living vertebrates have jaws. The few that don't — the lampreys and hagfish — are so specialized in other ways that understanding how jaws evolved is problematic. Fossils can provide some clues. Synchrotron radiation X-ray tomography of the heads of fossil galeaspids, extinct jawless vertebrates more closely related to living jawed vertebrates than to living jawless vertebrates, reveals an intriguing intermediate form. Modern jawless fishes, and most fossil ones, have a single, median nostril, but galeaspids had paired nasal sacs, as in jawed vertebrates, freeing up the centre of the 'face' as a field in which jaws could develop. Most living vertebrates are jawed vertebrates (gnathostomes), and the living jawless vertebrates (cyclostomes), hagfishes and lampreys, provide scarce information about the profound reorganization of the vertebrate skull during the evolutionary origin of jaws 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 . The extinct bony jawless vertebrates, or ‘ostracoderms’, are regarded as precursors of jawed vertebrates and provide insight into this formative episode in vertebrate evolution 8 , 9 , 10 , 11 , 12 , 13 , 14 . Here, using synchrotron radiation X-ray tomography 15 , 16 , we describe the cranial anatomy of galeaspids, a 435–370-million-year-old ‘ostracoderm’ group from China and Vietnam 17 . The paired nasal sacs of galeaspids are located anterolaterally in the braincase, and the hypophyseal duct opens anteriorly towards the oral cavity. These three structures (the paired nasal sacs and the hypophyseal duct) were thus already independent of each other, like in gnathostomes and unlike in cyclostomes and osteostracans (another ‘ostracoderm’ group), and therefore have the condition that current developmental models regard as prerequisites for the development of jaws 1 , 2 , 3 . This indicates that the reorganization of vertebrate cranial anatomy was not driven deterministically by the evolutionary origin of jaws but occurred stepwise, ultimately allowing the rostral growth of ectomesenchyme that now characterizes gnathostome head development 1 , 2 , 3 .