The origin of the gnathostome body plan: how the Evo-Devo models intersect with ostracoderm fossils
by Zhikun Gai | Philip Donoghue | Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China | Bristol Palaeobiology Group, School of Earth Sciences, University of Bristol, Bristol BS8 1TQ, UK
Abstract ID: 63
Event: The 3rd AsiaEvo Conference
Topic: Early evolution of vertebrates from evo-devo and paleontological perspectives
Presenter Name: Zhikun Gai

The assembly of the gnathostome body plan has been interpreted as a serial sequence of evolutionary and developmental innovations over the fundamental vertebrate body plan shared with cyclostomes. However, divining their evolutionary origins is more challenging because the fundamental lineages of living vertebrates, the gnathostomes (jawed vertebrates) and cyclostomes (jawless vertebrates) are extremely divergent, relying heavily on insights from developmental models, such as through comparative analyses of craniofacial development from hagfishes, lampreys and jawed vertebrates. Nevertheless, there is a rich fossil record of jawless vertebrates known as the ostracoderms, from the Ordovician to Devonian periods (from 538 to 365 million years ago). The ostracoderms are related by degree to the living jawed vertebrates that effectively record the sequential evolution of the gnathostome bodyplan. In the past twelve years, some major morphological transitions from jawless to jawed vertebrates have been revealed by discoveries of extinct galeaspid ostracoderms from China, which have provided corroborative evidence for models of developmental evolution. For example, using synchrotron radiation X-ray tomographic microscopy, we have shown that the paired nasal sacs and hypophyseal duct in galeaspid Shuyu are independent of each other, as in gnathostomes but cyclostomes or the osteostracan ostracoderms, which is the condition that current developmental models regard as a pre-requisite for the development of jaws. The first articulated galeaspid remains of Tujiaaspis from the Silurian of China, reveal a pair of skeletal ventrolateral fins, which are compatible with aspects of the fin-fold hypothesis for the origin of vertebrate paired appendages. Here, we further show that the prechordal cranium and all of the gill arches (splanchnocranium) including mandibular and hyoid arches in galeaspids are incorporated medially into the parachordal cranium to form a single massive endoskeletal ossification. This pattern is compatible with aspects of the intermediate stage of branchiomere development in gnathostomes in which neural crest cells have a specific median relocation to form a cylindrical wall surrounding a mesodermal core. A hollowed cylinder-shaped branchial chamber enclosing gills and their associated musculature and vessels are widely seen in Silurian-Devonian armored ostracoderms (e.g. osteostracans, heterostracans). Therefore, the cylindrical branchial chamber in jawless ostracoderms probably represents an intermediate condition between the lateral in lampreys and medial location of gills aches in jawed vertebrates.