The Evolution of the Spiracular Region From Jawless Fishes to Tetrapods

The spiracular region, comprising the hyomandibular pouch together with mandibular and hyoid arches, has a complex evolutionary history. In living vertebrates, embryonic may disappear in adult, develop into small opening between palatoquadrate hyomandibula containing single gill-like pseudobranch, or create middle ear cavity, but it never develops fully formed gill two hemibranchs. belief that complete must be ancestral condition led some 20th century researchers to search for such arches early jawed vertebrates. This hypothesized state was named aphetohyoidean condition, so far not been verified any fossil; supposed examples, as acanthodian Acanthodes symmoriid chondrichthyans, have reinterpreted discounted. Here we present first confirmed example of vertebrate, galeaspid (jawless stem gnathostome) Shuyu . Comparisons other groups jawless gnathostomes, osteostracans heterostracans, indicate they also probably possessed full-sized gills this thus primitive gnathostome group. contrasts cyclostomes, which are strongly modified is lost adult. While no truly found recently reported presence acanthodians pseudobranchs suggests two-step process whereby initial miniaturization followed, independently chondrichthyans osteichthyans, by loss anterior pseudobranch. On basis these findings an overview evolution among