Novel assembly of a head-trunk interface in the sister group of jawed vertebrates.

The standard scenario for the origin of jawed vertebrates depicts a transition from benthic grazers to nektonic predators, facilitated by a suite of anatomical innovations, including elaborate sensory systems, a high-flow heart and the integration of jaw-opening muscles with the craniothoracic hinge. However, the lamprey-like internal anatomy reconstructed for osteostracans, the sister group of jawed vertebrates, seem to lack these gnathostome traits, implying a morphological gap despite phylogenetic proximity. Here, using synchrotron-based X-ray microtomography on the model osteostracan Norselaspis glacialis, we reveal derived gnathostome traits straddling a uniquely ossified head-trunk interface in this jawless fish. The inner ear of Norselaspis shows sensory elaborations (enlarged pars inferior and sinus superior) acquired well before the origin of jaws. As in crown gnathostomes, paired venous drainage channels blood into a high-volume cardiac tract. We also confirm a feature not yet demonstrated in any other vertebrate, to our knowledge: the most anterior trunk nerve extends its single trunk to the pectoral fin. In this respect, our reconstruction challenges the hypotheses that the gnathostome shoulder evolved from the gill apparatus. Our observations highlight Norselaspis as a prelude to the intercalation of the muscular neck and throat that would power the early jaw apparatus. Therefore, the vertebrate jaw-often considered the functional driver for 'gnathostome' innovations-evolved instead as a follower to the sensory enhancement, increased cardiac output and greater locomotory control now inferred in the jawless sister group.