古代の無顎魚が進化の飛躍の鍵を示す(Ancient fossil fish reveals key step before evolutionary boom)

2025-08-12 シカゴ大学(UChicago)

Digital 3D image of the Norselaspis skull showing the brain and inner ear.Image courtesy of Michael Coates

<関連情報>

• https://news.uchicago.edu/story/ancient-fossil-fish-reveals-key-step-evolutionary-boom
• https://www.nature.com/articles/s41586-025-09329-9

顎を有する脊椎動物の姉妹群における頭部と体幹の接合部の新規な組み立て Novel assembly of a head–trunk interface in the sister group of jawed vertebrates

Tetsuto Miyashita,Philippe Janvier,Kristen Tietjen,Felisa Berenguer,Sebastian Schöder,Federica Marone,Pierre Gueriau & Michael I. Coates
Nature  Published:06 August 2025
DOI:https://doi.org/10.1038/s41586-025-09329-9

Abstract

The standard scenario for the origin of jawed vertebrates depicts a transition from benthic grazers to nektonic predators^1,2,3, 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^4,5,6,7. However, the lamprey-like internal anatomy^{8,9,10,11,12,13} 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^14,15,16 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^1,2,3—evolved instead as a follower to the sensory enhancement, increased cardiac output and greater locomotory control now inferred in the jawless sister group.