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

Nature volume 645, pages 686–691 (2025)Cite this article

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Abstract

The standard scenario for the origin of jawed vertebrates depicts a transition from benthic grazers to nektonic predators1,2,3, 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 hinge4,5,6,7. However, the lamprey-like internal anatomy8,9,10,11,12,13 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 hypotheses14,15,16 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’ innovations1,2,3—evolved instead as a follower to the sensory enhancement, increased cardiac output and greater locomotory control now inferred in the jawless sister group.

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Fig. 1: Internal anatomy of the jawless stem gnathostome N. glacialis based on a synchrotron-based X-ray microtomography scan.
Fig. 2: The circulatory system of N. glacialis reveals derived features shared with jawed vertebrates at the head–trunk interface.
Fig. 3: The pectoral fin attachment of N. glacialis reveals a unique state of the head–trunk interface with the complete absence of neck or hypobranchial musculature.
Fig. 4: Key gnathostome traits evolved early in jawless fishes.

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Data availability

Volumetric image stacks of MNHN-F-SVD3221, resulting from the synchrotron scan at the X02DA TOMCAT beamline of the Swiss Light Source, are deposited at MorphoSource under project ID 000683199 (https://www.morphosource.org/projects/000683199). Part (Media ID: 000683250; https://n2t.net/ark:/87602/m4/683250) and counterpart (Media ID: 000683605; ark:/87602/m4/683605) are available for download as separate objects. Volumetric image stacks of the comparative specimens are also deposited at MorphoSource. Media ID 000681198: I. bdellium, Ohio lamprey, head: https://n2t.net/ark:/87602/m4/681198; Media ID 000681285: C. plagiosum, bamboo shark, head: https://n2t.net/ark:/87602/m4/681285. Rendered surface mesh files for all three specimens, along with the supplementary video to enhance the reconstructed internal anatomy of N. glacialis (MNHN-F-SVD3221), are available at Figshare (https://doi.org/10.6084/m9.figshare.27905757)54 as data supplements.

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Acknowledgements

We thank G. Clément and A. Pradel for facilitating the loan of MNHN-F-SVD3221, the Paul Scherrer Institute for provision of beamtime at the TOMCAT beamline X02DA of the SLS, D. Haberthür and M. Stampanoni for assistance during data collection, S. Cohen for the stacking and pre-processing of the series of tomographic volumes and M. Brazeau, A. Caron, T. Hirasawa, J. Long, R. Sansom, K. Trinajstic, Y. Zhu for discussion. Funding was provided by the Chicago Fellows Program, Canadian Museum of Nature Research and Collection Grant, NSERC DG RGPIN-2021-04327 (T.M.), NSF DEB-1541491 (M.I.C.), the Museum national d’Histoire naturelle, Paris, and the CNRS (P.G.).

Author information

Authors and Affiliations

  1. Beaty Centre for Species Discovery, Canadian Museum of Nature, Ottawa, Ontario, Canada

    Tetsuto Miyashita

  2. Department of Biology, University of Ottawa, Ottawa, Ontario, Canada

    Tetsuto Miyashita

  3. Department of Origins and Evolution, Muséum national d’Histoire naturelle, Paris, France

    Philippe Janvier

  4. Biodiversity Institute and Natural History Museum, University of Kansas, Lawrence, KS, USA

    Kristen Tietjen

  5. Institut photonique d’analyse non-destructive européen des matériaux anciens, Ministère de la Culture, UVSQ, MNHN, Université Paris-Saclay, CNRS, Saint-Aubin, France

    Felisa Berenguer & Pierre Gueriau

  6. Synchrotron SOLEIL, L’Orme des merisiers, Gif-sur-Yvette, France

    Sebastian Schöder

  7. Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland

    Federica Marone

  8. Institute of Earth Science, University of Lausanne, Lausanne, Switzerland

    Pierre Gueriau

  9. Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL, USA

    Michael I. Coates

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  1. Tetsuto Miyashita
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  2. Philippe Janvier
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Contributions

T.M. designed the study, performed the morphological analysis, prepared the figures and wrote the paper. P.J. conceptualized the study and contributed to the morphological analysis and drafts of the paper. K.T. performed the computed tomography scan of Chiloscyllium and Ichthyomyzon, provided all described three-dimensional segmentation and renderings and designed the figures. F.B., S.S. and F.M. performed the synchrotron acquisition. P.G. identified MNHN-F-SVD3221, contributed to the conceptualization of the study, pre-processed the microtomography data and provided a draft rendering of the dataset. M.I.C. coordinated the study and contributed to the morphological analysis and drafts of the paper.

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Correspondence to Tetsuto Miyashita.

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The authors declare no competing interests.

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Nature thanks Per Ahlberg, Tatsuya Hirasawa, Yasuhiro Oisi, Min Zhu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Comparison of previous and new reconstructions of the jawless stem gnathostome Norselaspis glacialis.

The previous reconstructions11,12,13 are based on serial grinding sections (and the specimens were thus lost), whereas the new reconstruction was generated from a non-destructive Synchrotron scan of MNHN-F-SVD3221. (a, b) MNHN-F-SVD3221, part (a) and counterpart (b), scanned separately. (c, e, g, i, k) The head shield of Norselaspis glacialis as reconstructed by Janvier11,12,13 in left lateral view (c), sagittal section (e), dorsal view (g), ventral view (i), and posterior view (k). (d, f, h, j, l) The head shield of Norselaspis glacialis (MNHN-F-SVD3221) in left lateral view (d), sagittal section mirrored horizontally to show the left half for consistency (f), dorsal view (h), ventral view (j), and posterior view (l). Overall, MNHN-F-SVD3221 has much taller vertical profile compared to the previous interpretations, which were likely influenced by taphonomic deformation. Our reconstruction of MNHN-F-SVD3221 shows that the intramural cavity is closed dorsally by a bony ceiling (l). (m, n) Comparison of the previous11 (m) and current (n) reconstructions of the pectoral fenestra in left lateral view. Note the absence of the articular surface in the current reconstruction. (o, p) Comparison of a previous11 (o) and the current (p) reconstructions of the pericardial structures in left dorsolateral (o) and dorsal (p) views, respectively. The common cardinal veins are paired under the new reconstruction, contrary to the midline Cuvierian duct in the previous reconstruction11. (q, r) Comparison of another previous13 (q) and the current (r) reconstructions of the pericardial anatomy in anterior view. Under the new reconstruction, the paired common cardinal veins drain from ventrally, not from dorsally. The sinus venosus sits above the atrium, not behind it. (s, t) Osteological evidence for the previous11,12,13 (s) and current (t) reconstructions of the pericardial anatomy, showing the pericardial region of the postbranchial wall in anterior view. In the previous reconstruction11,12,13 (s), a broken ceiling of the intramural cavity was interpreted as a fenestra for venous drainage. The new reconstruction (r, t) shows that the cavity was closed dorsally, and the venous blood (blue broken arrows) drained bilaterally through the paired common cardinal veins into the sinus venous that occupied the dorsal part of the intramural cavity. c, e, g, i, k, m, o, q, s, Illustrations of the previous reconstruction were re-scanned from the original drawings in ref. 11, Institut des Sciences de l’Évolution de Montpellier (ISEM), ref. 12, Muséum national d’Histoire naturelle (MNHN), and ref. 13, Zoological Society of London (ZSL), and modified by T.M. and P.J. Panels m–t not to scale. For the key to Roman numerals, see Fig. 1.

Extended Data Fig. 2 Comparative models for anatomical interpretation of the skull of Norselaspis glacialis.

(a–f) The chondrocranium and associated systems of an adult lamprey (cyclostome), Ichthyomyzon bdellium, in left lateral view (a), dorsal view (b), ventral view (c), sagittal section showing the right half (d), anterior view (e), and posterior view (f). (g–l) The chondrocranium and associated systems of a bamboo shark (a crown gnathostome), Chiloscyllium plagiosum, in left lateral view (g), dorsal view (h), ventral view (i), sagittal section showing the right half (j), anterior view (k), and posterior view (l). Except about the single nasohypophyseal canal, Norselaspis generally conforms with the traits shown in a jawed gnathostome than those in a cyclostome.

Extended Data Fig. 3 Cranial endocast of Norselaspis glacialis.

(a–e) Cranial endocast of MNHN-F-SVD3221 rendered from a synchrotron-based X-ray microtomography scan in dorsal (a), ventral (b), anterior (c), right lateral (d), and left lateral (e) views. (f, g) Cranial endocast reconstructed from serial grinding section11 for comparison, in dorsal (f) and ventral (g) views. The original drawings were re-scanned and modified by the first and second authors. Although comparable overall, note several important differences between the two interpretations such as: 1 = proximity between the ophthalmic (CN V1) and maxillary (CN V2) branches of the trigeminal nerve; 2 = lateral head vein originating just lateral to the canal for the facial nerve (CN VII); 3 = facial nerve extending from the anterior margin of the vestibular fenestra; 4 = glossopharyngeal nerve (CN IX) sharing a root with the vagus nerve (CN X); 5 = brachial nerve having distinct root from the vagus nerve; 6 = mesencephalic region between the orbits; 7 = telencephalic (cerebral) region rounded anteriorly; and 8 = proportions of the semicircular canals. These differences are discussed in Supplementary Information. Images in f and g adapted from ref. 11, ISEM.

Extended Data Fig. 4 Comparative models for anatomical interpretation of the central nervous system of Norselaspis glacialis.

(a–e) The central nervous system and associated systems of an adult lamprey (cyclostome), Ichthyomyzon bdellium, in left lateral view with (a) or without skeletal rendering (b), in dorsal view (c), and in ventral view with (d) and without associated structures (e). (f–j) The chondrocranium and associated systems of a bamboo shark (a crown gnathostome), Chiloscyllium plagiosum, in in left lateral view with (f) or without skeletal rendering (g), in dorsal view (h), and in ventral view with (i) and without (j) associated structures with semi-transparent skeletal rendering.

Extended Data Fig. 5 Comparative anatomy of inner ear across major vertebrate lineages.

(a–f, v–x) A jawless stem gnathostome Norselaspis glacialis (MNHN-F-SVD3221). (g–m) An adult lamprey (cyclostome) Ichthyomyzon bdellium. (n–u, y) A bamboo shark (a crown gnathostome), Chiloscyllium plagiosum. (a, b) Left inner ear of Norselapis (MNHN-F-SVD3221) in lateral (a) and medial (b) views. (c–f) Inner ears of Norselapis (MNHN-F-SVD3221) in dorsal (c), ventral (d), anterior (e), and posterior (f) views. (g–i) Left inner ear of Ichthyomyzon in lateral (e), medial (f), and dorsal (g) views. (j, k) Left inner ear of Ichthyomyzon after virtual sagittal section to show medial (j) and lateral (k) halves in interna view. (l, m) Left inner ear of Ichthyomyzon after virtual horizontal section to show dorsal (l) and ventral (m) halves in internal view. (n–s) Left inner ear of a bamboo shark Chiloscyllium plagiosum in lateral (n), medial (o), dorsal (p), ventral (q), anterior (r), and posterior (s) views. (t, u) Left inner ear of a bamboo shark Chiloscyllium plagiosum in sagittal (t) and horizontal (u) sections to reveal the medial and ventral halves, respectively. (v, w) Inner ears of Norselapis (MNHN-F-SVD3221) in dorsal (v) and posterior (w) views. With the skeleton rendered semi-transparent, our visualization identified otoconia in this animal. (x, y) Left inner ears of Norselaspis (x) and Chiloscyllium (y), respectively, in lateral view for comparison of otoconial mass. (z) Otoconia (yellow arrowheads) shown in reconstructed tomographic slices (horizontal sections) for the posterior part of the braincase of Norselaspis from ventral (z1) to dorsal (z3).

Extended Data Fig. 6 Comparative anatomy of extraocular muscles in vertebrates.

(a–e) The left orbital cavity of Norselaspis gracialis (MNHN-F-SVD3221) reconstructed from a synchrotron-based X-ray microtomography scan in dorsolateral (a), lateral (b), anterior dorsolateral (c), posterior dorsolateral (d), and ventrolateral (e) views. (f–j) The extraocular muscles and their morphological correlates in the left orbital region of an adult lamprey (cyclostome) Ichthyomyzon bdellium in lateral (f), dorsolateral (g), dorsal (h), anterior laterodorsal (i), and posterior dorsolateral (j) views. (k–m) The extraocular muscles as attached to the left eye of Ichthyomyzon bdellium in medial (k), dorsal (l), and ventral (m) views. (n–r) The extraocular muscles and their morphological correlates in the left orbital region of a bamboo shark (gnathostome) Chiloscyllium plagiosum in lateral (n), dorsolateral (o), ventrolateral (p), anterolateral (q), and posterolateral (r) views. (s, t) The extraocular muscles as attached to the left eye of Chiloscyllium plagiosum in lateral view with semi-transparent eye (s) and in medial view (t). (u–x) Schematic comparison of the extraocular muscles in the left orbits in approximate relative position: (u) an osteostracan based on Norselaspis and boreaspidids31; (v) a placoderm based on Brindabellaspis, Murrindabellaspis, Romundina, and others30; (w) a lamprey; (x) a crown gnathostome. Schematic diagrams are original drawings by T.M. based on the described anatomy in cited resources. All panels: anterior to the left. Colour codes: yellow=cranial nerve; orange=oculomotor innervation (CN III); magenta=trochlear innervation (CN IV); purple=abducens innervation. Panels u–x not to scale. For the key to Roman numerals, see Fig. 1.

Extended Data Fig. 7 Internal anatomy of the postbranchial wall in the osteostracan Norselaspis glacialis reveals crown gnathostome-like traits of the head-trunk interface.

(a–d) The postbranchial wall of a jawless stem gnathostome Norselaspis glacialis (MNHN-F-SVD3221) reconstructed with solid (first row) and semi-transparent (second row) skeletal correlates and without them (third row) in dorsal (a), ventral (b), anterior (c), and posterior (d) views. The upper ≠head shield is removed. (e) Digital dissection of the postbranchial wall of Norselaspis glacialis (MNHN-F-SVD3221) in dorsal view, with the roof of the intramural cavity removed (e1), with further removal of the common cardinal veins and sinus venosus (e2), with horizontal section at a lower level, revealing the heart (e3), and with restoration of the venous structures (e4), progressing from left to right. (f) Sagittal section of the postbranchial wall of Norselaspis glacialis (MNHN-F-SVD3221) in medial view, mirrored to show the better-preserved left half, with bare bones with a magnified panel (f1) and with the pericardial and cardiac structures restored (f2). (g) Anatomy of the postbranchial wall of Norselaspis glacialis (MNHN-F-SVD3221) in left lateral view, with semitransparent skeleton (g1) and with internal soft tissues only in which the venous structures are rendered semitransparent (g2). (h–l) The heart and venous drainage of Norselaspis glacialis (MNHN-F-SVD3221) in dorsal (h), ventral (i), anterior (j), posterior (k), and left lateral view (l). Panels a–f are to scale; panels h–l are also to scale. For the key to Roman numerals, see Fig. 1.

Extended Data Fig. 8 The circulatory system of an adult lamprey Ichthyomyzon bdellium (cyclostome) as a comparative model for anatomical interpretation of that of Norselaspis glacialis.

Reconstruction of the circulatory structures with (left) and without (right) morphological correlates in (a) left lateral view to show the left side, (b) sagittal section to show the right side, (c) dorsal view, and (d) ventral view with an additional subpanel (d3) to show the major vessels on the cranial floor under the brain. (e) Pericardium in anterior view with (e1) and without (e2) the heart and arteries, revealing the sinus venosus. (f) Pericardium in posterior view with (f1) and without (f2, 3) the branchial basket and with (f1, 2) and without (f3) the esophagus, axial column, and main chondrocranium.

Extended Data Fig. 9 The circulatory system of a bamboo shark Chiloscyllium plagiosum (crown gnathostome) as a comparative model for anatomical interpretation of that of Norselaspis glacialis.

Reconstruction of the circulatory structures: (a) in left lateral view with (a1) and without (a2) skeletal correlates, and (a3) with a focus on the pericardial structures (semi-transparent); (b) in right lateral view with solid (b1) or with semi-transparent (b2) skeletal correlates, and (b3) with a focus on the pericardial structures; (c) in dorsal with solid (c1) or with semi-transparent (c2) skeletal correlates, and (c3) with a focus on the pericardial structures; (d) in ventral view with solid (d1) or with semi-transparent (d2) skeletal correlates; (e) in anterior view (e1) with a focus on the pericardial structures, and with (e2) or without (e3) the skeletal correlates; and (f) in posterior view with solid (f1) or with semi-transparent (f2) skeletal correlates.

Extended Data Fig. 10 Anatomy of the postbranchial wall and pectoral fin attachment in Norseaspis glacialis reveals a unique patterning of the head-trunk interface in this sister group taxon to jawed vertebrates.

(a, b) The left pectoral fenestra of Norselaspis (MNHN-F-SVD3221) in posterolateral (a) and posterior (b) views. (c) The left side of the postbranchial wall of Norselaspis (MNHN-F-SVD3221) in posteromedial view. (d) The left pectoral fenestra of Norselaspis (MNHN-F-SVD3221) in posterolateral view, reconstructed with the associated soft tissues only (d3) and with solid (d1) or semi-transparent (d2) skeletal rendering. (e) The postbranchial wall of Norselaspis (MNHN-F-SVD3221) with the associated soft tissues in posterior view. (f) The path of the brachial nerve (the most anterior spinal nerve) in Norselaspis (MNHN-F-SVD3221) in posterior view from the right side, with semi-transparent skeletal rendering. Note that the nerve extends in a single canal without any evidence of branching and enters the pectoral fenestra. (g) Positional relationships of the internal soft tissues reconstructed in Norselaspis (MNHN-F-SVD3221) in dorsal view, with semi-transparent skeletal rendering. (h) Selection of tomograms through MNHN-F-SVD3221 (sampled from the planes shown in the upper inset) showing that the pectoral fenestra consists of a smooth, finished perichondral bone with no evidence of endoskeletal joint or any articular structures. The unfinished cartilaginous interface for an endoskeletal joint11,12,13 is likely a taphonomic artifact. Therefore, the earliest endoskeletal joints likely evolved in the pharyngeal skeletons of jawed vertebrates. (i–m) Schematic drawings of the head-trunk interface and its peripheral structures in Norselaspis (i, k, l) and an idealized crown gnathostome after Chiloscyllium plagiosum (j, m). Only structures relevant to our interpretation of the head-trunk interface are illustrated. Pre- and post-trematic branches are simplified into single trunks; so are the motor and sensory roots. Number of somitic segments is reduced for convenience. Original drawings by T.M. Not to scale. (i, j) Peripheral structures of the head-trunk interface in an osteostracan (i) and in a crown gnathostome (j) in left lateral view. (k) Pericardial region of an osteostracan magnified from panel (i). Venous flow directions are shown in blue broken arrows. The main drainage is to the anterior through the midventral fenestra. After becoming confluent with the ventral jugular vein, venous blood is collected by the paired common cardinal veins to the sinus venosus posterodorsally. (l, m) Simplified vascular anatomy of Norselaspis (l) and an idealized crown gnathostome after Chiloscyllium plagiosum (m) in dorsal view. The heart and ventral arterial system are omitted. For the nervous system, only the last branchial and cardiac branches of the vagus nerve and the brachial nerve are shown in Norselaspis (l). Roman numerals indicate cranial nerves. Refer to Fig. 1 for captions except for the spinal nerves CN XI (accessory nerve) and CN XII (hypoglossal nerve) in panel (j).

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Visual summary of reconstructed internal anatomy of N. glacialis.

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Miyashita, T., Janvier, P., Tietjen, K. et al. Novel assembly of a head–trunk interface in the sister group of jawed vertebrates. Nature 645, 686–691 (2025). https://doi.org/10.1038/s41586-025-09329-9

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