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Efforts to understand the evolution of the vertebrate head have been hampered by the strikingly different anatomies encountered in cyclostomes and gnathostomes. In living cyclostomes, the neurocranium comprises an open framework of cartilages holding the brain and a feeding apparatus consisting of a symmetrical set of paired and midline cartilages9,10. In gnathostomes, the brain is instead enclosed by a single neurocranial unit that encases the brain and nasal capsules, with paired mandibular and hyoid arches forming the feeding apparatus11. In the absence of a suitable extant outgroup, the fossil record is crucial to polarize cyclostome and gnathostome character states and reveals character states in extinct taxa that can be used to test transformational hypotheses2,7,12,13.

However, the fossil record does little to bridge the two states, with major phylogenetic and temporal sampling gaps in early vertebrate cranial anatomy. Three-dimensional remains of galeaspids, osteostracans and placoderms, the Silurian and Devonian stem-group gnathostomes most closely related to the crown group, suggest that the plesiomorphic state for gnathostomes is a single, endocranial unit formed by the neurocranium and splanchnocranium, which encloses the brain and pharynx and fills the connective tissue compartments between them and the dermal skeleton1,7,14,15,16. However, in the multiple other groups of Palaeozoic vertebrates all that is known of the cranial anatomy comes from difficult-to-interpret two-dimensional fossils4,6,17,18 or is inferred on the basis of the dermal skeleton19,20,21. Although flattened remains exist of the crania of some Lower Cambrian vertebrates4,6, an expanse of time separates these from the Silurian galeaspids preserving three-dimensional neurocrania7,22,23. Of those few Ordovician vertebrate taxa that are known from dermal remains24, nothing is known of the endocranial anatomy.

The enigmatic Eriptychius americanus, from the Sandbian (458.4–453.0 million years ago) Harding Sandstone, is one of only four Ordovician stem-group gnathostome taxa known from articulated remains25,26,27,28,29,30,31,32,33,34,35 and the only one in which putative cartilages have been identified8. In this study, we use computed tomographic methods to image an articulated specimen of E. americanus, PF 1795, and confirm that these cartilages represent the earliest-known three-dimensionally articulated neurocranium in a fossil vertebrate. The cranial cartilages of Eriptychius are anatomically dissimilar from the crania of both cyclostomes and gnathostomes and instead represent a distinct pattern of the vertebrate head skeleton.

Systematic palaeontology

Subphylum: Vertebrata Lamarck, 1801

Eriptychius americanus Walcott, 1892

Eriptychius americanus Walcott, p. 167, plate 4, figures 5–11

Type material. Seven isolated fragments of dermal bone embedded in matrix, collected by C. D. Walcott from the Harding Quarry, Cañon City, Colorado, form the syntype USNM V 2350 in the collections of the Smithsonian National Museum of Natural History in Washington, DC, USA.

Emended diagnosis. Agnathan with mesomeric dermal tesserae and scales formed from acellular bone overlain by ornament formed from coarse wide-calibre tubular dentine. Body scales covered in multiple elongate ridges. Antorbital neurocranium comprising symmetrical set of elements containing numerous large canals internally, endoskeleton closely associated with but not fused to the surrounding dermal skeleton. Shared with Astraspis, arandaspids, other ‘ostracoderms’ excluding heterostracans: multiple branchial openings. Shared with Astraspis and tessellate heterostracans: dermal head skeleton formed from dorsal and ventral ‘headshields’ of ornamented tesserae. Differs from Astraspis in that ornament of dermal skeleton comprises ridges and presence of coarse tubular dentine.

Description. Computed tomography scanning of the part and counterpart of PF 1795 (here termed PF 1795a and b, respectively; Methods) confirms the identity of this material as a partially articulated Eriptychius head8, including components of both the dermal skeleton and endoskeleton (Fig. 1, Extended Data Figs. 1–3 and Supplementary Video; see Supplementary Information for comments on histology). The articulated individual is confined to the near surface of the matrix; below it is a mash of isolated elements typical of the Harding Sandstone bone beds including additional tesserae referable to Eriptychius that do not seem to be associated with the articulated specimen (Extended Data Fig. 1). Denison8 described several large elements of globular calcified cartilage as part of the internal skeleton of Eriptychius and we have been able to distinguish ten separate cartilages in total comprising the endoskeletal cranium (Figs. 1 and 2 and Extended Data Figs. 2 and 4).

Fig. 1: Overview of Eriptychius americanus PF 1795.
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a,b, Photographs of part PF 1795a, which had the split face set in epoxy and was manually prepared (a), and its counterpart PF 1795b, which remains in rock matrix (b). Both are shown in an anatomically ventral view. c,d, Digital model of computed tomographic data of the combined part and counterpart with most of dermal skeleton rendered transparent: anatomical ventral view (corresponding to the visible area of the part in epoxy) (c) and anatomical dorsal view (buried in matrix in the counterpart) (d). Colour scheme for renders: blue-greys, cranial cartilages (matching the detailed scheme in Fig. 2); transparencies, the dermal skeleton; orange, branchial plates; red, orbital plates. Anterior to top in a–d. ant. tess., anterior tesserae; artic. vent. tess., articulated ventral tesserae; branch. plate, branchial plate; cran. cart., cranial cartilages; disp., displaced; frag., fragment; L., left; orb. cart., orbital cartilage; orb. plates, orbital plates; R., right; vasc., vasculature; ?, probable. Scale bar applies to all panels.

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Fig. 2: The neurocranial cartilages of E. americanus PF 1795 based on computed tomography data.
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a–c, Cranial cartilages in estimated life position, with cartilages coloured in pairs in dorsal (a), ventral (b) and anterior (c) view. d,e, Mediolateral cartilages A in dorsal view (d) and median dorsal cartilage in ventral view (e) rendered with a vertical height map texture to emphasize the surface topology. f, Reconstruction of the forebrain relative to the cranial cartilages using a lamprey as a model9,52, shown in dorsal view. g, Cartilages in dorsal view, rendered transparent to show internal vasculature (red). h,i, Cartilages in preserved position in anterior view with dermal skeleton shown (h) and removed (i). Colours in a,b,c,f,h,i as in Fig. 1 with the following additions. Green, dermal skeleton. Red dashed line represents inferred position of mouth in c,h,i. In d and e lighter colours denote areas closer to the camera. Abbreviations as in Fig. 1 with the following additions: antorb. proc, antorbital process; ext. vasc. op., external vascular openings; forebr., forebrain; lat., lateral; medlat. cart, mediolateral cartilage; med. dors. cart, median dorsal cartilage; med., medial; med. vent. cart., median ventral cartilage; med. vent. ridge, median ventral ridge; olf. bulb, olfactory bulb; pin., pineal organ; pin. op., pineal opening; vent., ventral. Scale bar in a is shared by b,c; scale bar in d is shared by e.

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Six cartilages were identified on the split surface of PF 1795a by ref. 8 (Extended Data Fig. 2) and the remaining four are buried within the matrix of PF 1795b. The cartilages are closely wrapped by articulated squamation anteriorly, dorsally and ventrally and to one side (Extended Data Figs. 3 and 4); however, there is a clear separation between dermal and endoskeletal elements, unlike in galeaspids and osteostracans36. This squamation comprises a range of dermal element types including the scale types identified by ref. 8 and scales with an anteroposteriorly oriented ornament from farther back on the head. It also includes small, curved orbital plates (Fig. 1c,d and Extended Data Fig. 3c–e) and several plates similar in morphology (Fig. 1c,d and Extended Data Fig. 3f–i) to an isolated Eriptychius ‘branchio-cornual plate’ identified by ref. 37 (plate 2, 4–7). There is no obvious pineal dermal plate.

Denison identified two cartilages as the orbital cartilages on the basis of concave posterior faces8. Our scan data confirm that these represent cross-sections through large fossae that we interpret as forming the anterior wall of the orbits. These fossae are flanked laterally by an antorbital process (Fig. 2a,b and Extended Data Fig. 5) that suggests a dorsolateral orientation of the orbit. A smaller ventral fossa on each orbital cartilage below the orbit may have provided a location for muscle attachment (Fig. 2). One orbital cartilage is posteriorly displaced, along with elements of anterior squamation including a rostral scale (Fig. 1 and Extended Data Fig. 3) and has rotated by about 180° in the sagittal axis; when rotated back into position it aligns with the orbital plates (Fig. 1). The anteriormost branchial plate lies posterolateral to the other orbital cartilage (Fig. 1 and Extended Data Fig. 3), suggesting that the relative positions of the orbits, otic region and branchial arches were similar to those inferred in heterostracans2,19, although this is impossible to judge exactly because of the collapse of the dermal skeleton. The absence of anything assignable to the branchial skeleton suggests that the branchial arches were not incorporated into a single mineralization with the neurocranium, a major difference with respect to galeaspids and osteostracans7,15.

The remaining cartilages lie between the two orbital cartilages, although they have slumped slightly and been pulled posteriorly on one side with the displaced orbital cartilage (Figs. 1 and 2a–c and Extended Data Fig. 4). There are three paired cartilages arranged symmetrically across the midline—two dorsal (termed mediolateral A, B) and one ventral (termed mediolateral C)—along with one unpaired midline cartilage dorsally and one ventrally. Of the two unpaired cartilages, the smaller dorsal element is kite-shaped and has both the dorsal and ventral surfaces punctured by large medial foramina that we consider likely to be the pineal opening (Fig. 2a,e). These dorsal and ventral foramina do not exactly line up anteroposteriorly but appear to communicate with a common large space inside the cartilage (Fig. 2g and Extended Data Fig. 6). This kite-shaped cartilage is preserved overlying the mediolateral cartilages A and its ventral surface bears a median ridge with a shallow depression on either side (Fig. 2a,e). Together with shallow depressions on the dorsal surface of mediolateral cartilages A, these depressions frame paired fossae which we interpret as having accommodated part of the forebrain, possibly the olfactory bulbs (Fig. 2d–f), an interpretation consistent with their position relative to the orbits and putative pineal opening. Thus, we infer this to be the dorsal side of the animal and term this the median dorsal cartilage. The larger median ventral cartilage underlies the mediolateral cartilages.

All cartilages are pervasively penetrated by canals (Fig. 2g and Extended Data Fig. 6). In the larger cartilages, that is, the orbital cartilages and the mediolateral cartilages A, this tends to follow the pattern of a larger trunk entering the cartilage from the posterior side before splitting into smaller branches that open to the surface. The pattern is not exactly bilaterally symmetrical in the paired and unpaired elements but does follow a similar organization with the trunk canal entering at equivalent points. These canals could plausibly have carried sensory rami onto the surface of the head; for example, the superficial ophthalmic nerve in the case of the canals in the orbital cartilages. However, the canal openings are not restricted to surfaces where the cartilages contact the dermal skeleton. On the basis of this and the lack of sensory canal openings in the head tesserae, they may have carried vasculature instead or as well. In living chondrichthyans, canals carry vasculature and transport prechondrocytes into the cartilage from the perichondral surface. This could implicate the canals in Eriptychius in both cartilage maintenance and interstitial cartilage growth, although in modern chondrichthyans the width of these canals are an order of magnitude smaller than in Eriptychius38,39. Although canals have not been explicitly reported in other early vertebrate cartilages, sections through galeaspid cartilage suggest that this tissue has a degree of vascularity36.

In concert with the displacement of the cartilages, the dermal squamation has undergone postmortem collapse. We interpret the articulated area of squamation as having covered the right side of the head, around the region of the right orbit and the right side of the mouth, as well as the areas dorsal and ventral to the mouth (Fig. 1 and Extended Data Fig. 3). On the basis of this interpretation, the cartilages would have comprised the preorbital region of the head, surrounding the mouth (Fig. 2c,h,i). The articulated dorsal and ventral patches of squamation indicate that the mouth opened between the cartilages, bordered by the ‘rostral’ plates8 and was oriented supraterminally (Fig. 2c,h,i), unlike most jawless stem-gnathostomes (notable exceptions being Doryaspis40 and Drepanaspis41). The fact that the cartilages are separate may mean that some movement of the oral endoskeleton was possible, although this would have presumably been limited by their close relationship with the squamation.

The evolution of vertebrate crania

The cranial cartilages of Eriptychius have no obvious homologue in the head of any known extant or extinct vertebrate (Fig. 2 and Extended Data Fig. 4). The most obvious comparison is with the numerous paired and midline cartilages that comprise the complex feeding apparatus of cyclostomes42,43 and possibly anaspid-like early vertebrates17. The preorbital region of Eriptychius is ostensibly similar in that it comprises numerous cartilages that bordered the mouth. Unlike cyclostomes, however, in which the entire brain is held in an open cartilaginous framework10, in Eriptychius the forebrain at least, as well as the orbits, were bounded by these cartilages. The only large cartilage that is not associated with the forebrain or orbit, the median ventral cartilage, is closely associated with the dermal skeleton, suggesting that mobility would have been restricted. Although it is impossible to rule out movement of the smaller mediolateral cartilages B and C, a cyclostome-like mobile feeding apparatus seems unlikely.

The preserved cranial cartilages of Eriptychius instead seem to have formed a static structure, which, in this sense, is more comparable to neurocrania in known jawless stem-group gnathostomes. In these taxa, osteostracans and galeaspids, endoskeletal tissues fill the connective tissue space between the dermal skeleton and the brain, where they surround the mouth and pharynx and buttress the head1,7,14. The cartilages in Eriptychius are similar in that they fill out the head and closely support the dermal skeleton. Unlike osteostracans and galeaspids, however, the neurocranial cartilages are separate from one another and there is no evidence for any mineralization from the level of the orbits posteriorly or for the fusion of the splanchnocranium into a unit with the neurocranium. This could be taphonomic but parts of the dermal skeleton posterior to the orbits remain articulated in PF 1795. Instead, Eriptychius may have resembled heterostracans, in which the otic region of the brain and the pharynx are closely associated with the dermal skull roof19,44, suggesting that they were primarily supported by the dermal skeleton. The patterning of developmental cartilages in extant gnathostomes might suggest that the mineralization of this anterior region in Eriptychius was limited to the prechordal region of the neurocranium (the trabeculae cranii), which originate from neural crest3,12. However, it has been demonstrated that in lampreys the parachordals extend forward to form most of this region3,12, another possible origin for the cartilages in Eriptychius.

Eriptychius fills an important gap, both temporal and phylogenetic, in our understanding of the evolution of the vertebrate head. Our inclusion of Eriptychius in a recent phylogenetic matrix for early vertebrates45,46 recovers it within or in a polytomy with the vertebrate crown group and as a stem-group gnathostome in the Adams consensus of the parsimony analysis (Fig. 3 and Extended Data Figs. 7–9) which is consistent with previous phylogenetic analysis35. This phylogenetic placement would extend the condition in which endoskeletal tissues fill space in the neurocranium into the earliest, Ordovician, stem-group gnathostomes and show that this is not limited to taxa with an osteostracan/galeaspid morphology of a ventrally positioned mouth and dorsally located orbits. However, the fact that in Eriptychius the cartilages are not fused into a single unit around the brain suggests that these early gnathostome neurocrania calcified in several parts. An enclosing neurocranium, which in galeaspids and osteostracans is fused with the splanchnocranium, is a trait which unites galeaspids, osteostracans and mandibulate gnathostomes to the exclusion of Eriptychius and cyclostomes. Although it is possible to identify broad similarities, the substantial difference between the neurocranial anatomy of the Ordovician Eriptychius to either cyclostomes or gnathostomes helps to explain why it has been so difficult to draw direct anatomical comparison between the skulls of the two living vertebrate groups2.

Fig. 3: The neurocrania of vertebrates in a phylogenetic framework.
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a, Haikouichthys, adapted from ref. 3, CC-BY 4.0. b, Petromyzon. c, Generalized cyathaspid, adapted from ref. 20, Springer Nature. d, Sacabambaspis, reproduced from ref. 31, copyright ©1993 Elsevier Masson SAS, all rights reserved (but see ref. 53 for alternative interpretation). e, Eriptychius. f, Shuyu, adapted from ref. 7, Springer Nature. g, Dicksonosteus adapted with permission from ref. 14,Muséum national d’histoire naturelle, Paris. Phylogenetic scheme is the Adams consensus tree described in the text, bold taxon names represent those depicted above. Coloured regions illustrate positions of key sensory organs: yellow, eyes; blue, pineal. Light grey lines represent body outlines, black lines represent dermal body armour, light grey regions with dark grey borders represent cranial mineralizations, brown regions represent the brain (b), imprints of the brain in the dorsal headshield (c) or the endocast (f,g). Square brackets represent the preorbital part of the head. Petromyzon redrawn from Sketchfab (https://sketchfab.com/3d-models/lamprey-inside-c9f05e3f00c94d929e7ed018fac6d782) under a CC BY 4.0 licence. Abbreviations: CG, crown group; mand. gnath., mandibulate gnathostomes; TG, total group. *‘Crown-group cyclostomes’ represents the Petromyzontidae and Myxinoidea total groups and Gilpichthys, which was recovered in a polytomy with those two groups. **Cyclostome and gnathostome total groups in this topology recovered in a polytomy with Metaspriggina and (Haikouichthys + Myllokunmingia).

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Eriptychius provides the earliest direct evidence for a prechordal endocranium in a vertebrate. This was also likely to have been the case in the contemporary Astraspis and the later heterostracans based on the lateral positions of the orbits33,47,48 (Fig. 3d,e). This contrasts with Sacabambaspis, in which the orbits are placed at the extreme anterior margin of the headshield30 (Fig. 3c) comparable to those of putative stem-group vertebrates4 such as Haikouichthys, Metaspriggina and conodonts4,5,6,49 (Fig. 3a) as well as in the naked anaspids Jamoytius and Euphanerops50,51. In the past, this unusual anatomy has usually been dismissed as a specialization on the basis of interpretations of Sacabambaspis in a heterostracan light (for example, refs. 27,31 and supplementary appendix (p35) of ref. 45). The discovery of this preorbital neurocranium in Eriptychius and the movement of early vertebrate taxa around the vertebrate crown node in recent phylogenies45,46 should prompt reconsideration of whether differences in orbital placement in Ordovician vertebrates instead reflect a more fundamental anatomical difference in cranial organization.

Methods

Specimen

E. americanus PF 1795 is held at the Field Museum, Chicago, USA. It comprises two pieces, part and counterpart, one of which had the split face set in epoxy and was manually prepared out of the matrix from the other side8. Denison8 figured only the part in epoxy (before and after preparation) and referred to both part and counterpart as PF 1795: here, we term the part in epoxy PF 1795a and the unprepared counterpart PF 1795b for the sake of clarity (Fig. 1).

Geological setting

The Harding Sandstone (Sandbian, Upper Ordovician) is a thin unit of interbedded mudstones and sandstones that extends around the Cañon City embayment in the frontal range of the Rockies. The sequence was first studied in detail by ref. 54 focusing on the presence of Ordovician vertebrates with periodic attention paid subsequently. Sedimentologically, the sequence records shallow marine deposition within a microtidal lagoonal setting with localized estuarine input55. The extensive bone beds that occur through the Harding represent winnowed shoreface deposition with specimen PF 1795 described here thought to come from an equivalent or potentially the same horizon to the articulated specimen of Astraspis desiderata that represents a shoreface strandline33.

Scanning

Computed tomography was carried out at the University of Chicago on a Phoenix v|tome|x with a dual 180 tube. PF 1795a was scanned at a voltage of 100 kV and current of 370 μA with a 0.1 mm Cu filter, for 1,800 projections, achieving a voxel size of 34.17 μm. PF 1795b in the matrix was scanned at a voltage of 110 kV and current of 300 μA with no filter, for 2,000 projections, achieving a voxel size of 44.8940 μm. Both datasets were segmented in Mimics v.25 (materialize) to create three-dimensional meshes using manual segmentation with some interpolatory functions (‘3D interpolate’ and ‘Multiple Slice Edit’). These were exported in the PLY format and then visualized in Blender (blender.org) v.3.3.0. An additional, higher resolution scan of PF 1795a was carried out at the University of Bristol in an effort to better visualize the endoskeletal tissue; this was carried out at a voltage of 120 kV and current of 119 μA with no filter obtaining 3,141 projections with a voxel size of 14.72 μm.

Phylogenetic analysis

The phylogenetic analysis was conducted on the basis of the matrix of ref. 45, with minor modifications focussed on Ordovician vertebrates. Eriptychius was added, astraspids recoded as Astraspis and arandaspids as Sacabambaspis. We also revised the codings of heterostracan taxa, revising Athenaegis and splitting Heterostraci into Anglaspis and Errivaspis. Changes are listed and justified in the Supplementary Information. This resulted in a matrix including 54 taxa and 167 morphological characters, which we analysed using parsimony and Bayesian analyses. Parsimony analysis was carried out in TNT v.1.5 using a parsimony ratchet and TBR branch swapping with 10,000 replicates, holding 100 trees from each iteration, with the constraint (Hemichordata (Cephalochordata (Tunicata + all other taxa))) and Hemichordata set as the outgroup. This resulted in 1,951 equally parsimonious trees with a length of 351. Bayesian analysis was carried out in MrBayes v.3.2.7, a flat (uniform) prior was used with an Mkv model and a gamma-distributed rate parameter. Hemichordata was set as the outgroup and total-group vertebrates were constrained to be monophyletic. We carried out the search for 10,000,000 generations, sampling a tree every 1,000 generations and calculated a majority rule consensus tree with a relative burn-in of 25%.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.