Main

Study of tetrapodomorph skulls, fins, axial skeleton and scalation has revealed the ways that feeding, respiration and appendage-based locomotion changed as fish shifted from aquatic to terrestrial lifestyles15,16. P. rhombolepis1,2,3, T. roseae4,5,6,7,8 and E. watsoni9 hold a special place in these analyses, showing a combination of plesiomorphic and apomorphic features that give insight into a sequence of anatomical changes in the origin of limbed taxa (that is, those in possession of digited appendages and lacking dermal rays). Now missing, however, is an understanding of the morphological, functional and ontogenetic diversity of the finned tetrapodomorphs most closely related to limbed forms. This is unfortunate, as isolated or fragmental specimens have controversially hinted at a wider range of diversity than is observed in more complete material10,11,12,13,14.

Here we describe a new finned tetrapodomorph that is closely related to T. roseae and E. watsoni. The new form shows an unexpected combination of characters, one that suggests a broad range in disparity among the closest finned relatives of limbed forms. The specimen was collected 1.5 km east of the site that yielded T. roseae, but from a slightly lower horizon in the Fram Formation of southern Ellesmere Island, Nunavut, Canada. We describe this new taxon and present a phylogenetic analysis to reveal its implications for understanding the evolution of the nearest relatives of limbed tetrapodomorphs. Comparison of the new taxon to other Frasnian-age forms allows a reinterpretation of isolated elements of previously uncertain affinity, thus, indicating a more widespread and diverse assemblage of tetrapod relatives than previously recognized.

Geological framework

Embry and Klovan17 described the type section of the Fram Formation from a drainage feeding the eastern arm of Bird Fiord on southern Ellesmere Island. They indicate an early to middle Frasnian age for the Fram Formation on the basis of palynological spot samples, which were collected from near the base, the middle and top of the formation17. The Nunavut Paleontological Expeditions collected vertebrate remains from 2000 to 2008 at 16 sites from the Fram Formation within the type section. The holotype of T. roseae (NUFV 108), as well as all other T. roseae specimens, were collected from site NV2K17, which occurs in silty overbank floodplain deposits18 at 533 m above the base of the measured type section of Embry and Klovan17. The specimen discussed here (NUFV 137) was collected at site NV0401 (77° 10.235′ N, 86° 11.279′ W) from lower in the same section and 1.5 km from NV2K17 (Fig. 1a,b and Extended Data Fig. 1). Site NV0401 is about 453 m above the base of the type section and occurs in a medium-grained sandstone. The surface-collected NUFV 137 is the only specimen found at the site. NUFV 137 is older than T. roseae and was collected from a different facies in the floodplain deposits of the Fram Formation.

Fig. 1: Locality and holotype of Qikiqtania wakei gen. et sp. nov.
figure 1

a, Specimen NUFV 137 was discovered on southern Ellesmere Island, Nunavut, Canada. b, The site, NV0401, lies 80 m below NV2K17, the site where T. roseae was discovered. c, Materials were µCT scanned and are shown here in dorsal aspect. General body shape based on specimen MHNM 06-2067 of E. watsoni9. Scale bar, 10 cm. Abbreviations: Fm, formation.

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Systematic palaeontology

Sarcopterygii19 Romer, 1955

Tetrapodomorpha20 Ahlberg, 1991

Elpistostegalia21 Camp and Allison, 1961

Qikiqtania wakei gen. et sp. nov.

Locality. Canada, Nunavut, southern Ellesmere Island, near the eastern arm of Bird Fiord, Nunavut Paleontological Expedition site NV0401, 77° 10.235′ N, 86° 11.279′ W.

Geological setting. Fram Formation (Upper Devonian, early Frasnian Stage).

Etymology. Qikiqtania (pronounced ‘kick-kiq-tani-ahh’) is derived from Inuktitut word Qikiqtaaluk/Qikiqtani, the traditional name for the region where the fossil site occurs. The species designation is in memory of David Wake, an eminent evolutionary biologist and transformative mentor, late of the University of California at Berkeley.

Holotype. Nunavut Fossil Vertebrate Collection (NUFV) 137.

Material. The description is based on a specimen from the NV0401 site that preserves the lower jaws, partial left upper jaw and palate in articulation, gulars, ceratohyals, an articulated left pectoral fin and articulated scales from the dorsal midline, flank and lateral line series (Fig. 1c, Extended Data Fig. 2 and Supplementary Video 1). The jaw material was physically prepared at the Academy of Natural Sciences of Drexel University. Computed tomography scans were collected at The University of Chicago’s PaleoCT scanning facility (Supplementary Table 1). The specimen will be housed at the Canadian Museum of Nature, Ottawa, Ontario, until such time as research and collections facilities are available in Nunavut.

Diagnosis. Elpistostegalian tetrapodomorph characterized by the following unique combination of characters: dorsoventral asymmetry in pectoral fin lepidotrichia (also present in T. roseae) and possession of a boomerang-shaped humerus lacking ventral ridge and associated foramina and ectepicondyle (distinct from P. rhombolepis, E. watsoni, T. roseae and more crownward tetrapods).

Description

Upper jaw and palate

Rostral elements of the upper jaws and palate, including portions of the ectopterygoid, dermopalatine, vomer, premaxilla and maxilla are preserved (Fig. 2a,b, Extended Data Fig. 2 and Supplementary Video 2). These elements are primarily from the left side and preserved in articulation with the lower jaws. The vomer is broad, fanged and forms the posterior wall of the palatal fossa with a row of smaller teeth. Fangs and a row of smaller teeth are also present on the dermopalatine and ectopterygoid. An expanded mesial surface of the dermopalatine lacks teeth and overlaps slightly with the vomer, similar to T. roseae8, forming the mesial and posterior margin of the choana. The anterolateral wall of the choana is formed by a simple, smooth articulation of the premaxilla and maxilla. Maxillary teeth are smaller than the premaxillary teeth. In their respective tooth rows, maxillary and premaxillary teeth are uniform in size.

Fig. 2: The feeding apparatus of Q. wakei.
figure 2

Volume renderings of µCT scans of the lower jaw and extra fragments reconstructed in their natural positions. a, Dorsal view of the lower jaws, ceratohyal, gular plates, premaxilla and palate. Scale bar, 1 cm. b, Ventral view with premaxillary and palatal elements displaced so ventral surfaces are visible. c, Left lower jaw, dorsal. d, Left lower jaw, medial. e, Right lower jaw, ventral. f, Right lower jaw, lateral. Abbreviations: ac, anterior coronoid; acf, anterior coronoid fang; ch, ceratohyal; cho, choana; d, dentary; df, dentary fang; dpf, dermopalatine fang; ecf, ectopterygoid fang; g, gulars; mc, Meckel’s cartilage; mcf, Meckelian canal foramen; mx, maxilla; pa, prearticular; pc, posterior coronoid; pcf, precoronoid fossa; pmx, premaxilla; pspl, postsplenial; sbp, submandibulo-branchiostegal plate; psf, post-symphyseal flange; vf, vomerine fang.

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Lower jaw

The lower jaws of Q. wakei are preserved in articulation anterior to the adductor chamber, including the dentary, infradentaries, coronoids and prearticular (Fig. 2). The symphysis is relatively smooth, not interdigitating. Large fangs with plicidentine infolding are present on the dentary, anterior coronoid and middle coronoid. Rows of smaller dentition are also present on the coronoids and dentary, including evidence of an auxiliary lateral tooth row on the dentary. The prearticular has a broad shagreen field of denticles that is raised adjacent to coronoids, and the denticles possess a distinct dorsoventral gradient in size. The adsymphyseal is missing, but small teeth embedded in the matrix of the precoronoid fossa suggest it was present in life.

Infradentaries are identifiable by the presence of the mandibular canal and postsplenial pit line. The mandibular canal is an open groove along most of its length, but in areas of the most intact preservation it takes the form of discrete pits the bone surface. The splenial has a larger postsymphyseal flange than in T. roseae but has a similar articulation with the prearticular4. Boundaries between the infradentaries are obscured by overlying dermal sculpting and are difficult to distinguish in computed tomography cross-section.

The Meckelian canal contains only partially ossified Meckelian bone along its length, but evidence of Meckelian ossification extends from the symphysis to the posterior coronoids. The canal is exposed lingually ventral to the prearticular and, in areas of intact ossification, Meckelian fenestra are bordered dorsally by Meckelian bone and ventrally by infradentaries.

Gular plates and ceratohyal

Fragments of a principal and median gular plate are preserved, along with a series of submandibulo-branchiostegal plates (Fig. 2a,b). A straight, grooved ceratohyal lies immediately adjacent to the left lower jaw. A small anterior fragment of the right ceratohyal is preserved adjacent to the right lower jaw19,20,21.

Pectoral fin

The left pectoral fin includes the humerus, ulna, radius, intermedium, third mesomere, third radial, fin web and associated scales (Fig. 3a,b and Supplementary Video 3). The fin is embedded in matrix with the proximal articular surface of the humerus and the posterior distal fringe of the fin web exposed at the edges of the block (Extended Data Fig. 3a). Three endoskeletal elements contact the humerus. Two have robust proximal articular surfaces and are identified as the radius and ulna. The third, which lies between and slightly dorsal to them, is identified as the intermedium proximally displaced during preservation, although its shape is difficult to assess because of its position relative to other elements (Fig. 3c,d and Supplementary Discussion).

Fig. 3: Left pectoral fin of Q. wakei.
figure 3

Volume renderings of µCT scans of the fin with scales removed. a,b, Dorsal (a) and ventral (b) views of the fin with endoskeleton in grey and dermal rays in orange. Dotted lines indicate the boundary between ulna and ulnare. The dashed line indicates position of cross-section in e, which is oriented orthogonal to the plane of the fin web. c, Endoskeleton viewed from the proximal side with humerus removed. d, Reconstruction of endoskeletal elements with estimated boundary between the radius and intermedium. Scale bar, 1 cm. e, Cross sections of the fin rays, showing asymmetry in the size of dorsal and ventral hemitrichia. f–k, Humerus in dorsal (f), pre-axial (anterior) (g), ventral (h), postaxial (posterior) (i), proximal (j) and distal (k) perspectives. Scale bar, 1 cm. Proximal is up in f–i. Dorsal is up in j and k. Abbreviations: ar, anterior radial; cap, caput humeri; h, humerus; ir, intermedium; r, radius; rf, radial facet; m3, third mesomere, u, ulna; ul, ulnare; uf, ulnar facet.

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The fin is characterized by ventralward curvature of the radius and asymmetry in the lepidotrichia, in which dorsal hemitrichia have a greater cross sectional area than ventral hemitrichia, as in T. roseae (Fig. 3e and Extended Data Fig. 3c,d)7. Roughly 30 lepidotrichia are preserved. Similar to other finned tetrapodomorphs, rays are more robust anteriorly and more gracile posteriorly and rays are more terminally positioned on the posterior side7.

The humerus is boomerang-shaped and lacks numerous characteristic elpistostegalian features, notably a humeral ridge and associated foramina, ectepicondylar process, prominent entepicondyle and distinct articular surfaces for the ulna and radius (Fig. 3f–k). The ulna lacks a postaxial process and distally would have articulated with the intermedium and ulnare. The fin is gracile as compared to other elpistostegalians. The anteroposterior width of the humerus is narrower than the humeri of T. roseae5 and E. watsoni9 and more similar to P. rhombolepis3. The shallow dorsoventral depth of the fin might reflect compression; however, articular surfaces of the ulna and radius are similar in their geometry to three-dimensionally preserved specimens of T. roseae specimens (NUFV 108, 109, 110), suggesting that morphology was narrow in life (Supplementary Discussion).

Scalation

Scales are preserved from the trunk, including dorsal midline and flank, the pectoral fin and the lateral line series (Extended Data Fig. 4). Scalation is broadly similar to other finned elpistostegalians7,9,22. Scales are rhomboid in shape with the free surface sculpted and a smooth internal surface that often bears a ventral keel (Extended Data Fig. 4a–c). On the trunk, scale rows extend posterolaterally from the dorsal midline, with individual scales partially covering the scale that follows in the row and also the scale of an adjacent posterior row (Extended Data Fig. 1d,e). Pectoral fin scales are smaller than those of the flank and show variation in their morphology (Extended Data Fig. 4f–m). Lateral line scales are preserved from the left flank and show a completely enclosed tube with anterior suprascalar and posterior infrascalar pores enlarged relative to the diameter of the canal, and a small pore midway along the length of the scale connecting the canal to the external environment (Extended Data Fig. 4n–r).

Phylogenetic relationships

The phylogenetic position of Q. wakei was analysed by maximum parsimony and undated Bayesian approaches, which were applied to a matrix of 13 taxa and 125 characters primarily assembled from previous publications (see Supplementary Information for detailed description of phylogenetic data)9,23,24. Both methods robustly recover Q. wakei as crownward to P. rhombolepis and, thus, as an elpistostegalian closely related to limbed tetrapods (Fig. 4). The analyses differ in their relative placement of Q. wakei, T. roseae, E. watsoni; a strict consensus tree of the 28 shortest trees recovered from maximum parsimony analyses shows an unresolved polytomy, whereas Bayesian analysis finds weak support for a sister relationship between Q. wakei and T. roseae with E. watsoni positioned more crownward. This is similar to other recent phylogenetic analyses of stem tetrapods, which have robustly recovered Tiktaalik and Elpistostege as outgroups to digited forms, although support for their relative positions is not strong9,23,25.

Fig. 4: Phylogenetic analysis.
figure 4

a, Strict consensus tree from the maximum parsimony analysis with Bremer decay (D) and bootstrap support values. b, Majority-rule tree from undated Bayesian analysis with posterior probabilities. Both analyses recover a basal polytomy; Megalichthys is shown as the outgroup, consistent with other studies9,23,25,36.

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Discussion

Q. wakei reveals a combination of characters unique among stem tetrapods. The pectoral fin, lacking a postaxial process on the ulnare and showing accentuated hemitrichial asymmetry, is clearly elpistostegalian5,7. Yet, the morphology of the humerus is unlike others described. With the absence of a ventral ridge or ectepicondylar process and in possession of a general boomerang shape, it is more similar to the humerus previously attributed to the tetrapod, Elginerpeton pancheni10, than to any other Devonian taxon (Fig. 5). That specimen, GSM 104536, from Scat Craig in Scotland, is an isolated bone from a coeval deposit in Laurentia that generated debate as to whether it was from a tetrapod or whether it was even a humerus at all10,11,12,13,14. The similarity to Q. wakei suggests that GSM 104536 is indeed a humerus but belongs to a finned elpistostegalian, not a limbed tetrapod.

Fig. 5: Humeri at the fin-to-limb transition.
figure 5

For consistency of orientation between species, several specimens have been reflected, so that each is represented as being from the right side. Illustrations are based on previously published descriptions: Eusthenopteron28, Panderichthys2,3, Tiktaalik5, Elpistostege9, Acanthostega37, Ichthyostega30 and GSM 104536 (refs. 10,14). Abbreviations: ect, ectepicondyle; ent, entepicondyle; hr, humeral or ventral, ridge.

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The morphology of the Q. wakei humerus is distinctive among stem tetrapods. Indeed, the lack of muscular processes on the humerus for flexors and extensors at the shoulder and elbow, the terminal position of the facets for the radius and ulna, and the relatively large surface area of the fin web suggest that the fin of Q. wakei is less suited for walking, trunk lifting and station holding in water than it is for a range of swimming behaviours13. With its gracile form and lacking many of the known main osteological correlates of muscular attachment26, the pectoral fin of Q. wakei represents a strategy of controlling hydrodynamic forces not seen in other stem tetrapods. As these features are not seen in tristichopterids, osteolepids or rhizodontids, they probably arose as apomorphies in elpistostegalians.

The holotype of Q. wakei is estimated to be a standard length of 75 cm (calculated from the proportions of E. watsoni specimen MHNM 06-2067 (ref. 9) scaled to the length of the lower jaw), making it smaller than other described elpistostegalians. The ontogenies of Eusthenopteron foordi and T. roseae provide evidence that, despite its relatively small size, the unique humeral morphology of Q. wakei reflects phylogenetic signal and not developmental stage. E. foordi individuals are described spanning more than 40-fold variation in size27, and across a broad range of sizes uniformly retain a ventral ridge, entepicondylar process and orientations of facets for articulation with the radius and ulna28,29. T. roseae, which is known from humeri ranging twofold in size, show a similar pattern, preserving these features across this size range, although overall proportions might vary5,7. Thus, major ontogenetic shifts in limb skeletal anatomy of Ichthyostega and Acanthostega, indicated to correspond to aquatic subadults transitioning to more terrestrial adult lifestyles using appendage-based substrate support, are derived for limbed forms30. Finned tetrapodomorphs, by contrast, are predicted to show more minor changes in the proportions of endoskeletal, and potentially dermal, components of their paired fins7.

With two elpistostegalian genera now known from nearby localities in Canadian Arctic and others from Quebec9, Latvia31,32 and potentially Russia33, Australia34 and Scotland10, the group probably has a wide distribution by the Frasnian Stage of the Late Devonian. This broad biogeographic range, coupled with the morphological disparity revealed by Q. wakei, hints at a wider diversity of elpistostegalians than known at present, with the closest relatives of tetrapods adapting in new ways to benthic, littoral and open water habitats by the Late Devonian25,35.

Methods

Computed tomography scanning

µCT scans were collected at The University of Chicago’s PaleoCT scanning facility using a GE Phoenix v|tome|x 240 kV/180 kV scanner (http://luo-lab.uchicago.edu/paleoCT.html). Scan parameters are reported in Supplementary Table 1. µCT data were reconstructed with Phoenix Datos|x 2 (v.2.3.3), imported to VGStudio Max (v.2.2) for cropping and exportation as a 16-bit tiff stack. Tiff stacks were segmented and visualized in Amira v.20.2 (FEI Software). For some scans, to accommodate for computational challenges that arise from large file sizes, data were converted to 8-bit files for segmentation; in such cases, after segmentation the renderings were generated from the original 16-bit files. Animations were generated by exportation tiff stacks from Amira and then edited with Adobe Premiere (v.13.12). High-resolution versions of images from all figures are provided in the Supplementary Data.

Phylogenetic analyses

We investigated the phylogenetic position of Q. wakei using a phylogenetic data set of 13 taxa and 125 characters (see Supplementary Information for a detailed description of the phylogenetic matrix). All characters were treated as equally informative, and we assumed unordered evolution among states.

Maximum parsimony analyses were performed using PAUP* (v.4.0a168)38. The branch and bound method for searching tree space was used with no topological constraints. A total of 28 most-parsimonious trees were recovered (tree length 151 s). The trees are summarized as a strict consensus tree (Fig. 4) and as an Adams consensus tree (Extended Data Fig. 5a). Clade support was estimated using two approaches: Bremer decay values39, calculated with AutoDecay (v.5.06)40 and non-parametric bootstrapping, calculated in PAUP* with 500 replicates (Fig. 4 and Extended Data Fig. 5b). Apomorphies of nodes in the strict consensus tree were identified using the function ‘apolist’ in PAUP*, which returns characters optimized under both accelerated transformation (ACCTRAN) and delayed transformation (DELTRAN) conditions (Extended Data Fig. 5c).

Undated Bayesian analyses were performed using MrBayes (v.3.2.7a)41. Analyses were run for five million generations with four runs of four chains sampling every 5,000 generations and a burn-in of 20%. Megalichthys was designated as an outgroup, consistent with other studies9,23,25,36.

Convergence was assessed with diagnostics reported by MrBayes (average s.d. of split frequencies <0.02, potential scale reduction factors was 1, effective sample sizes >200). Results are summarized by a majority-rule consensus tree of postburn-in trees (Fig. 4).

For both maximum parsimony and Bayesian analyses, executable files, log files, and individual trees that contribute to the summary trees are included as supplementary files (Supplementary Data). Custom R (v.3.6.1) code used for the calculation of Bremer decay values and for visualization of phylogenies are available at https://github.com/ThomasAStewart/Qikiqtania. All code is archived at Zenodo (https://doi.org/10.5281/zenodo.6557684).

Reporting summary

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