The axial skeleton of the Devonian tetrapodIchthyostegaPer Erik Ahlberg1, Jennifer A. Clack2 & Henning Blom1,2

Ichthyostega was the first Devonian tetrapod to be subject to awhole-body reconstruction1–3. It remains, together with Acanthos-tega4, one of only two Devonian tetrapods for which near-complete postcranial material is available. It is thus cruciallyimportant for our understanding of the earliest stages of tetrapodevolution and terrestrialization. Here we show a new reconstruc-tion of Ichthyostega based on extensive re-examination of originalmaterial and augmented by recently collected specimens. Ourreconstruction differs substantially from those previously pub-lished and reveals hitherto unrecognized regionalization in thevertebral column. Ichthyostega is the earliest vertebrate to showobvious adaptations for non-swimming locomotion. Uniquelyamong early tetrapods, the presacral vertebral column showspronounced regionalization of neural arch morphology,suggesting that it was adapted for dorsoventral rather than lateralflexion.A complete skeletal reconstruction of Ichthyostega was first pre-

sented in 1955 (ref. 2) and republished with a more detailedcommentary in 1980 (ref. 3). Jarvik’s final reconstruction in 1996(ref. 5), differs from the previous version only in a few details such asthe relatively smaller shoulder girdle5. Both reconstructions show anundifferentiated presacral vertebral column, with similar posteriorly

inclined neural arches from the occiput to the pelvis, and posteriorto the pelvis a very gradual transition into the more slenderbut otherwise comparable neural arches of the tail (Fig. 1b). How-ever, the 1996 monograph5 contains evidence of problems with thisinterpretation: the detailed reconstruction of two vertebrae in Jar-vik’s Fig. 34 shows a neural arch morphology that is not representedanywhere in the complete postcranial reconstruction, and the sameapplies to the specimen photographs in plates 34:1 and 34:2. Recentwork has also cast doubts on the relative proportions of the parts ofthe skeleton6, and the paddle like hindlimb with its unique pattern ofseven digits has been described previously7.We have collated all available specimens of the vertebral column of

Ichthyostega, in order to arrive at a longitudinally near-completereconstruction (Fig. 1a). Other than a short gap in the proximal partof the tail—possibly spanned by a series of haemal arches—thereconstructed column is complete from occiput to tip of tail. Nosingle specimen shows a complete column. However, the neck, trunkand immediate postsacral region are represented by a series of partialspecimens, the preserved regions of which overlap substantially alongthe body axis (Fig. 2; see Methods and Supplementary Information).The correct relative sizes of the limb girdles and head can also beinferred from this material. All ribs are represented, complete to the

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Figure 1 | Reconstructions of Ichthyostegaand Acanthostega. a, New reconstructionof Ichthyostega, most anterior cervicalvertebrae (obscured by skull in lateral view)in grey outline. The reconstruction shows amaximally extended presacral column. Notethe revised dentition, orientation of thelimb elements and proportional differences(larger limb girdles and shorter tail)compared with b (ref. 5). The forearm isshown close to the beginning of thepowerstroke; the manus remains unknown.Neural spines of the cervical and anteriorcaudal regions are also unknown. Regionsare as follows: cervical (1), thoracic (2),lumbar (3) and sacral (4); unlabelled,caudal. Hindlimb based on ref. 7 and newobservations. b, Previous reconstruction ofIchthyostega from ref. 5. c, Lateral viewreconstruction of Acanthostega based onunpublished illustrations by Coates and refs4 and 16. Grey shading in a and c showswhat are considered to be the sacral neuralspines. Scale bars, 100mm.

1Subdepartment of Evolutionary Organismal Biology, Department of Physiology and Developmental Biology, Uppsala University, Norbyvagen 18A, 752 36 Uppsala, Sweden.2Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.

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tips apart from two or three in the mid-trunk region. Neural archesare known and essentially complete from the middle of the ribcage tothe tip of the tail, except for the previously mentioned gap in the tailbase. The intercentra are less well understood, as they are oftenconcealed by other elements, but examples can be seen at intervalsthroughout the column and these do not appear to differ greatlyfrom the published descriptions5. We have not observed any indis-putable pleurocentra, with the possible exception of some poorlyossified elements in MGUH (Geological Museum, Copenhagen) VP(Vertebrate Palaeontology) 6140 (Fig. 2). The shape of the neuralarch bases provides room for a series of rounded pleurocentra asreconstructed by Jarvik5, but it seems that these elements were in facteither unossified or absent. The most important gaps in our data arethe incomplete tail base and an almost total lack of informationabout the cervical neural arches.The vertebral column differs radically from Jarvik’s interpretation.

In complete contrast to previous reconstructions, we find that theneural arches are regionalized into discrete thoracic, lumbar, post-sacral and caudal morphologies. Thoracic neural arches are straightand posteriorly inclined; these seem to be the model for the uniformpresacral neural arches reconstructed by Jarvik5, but they are taller

and have rounded rather than square tips. Jarvik’s reconstructedmorphology appears to be based directly on MGUH VP 6115, inwhich the tips of the thoracic neural arches are all broken off at asingle horizontal plane. Posteriorly the thoracic arches grade rapidlyinto the upright, anteriorly concave arches of the lumbar region, andbecome progressively anteriorly inclined nearer the pelvis. Theneural arch of the sacral vertebra has the same procumbent mor-phology, but the first four postsacrals have broad fan-shaped archesthat effect a rapid transition in orientation from anteriorly toposteriorly inclined (Fig. 3). The latter morphology, slender andanteriorly concave in contrast to the thoracic neural arches, gradessmoothly into the caudal arch morphology. There is also variation inneural spine height, with the thoracic and postsacral vertebrae beingsignificantly taller than the posterior lumbar vertebrae. The differentspecimens agree well in general structure and show little individualvariation. The only significant exception is the lumbar columnfragment MGUH VP 6140 (Fig. 2c, d), that seems to lack the swifttransition from recumbent to procumbent neural arch orientationseen in, for example, MGUH VP 6054 (Fig. 3). However, MGUH VP6140 is too short to determine whether this really reflects a majordifference in morphology. The presence of broad flanged ribs in thespecimen demonstrates that it represents Ichthyostega rather thansome other taxon.In addition to the differences in neural arch shape, there is strong

regional variation in the development of muscle attachments on thearches. The thoracic, postsacral and caudal arches have no visiblemuscle scars, but some of the anterior lumbar arches carry largejagged outgrowths that must represent ossified attachments for themyosepta of the epaxial musculature. These can be seen in several

Figure 2 | Specimens of Ichthyostega. a, MGUHVP 6115 in left lateral viewwith forearm articulated as close to life position as practical (humerus andulna are separate elements, radius is impacted onto the ventral surface of thehumerus, attitude is slightly elevated from natural position). Note therelative size of the shoulder girdle corresponds with that in Fig. 1a but notFig. 1b. b, Thoracic region of above specimen viewed in right dorsolateralaspect to show backward sloping neural spines. c, MGUH VP 6140, isolatedlength of presacral column, whitened latex peel figured in reverse, anterior tothe left, showing alternating pattern of narrow and broad neural spines, thelatter with muscle scars and processes overlapping onto next most anteriorspine. Thoracic region flanged ribs seen anteriorly locate this specimen inthe column relative to other specimens. d, Interpretive drawing of c. Notethat the pleurocentra as interpreted by Jarvik have been indicated, thoughwe are not convinced of their identity. e, Schematic representation of keyMGUH VP specimens used for the reconstruction. Scale bars, 10mm. Theabbreviation scapcor indicates scapulocoracoid.

Figure 3 | MGUH VP 6054, vertebral column, pelvis and hindlimb ofIchthyostega. a, Specimen, in left dorsolateral view, partially assembledshowing presacral column as bone, and pelvis, postsacral neural spines andparts of the hind limb as natural mould. Black outline shows the position ofthe block figured in c and d. b, Composite drawing compiled fromobservations of all parts of the specimen, not all of which are figured here orare easily visible in photographs. It shows the neural spines, presacral ribsand parts of the pelvis. The left ilium is indicated by dark shading and theright (concealed) ilium is indicated by light shading. Note that the presacralneural spines are deflected to the right, whereas the postsacral spines aredeflected to the left, with the point of dislocation indicated by a heavy line.c, Block preserving the postsacral region of neural spines, postsacral ribs andthe left postiliac process. d, Whitened latex peel of c, shown in reverse forease of comparison with b. c is a dome-shaped block making the elementsappear foreshortened. The peel has been relieved to reduce the three-dimensionality. The starred neural spine in b and d is the broadest, but in d itcan clearly be seen to be postsacral. The sacral spine is probably thatimmediately posterior to the break. Scale bars, 10mm.

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specimens, such as MGUH VP 6139 (Fig. 4) and 6054, but they arebest preserved in MGUH VP 6140 (Fig. 2). In this individual theattachments are strongly developed on every other neural arch, andthese bony outgrowths extend forward to overlap the next anteriorneural arch laterally. We discuss the functional implications of thispattern below. Acanthostega also has ossified muscle attachments onsome neural spines (Fig. 1c), but these occur throughout thepresacral column and do not overlap adjacent vertebrae4.The ribmorphology of Ichthyostega differs from that reconstructed

by Jarvik, although less dramatically than the neural arches. MGUHVP 6139 shows a series of four cervical ribs that lack flanges andbecome progressively shorter towards the anterior; whereas Jarvikreconstructed no neck at all and showed the longest thoracic ribattaching to the first vertebra (Fig. 1b). The longest of the flangedthoracic ribs is the sixth or seventh (not the first), and the posteriortransition into short unflanged lumbar ribs is sharper than in the oldreconstructions. There are also fewer flanged ribs in the series thanreconstructed by Jarvik. Together, these features give quite a differentoutline to the ribcage. The thoracic ribs are only gently curved, andthe distal ends of the left and right ribs often meet in the midline(for example in MGUH VP 6115 and 6139). This considered along-

side the comparably modest curvature of the cleithrum andscapulocoracoid (for example, Jarvik’s plate 45:4, 5; ref. 5), suggestsan upright oval body cross-section. The curiously large postsacralribs were figured accurately by Jarvik.Our body reconstruction (Fig. 1a) incorporates recent data on the

morphology and orientation of the limbs6–8 as well as a reassessmentof the dentition. Its distinctive morphology differs greatly not onlyfrom Jarvik’s reconstructions but from other early tetrapods such asAcanthostega4 (Fig. 1c). Ichthyostega presents the earliest example of adifferentiated presacral vertebral column in a tetrapod—cervical,thoracic, lumbar, sacral and caudal regions can be distinguished.This morphology further implies a differentiation of the axialmusculature unexpected in such a primitive tetrapod, and raisesquestions about the locomotory mechanics and mode of life ofIchthyostega.We hypothesize that this unique morphology reflects a loss of

horizontal flexion in the presacral column and possibly the acqui-sition of limited vertical flexion in the lumbar region. The deeplyoverlapping thoracic ribs probably did not permit either lateral ordorsoventral flexion of the thorax. In the lumbar region, the presenceof expanded muscle scars that cause some neural arches to overlapadjacent ones (Fig. 2c, d) argues against lateral flexion, andalso demonstrates the presence of a strongly developed epaxialmusculature. The procumbent orientation of the lumbar neuralspines may be an indication of vertical flexion, although the preciserelationship between spine orientation and flexion remains uncleareven inmammals (some of which have a remarkably Ichthyostega-likepattern of neural spine morphologies), and requires further investi-gation. The zygapophyses of Ichthyostega are rarely well preserved,but MGUH VP 6139 (Fig. 4d) shows them to be steeply oriented andcertainly not expanded horizontally. We infer that the whole pre-sacral vertebral column was incapable of significant lateral flexion,but that a modest degree of vertical flexion may have occurred in thelumbar region.If we are correct and the axial morphology did not permit lateral

trunk flexion in Ichthyostega, two gaits seem theoretically possible:‘walking’ with diagonally synchronized limb movements and rigidelevated trunk; and a bilaterally symmetrical ‘shuffling’ or‘inchworm’ movement, with vertical flexion of the lumbar regionas part of the powerstroke for the forelimb and recovery stroke for thehindlimb, and lumbar extension contributing to the converse. Bothmay have been used at different times. These locomotory hypothesesremain to be tested by further analysis of limbs and joint surfaces.Acanthostega, in contrast to Ichthyostega, had a laterally flexible

trunk and appears better adapted for tail-propelled swimming. Thelimbs and limb girdles of Ichthyostega are also proportionately muchlarger than those of Acanthostega (Fig. 1a, c). Coupled with differ-ences in the dentition (recurved sectorial teeth in Ichthyostega,conical piercing teeth in Acanthostega) and body size (Ichthyostegais about 30% larger than Acanthostega), plus the fact that the twogenera rarely occur together in the same locality within the Green-land sediments9, this points to a clear ecological separation betweenthese primitive tetrapods.Despite its peculiarities, which includes an ear specialized for

underwater hearing10, Ichthyostega is not completely unique. Aribcage of flanged ribs followed by a more flexible lumbar region isalso seen in the Early Carboniferous stem-group tetrapods Pederpesand Whatcheeria11–13. Recent phylogenetic analyses placing Ichthyos-tega immediately below Tulerpeton and these Carboniferous forms14

suggest that such similarities may be homologies reflecting thegradual terrestrialization of the tetrapod stem group. However, thezygapophyses of Pederpes are horizontal13 in contrast to the steeplyangled orientation in Ichthyostega, suggesting a very different modeof locomotion in the two. The variable neural spine heights ofIchthyostega (although not the diversified neural spine mor-phologies) are matched by the Carboniferous crown-group tetrapodProterogyrinus15 and may relate to terrestrial locomotion. Gradually,

Figure 4 | MGUH VP 6139, skull, shoulder girdle, thoracic and presacralregion of Ichthyostega. a, Specimen part-assembled with skull and thoracicregion placed on the natural mould of the right side, in which rib heads andsome neural arches are just visible beneath. b, Whitened latex peel shown inreverse of basal block shown in a. White arrows indicate three neural archesenlarged in d. c, Composite drawing incorporating information from bothright and left sides, showing position of skull, shoulder girdles, rib cage andneural spines. Arrows indicate three neural arches enlarged in d. Note thesteeply inclined angle of the zygapophyes and the alternation of spineanteroposterior length. d, Enlarged neural arches, aligned with ventral partof each arch vertical to highlight differences in neural spine morphology.Scale bars, 10mm.

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Ichthyostega is emerging from its iconic but isolated position as ‘theearliest tetrapod’ to occupy a recognizable place in the developingpicture of early tetrapod diversity and evolution. Ichthyostega appearsto be an early and ultimately unsuccessful attempt at adapting thetetrapod body plan for terrestrial locomotion, divergent but not veryremote from the lineage that successfully solved these adaptiveproblems and ultimately gave rise to all living tetrapods.

METHODSSpecimens and techniques used to compile the reconstruction. All specimenscarryMGUHprefixed numbers plus either VPor f.n. (field number).Most of thespecimens exhibit some degree of vertical compression because of sedimentarycompaction (resulting, in morphological terms, in dorsoventral, lateral oroblique compression depending on the orientation of the body within thesediment) but none shows any sign of oblique shear. The observed differences inneural spine orientation relate directly to differences in morphology, and are notaccompanied by distortions in adjacent structures such as the ventral parts of theneural arches or the rib heads. We are thus confident that the morphologicalvariation is real and not a product of post-mortem distortion by geologicalprocesses.VP 6054. Block containing the natural mould of a presacral column, pelvicgirdle, both hindlimbs, sacral region and first few caudal vertebrae (plate 34 inref. 5). The specimen was broken and somewhat twisted in the immediatelypresacral region. In many parts, some etched with dilute HCl and peeled withlatex. Sederholm Bjerg, profile 4, 1,174m, collected in 1934.VP 6109. Block showing one hindlimb, parts of the pelvis and postsacral columnand complete caudal series (plates 37, 38, 65 and 66 in ref. 5). N. Celsius Bjerg,topmost coulisse, collected in 1948.VP 6138. Block containing obliquely compressed skull, shoulder girdles, bothforelimbs and parts of their carpal regions, parts of cervical region (as naturalmoulds or embedded bone), complete thoracic region and one associatedhindlimb. In many parts, with skull, both forelimbs and ribcage partly etchedand peeled as above. Sederholm Bjerg, profile 4, 1,174m, collected in 1949.VP 6139. Block containing laterally compressed partial skull and shoulder girdle,cervical series (mostly embedded in matrix), thoracic region and part ofpresacral column (plate 24 in ref. 5). In many parts, with main block etchedand peeled as above. Sederholm Bjerg, profile 4, 1,174m, collected in 1949.VP 6140. Length of isolated vertebrae that was etched and peeled as above (partfigured in plate 34.3 in ref. 5). Sederholm Bjerg, profile 4, 1,174m, collected in1949.VP 6115. Posterior half of skull, both forelimbs and shoulder girdles, somewhatcompressed, left with radius and ulna in articulation, thoracic region (plates 42,45, 46, 51, 53 and 54 in ref. 5). Associated with a hindlimb and partial columnthat is probably not from the same individual. S. SmithWoodward Bjerg, 370m,collected in 1948.VP 6156. Complete tail in part and counterpart (plate 40 in ref. 5). SederholmBjerg, profile 4, 1,174m, collected in 1949.f.n. 1394. Hindlimb, pelvis and part of postsacral and caudal series. Surfaceprepared mechanically. Figured in ref 7. Gauss Halvø, 90m, collected in 1987.f.n. 300. Two individuals in the same block, one with humerus, shoulder girdles,complete column and ribs up to sacral region, pelvis and hindlimb. Partly surfaceprepared mechanically. S. Celsius Bjerg, 500m, collected in 1998.

Note that 6054, 6138, 6139, 6140 and 6156 derive from the same ‘in situ’horizon.

Received 24 February; accepted 6 June 2005.

1. Jarvik, E. On the fish-like tail in the ichthyostegid stegocephalians. Meddelelserøm Grønland 114, 1–-90 (1952).

2. Jarvik, E. The oldest tetrapods and their forerunners. Sci. Mon. 80, 141–-154(1955).

3. Jarvik, E. Basic Structure and Evolution of Vertebrates Vol. 1 (Academic, NewYork, 1980).

4. Coates, M. I. The Devonian tetrapod Acanthostega gunnari Jarvik: postcranialanatomy, basal tetrapod relationships and patterns of skeletal evolution. Trans.R. Soc. Edinb. Earth Sci. 87, 363–-421 (1996).

5. Jarvik, E. The Devonian tetrapod Ichthyostega. Fossils and Strata 40, 1–-206(1996).

6. Coates, M. I. & Clack, J. A. in Studies on Early Vertebrates (eds Arsenault, M.,Lelievre, H. & Janvier, P.) 373–-388 (Bulletin du Museum national d’histoirenaturelle, Paris, 1995).

7. Coates, M. I. & Clack, J. A. Polydactyly in the earliest known tetrapod limbs.Nature 347, 66–-69 (1990).

8. Clack, J. A., Blom, H. & Ahlberg, P. E. New insights into the postcranial skeletonof Ichthyostega. J. Vert. Paleontol. Abstr. 23, 41A (2003).

9. Blom, H., Clack, J. A. & Ahlberg, P. E. Localities, distribution and stratigraphicalcontext of the Late Devonian tetrapods of East Greenland. Meddelelser ømGrønland Geosci. 43, 4–-50 (2005).

10. Clack, J. A. et al. A uniquely specialised ear in a very early tetrapod. Nature425, 65–-69 (2003).

11. Clack, J. A. An early tetrapod from Romer’s Gap. Nature 418, 72–-76 (2002).12. Lombard, R. E. & Bolt, J. R. A new primitive tetrapod, Whatcheeria deltae, from

the Lower Carboniferous of Iowa. Palaeontology 38, 471–-494 (1995).13. Clack, J. A. & Finney, S. M. Pederpes finneyae, an articulated tetrapod from the

Tournaisian of western Scotland. J. Syst. Palaeontol. 2, 311–-346 (2005).14. Ruta, M., Coates, M. I. & Quicke, D. L. J. Early tetrapod relationships revisited.

Biol. Rev. 78, 251–-345 (2003).15. Holmes, R. The Carboniferous amphibian Proterogyrinus scheeli Romer, and the

early evolution of tetrapods. Phil. Trans. R. Soc. Lond. B 306, 431–-524 (1984).16. Clack, J. A. The dermal skull roof of Acanthostega, an early tetrapod from the

Late Devonian. Trans. R. Soc. Edinb. 93, 17–-33 (2002).

Supplementary Information is linked to the online version of the paper atwww.nature.com/nature.

Acknowledgements We thank the Geological Museum and University ofCopenhagen for access to their Ichthyostega material, and G. Cuny and D. Harperfor their help and support during our visits. We would also like to acknowledgethe late E. Jarvik and S. Bendix-Almgreen, both of whom gave us access tospecimens then in their care. Preparation of MGUH f.n. 300 was carried out byS. Finney. M. Coates kindly permitted us to use one of his unpublishedAcanthostega reconstructions for part of Fig. 1c. Most of our project wassupported by an NERC grant with additional funding for H.B. provided by theIsaac Newton Trust Fund and by a project grant and salary fromVetenskapsradet (the Swedish Research Council) for P.E.A.

Author Information Reprints and permissions information is available atnpg.nature.com/reprintsandpermissions. The authors declare no competingfinancial interests. Correspondence and requests for materials should beaddressed to P.E.A. (per.ahlberg@ebc.uu.se).

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