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Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes


Acanthodians, an exclusively Palaeozoic group of fish, are central to a renewed debate on the origin of modern gnathostomes: jawed vertebrates comprising Chondrichthyes (sharks, rays and ratfish) and Osteichthyes (bony fishes and tetrapods)1,2,3,4,5,6. Acanthodian internal anatomy is primarily understood from Acanthodes bronni2,7,8,9,10 because it remains the only example preserved in substantial detail, central to which is an ostensibly osteichthyan braincase1,2,7. For this reason, Acanthodes has become an indispensible component in early gnathostome phylogenies1,11,12,13,14,15,16,17. Here we present a new description of the Acanthodes braincase, yielding new details of external and internal morphology, notably the regions surrounding and within the ear capsule and neurocranial roof. These data contribute to a new reconstruction that, unexpectedly, resembles early chondrichthyan crania. Principal coordinates analysis of a character–taxon matrix including these new data confirms this impression: Acanthodes is quantifiably closer to chondrichthyans than to osteichthyans. However, phylogenetic analysis places Acanthodes on the osteichthyan stem, as part of a well-resolved tree that also recovers acanthodians as stem chondrichthyans and stem gnathostomes. As such, perceived chondrichthyan features of the Acanthodes cranium represent shared primitive conditions for crown group gnathostomes. Moreover, this increasingly detailed picture of early gnathostome evolution highlights ongoing and profound anatomical reorganization of vertebrate crania after the origin of jaws but before the divergence of living clades.

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Figure 1: Acanthodes bronni cranial reconstruction.
Figure 2: Acanthodes braincase: details of braincase morphology.
Figure 3: PCO of early gnathostome character data.
Figure 4: Results of phylogenetic analysis, and early gnathostome braincases preceding conditions in modern jawed vertebrates.


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We thank R. Paton, Z. Johanson, M. Richter, J. Clack, D. Unwin and W. Simpson for specimen loans and collections access; M. Friedman, M. Brazeau, G. Hanke and J. Long for discussions on early gnathostome cranial anatomy. Financial support for this work was provided by Natural Environment Research Council (UK) studentship GT4/97/183ES, and grant DEB-0917922 from the National Science Foundation (USA) (to M.I.C.).

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S.P.D. completed the original data collection and initial analysis. S.P.D. and M.I.C. contributed to anatomical analysis, initiated the project and assembled the comparative data set. J.A.F. performed quantitative phenetic analyses. M.I.C. and J.A.F. contributed to phylogenetic analysis and figure preparation. All authors contributed to manuscript preparation.

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Correspondence to Michael I. Coates.

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The authors declare no competing financial interests.

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Davis, S., Finarelli, J. & Coates, M. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486, 247–250 (2012).

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