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.
Access optionsAccess options
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
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.).
This file contains Supplementary Figures 1-20, Supplementary Tables 1-8, Supplementary Notes 1-2 and Supplementary References – see Contents list for details.
About this article
Electrophysiological measures of temporal resolution, contrast sensitivity and spatial resolving power in sharks
Journal of Comparative Physiology A (2017)