Author: ["Michael I. Coates","Robert W. Gess","John A. Finarelli","Katharine E. Criswell","Kristen Tietjen"]
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Abstract
The chimaeroids are one of the four principal divisions of the living jawed vertebrates and their evolutionary origins have been hard to discern; here, the study of a skull of the extinct shark Dwykaselachus shows that the chimaeroids nest among the once fairly common and widespread symmoriiforms. The chimaeroids, or Holocephali, are a major group of cartilaginous fishes obscurely related to sharks and rays—obscurely because chimaeroids are very distinctive in their appearance and are difficult to place in evolutionary context. They have eyes so large that the orbits distort the shape of the brain, and they have strange, distinctive teeth. Michael Coates et al. have been studying a skull of the extinct shark Dwykaselachus from the Permian of South Africa. Although the outside of the skull resembles a kind of prehistoric shark known as a symmoriiform, scanning reveals that the braincase is characteristically chimaeroid. That means that the chimaeroids can be placed among the once fairly common and widespread symmoriiforms, and represent the end-stage of a long period of symmoriiform evolution going back to the Devonian. Chimaeroid fishes (Holocephali) are one of the four principal divisions of modern gnathostomes (jawed vertebrates). Despite only 47 described living species1, chimaeroids are the focus of resurgent interest as potential archives of genomic data2 and for the unique perspective they provide on chondrichthyan and gnathostome ancestral conditions. Chimaeroids are also noteworthy for their highly derived body plan1,3,4. However, like other living groups with distinctive anatomies5, fossils have been of limited use in unravelling their evolutionary origin, as the earliest recognized examples already exhibit many of the specializations present in modern forms6,7. Here we report the results of a computed tomography analysis of Dwykaselachus, an enigmatic chondrichthyan braincase from the ~280 million year old Karoo sediments of South Africa8. Externally, the braincase is that of a symmoriid shark9,10,11,12,13 and is by far the most complete uncrushed example yet discovered. Internally, the morphology exhibits otherwise characteristically chimaeroid specializations, including the otic labyrinth arrangement and the brain space configuration relative to exceptionally large orbits. These results have important implications for our view of modern chondrichthyan origins, add robust structure to the phylogeny of early crown group gnathostomes, reveal preconditions that suggest an initial morpho-functional basis for the derived chimaeroid cranium, and shed new light on the chondrichthyan response to the extinction at the end of the Devonian period.Cite this article
Coates, M., Gess, R., Finarelli, J. et al. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature 541, 208–211 (2017). https://doi.org/10.1038/nature20806 