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ENDOSKELETAL STRUCTURE in CHEIROLEPIS (OSTEICHTHYES, ACTINOPTERYGII), an EARLY RAY-FINNED FISH by SAM GILES1, MICHAEL I

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ENDOSKELETAL STRUCTURE in CHEIROLEPIS (OSTEICHTHYES, ACTINOPTERYGII), an EARLY RAY-FINNED FISH by SAM GILES1, MICHAEL I [Palaeontology, Vol. 58, Part 5, 2015, pp. 849–870] ENDOSKELETAL STRUCTURE IN CHEIROLEPIS (OSTEICHTHYES, ACTINOPTERYGII), AN EARLY RAY-FINNED FISH by SAM GILES1, MICHAEL I. COATES2,3, RUSSELL J. GARWOOD4,5, MARTIN D. BRAZEAU6,ROBERTATWOOD7, ZERINA JOHANSON8 and MATT FRIEDMAN1 1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK; e-mails: [email protected], [email protected] 2Department of Organismal Biology and Anatomy, University of Chicago, 1027 E. 57th Street, Chicago, IL 60637, USA; e-mail: [email protected] 3Committee on Evolutionary Biology, University of Chicago, 1025 E. 57th Street, Chicago, IL 60637, USA 4School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK; e-mail: [email protected] 5The Manchester X-Ray Imaging Facility, School of Materials, The University of Manchester, Manchester, M13 9PL, UK 6Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, SL5 7PY, UK; e-mail: [email protected] 7The Joint Engineering and Environmental Processing Beamline, Diamond Light Source, The Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK; e-mail: [email protected] 8Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK; e-mail: [email protected] Typescript received 17 March 2015; accepted in revised form 30 May 2015 Abstract: As the sister lineage of all other actinopterygians, considered uncharacteristic of early actinopterygians, includ- the Middle to Late Devonian (Eifelian–Frasnian) Cheirolepis ing an uninvested dorsal aorta and imperforate propterygium, occupies a pivotal position in vertebrate phylogeny. Although and corroborate the early divergence of Cheirolepis within the dermal skeleton of this taxon has been exhaustively actinopterygian phylogeny. These traits represent conspicuous described, very little of its endoskeleton is known, leaving differences between the endoskeletal structure of Cheirolepis questions of neurocranial and fin evolution in early ray-finned and Mimipiscis. Additionally, we describe new aspects of the fishes unresolved. The model for early actinopterygian anat- parasphenoid, vomer and scales, most notably that the scales omy has instead been based largely on the Late Devonian display peg-and-socket articulation and a distinct neck. Col- (Frasnian) Mimipiscis, preserved in stunning detail from the lectively, these new data help clarify primitive conditions Gogo Formation of Australia. Here, we present re-examina- within ray-finned fishes, which in turn have important impli- tions of existing museum specimens through the use of high- cations for understanding features likely present in the last resolution laboratory- and synchrotron-based computed common ancestor of living osteichthyans. tomography scanning, revealing new details of the neuro- cranium, hyomandibula and pectoral fin endoskeleton for the Key words: computed tomography, Devonian, neurocrani- Eifelian Cheirolepis trailli. These new data highlight traits um, palaeontology. R AY- FINNED fishes (Actinopterygii) account for nearly et al. 2013). Further uncertainty surrounds the affinities half of living vertebrate diversity (Nelson 2006; Faircloth of Dialipina Schultze, 1968 and Ligulalepis Schultze, 1968, et al. 2013), but understanding of their early evolution is both originally described as actinopterygians (Ligulalepis: substantially incomplete. Despite a probable date of diver- Basden and Young 2001; Dialipina: Schultze and Cumbaa gence from Sarcopterygii of around 420–430 Ma (Zhu 2001) but now more commonly recovered as stem oste- et al. 2009; Broughton et al. 2013), no unequivocal actino- ichthyans (Friedman 2007; Brazeau 2009; Davis et al. pterygians are known from the Silurian. Scale taxa such 2012; Zhu et al. 2013; Dupret et al. 2014; Giles et al. as Lophosteus Pander, 1856, Andreolepis Gross, 1968 and 2015a). Of the handful of articulated actinopterygians Naxilepis Wang and Dong, 1989, once considered to be known from the Devonian, the majority are described primitive actinopterygians (Gross 1968; Schultze 1977; exclusively from their dermal skeletons, with only limited Janvier 1978; Wang and Dong 1989; M€arss 2001), are reports of endoskeletal remains (Gardiner and Schaeffer now thought to branch from the osteichthyan stem 1989). Two important exceptions are the early Frasnian (Botella et al. 2007; Friedman and Brazeau 2010; Zhu Mimipiscis toombsi (Gardiner and Bartram, 1977) and © 2015 The Authors. doi: 10.1111/pala.12182 849 Palaeontology published by John Wiley & Sons Ltd on behalf of The Palaeontological Association. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 850 PALAEONTOLOGY, VOLUME 58 Moythomasia durgaringa Gardiner and Bartram, 1977, (1864) description of micrometric squamation in the described in great detail from multiple three-dimensional, Carboniferous ‘palaeoniscoid’ Myriolepis clarkei Egerton, acid-prepared specimens from the Gogo Formation, Wes- 1864. The structure of the fins, shoulder girdle and skull tern Australia (Gardiner and Bartram 1977; Gardiner bones led Traquair (1875) to align Cheirolepis with taxa 1984; Choo 2011). Given its exceptional preservation, that are now assigned to Actinopterygii, a position uni- Mimipiscis Choo, 2011 has understandably become the versally accepted since. More recently, cladistic analyses key exemplar for the primitive actinopterygian conditions have consistently resolved Cheirolepis as the sister taxon (Gardiner 1984; Gardiner and Schaeffer 1989; Coates of all other ray-finned fishes (Gardiner 1984; Coates 1999; Cloutier and Arratia 2004; Gardiner et al. 2005; 1999; Cloutier and Arratia 2004; Gardiner et al. 2005; Friedman and Blom 2006; Long et al. 2008; Choo 2011; Friedman and Blom 2006; Friedman 2007; Friedman Friedman 2015). et al. 2007; Long et al. 2008; Brazeau 2009; Swartz 2009; Cheirolepis Agassiz, 1835 is the earliest occurring taxon Zhu et al. 2009; Choo 2011; Davis et al. 2012; Giles that can be unequivocally placed within Actinopterygii. It et al. 2015a). As only a small part of the endoskeleton of is represented by articulated specimens from the late Cheirolepis has been described (Fig. 1; Pearson and Wes- Eifelian of Scotland, the Givetian of Nevada (Reed 1992; toll 1979), very limited comparisons can be drawn with Arratia and Cloutier 2004) and the Frasnian of Canada, other well-known early actinopterygians such as Mimipi- as well as by scales from the Givetian of Germany (Gross scis and Moythomasia. This makes it impossible to under- 1973) and Eifelian–Givetian of Belarus, Latvia and Estonia stand the evolution of key actinopterygian characters (Blieck and Cloutier 2000; Mark-Kurik 2000; Luksevics during the early history of the group, particularly endo- et al. 2010). As part of his original description, Agassiz skeletal structures with a major impact on osteichthyan, (1835) erected three species on the basis of Scottish mate- and gnathostome, phylogeny more generally. The paucity rial: the type species C. trailli from Orkney, C. uragus of endoskeletal data outside of Mimipiscis and Moythom- from Gamrie and C. cummingae from Cromarty. A fur- asia has led to a situation where early actinopterygian ther three species were described by M’Coy (1848): relationships are investigated almost exclusively on the C. velox, C. macrocephalus (both from Orkney) and basis of dermal characters (Gardiner and Schaeffer 1989; C. curtus, from Lethen Bar. These species were subse- Cloutier and Arratia 2004; Friedman and Blom 2006; quently revised by Egerton (1860) and Traquair (1888), Long et al. 2008; Swartz 2009; Choo 2011), in stark con- with only C. trailli retained. Whiteaves (1881) first trast to the more comprehensive character sets used when reported Canadian material from the Frasnian Miguasha examining early gnathostome and sarcopterygian interre- Lagerst€atte and assigned it to the new species C. canaden- lationships (Zhu and Yu 2002; Friedman 2007; Brazeau sis. An additional species, C. schultzei, was erected by 2009; Zhu et al. 2009; Davis et al. 2012; Zhu et al. 2013; Arratia and Cloutier (2004) for specimens from the Giles et al. 2015a). Attempts to expand the actinoptery- Denay Limestone of Nevada, first reported by Reed gian character set (Coates 1998, 1999; Hamel and Poplin (1992) as Cheirolepis cf. C. canadensis. C. trailli and 2008; Giles and Friedman 2014) have increased the num- C. canadensis were comprehensively reviewed by Pearson ber of informative endoskeletal characters, but these are and Westoll (1979) and Arratia and Cloutier (1996). yet to be incorporated into many analyses due to the The affinities of Cheirolepis with bony fishes generally, large amounts of missing data associated with taxa and actinopterygians specifically, have not always been known principally from dermal material (Choo 2011). apparent. On the basis of its micromeric, non-overlap- Here, we use computed tomography (CT) to examine ping scales, Agassiz (1835) grouped Cheirolepis with endoskeletal anatomy in Cheirolepis. This study uses Acanthodes Agassiz, 1833 and Cheiracanthus Agassiz, material previously noted as preserving endoskeletal struc- 1835 in his Acanthodidae, a placement upheld by M’Coy tures, but which could only be described on the basis of (1855) and Egerton (1860). Dissenters from this view
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