An exceptional 3D coelacanth (Osteichthyes: Sarcopterygyii) from the Devonian of Australia and interpretation of fossil cranial endocasts

Coelacanths are a clade of the ‘lobe-finned’ fishes (Sarcopterygii), alongside lungfishes and tetrapods. They are iconic sarcopterygian fishes which peaked in diversity during the Carboniferous Period around 330 million years ago, but steadily declined with only two species alive today (Latimeria chalumnae and L. menadoensis). Most early coelacanth fossils are preserved as compressed specimens and details of their internal cranial anatomy were derived from serial wax sections of one taxon, Diplocercides, from the Late Devonian of Germany (Stensiö 1963). Here we present two specimens of the first exceptionally preserved 3D coelacanth from the Late Devonian (Frasnian) Gogo Lagerstätte of northern Western Australia, which reveals for the first time the cranial anatomy of an early coelacanth elucidated using modern micro-CT tomography (μCT).With advances in tomographic scanning techniques and software for 3D analysis and visualisation, more internal cranial anatomy and endocast data from fossil taxa are informing debates surrounding evolutionary problems of early jawed vertebrates (e.g. Lu et al. 2012; Clement & Ahlberg 2014; Giles et al. 2015; Dupret et al. 2017). Coelacanth endocasts are exceedingly rare. The shape of the cranial cavity housing the brain is only known in the extant genus, Latimeria (Millot and Anthony 1958) and one Devonian taxon Diplocercides (Stensiö 1963). The new Gogo coelacanth herein is the second fossil coelacanth for which the neurocranium is substantially known in 3D from which a cranial endocast can be generated. Despite having been known to science for over 125 years, the spatial relationship between the brain and braincase in a related group, the lungfishes, was only recently described and quantified (Clement et al. 2015). Through the application of virtual microtomography and 3D rendering software, brain-neurocranial spatial information can be visualised and analysed, and used to aid interpretation of fossil endocasts. We herein present the recent ”brain-warp” 32 neural reconstruction technique, as used on the Australian lungfish for both juvenile (Clement et al. 2016) and adult specimens (new data). Furthermore, we discuss how brain-endocranial spatial data changes through ontogeny, and how it can be used to help interpret endocasts of fossil sarcopterygians. Potential application and limitations of this technique for use on coelacanths and other taxa are discussed.